RADIO TELEMETRY APPLICATIONS MANUAL

RADIO TELEMETRY

APPLICATIONS MANUAL

7000 0000

© Copyright Wood & Douglas 1996

Version: 2.06Issue Date: October 1999

LATTICE HOUSEBAUGHURST ROAD

BAUGHURST, TADLEYHAMPSHIREUK RG26 5LP

Tel: +44 (0)118 981 1444 Internet :www.woodanddouglas.co.ukFax:+44 (0)118 981 1567 E-Mail : [email protected]

THIS PAGE IS INTENTIONALLY BLANK

This manual has been written with the intention ofproviding the non-specialist to the world of radio

communications with an insight into this sometimescomplex subject.

Whilst this manual is not intended to be a source ofreference for radio engineering practise, Wood & Douglashope that with the aid of plain language and supportingillustrations some of the more frequently misunderstoodconcepts in the field of radio telemetry may be clarified.

CONTENTS

1.0 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1 THE COMPANY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2.0 TYPICAL APPLICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.1 OPTIONS FOR THE OEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.1.1 OWN DEVELOPMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.1.2 MODIFICATION OF `OFF-THE-SHELF' WALKIE TALKIES . . . . . . . . . . . . 42.1.3 OEM MODULES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.1.4 CUSTOM MODULES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3.0 UNDERSTANDING THE TECHNICAL TERMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73.1 BASIC SYSTEM BLOCKS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.1.1 CARRIER WAVES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73.1.2 AM MODULATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83.1.3 AM DEMODULATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83.1.4 FM MODULATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83.1.5 FM DEMODULATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83.1.6 BANDWIDTH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.2 SOME OTHER RF TERMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.2.1 TRANSMIT ENABLE TIME (POWER ON) . . . . . . . . . . . . . . . . . . . . . . . . 113.2.2 FREQUENCY STABILITY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.2.3 SELECTIVITY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.2.4 SPURIOUS OUTPUT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.2.5 ERROR CORRECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.2.6 MEASUREMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4.0 HOW DOES RADIO WORK? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134.1 BASIC RF PROPAGATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134.2 WHICH FREQUENCY DO I USE? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144.3 HOW FAR CAN I TRANSMIT? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154.4 FACTORS AFFECTING PERFORMANCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164.5 CALCULATION OF RANGE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

4.5.2 DETERMINE THE RADIO HORIZON . . . . . . . . . . . . . . . . . . . . . . . . . . . 174.5.3 FIND THE TRANSMITTER POWER FACTOR . . . . . . . . . . . . . . . . . . . . . 194.5.4 ANTENNA HEIGHT FACTOR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204.5.5 DETERMINE TERRAIN CORRECTION FACTOR . . . . . . . . . . . . . . . . . . . . 214.5.6 CALCULATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

5.0 WHICH ANTENNA SHOULD I USE? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235.1 NON-DIRECTIONAL ANTENNA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

5.1.1 HALF-WAVE DIPOLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245.1.2 QUARTER-WAVE WHIP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245.1.3 HELICAL STUB ANTENNA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255.1.4 CO-LINEAR ANTENNA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

5.2 PORTABLE ANTENNA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265.2.1 GROUND PLANES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265.2.2 END-FED DIPOLE FOR PORTABLES . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

5.3 DIRECTIONAL ANTENNA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275.3.1 THE YAGI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275.3.2 MIXED ANTENNA WORKING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

5.4 OTHER ANTENNA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295.4.1 PLATE ANTENNA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295.4.2 DISH ANTENNA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

5.5 THE DECIBEL (dB) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305.5.1 ANTENNA GAIN AND THE dB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

6.0 CONNECTING THE ANTENNA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 336.1 FEEDER CABLE TYPES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

7.0 USING RADIO LINKS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357.1 REGULATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357.2 APPLYING W&D MODULES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357.3 INTERFACE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

7.3.1 ANALOGUE INTERFACE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367.3.2 DIGITAL INTERFACE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

7.4 MODEM OPERATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367.5 GMSK SIGNALS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367.6 RECEIVER OUTPUT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377.7 PHYSICAL CONNECTIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377.8 ANTENNA CONNECTIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377.9 VIBRATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 387.10 POWER SUPPLY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 387.11 ENCLOSURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 397.12 OUTPUTS AND INPUTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

8.0 INTERFERENCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 418.1 INTERFERENCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 418.2 NON RADIO SOURCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 418.3 INTERNAL SOURCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

8.3.1 REFERENCE CLOCKS IN ASSOCIATED EQUIPMENT . . . . . . . . . . . . . . . 418.3.3.2 RADIO RELATED SOURCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 428.3.3 POWER SUPPLY RIPPLE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

8. 4 IN BAND INTERFERENCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 428.4.1 ON CHANNEL INTERFERENCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 438.4.2 ADJACENT CHANNEL INTERFERENCE . . . . . . . . . . . . . . . . . . . . . . . . . 438.4.3 INTERMODULATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 448.4.4 DE-SENSITISATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

8.5 OUT OF BAND EFFECTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 468.5.1 IMAGE REJECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

8.6 SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

9.0 MODULATIONS AND MODEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 499.1 WHAT TYPE OF SIGNALS CAN I TRANSMIT? . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

9.9.1.1 ANALOGUE DATA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 499.1.2 DIGITAL DATA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

9.2 USE OF MODEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 519.2.1 MODEM TYPES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 529.2.2 SINGLE OR TWO WAY COMMUNICATION . . . . . . . . . . . . . . . . . . . . . . 52

9.2.3 RELIABILITY AND SPEED OF TRANSMISSION . . . . . . . . . . . . . . . . . . . 529.2.4 INTELLIGENCE OF USER INTERFACE . . . . . . . . . . . . . . . . . . . . . . . . . . 539.2.5 TYPE OF DATA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 539.2.6 INTELLIGENT MODEM FUNCTIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . 549.2.7 INTERFACING REQUIREMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 569.2.8 HAYES© COMPATIBLE MODEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

10.0 OPERATING MODES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5710.1 SIMPLEX AND DUPLEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

10.1.1 DUPLEX RADIO LINKS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5810.1.2 SEMI DUPLEX RADIO LINKS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5810.1.3 SIMPLEX RADIO LINKS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

11.0 LICENCING INFORMATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

12.0 GLOSSARY OF TERMINOLOGY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

INDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

APPENDIX A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .68

InsertionsFIGURES

Figure 1 Example of a carrier wave . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Figure 2 AM modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Figure 3 FM modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Figure 4 Bandwidth & effects of filtering (25KHz channelling) . . . . . . . . . . . . . . . . . . . . . . . . . 10Figure 5 Line of sight communication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Figure 6 UHF telemetry band channel vs. frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Figure 7 Sites below the radio horizon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Figure 8 Determining the radio horizon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17Figure 9 Transmitter power factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18Figure 10 Antenna height factor nomogram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19Figure 11 Terrain correction factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20Figure 12 Calculating the range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21Figure 13 Fundamental half wave construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24Figure 14 Construction and effect of Co-linear dipole . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25Figure 15 Vertically polarised Yagi antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27Figure 16 Outline of typical plate antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28Figure 17 Basic construction of typical dish antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28Figure 18 Attenuation effect when using feeder cables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31Figure 19 Interference caused by third order intermodulation . . . . . . . . . . . . . . . . . . . . . . . . . . . 42Figure 20 Simple illustration of serial vs. parallel data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48Figure 21 The most commonly used form of RS232 interface . . . . . . . . . . . . . . . . . . . . . . . . . . 48

TABLES

Table 1 Factors affecting radio link performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Table 2 Typical gains of UHF Yagi antennas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30Table 3 Decibel conversion table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30Table 4 RF cable characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

EQUATIONS

(1) Calculation for occupied bandwidth - AM modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9(2) Calculation for occupied bandwidth - FM modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9(3) Range scale calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20(4) decibel calculation for power values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 (5) decibel calculation for voltage values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29(6) Lower third-order intermod . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42(7) Upper third order intermod . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Wood & Douglas Ltd Applications Manual INTRODUCTION

1

Section 1Introduction

Wood & Douglas Ltd is an independent UK company dedicated to the design and manufacture of highquality RF designs for the telemetry, security and broadcast markets world-wide.

The information given in this manual is Copyright Wood & Douglas and is available to Wood & Douglascustomers in response to the expressed need for applications information regarding the range of productssupplied. The intention is therefore to provide the reader with a basic understanding of RF terminology plusan insight into the use of Wood & Douglas products in general. This will enable those users with little RFexperience to gain maximum benefit from the available hardware. (A glossary of RF terms is included atthe rear of this manual).

The contents of this manual must not be copied or reproduced without the permission of Wood & Douglas.The company reserves the right to amend or change specifications of its products without prior notice inaccordance with the company policy of continued product improvement.

Wood & Douglas are attempting (as far as possible) to make the acquisition and use of radio links as simpleas purchasing any other OEM component. This manual is intended to answer the most commonly askedquestions, and to unravel some of the underlying 'mysteries' of RF technology generally.

Any comments or suggestions from customers regarding ways in which we could improve this manualwould be most welcome.

1.1 THE COMPANY

Wood & Douglas Ltd are an independent UK manufacturer and design facility specialising in the field ofradio frequency (RF) engineering. Currently located in Baughurst near Aldermaston, the company employsaround 35 full-time members of staff engaged in the process of design, manufacture and support of itsradio-based products.

Having begun trading in 1976, Wood & Douglas catered for the needs of the radio amateur by providinga range of products designed specifically for them. From small beginnings the company began to grow andlater moved to premises on Youngs Estate near the AWE in Aldermaston. There they became involved withindustrial and commercial customers whose requirements were for radio-based solutions to communicationproblems. With this move into commercial design and manufacturing, and with growing awareness of thecompany by potential industrial users, Wood & Douglas soon found themselves supplying RF hardware tothe broadcast industries, notably the Independent Broadcasting Authorities who were updating andequipping new studios and OB (outside broadcast) facilities.

The company has been involved in the design and manufacture of such diverse products as studio talk-backequipment (destined for the BBC), as well as major turnkey projects for NATO and various foreignbroadcasting authorities. The purpose-built headquarters which the company has occupied at Lattice Housesince the beginning of 1990 now provides the base from which the world-wide manufacturing and salesof Wood & Douglas products is managed.

Using state-of-the-art technology, including CAD design and surface-mount printed circuit board assembly,the company now has a wide range of designs for telemetry, broadcast and security communication linksup to 10GHz, plus an increasing number of 'standard' modules to suit the diverse and sometimes unusualcustomer requirements.

INTRODUCTION Wood & Douglas Ltd Applications Manual

2


Wood & Douglas Ltd Applications Manual TYPICAL APPLICATIONS

3

Section 2Typical applications

The actual number of applications for which some form of radio-based link could be utilised is simply toovast to numerate. The benefits to be gained from the use of a 'wireless' transmission media between two(or more) points are enormous.

The primary attraction of any radio device lies in the ability to send and receive information without theneed for costly inter-connecting wires, thereby offering the advantages of mobility plus extended range ofoperation.

This ability makes the use of radio links especially attractive to the industrial user, where the use of wiresor cables could prove a hindrance and even dangerous in certain circumstances.

The ability to be fully mobile in use, or simply to provide temporary installations, enhances the attractionof radio solutions still further. The facility for adding to existing installations also contributes to the overallcost effectiveness of the true 'wireless' solution to communication problems.

Modern technology has meant that the size of most items of electronic equipment (including radios), hasbeen decreasing over the years. The following industries have all benefited from the purchase of Wood &Douglas RF designs in the past:

Radio & TV broadcast, the water supply industry, crane manufacturers, hydrographic survey, securitycompanies, industrial process control, burglar alarm manufacturing, the stock market, sporting gamesmanufacturers, electronic displays and scoreboards, SCADA and telemetry manufacturers, pleasure boatsecurity, load cell manufacturers, automated warehouse equipment, the coal industry, offshore dredgingcompanies, meteorology equipment manufacturers, oil and gas companies, bomb disposal specialists,theatre companies plus many others.

Wood & Douglas radio links may be used to convey analogue signals (tones, frequencies etc) or with theaddition of simple circuitry they may be adapted to work with digital signals such as the now familiarRS232 serial data ports found on most PCs and other small computers.

In fact almost any type of signal can be accommodated with the provision of suitable interface circuitrybetween the user and the transmitter and the output of the radio receiver itself. The most common typesof signal encountered are 4/20mA current loops, pulse outputs (such as found in meteorological equipment)and voltage outputs.

Nearly all of the radio modules supplied by Wood & Douglas have provision for either analogue or digitalinputs to be selected by the user. Radio links are often used to control other items of machinery, forexample cranes or electrically driven vehicles. As an alternative the use of Wood & Douglas ̀ off-the-shelf'designs may provide complete remote control systems for video cameras, robots etc.

As well as data communications and control applications, the company offers a range of standard designsfor specialised applications where high quality video transmission is required. Typically operating in the1GHz to 3GHz and 10GHz range, these products offer a cost-effective and compact solution for a largenumber of specialist applications, mainly in the military and security markets. (Please contact the salesoffice for more information).

TYPICAL APPLICATIONS Wood & Douglas Ltd Applications Manual

4

2.1 OPTIONS FOR THE OEM

When developing OEM products employing radio links the user must decide which of the following routesto take in bringing the finished product to the market place:-

1) Develop the product entirely `in-house'.2) Make use of an existing 'off-the-shelf' walkie-talkie design, and add his own modifications.3) Buy standard OEM radio modules.4) Commission his own custom design thereby fulfilling his needs exactly.

Taking these options in more detail:-

2.1.1 OWN DEVELOPMENT

Radio is still regarded by some as a `black art'. As the frequencies involved move up into the TV bandsand even higher into the world of satellite communications, this pre-conception becomes even stronger.Manufacturers wishing to provide their own RF links are faced not only with the prospect of recruitinghighly specialised RF engineers, but also the provision of expensive test equipment, thereby depriving theirbusiness of much needed capital.

Having completed the initial design & development, other equally costly skills are required in order to bringthe product into a pre-production stage. Having reached this stage, the design must be capable ofreproducible and acceptable volume production, i.e., the build quality must not vary and the engineersintended design must be easily achievable under production constraints. Any variations in build standardno matter how small (even a difference in lead length), can affect the dynamic performance of the finishedarticle when dealing with high frequency radio circuits.

The foregoing considerations tend to preclude most commercially orientated companies from undertakingtheir own RF design and manufacture. These companies usually prefer to seek the services of firms suchas Wood & Douglas.

2.1.2 MODIFICATION OF `OFF-THE-SHELF' WALKIE -TALKIES

The use of bought-in walkie-talkies, either due to cost or time considerations, may appear to offer theperfect solution to manufacturers seeking a suitable radio link for their own products. At first glance itwould seem to bypass most, if not all, of the problems regarding suitably-skilled personnel. In practicehowever, RF engineering effort (plus associated test equipment) would still be required in order to carry outthe modifications necessary in order to customise the product. Remember that the walkie-talkie will havebeen designed to meet the needs of customers requiring hand-held voice communications. This is anapplication which is not closely related to data telemetry.

The case will most probably have been designed for hand-held use and will not prove easily adaptable forincorporation into other housings or modules. It will offer facilities of little or no interest to the OEM user,and will undoubtedly consume more power than would otherwise be required by a dedicated OEM telemetrymodule.

Finally, and perhaps most importantly, the cannibalised radio circuitry would no longer meet the statutoryspecifications, and would constitute a breach of the licencing laws in almost every country in the world ifoffered for sale or used operationally. Even if the converted design were submitted for subsequent TypeApproval, this would prove very costly both in terms of administration and time scales, in addition to thefees required for each design submitted.

Wood & Douglas Ltd Applications Manual TYPICAL APPLICATIONS

5

2.1.3 OEM MODULES

Wood & Douglas have specialised in the field of OEM radio frequency design and manufacture since 1976and are now the UK's leading source of OEM modules for RF applications. The wide range of universaldrop-in modules provides a total solution to the OEM manufacturer wishing to incorporate RF data linkswithin his own product range.

In almost every case the modules supplied will carry Type Approval for one or more countries world-wideincluding the European market. This will be included within the selling price as standard. In all aspectsthe modules are designed to be `user-friendly' . This eliminates the need for the customer to provide hisown specialist staff and in-house test and alignment facilities.

2.1.4 CUSTOM MODULES

Wood & Douglas OEM modules provide a universal solution to an infinite number of user applications byincorporating many years of practical experience in the design & manufacture of such items. Feedbackfrom customers both past and present enables Wood & Douglas to build-in many of the features mostfrequently sought after by its OEMs. This further enhances these flexible designs.

There may be times when even a standard W&D module cannot be `squeezed' into the available space orperhaps the signals available do not quite match the required inputs. It is at these times that the collectiveengineering experience of the company can be utilised to provide a custom solution to the problem. Theresult will be an RF engineering solution exactly matching the client’s electrical and mechanicalrequirements.

A fundamental policy of Wood & Douglas is to appreciate the needs of the customer and to fulfil theseneeds first and foremost and not to dictate how the customer designs his end product. In this way it isthe company's intention that the customer should obtain maximum benefit from using the flexible andpowerful medium of radio linking.

TYPICAL APPLICATIONS Wood & Douglas Ltd Applications Manual

6


Wood & Douglas Ltd Applications Manual UNDERSTANDING THE TECHNICAL TERMS

7

Figure 1 Example of a carrierwave

Section 3Understanding the technical terms

The concept behind Wood & Douglas OEM modules is to distance the end-user as far as possible from thetechnicalities of radio engineering. This reduces the RF aspect of his project to a `black box' exercise.In many ways the process can be likened to computer software programs. These assist the user with therunning of the computer by simplifying commands with the use of mouse driver routines and iconsrepresenting the available features.

This manual should aid the process of integrating radio by explaining in simple terms how to gain maximumbenefit from radio links. Engineers and technologists of all persuasions tend to indulge in the use of`jargon' relevant to their personal interests. It is the intention of this manual to dispel some of thesemysteries. This will enable the reader to acquire sufficient information to integrate the use of RF moduleswithin the area of interest. It should make the process more straightforward and rewarding process.

3.1 BASIC SYSTEM BLOCKS

Sending information from point A to point B (using radio) requires two distinct and quite separate blocks;a transmitter and a receiver. The data or information in question is connected to the transmitter and thenreproduced by the radio receiver at the distant end. Just how does this happen? Let us begin by brieflyexamining some basic theory.

3.1.1 CARRIER WAVES

Transmitters generate a continuous signal termed a carrier (or bearer) which,as the name implies is used to ̀ carry' information. In practice this is producedby a high frequency oscillator operating at a known frequency within a pre-defined band. A typical UK radio telemetry system, for example, can operateat 458MHz and hence produces a carrier at this frequency. This signal leavingthe transmitter via the aerial constitutes the basic carrier signal which iscommon to all radio systems. If we were to tune a radio receiver to thisfrequency we would hear - nothing!

So why if we are transmitting do we hear nothing? When you alter the tuningof your FM (VHF) radio at home, you may notice a loud hiss as the dial is turned between the various radiostations. When the radio is properly tuned to a station however, all of the noise disappears and only theprogram is heard. Remember what happens though should the announcer pause during a news broadcast;what do you hear then? - that's right nothing!

The silence indicates that your receiver is still tuned to the carrier frequency which is transmitted for aslong as the station is broadcasting, even though the announcer is not actually speaking at the time. If theradio station were to suddenly switch off its transmitter, then the silence would be broken by the same loudhiss you heard when tuning the radio between stations. This hiss is also referred to as `white' or randomnoise, and is the culmination of all naturally occurring radio frequency signals that are constantly presentin and around the Earth's atmosphere, plus those generated by the radio circuits themselves.

In order that we can hear our announcer (or in our case convey information from point A to point B), wemust find some method of merging our information with the carrier wave leaving our transmitter. In simpleterms therefore we electrically mix the announcer’s voice and the carrier wave together to produce acomplex program signal; in this manner we are said to be modulating the carrier.

UNDERSTANDING THE TECHNICAL TERMS Wood & Douglas Ltd Applications Manual

8

Figure 2 AM modulation

Figure 3 FM modulation

3.1.2 AM MODULATION

The two simplest forms of modulation are amplitude modulation (AM) and frequency modulation (FM).Amplitude modulation varies the vertical component (or height) of the waveform, whereas frequencymodulation affects the horizontal axis (or frequency) of our transmitted waveform.

Figure 2 shows our same carrier wave from the previous section butwith the addition of amplitude modulation. Notice how the height of thewaveform varies. In a very simple form, amplitude modulation effectscan be produced by nothing more sophisticated than turning a carrierwave on and off at the transmitter.

Taking this process a step further, if instead of our signal being eitherwholly on or off we could convey a number of intermediate pointsbetween these two extremes, we would then have a means ofrepresenting a constantly changing waveform, or what is morecommonly referred to as an analogue voltage. Speech and music areexamples of analogue voltages and hence we have the ability to transmitthese signals using amplitude modulation techniques.

3.1.3 AM DEMODULATION

Because this manual is concerned with the subject of radio telemetry communications (which rarely, if ever,uses AM techniques), we will not attempt to explain AM demodulation here. The reader should simply beaware of the two types of modulation in order to grasp a basic understanding of radio technology ingeneral.

3.1.4 FM MODULATION

FM is the term used when our modulating signal (the programmeannouncer’s voice for example) is mixed with a carrier wave so as tocause the frequency of the carrier wave to vary as opposed to itsamplitude. The variation is not very great, typically a difference of3kHz in 458MHz is all that is necessary for satisfactory FMtransmission. This `variation' is termed the deviation of the carriersignal and is a measure of how much it deviates above and below thenominal un-modulated frequency.

As an example of an FM broadcast, the nominal carrier frequency isfirst decreased by 3kHZ and then increased by 3kHz representing atwo state code which could, for example, be a series of `1's and`0's. The carrier frequency is altered in direct sympathy with the modulating signal, thereby conveyingthe required information.

3.1.5 FM DEMODULATION

The design of an FM receiver is more sophisticated than that of an AM receiver and reflects the superiorperformance characteristics inherent in FM transmissions in general. Because we are dealing with changesin frequency (and not amplitude), the FM receiver is designed to eliminate even the smallest change inamplitude in order that it may detect changes in frequency. This is achieved in practice by heavily over-amplifying the incoming signal to a point where the upper and lower peaks of the alternating signal becomeflattened due to the excessive amplification. Because we have used so much amplification, the rising andfalling sections of the original waveform now appear very fast and much straighter due to the exaggerationof the vertical axis.


9

(1) Calculation for occupied bandwidth - AM modulation

(2) Calculation for occupied bandwidth - FM modulation

As we are only interested in the frequency of our waveform, the actual height and/or shape of the signalis relatively unimportant as we have now re-established the period (frequency) of the transmitted waveformalmost exactly. This phenomena will always give FM a clear advantage over AM methods provided we canalways amplify the incoming signal sufficiently for this effect to take place.

These then, are the fundamental concepts of AM and FM transmissions. All other modulating techniquesare simply variations or combinations of these two methods. Both possess advantages and disadvantagestoo numerous to mention here; suffice to say that FM generally offers the better environment for higherquality radio transmission within a given bandwidth and signal to noise ratio.

3.1.6 BANDWIDTH

There is often confusion amongst Wood & Douglas customers when discussing bandwidths, especiallywhen considering the occupied bandwidth of a system as opposed to the simple RF bandwidth. In anattempt to clarify matters we must first examine some fundamental concepts.

Audio bandwidth is simply defined as the audio frequency response characteristic which would be quotedfor instance by a salesman when purchasing new hi-fi equipment. The type of radio circuits employed fortelemetry purposes will typically have an audio response similar to the telephone i.e., around 300Hz to3kHz. This may be extended to below 100Hz and above 5kHz where higher performance is required. Anydiscussion regarding bandwidths is normally centred around the -3dB roll-off points of the responsecharacteristic, and is, in effect, the range of frequencies that any system passes before significantdeterioration of the response occurs.

When discussing radio transmission however, the term 'RF bandwidth' is no longer adequate to describesuch matters. Rather we should talk of the occupied bandwidth in this context as it encompasses not onlythe audio bandwidth of the signal being handled, but also pays due regard to the method of modulationused by the transmitter. This is quite different and must be considered distinct from the simple term 'RFbandwidth' which we use when discussing that characteristic of a radio circuit such as a tuned amplifieror a filter.

There are two basic formulae used for calculating the occupied bandwidth of a transmission; one for AMand one for FM modulation.

The issue becomes a little more complicated, however, when we introduce the concept of channel spacing,this term being the frequency increment established between adjacent radio channels in a band. Withinthe UK UHF telemetry bands, for example, two channel spacings are stipulated; one of 25KHz and the otherat 12.5KHz which effectively doubles the number of available channels within a given frequency band.(See Figure 6 for details of the channel spacing and frequency allocations in the UHF band).

Within the channel spacing therefore, we must accommodate three important characteristics if we are toestablish reliable communications:-

(1) The occupied bandwidth of the transmission (as explained above).(2) Any drift in the carrier's fundamental frequency setting.(3) The ability of the receiver to filter out adjacent channel transmissions.


1 Drift beyond our stated limits will not prevent reception, but will cause distortion in the receiver due to the slopes ofthe filter response at the edges of the passband.

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Figure 4 Bandwidth & effects of filtering (25kHz channelling)

The first two items should now be reasonably clear; however, the third item warrants closer attention.

In theory each channel provides 25kHz (or 12.5kHz) of spectrum which our signal may occupy. If this werethe case, all channels would follow on one from another without any free spectrum between them; thiswould make it impossible for any receiver (no matter how well designed) to be able to fully distinguish onechannel from the next. We must obviously allow some 'slack' between channels, this being achieved byfiltering, which restricts the width of the signal and allows the receiver to discriminate between signals onadjacent channels.

There has to be a trade-off in filter circuit design and implementation between the degree of adjacentchannel rejection and the bandwidth of the signal to be passed. Ideally we want our filters to have a flatresponse to the wanted frequencies and then to cut-off ‘vertically’ at the outer limits. In practice, of coursethis cannot be achieved, and filters tend to provide a sloping response rather than a sharp cut-off.

What we are saying, ineffect, is that due to theeffects of the filterslopes, our availablebandwidth will be lessthan ideal, this being int he i n t e r e s t s o fm a i n t a i n i n g g oodrejection of the adjacentchannels. Typically thisresults in a nominal25kHz channel having apassband (at ±3dB) afterfiltering of around15kHz! (Similarly as t anda rd 12 .5kHzchannel would bereduced to 7.5kHz).Rejection of adjacentchannels occurs at>60dB down the filter slope characteristic as can be seen in Figure 4, thus enabling the receiver todiscriminate between channels.

Using our formula from equation (2) for calculating the occupied bandwidth and given a 3kHz audio signalwith 3kHz peak deviation, we obtain an occupied bandwidth of 12kHz which, in the case of 25kHz channelspacing, leaves a mere 1.5kHz allowance for drift1. A very fine margin indeed as shown in Figure 4 above!

To summarise therefore, we should always maintain a distinction between the following definitions:-

AF Bandwidth C Frequency range of the audio or modulating signals passedRF Bandwidth C Range of the radio frequency signals capable of being passedOccupied Bandwidth C A combination of the AF signals plus the carrier waveChannel Spacing C The spacing necessary between RF channels to minimise

interference.


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3.2 SOME OTHER RF TERMS

3.2.1 TRANSMIT-ENABLE TIME (POWER-ON)

A radio transmitter requires a certain amount of time to reach its specified operating levels. This is mainlydue to the high frequency oscillator from which carrier wave output is ultimately derived which needs tostabilise at its predetermined frequency. As an example, the RF stages in a transmitter would be expectedto be within specified limits after 50ms.

3.2.2 FREQUENCY STABILITY

This is a measure of the ability of the carrier oscillator (and other parts of the circuit) to maintain thespecified frequency, having gone through the start-up delay and reached normal operating temperatures.Ideally the frequency should not drift up or down with time or with variations in the operating temperature.In practice, drift cannot be totally eliminated; consequently limits are set within the Type ApprovalSpecifications which typically allow for ±2kHz drift for a 25kHz radio channel operating at 458MHz.

Should a transmitter drift off frequency to any large extent, then failure of the radio link would result asthe receiver would (in theory) still be operating on and expecting a signal at the original frequency.

3.2.3 SELECTIVITY

This term refers to the receiver's ability to ignore unwanted signals which may be present on a frequencyclose to the operating frequency, e.g., the next designated channel. Selectivity is a measure of theperformance of the adjacent channel filtering circuits which, in a high quality receiver such as thosesupplied by Wood & Douglas, will include a crystal filter. Such filters provide very steep ̀ slopes' typicallyproviding 60dB (or more) rejection of signals on adjacent channels and beyond.

3.2.4 SPURIOUS OUTPUT

All radio circuits, no matter how well designed, will generate unwanted signals at frequencies other thantheir operating frequencies. If these other frequencies are actually radiated away from the circuit withsufficient power they may cause problems for other users and generally lead to an unwanted clogging ofthe air waves. These outputs are known as spurious outputs and must be strictly controlled by thedesigner in order to minimise their harmful effects. For example, MPT1329 requires that spurious signalsare maintained 81dB or more below the 458MHz carrier level of 500mW, which means that spurioussignals must be less than 4 nano Watts (4 x 10 -9W).

3.2.5 ERROR CORRECTION

Error correction implies the use of computer intelligence or some other means of making decisions basedupon a series of tests. The reader will therefore appreciate that a radio communications link itselfpossesses no means of performing this role and any error correction required by the user must be furnishedas an external feature to the radio system. This is dealt with in more detail in the modem section later inthis manual.


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3.2.6 MEASUREMENTS

All radio related measurements have units of measure but also have values expressed as relative values indecibel terms referenced to a known level. The most common reference is dB relative to 1milliwatt (mW)which is written on data sheets etc as dBm. This term will be used for both transmitter output levels andreceiver sensitivities. A 500mW output transmitter is the same as one having an output of +27dBm. Areceiver having a sensitivity of 1uV p.d. is the same as one at -107dBm.

The big advantage through using levels in dB relative terms, is that calculations such as propagation andtechnical performance can be quickly checked through simple additions and subtractions now that figuresare logarithmically based.

Wood & Douglas Ltd Applications Manual HOW DOES RADIO WORK?

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Figure 5 Line of sight communication

Section 4How does radio work?

4.1 BASIC RF PROPAGATION

Most of the headings chosen throughout this manual can (and do!) occupy entire volumes in their ownright; but as the purpose of this document is to provide a basic insight for inexperienced users of radioequipment, this section (like the others) will be restricted in content.

Radio waves propagate (travel) by various means: low frequency waves can travel great distances becausethey are able to pass between the atmosphere and the ground (similar to a tunnel effect) due to their longwavelength. Higher frequency transmissions can also travel comparatively long distances as they arereflected off the upper atmosphere and back down to earth and with sufficient power may be re-reflectedseveral times before being attenuated beyond use.

Radio waves in theVHF, UHF andSHF bands, i.e., allof the bands usedby Wood &D o u g l a s f o rt e l e m e t r ypurposes, travel instraight lines frompoint to point, at

least as far as this document is concerned. That is to say they are not reflected by the atmosphere, norare they propagated in any way other than by direct 'line of sight'. From an operational standpointtherefore, radio telemetry systems can be considered analogous to light waves when considering theirpropagation properties.

The phrase 'line of sight' often leads to confusionamongst users, because if taken too literally itimplies that both the transmitting and receivingantennas must be in clear view of each other.Whilst this is a highly desirable situation and indeedabsolutely necessary condition when planning longrange point-point systems, it is not always essentialto have an absolutely unobstructed path betweentransmitting and receiving stations for telemetryapplications.

UHF signals perform well inside buildings and inmany practical applications involving radiotelemetry, it would be quite impossible to providecontinuous clear paths between transmitter (TX) andreceiver (RX) units. Provided the range is not excessive most telemetry products will provide goodcoverage without true line of sight; indeed, many customers have reported their surprise at how well Wood& Douglas products perform under these conditions.

HOW DOES RADIO WORK? Wood & Douglas Ltd Applications Manual

2 Users with specific enquiries should contact the DTI directly as policies are under constant review.

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UK UHF TELEMETRY BAND (458MHz)

12.5kHz (only) 12.5kHz & 25kHz

458.5125 458.525

458.5375 458.550

458.5625 458.575

458.5875 458.600

458.6125 458.625

458.6375 458.650

458.6625 458.675

458.6875 458.700

458.7125 458.725

458.7375 458.750

458.7625 458.775

458.7875 458.00

458.8125 (458.825)*

(458.8375)* 458.850

458.8625 458.875

458.8875 (458.900)*

458.9125 458.925

458.9375

32 x 12.5kHz 15 x 25kHz

Figure 6 UK UHF telemetry band channel frequencies

4.2 WHICH FREQUENCY SHOULD I USE?

The choice of which frequency is used will largely depend on the application involved, rather than any otherprimary consideration. In other words, most country’s regulatory bodies have set aside certain sectionsof the radio spectrum for specific uses and anyone wishing to implement a particular service will berestricted to the use of those designated frequency bands.

In the U.K. for example, the DTI has approvedcertain frequencies for use in telemetry andtelecommand applications. The term telemetryas defined by the DTI is "The use oftelecommunications for automatically indicatingor recording measurements at a distance fromthe measuring instrument". Telecommand isdefined thus: "The use of telecommunicationsfor the transmission of signals to initiate,modify or terminate functions of equipment ata distance".2

Other frequencies are given over to suchapplications as security alarms, baby alarms,personal alarms for the elderly, radio paging,pleasure boat security, garage door controlsystems etc. Details of all of these (and more)may be obtained directly from the DTI. Eachcategory is given an MPT prefix and areference number which indicates the specificrequirements of the frequency use. Typicallymany of Wood & Douglas's own telemetryproducts are approved by the DTI underMPT1329 as is explained later in this manual.

The U.K. telemetry bands are available in bothVHF and UHF, (MPT1328 and MPT1329respectively). UHF signals will (as a rule ofthumb), not travel as far as VHF signals. Thisis because higher frequencies are attenuatedmore readily during transmission than lowerones.

Figure 6 shows all of the available channel frequencies within the 458.500 to 458.950MHz UK telemetryband. When using 25kHz channel spacing only frequencies listed in the right hand column are available.When using 12.5kHz channel spacing then the entire list of frequencies may be used except for thosemarked with an asterisk (*). These three frequencies (458.825MHz, 458.8375MHz and 458.900MHz arereserved for use under MPT1361 (which relates to fixed, transportable and mobile alarms) and may not beused for telemetry under MPT1329. Note also that use of the two frequencies at the extremities of theband is not permitted, i.e., 458.500 and 458.950MHz.


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Figure 7 Sites below the radio horizon

Equipment working in the VHF bands is plentiful and the amount of traffic on these bands is heavy to saythe least! The same is rapidly becoming the case with UHF allocations, with the result that even higherfrequencies are being sought in order to cater for the increasing demand for radio telemetry services.

UHF equipment is ideally suited to 'indoor' use (i.e., warehouse automation systems, TV studios, factoriesetc), because the ability of the relatively short wavelength signals to get around obstacles and otherobstructions makes it an effective medium for such applications. The antennas required for use at thesehigher frequencies can be very compact indeed and in some cases be accommodated within the equipmenthousings themselves. (See also section 5 for detailed information regarding antennas).

Generally speaking then, higher frequencies provide greater immunity from man-made noise (i.e.,generators, welding plant etc) and hence better quality of transmission.

Recently, during 1988, a new band between 868-870MHz has been allocated for use in the UK. This isa pan-European band which eventually should be useable in all countries within the EC. There is a UK‘band-plan’ which is detailed in Appendix A at the rear of this manual.

4.3 HOW FAR CAN I TRANSMIT?

The distance over which any radio signal is ableto travel depends upon many physical factorsand it is impossible to predict with absolutecertainty how far a signal will carry before itbecomes unusable. This assessment of rangeis known as link performance. Where the abilityto predict link performance becomes criticaltherefore, it is recommended that a detailedsite survey be commissioned in order toexamine in greater detail the relevant localconditions affecting the transmission path.Only in this way can any accurate estimates be made as to the technical requirements for a particular installation.

A primary consideration when planning any radio communications link is the radio horizon. For practicalpurposes all of the radio signals we will be discussing throughout this document will be travelling in straightlines, i.e., the radio waves will not follow the earth's curvature and hence there will be a finite distanceover which we can transmit a radio signal due to this effect.

From the figure on the left it should be apparent that the higherthe transmitting and receiving antennas, the greater thetheoretical range will be. The radio horizon should always bethe first calculation checked when considering a particular site.(This subject is covered in greater detail in section 4.5.2).Sufficient antenna height should be provided in order to ensurethat the radio wave has a clear line of sight over the ground tothe distant antenna.


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Transmitter powerHeight of transmitter antennaLength of feeder cables usedHeight of receiving antenna

Types of antenna usedFrequency used

Table 1 Some factors affecting radio link performance

4.4 FACTORS AFFECTING PERFORMANCE

Ignoring for the moment the quality of the radio equipment used, the radio link’s performance (i.e., thedistance over which signals will be received reliably), is a function of many variables; some of which arelisted over :

.

It should be apparent to the reader that radio propagation is no simple matter and as with so many otherthings in life, the overall radio system performance will only be as effective as the weakest link in the chain.Some of the factors mentioned above will be beyond the control of the user, for example the weatherconditions, the RF power permitted (very often dictated by the authorities) and even the height of theantennas may be outside the direct control of the operator.

Generally speaking therefore, one must always strive to provide the best possible operating environment,by ensuring that antennas are mounted as high as possible. Avoid using feeder cables that areunnecessarily long. Each additional metre of cable used increases the losses in the system, therebydegrading the overall link performance. (See section 6 for more information regarding the use of feedercables).

4.5 CALCULATION OF RANGE

Various methods exist for calculating the theoretical range over which radio waves will travel successfully.Any calculation will only be proven once the RF hardware is actually installed and tested at the site ofoperations. As a guide however, the procedure below has been found to give a reliable estimate oflink/range performance:

The nomograms provided will assist in the following calculations. To use the nomograms simply place aruler or straightedge at the required crossing points on the vertical axis and read off the unknown value.

4.5.1 Before using the nomograms however, the following data must be obtained. (N.B. all heightmeasurements for the antennas are related to height above ground in feet).

1) transmitter antenna height in feet (Tht)2) receiver antenna height in feet (Rht)3) transmitter antenna gain in dB (Tgn)4) receiver antenna gain in dB (Rgn)5) transmitter feeder cable loss in dB (Tfd)6) receiver feeder cable loss in dB (Rfd)7) transmitter power output in Watts (Tpw)8) maximum range required of system (Rmx)


17

The gain of the antenna used at the transmitting and receiving sites should be obtained from the antennamanufacturers own data sheets. Alternatively this may be estimated by reference to Table 2. The basichalf wave dipoles mentioned in previous sections normally have unity gain (0dB) and basic co-linearantennas around +3 to +6dB. Small helical stub antennas will have less than unity gain and in practicetheir performance is difficult to assess as they rely on a ground plane to be provided externally. Theseantennas are not generally recommended for anything other than short range use. (See section 5 fordetailed information regarding the selection and use of antennas).

4.5.2 DETERMINE THE RADIO HORIZON

Before making any calculations we should first ensure that the intended sites are within the 'radio horizon'.If the intended range is beyond the radio horizon (and remember the radio horizon is always less then thevisible horizon), then no matter how good or how powerful our transmitter may be, we will not obtainsatisfactory communications.

Knowing the heights of the two antennas as in (1) and (2) above and using the nomogram opposite we canestimate the radio horizon.

For example: The height of the transmitting antenna is 100 feet and that of the receiving antenna is 50feet. Placing a ruler on these two values on the two outer columns and reading off the middle columnindicates a radio horizon of 24 miles. (N.B. This is closer than the visible horizon).


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Figure 8 Determining the radio horizon

This indicates that the maximum distance a radio signal could travel between two stations of the heightsgiven in the above example is 24 miles. It does not mean that a signal will necessarily achieve this inpractice.

Provided the required maximum range (Rmx in (8) above), is less than 24 miles we can now proceed tocalculate the range factor.


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Figure 9 Transmitter power factor

4.5.3 FIND THE TRANSMITTER POWER FACTOR

If we now ascertain the output power of the transmitter Tpw (from the manufacturers data sheet) thetransmitter power factor may be obtained from the figure below. As an example, a Wood & DouglasST450 UHF telemetry transmitter module has an output of 500mW which, reading directly from Figure 9,gives a power factor of +27dBm.


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Figure 10 Antenna height factor nomogram

4.5.4 ANTENNA HEIGHT FACTOR

Using the same antenna heights as for paragraph 4.5.2 (radio horizon), apply them to the antenna heightnomogram Figure 10 opposite.

i.e., Tht=100 feet, Rht=50 feet; thus from Figure 10 the antenna height factor = 47dB.


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Figure 11 Terrain correction factor

(3) Range scale calculation

4.5.5 DETERMINE TERRAIN CORRECTION FACTOR

Scaling off horizontally from the 460MHz frequency axis at the 90% point (It is suggested that 90% beused to obtain the most realistic estimate) we obtain a terrain correction factor of 35dB.

This figure is basically an estimate of the losses sustained by our radio signal when passing over 'anaverage path' in conjunction with the percentage of good data we can expect at the distant point.

As an example: If we have an ST450 transmitter operating on 458MHz (460MHz), referring to Figure 11and moving vertically upwards to the 90% crossing point, we obtain a correction factor of 35dB.

4.5.6 CALCULATION

We are now able to calculate the overall range scale factor from the following formula:

Example:

Assume that both the transmitter and receiver are provided with UR67 low loss feeder cable; from the tablein section 6.1 we can estimate a loss of around -1.6dB when using a 10 metre length of the cable. If thetransmitter is provided with a unity gain dipole and a +6dB gain Yagi is used for the receiver, then thisbecomes:

- Rscale = 27 + 0 - 1.6 + 6 - 1.6 - 35 - 47 Rscale = 52.2dB


3 in general however, W&D do not recommend attempting the use of paths which give less than 1 microvolt, in orderto maintain a margin of safety in use.

22

Figure 12 Calculating the range

Having calculated our Rscale factor of 52dB, we locate the diagonal axis corresponding to 52dB. Knowingthe maximum distance over which we are required to transmit (let us suppose a range of 15 miles), welocate the crossing point of 52dB and 15 miles and working to the right (receiver input), we obtain anestimated signal strength (at the receiver antenna) of 2.7 microvolts over the required distance.

Alternatively, scaling directly across to the left hand Y axis marked Receiver Input Level, we find a figureof -98dBm which is a form more commonly used by RF engineers when discussing such matters. Thetypical sensitivity of a Wood & Douglas UHF receiver is around 0.3 microvolts and we therefore havesufficient signal in hand at 15 miles to obtain a useable transmission path.3

Propagation Programs

A number of simple PC based programs are available from Wood & Douglas to assist in path losscalculations. These can be obtained from the Sales Office or downloaded from our Internet site(http://www.woodanddouglas.co.uk)

Wood & Douglas Ltd Applications Manual WHICH ANTENNA SHOULD I USE?

23

Section 5Which antenna should I use?

The subject of antennas for communication purposesis a complicated one to say the least! The followingsection attempts to simplify the process of selectingthe right antenna by explaining some basic concepts.

We have all seen the metal coat hanger protrudingfrom the wing of the old family saloon and whilst thismay pick up Radio 1 well enough for most people,where we are dealing with very low transmitterpowers (Radio 1 uses around 150,000 Watts asopposed to our 0.5 Watts!) and possibly difficultterrain, the aerials we use must be a little moresophisticated than a piece of 10 gauge steel wire.

Aerials (also referred to as antennas) fall into one of two categories; (i) non-directional and (ii) directional.Non-directional antennas are simple devices which transmit (and receive) signals equally in a horizontalcircular plane around the antenna (i.e., 360 degrees). A directional antenna as the term implies, is designedto radiate (or receive) energy only from the direction in which the antenna is pointing. This is accomplishedby focussing the signals handled by the antenna, a process which effectively amplifies the signal. (Theanalogy here is very similar to light, where a reflector placed around a bulb intensifies the light beam froma torch by focussing the light rays). Due to this phenomenon, it is said that this type of directional antennahas gain due to the increase in signal strength resulting from the focussing of the beam.

The final choice of antenna, together with its physical location, will greatly affect the overall operation ofany radio-based communications link. Always remember that any improvement (e.g., the use of an antennawith higher gain) at one end of a radio link enhances the overall performance of the entire system as it isthe combined effect that matters.

The following sections will assist users in deciding which type of antenna will be required for individualsituations.

5.1 NON-DIRECTIONAL ANTENNA

This type of antenna is more commonly referred to as an omni-directional antenna and as mentioned aboveis able to work with signals in a circular plane surrounding the antenna. This 'all-round' ability makes theomni-directional device the obvious choice when communicating with a mobile station, or where forexample, a receiver is obtaining signals from a number of transmitter sites surrounding it.

Non-directional antenna can range from just a few centimetres in length, up to several metres dependingon the frequency of operation. A typical half-wavelength (also written as ½8) antenna for use with Wood& Douglas VHF (173MHz) telemetry modules would measure 0.75 metres in length, whilst a similarantenna for UHF (458MHz) telemetry would measure approximately 0.28 metres. These antennas willnormally have unity gain ( 0dBd), in other words they neither increase nor decrease the signal levels passingthrough them. This is acceptable in many applications, particularly as these antennas are low-cost,compact and simple to mount.

WHICH ANTENNA SHOULD I USE? Wood & Douglas Ltd Applications Manual

4 To calculate radio wavelength: Wavelength (metres) = Speed of propagation divided by frequency of operation. (Radiowaves travel at 300,000,000 metres/sec; frequency is expressed in Hz).

24

Figure 13 Fundamental half wave construction

quick summary: Gain ² Unity. Good general purpose antenna, omni-directionalcharacteristic, simple construction, cost effective. Used mainly as basestation antenna.

5.1.1 HALF-WAVE DIPOLES

A half-wave dipole is a convenient reference when measuring antenna gain. This has a nominal gain ofunity (0dB). However, its gain relative to an isotropic radiator is +2.15dB. It is important therefore, toestablish whether the gain of an antenna is specified relative to a half-wave (dBd) or relative to an isotropicradiator (dBi).

This can be expressed by the following equation:-

Gain (dBi) = gain (dBd) +2.15

Of all the dipole antennas, the half-wave device is the most significant and fundamental, as it provides thebasis for a large number of other antenna designs.

As an example, a very crude half-wave dipole couldtheoretically be constructed from two pieces of stiff wireplaced one above the other in a vertical plane. The RFsignal connection is then made to the inner ends of eachpiece of wire at the centre of the antenna; one feed tothe upper wire and one to the lower. If the length ofeach piece of wire is then cut to be one quarter of thewavelength4 in question, then the whole assembly willfunction as a half-wave dipole. (Antenna length isdirectly related to frequency of operation; the higher thefrequency the shorter the antenna becomes).

This antenna is said to be balanced because its twocomponent parts (the upper and lower sections) areequal in all respects. Not all antennas are balanced aswill be explained later.

When a half-wave dipole is mounted in the vertical position, the electric field radiated from it is also verticaland the antenna is said to be vertically polarised (horizontal polarisation is also possible). Antennamanufacturers’ data sheets will normally provide a graph showing the pattern of radiation from individualantennas. In the case of the half-wave dipole, the electrical radiation pattern can be likened to placing acar inner tube (or doughnut) over the antenna. There are no signals given off either directly above or belowthe antenna, but there is a continuous circle of propagation around the antenna. This is why the half-wavedipole is said to be omni-directional.

5.1.2 QUARTER-WAVE WHIP

Other derivatives of the half wave dipole have been developed to fulfil specific requirements. Where asmaller more compact device is required, a quarter-wave whip antenna is employed. This antenna isphysically smaller because one of the radiating elements as described above is removed, thus leaving onlya shorter single element of a quarter wave length. In addition the element is often constructed from fairlysmall diameter stiff wire (hence the name whip) and is therefore less obtrusive than the half wave dipole.


25

quick summary: Gain ² -3dBd. Being smaller than the half wave dipole, the quarter wavewhip may find use as a mobile antenna for vehicle use, or generally whereranges are not excessive.

quick summary: Gain ² less than unity. The helical antenna is a low-cost device suitablefor short (up to a few hundred metres) range; extremely compact in use,less prone to damage.

How does this antenna still operate with only a single element? In order for the quarter-wave dipole tooperate efficiently it requires a `ground plane' around the radiating element. This can be formed forexample by placing the antenna on a car roof, the flat metal surface acting as a ground plane which (as faras the antenna signals are concerned) acts in a similar manner to the second element of a half-wave dipole.This ground plane effect could also be provided by fixing the antenna to a metal enclosure or housing, thuscreating the same effect as the car roof. In theory the ground plane should have a radius of at least aquarter wavelength in order for the antenna to operate at maximum efficiency.

Some quarter-wave dipoles incorporate their own ground planes which take the form of four (or more)horizontal rods which usually screw into the base of the antenna. These horizontal elements create theeffect of a ground plane where none exists; i.e., where the antenna is to be fixed to a pole or mast clearof the ground and lacking in any other metallic surface directly beneath the antenna.

A quarter-wave whip has a gain of +3dB when used with an infinite ground plane; however, this is onlytrue when measured at an angle of about 40° relative to the ground plane, i.e., when transmitting, mostof the signal is radiated upwards rather than horizontally. In most applications the effective gain can beconsidered to be in the region of -3dBd.

5.1.3 HELICAL STUB ANTENNA

With the proliferation of so many hand-held radio terminals, walkie-talkies etc, a furtherrefinement of the quarter-wave antenna became necessary in order to provide a suitablycompact alternative.

Hence the helical stub antenna came into being, taking its name from the way in whichthe active element is formed within the device. As the name implies, the element ishelically wound around a vertical former, thus making the antenna very much shorterthan even the standard quarter-wave dipole antenna.

The advantage of helical stub antennas lies in their low cost and small physical size. Performanceconsequently may not match that of the larger dipoles but will prove extremely effective for short rangelow-cost operations.


26

Figure 14 Construction and effect of Co-linear antenna

quick summary: Gain ² Usually +3 to 8dBd depending on the number of dipoles used(equivalent to a small Yagi but retaining the 360 degree mode ofoperation). Normally used for permanent long range radio installationswhere omni-directional working is required, i.e., for use on a base station.

5.1.4 CO-LINEAR ANTENNA

Before leaving the subject of omni-directionaldipole antennas, mention should be made ofthe Co-linear antenna; a very useful device forcertain installations. During manufacture, anumber of half-wave dipoles are stackedvertically in a column and are then inter-connected. This gives the antenna gain relativeto a single half-wave dipole which means wecan use it to cover longer distances or uselower transmit power levels to achieve thesame distance.

How do we obtain this increase in gain? -Imagine an inflated balloon; if apply downwardpressure, the balloon expands sideways. Thevolume of air inside the balloon remains the same, it has simply been forced into a different orientation.A similar effect is produced in the Co-linear antenna by `stacking' dipole elements vertically so that theelectro-magnetic field produced is compressed in the vertical plane. This has the effect of making the fieldextend in a horizontal direction. It is this extension of the field that effectively creates an increase in thegain of the antenna.

5.2 PORTABLE ANTENNAS

In view of the current increase in demand for hand-held and portable radio terminals, this section has beenadded to provide guidance for users of such equipment.

Often the size of the equipment housing will dictate the size of the corresponding antenna. A miniaturehand-held transmitter weighing only a few grams is unlikely to be able to support a large bulky antenna.However there is often a requirement for high performance antennas with the slightly larger and moresubstantial types of hardware such as Wood & Douglas' own SurTel range of ruggedised data links.

5.2.1 GROUND PLANES

Hand-held and portable equipment is very often supplied with a helical stub or quarter-wave antenna whichis generally in keeping with both the physical size and the cost of such units. However these antennas donot perform particularly well as explained previously and require a ground plane (see section 5.1.2) if theyare to operate as intended. Remember that the ground plane can be provided by using a metal housing forthe equipment and mounting the antenna directly on top of the box. The ideal ground plane is one quarterwavelength in diameter around the antenna but is rarely achieved in practice. This can dramatically affectthe match and therefore the gain of stub and whip antennas with the exception of the end fed dipole.


27

quick summary: Gain ² Around 0dBd, providing superior performance to smaller quarter-wave and helical stub antennas. Still reasonably compact; ideally suitedto semi-portable style of operation whilst offering good performance.

Figure 15 Vertically polarised Yagi antenna

5.2.2 END-FED DIPOLE FOR PORTABLES

In situations where maximum performance is required from portable or semi-portable equipment(such as the Wood & Douglas SurTel range) then a range of antennas is available which aretermed End-Fed Half Wave Dipoles.

These high-quality flexible rubber-coated antennas are not only compact (less than 35cm at450MHz) but provide superior performance when compared with most quarter-wave and helicalstub antennas. By virtue of their design these antennas do not require a ground plane forsatisfactory operation. Supplied with a variety of connectors ranging from BNC or TNC to thestandard UHF N type, they will greatly enhance the performance of any portable equipmentcurrently using less efficient antennas or where a ground plane is not possible.

5.3 DIRECTIONAL ANTENNA

As explained at the beginning of this section, directional antennas are designed to operate only with signalsoriginating within a limited area and therefore will be employed where we wish to operate between fixedsites. It should be noted that because directional antennas provide gain (and sometimes comparatively highgain), they also, by their nature, provide a good deal of signal rejection to the rear of the antenna. In thisway, directional antennas can sometimes eliminate unwanted interference signals, or simply reduce the riskof picking-up unwanted transmissions from local sources. (This characteristic is referred to as the `front-to-back ratio' and is expressed in dB). The area of effective operation (i.e., the horizontal beamwidth),when using this type of antenna is typically in the region of 30 to 50 degrees, depending on the physicalconstruction.

These two features make the directional antenna a very useful device indeed, the main penalties being insize and cost.

5.3.1 THE YAGI

By far the most common type of directionalantenna is the Yagi (so called after itsinventor). These antennas are found incommon use as domestic TV aerials, with theirfamiliar long horizontal boom and smallperpendicular elements which typify the Yagidesign. (As a general rule the more elementsa Yagi has the greater is its gain).


5 The amount of power depends upon which frequency band and service is being used. Please consult the DTI for furtherinformation regarding permitted power levels.

6 Effective Radiated Power - The apparent power level leaving the antenna when measured in the direction of maximumfield intensity.

28

quick summary: Gain ² From 3dBd upwards. Provides good long range working betweenfixed stations plus rejection of unwanted signals from areas outside of theantenna’s beamwidth.

Use of Yagi antennas is possible with U.K. telemetry systems, but users should be aware that there arelimitations placed on the maximum amount of power that may be effectively transmitted by suchequipment.5 Any antenna having gain (Co-linear or Yagi), will increase the effective output power of anytransmitter connected to it. In other words, if a transmitter giving an output of 1 Watt is connected to aYagi antenna with a gain of 3dB then effective transmitted power is doubled to 2 Watts.

In the above example, the figure of 2 Watts is termed the effective radiated power (E.R.P.)6, of the systemas it is the apparent power leaving the antenna which is of interest, rather than the output power of thetransmitter feeding the antenna. In order to use antennas with gain, therefore, it will be necessary to firstreduce the output power of the transmitter so as to keep within the required limits for ERP which, in thecase of the 458MHz UK telemetry bands, is 500mW. It must be emphasised that there is no limitation asto the type of antenna connected to a radio receiver as this is a passive device picking-up signals, i.e., itsuse does not increase the amount of power leaving the transmitter.

The reader may well ask "Why use a Yagi if I first have to reduce the power of the transmitter?" Theanswer is simply that by reducing the transmitter power in order to maintain the required ERP, there is areduction in the current drawn by the transmitter. This is an important advantage to users of battery-powered installations.

Remember too that a Yagi will reject unwanted signals from any direction other than that intended and, asa bonus will reduce the possibility that you may cause interference to other users of similar systems nearby.

When installing a directional antenna care must be taken to ensure that it faces the right direction and thatthe elements are polarised correctly. The Yagi is installed so as to `point' towards the opposing station,i.e., with its shortest element at the front and the longest reflecting element to the rear. (The elementsbecome progressively shorter towards the front of the antenna). The Yagi can be mounted so that theelements are either horizontally or vertically orientated, in which case the device is said to be horizontallyor vertically polarised accordingly. There is little advantage to be gained between either method ofinstallation, but ensure that both/all stations within the transmission path have the same orientation. Thisvertical or horizontal orientation of the elements is referred to as the polarisation of the antenna.

From the above it should be clear that this type of antenna can only be used between two fixed points andnot where one or both stations are moving or where two or more signals are arriving from oppositedirections.


7 Within the same frequency band.

29

Figure 16 Outline of typicalplate antenna

quick summary: Primarily designed where a very low profile is required, i.e., high speedvehicles (trains), warehouse installations, covert security operations.

Figure 17 Basic construction oftypical dish antenna

5.3.2 MIXED ANTENNA WORKINGIt is perfectly acceptable to mix the different types ofantenna7 in order to create the best possible workingenvironment for a radio communications network. Wherecommunication is required between a Yagi and any of thevertically polarised antennas, i.e., omni-directional dipoles,then the user must install the Yagi with the elementsvertically aligned to correspond with the vertical orientationof the dipole/s. A very large signal loss will result if thisprecaution is not observed with maximum effect occurringwhen the Yagi elements are horizontal (i.e., at right anglesto the vertical dipole).

5.4 OTHER ANTENNA

Various other types of antenna are available and some of these are briefly mentioned here to acquaint thereader with their possible applications.

5.4.1 PLATE ANTENNA

A development from the aeronautical industry has been the plate antenna,which in its simplest form is a rectangular low-profile device withdirectional properties as opposed to the omni-directional dipoles discussedearlier. This antenna is unobtrusive due to its construction and hence findsgreat popularity with security applications, but will equally find use instudio talk-back systems because of its ability to provide almost 90 degreecoverage over a given area without being bulky or unsightly.

This means that large floor areas may be covered by radio link, i.e., in a studio or a warehouse environmentwhere the plate antenna is mounted in the roof or ceiling. This antenna may also be used to provide radiocoverage in confined narrow corridors, e.g., between tall shelving inside narrow storage areas.

5.4.2 DISH ANTENNA

Dish antennas are commonly used at microwave frequencies, usually 1GHz and above. The wavelengthsin this area of the radio spectrum are so short that radio transmissions begin to take on the characteristicsof light waves.

The dish (usually having a parabolic profile) focusses received signals onto(typically) a dipole element at the heart of the antenna, thus forming a highlyefficient device for radio transmission and reception.Wood & Douglas supply a number of video transmitters and receivers whichutilise microwave dish antennas.


30

quick summary: Gain ² (high). As used for modern satellite TV reception. Also used formicrowave frequency data links. Used by Wood & Douglas for videotransmission.

(4) decibel calculation for power values

(5) decibel calculation for voltage values

5.5 THE DECIBEL (dB)

The decibel (dB) is not a unit of absolute measurement, unlike the volt or the amp, but is in fact the ratiobetween two distinct levels of power, voltage or current and is commonly used in audio and RF engineering.The reader should have a basic understanding of this term in order to fully comprehend the basic aspectsof radio frequency engineering.

As an example; if we apply a signal of 1 Watt to the input of an amplifier and in so doing obtain an outputof 2 Watts, we can say that we have a gain of +3dB. (i.e., a doubling of our original power level).

Conversely, if we apply an RF signal at a level of 1 Watt to a filter network and then measure an outputof 0.5 Watt we are observing a loss of 3dB.

The formula for calculating the ratio in dB between two POWER levels is:

When calculating decibel ratios for VOLTAGE values the formula becomes:

NOTE: Power gain and voltage are not interchangeable unless the input and output impedances of theamplifier filter etc., are equal and matched.

As the reader will have noticed, the decibel is a logarithmic function; this means that we are able to simplyadd or subtract dB values when making calculations associated with radio engineering.

eg., A 3dB increase in transmitter power output together with a 10dBd gain antenna will yield a 13dBoverall increase in effective radiated power (i.e., 20 times) relative to using the original transmitter poweroutput with a half-wave dipole.

5.5.1 ANTENNA GAIN AND THE dB

As already mentioned the gain provided by a Yagi basically depends on the number of elements used in thedesign and therefore a 12 element Yagi will normally have greater gain than would, say, an 8 elementdesign. When discussing decibels and antenna gain, it is common to express the gain of a particularantenna in dBd (i.e., relative to a half wave dipole), which is taken as the unity gain reference point. (Somemanufacturers will quote dBi (isotropic), in which case simply deduct 2.15 to convert to dBd.


31

Number ofelements

Gain approximatelength of boom

4 +7.5dBd 1.2m

8 +10dBd 2.6m

12 +12dBd 3.0m

18 +15dBd 4m

Table 2 Typical gains of UHF Yagi antennas

To conclude this section, the table below gives selected dB levels vs. voltage and power which may behelpful. (NOTE: The voltage ratios expressed below are also applicable to current).

DECIBEL TABLE

Voltage ratio dB Power ratio

0.32 : 1 -10dB 0.10 : 1

0.56 : 1 -5dB 0.32 : 1

0.71 : 1 -3dB 0.50 : 1

0.89 : 1 -1dB 0.79 : 1

1.00 : 1 0dB 1.00 : 1

1.12 : 1 +1dB 1.26 : 1

1.41 : 1 +3dB 2.00 : 1

1.78 : 1 +5dB 3.16 : 1

3.16: 1 +10dB 10.0 : 1

105 : 1 +100dB 1010 : 1

Table 3 Decibel conversions

Notice especially the values for +3dB and -3dB. These are common reference points and represent doubleand half power relationships respectively. Note also that a 1dB loss is equivalent to about a 20% powerreduction. If one remembers the ratios for 1dB, 3dB and 10dB then many dB calculations become a matterof simple mental arithmetic.


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Wood & Douglas Ltd Applications Manual CONNECTING THE ANTENNA

8 The use of 50 ohms is an internationally accepted standard impedance which falls between the natural impedance ofa ½ wave dipole (70 ohms) and a ¼ wave whip (35 ohms).

33

Figure 18 Attenuation effect when using feeder cables

Section 6Connecting the antenna

This section explains how antennas may be connected to transmitters and receivers without losing moreof the signal than absolutely necessary.

A feeder is the cable used to connect a radio transmitter or receiver to anantenna or similar device. Whenever a radio signal is passed down a cablethere will be a loss in signal strength due to the attenuating effect of thecable. The degree of attenuation will depend on the type of cable employedand the frequency of the signal passing down the cable. The higher thefrequency of the signal, the greater is the attenuating effect of the cable.

Tests carried out on two short (10m and 25m) lengths of medium quality cabledemonstrate just how much signal can be lost in such short lengths of cable.

Both cables were cut to length and terminated with N type connectors. Using highly sophisticated RF testequipment the two cables were tested independently in order to measure the attenuation through the cable.(We should of course point out that the figures obtained included the losses introduced by the cable andthe two RF connectors attached to it).

Results for the 10 metre cable showed that when a 450MHz signal passes through the cable, a loss of1.8dB was experienced, which in practice would mean that the output of a 500mW transmitter iseffectively reduced to 354mW before it reaches the antenna! Similar tests carried out using the 25 metrecable gave a loss of 4.1dB, which would leave less than 200mW at our antenna when connected to a500mW transmitter.

The above examples graphically illustrate just how much signal is lost when using feeder cables and howvital it is to keep their lengths as short as possible. Attention to cable length is just as important on thebench as it is on an installation. Bench cables for goods inwards testing should be short and low loss ifmeaningful measurements are to be made. Generally the thinner the cable diameter, the higher will be theloss.

6.1 FEEDER CABLE TYPES

The cable used in the previous example is known as UR67 and is a medium quality low-loss coaxial cable.Other types of cable are available and a short list of types and basic characteristics is given below. Thesecables are of 50 ohm impedance8 and are intended for use with radio communications engineering:

CONNECTING THE ANTENNA Wood & Douglas Ltd Applications Manual

34

They should not be confused with the much cheaper (and vastly inferior) 75 ohm coaxial cable employedfor use with domestic TV receivers.

Prices will vary according to the supplier and lengths purchased, but most are readily available from theusual electronic component stockists.

TYPE OVERALLDIAMETER

ATTENUATIONPER 100ft

TYPICAL USE

UR43 0.2 " 9.2dB @400MHz

low quality cable - very shortinterconnection

UR76 0.2" 10dB @400MHz

low quality cable - very shortinterconnection

RG213 0.4" 6.5dB @400MHz

low/med quality cable general use

UR67 0.4" 4.7dB @400MHz

medium quality cable

LDF450(Heliax)

0.7" 1.5dB @400MHz

high quality cable - low loss - semirigid

Table 4 RF cable characteristics

Wood & Douglas Ltd Applications Manual USING RADIO LINKS

35

Section 7Using radio links

7.1 REGULATIONS

Use of the normal UHF and VHF telemetry bands does not bestow any rights, or assign any particularfrequency within a band to individual users; (including those who held licences under the older specMPT1309). Certain users such as the utilities are assigned quite separate frequencies as defined underMPT1411 where operation at up to 10 Watts is permitted.

Increasingly, the DTI is handing responsibility for 'housekeeping' and good order of the low power telemetrybands to the users themselves, this means that we must all act responsibly if we are to continue to enjoythe benefits that radio communications offers.

Provided that any radio telemetry hardware purchased is supplied by a reputable manufacturer and has beenawarded the approval certification stipulated by the DTI, the user may be confident that the operation ofthe equipment will interfere as little as possible with other users. This does not guarantee however, thatco-channel interference from other local transmitters on similar frequencies will not be a problem. If thisshould happen, then it may become necessary to alter the location of the radio installations, or perhaps toconsider a change in the operating frequency. Section 8 on interference should be consulted for this.

Manufacturers who comply with DTI regulations should quote an approval number for each product in theirrange. If in doubt, ask the manufacturer for the approval certificate number which is awarded uniquely tothat manufacturer for a specific product. (Variants are not accepted under existing approvals andmodifications such as alterations to the housing are not permitted).

De-regulated i.e., non-licensed devices (see also section 11) operate in specific bands which are availableto all users of the equipment. Generally, it is unlikely that a clash of frequencies will occur as there aremany channels available to manufacturers in these bands and the range of the transmitters is limited. Ifproblems are encountered therefore, the simple solution is to change the operating frequencies involved,a task which Wood & Douglas will undertake on request at minimal cost. The popular UK de-regulatedtelemetry bands are MPT1329 (UHF) and MPT1328 (VHF). Any equipment gaining approval under theseregulations does not require a licence in operation. Wood & Douglas have a number of standard modulesmeeting these specifications; details are available from the Sales Office on request.

Readers may also be interested to know that an independent body entitled the 'Low Power RadioAssociation' (LPRA) exists to cater specifically for the needs of those involved in such activities. (LPRA -01422 886950).

7.2 APPLYING W & D MODULES

Wood & Douglas produce a range of telemetry modules designed to allow the OEM a fast track minimuminvestment route to incorporating radio into his product.

The concept behind the modules has always been that the user should not need to internally adjust ormodify the units in anyway and by avoiding this need it is ensured that the radio satisfies type approval.

The modules are intended to be user friendly in application but a few points are worth making to assist theOEM.

USING RADIO LINKS Wood & Douglas Ltd Applications Manual

36

7.3 INTERFACE

Wood & Douglas modules are generally supplied having a choice of analogue or digital interfaces. Thedifference between the two is as follows :

7.3.1 ANALOGUE INTERFACE

This is an a.c. coupled modulation path through the transmitter. This is ideal for audio signals such as avoice, d.t.m.f. tones, general analogue information, modem tones etc. All such signals have one thing incommon in that if the mean d.c. content over a period is averaged out, it will tend to be zero. This isimportant because with an a.c. coupled modulation path, any integrated or average d.c. content to thesignal will cause an appropriate, albeit short term, change in the transmitter centre frequency which couldlead to information loss. This loss would occur at the receiver site where the receiver would no longer beexactly centred on the transmission being received.

If a digital, (e.g. TTL), signal were to be fed into this input, unless care had been taken with the type ofcoding used for the data, this effect would be obvious due to the unequal nature of the data ‘1’s and ‘0’scontent.

Note that the a.c. response of W&D modules is quite wide from typically 10Hz through to 3kHz makingthem suitable for a wide range of applications and signalling formats.

7.3.2 DIGITAL INTERFACE

This input is for data signals such as TTL levels. The input signal is limited and passes as a directly coupledsignal through to the modulating circuit. The radio signal is effectively jumping from one of two frequenciesdefined by a ‘one’ or a ‘zero’ input level. The d.c. integration effects observed in the analogue interfaceare not apparent due to the directly coupled nature of the input signal path. The apparent contradictionto this is that most receivers are a.c. coupled which potentially re-introduces the problem at the receiver.

If the physical range of an application is relatively short then the link will have an acceptable level of signalin hand and operation using direct digital input to the radios will be acceptable. If however the user wantsto achieve the best possible performance from the radio combination then operation through a modem isrecommended.

7.4 MODEM OPERATION

A separate section deals with this in more detail but suffice to say that a modem allows digital ‘one’s and‘noughts’ to be represented over the radio link by audio tones. Because the audio tones are sinusoidal theyhave a mean d.c. content of zero and therefore remove the problems associated with transmitting‘unbalanced’ data as detailed above. The tones are precisely generated and can therefore be accuratelydetected. This allows the radio to perform at signal levels very close to its natural sensitivity limit andtherefore give optimum performance and range. The modem output is now being fed to the transmitteras an analogue signal rather than a digital one.

7.5 GMSK SIGNALS

As the data rate increases the modem techniques used have to be changed to accommodate this. Thecurrent natural limit for data over a radio is 9600 baud within a 25kHz channel spacing system. This isachieved using a special modem technique called Gaussian Minimum Shift Keying. The GMSK signals aregenerated in special chip sets that are controlled with a microcontroller. When running GMSK it isrecommended that the GMSK modem output signals should be fed to the analogue input rather than thedigital input on a Wood and Douglas module. This apparently contradicts the data sheets associated withGMSK chip sets. In practice an audio passband of 7Hz to 7kHz is needed for GMSK making a.c. coupling


37

acceptable and so removing problems associated with drift which would be apparent with d.c. coupling.

It is important to increase the high frequency response of the radio to allow a 7kHz audio response. Thisin normal circumstances would contravene the type approval process. If a ‘normal’ 7kHz audio signal wereto be transmitted in a modified extended audio transmitter, the adjacent channel power would be excessive.GMSK techniques allow such signals to be processed and filtered to avoid this situation. The radio modulewill still need modification for this type of operation and the OEM should consult our Sales Office fordetails.

7.6 RECEIVER OUTPUT

The audio output from a Wood & Douglas radio module is usually at a low level of a few hundred millivoltspeak to peak. This will be taken directly from the discriminator or demodulator and will have had aminimum amount of processing prior to passing to the output connector. This allows the OEM maximumflexibility in the approach that is adopted to signal recovery. One or two radios in the Wood & Douglasrange have a data slicer included.

7.7 PHYSICAL CONNECTIONS

The majority of the modules in the W&D range have a 9 way ‘D’ type connector for all the interfaceconnections. The design of the module will have taken into account any filtering necessary to theseconnections to ensure compliance with spurious emissions and EMC regulations. The user should thereforenot need to take any further precautions against spurious emissions. The mating ‘D’ connector is notgenerally supplied.

Some modules have a simplified interface consisting of solder connections on feed through capacitors oran array of 0.1" pitch header pins.

7.8 ANTENNA CONNECTIONS

The favourite antenna connector supplied on W&D modules is the SMB, sometimes referred to as Sub-Miniature Bayonet. This is a small push on connector with a good RF characteristic to low GHzfrequencies. For those requiring a more rigid connection, an SMC connector can be fitted with a surcharge.This is a direct equivalent to the SMB but with a screw thread. It should be said that few difficulties havebeen experienced with the SMB series over many years of usage. Both of these connectors are smallenough to be mounted on the wall of standard W&D enclosures.

If a larger connector such as a BNC is needed (or TNC which is the threaded equivalent), quite often theonly well engineered way to implement this is on a flying lead. This lead can be cut to the users’ requiredlength. In these circumstances it is preferred to fit a BNC Bulkhead Jack. This gives a correctly terminatedshielded cable entry and allows rear mounting of the connector in the customers enclosure.

This raises an often overlooked, but important matter and that is how the OEM connects not only to theSMB connector but also how the connection is made to the ‘outside world’ antenna.

Unfortunately good wiring practices using insulating sleeves do not apply to coaxial terminations. For theSMB connection, a series of gland entry and crimp entry connectors are available from all the maincomponent distributors as both straight and right-angle entry. In practice the right angle crimp connectorsare the easiest to fit. An alternative is to buy an SMB to SMB ready made cable from a distributor and cutit in half to make two connecting cables. The free end of each will then need the antenna connector fitting.This could be a BNC, TNC or ‘N’ type according to the OEM’s taste. Note that only the ‘N’ type is trulywaterproof.

It is at this connector where problems arise due to two main influences :-


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a. Use of a ‘tail’ on the coax to make discrete connections to the connector inner and a solder tag onthe connector mounting.

b. Use of polythene dielectric coax (RG174) and its tendency to melt if excessively heated.

The combination of these two give rise to many service returns and ‘no fault found’ service reports. Indetail therefore, we suggest that the ‘tails’, if this is the method which has to be used, are kept to anabsolute minimum. At UHF this should be 5mm maximum. Adding insulating sleeves, whilst good wiringpractice, will increase the lengths necessary to make the connection. A far more elegant method overallis to use a proper cable entry bulkhead jack component as W&D would normally offer on flying leadconfigured products.

The problem of melting coax can be addressed by using RG316 coax which is made from PTFE materialsand does not melt. It is a direct physical equivalent of RG174. It should be noted that the ready made SMBto SMB cables previously mentioned use RG174 cable and appropriate care is needed when using theseleads. As an alternative, the OEM should consider ordering connecting leads in bulk from W&D’sproduction facility. The factory is equipped with a fully automated coax stripping machine for cablepreparation and a wide range of mating RF connectors bought in high volumes with attractive pricing.Simply call our Sales Office with your needs or send a fax showing a sketch of the lengths and terminationsneeded.

7.9 VIBRATION

The W&D modules are generally immune to vibration effects but should the application using the modulesexperience high levels of vibration this should be discussed with W&D technical support engineers.

7.10 POWER SUPPLY

The modules in the W&D range are designed to be relatively immune to power supply influences. Eachmodule will have a defined operating input voltage range and internal voltage stabilisers will regulate andprocess this input to the module requirements.

Two important points should be considered. Firstly if a switch mode power supply is being used, is thissufficiently well filtered to avoid clock noise being transferred into the radio module? It is difficult to makeintegrated regulators reject all possible input supply variations particularly as the frequency of theseincreases. The worst possible situation is where the SMPU clock is running at 12.5kHz or multiplesthereof. (See section 8 on Interference)

The second problem arises where a power supply being fed to a module runs with sufficient capacity onreceive but cannot withstand the sudden switch on surge when the transmitter is keyed. This can lead toserious interference potential. If in doubt monitor the rails and check. In the extreme this could cause asynthesised transmitter to momentarily kick out of lock.

7.11 ENCLOSURES

Wood & Douglas use tin plated steel for their module enclosures. In some instances tin plated brass can


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be offered where harsh corrosive environments are to be encountered.

The approval of the radio by the telecommunication authorities is based upon the module being in the styleof enclosure as documented at the time of approval. If the OEM modifies the radio enclosure in any wayon receipt of the goods from Wood & Douglas, then the approval is no longer valid. In the extreme casethis will be an offence under the Wireless Telegraphy Act or worldwide equivalent.

W&D technical support staff can advise on such issues if advice is needed.

7.12 OUTPUTS AND INPUTS

W & D modules will often have extra user facilities as part of the interface connections. These will include :-

Squelch Flag : an output from receivers that indicates when a signal is received. Some modules havea transistor driver included, some just a simple level change. Details are usually givenin the associated module data sheets. There is little standardisation within the industryas to whether an illuminated LED should show receipt of a signal or loss of signal!

RF Present : this is the output that gives a high level to indicate that RF is present at thetransmitter output. It is usually a signal derived from rectifying a little of theoutput RF energy but on synthesised equipment it could be taken from thesynthesiser status detector.

TXE : this is the W&D label given to transmitter enable or the push to talk line. Usuallyit has a ‘ground to activate’ sense.

‘S’ meter (RSSI) : this is an analogue output from receivers which will give an indication of therelative strength of received signals on the frequency of operation. The dynamicrange will be 40dB or better and will be logarithmic in characteristic. The exactcharacteristic will vary between module types but should be consistent within anyone type.


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Wood & Douglas Ltd Applications Manual INTERFERENCE

41

Section 8Interference

8.1 INTERFERENCE

In today’s crowded radio spectrum it is all too common for a radio user to experience interference with theirtransmissions. The sources and cause of interference can be a complex problem to solve. This sectiondetails potential sources and suggests possible solutions.

Interference can be grouped into the following types :-

(i) Non radio related sources external to the radio;(ii) Internal to the radio and associated electronics;(iii) ‘In-band’ radio interference;(iv) ‘Out of band’ radio interference

8.2 NON RADIO SOURCES

With regard to interference from non radio sources, i.e., man-made electrical and radio frequency noise,the following comments based on Wood & Douglas' own experience are offered.

In Europe (since 1992) more stringent demands have been made on radio equipment manufacturers in orderto satisfy the new EMC requirements. These are concerned mainly with the immunity of other externalequipment to the effects of interference and more specifically to the level of interference generated by radio(and computers etc.). Radio equipment manufacturers such as Wood & Douglas whose design techniquesare of a high standard have little trouble however, in satisfying these new controls.

Electromagnetic radiation from electrical supply cables, including high voltage grid lines, rarely if everbecomes a problem. Equipment operating in the VHF bands around say 173MHz is more prone tointerference from electrical sources than would be the case for UHF or microwave frequencies. Obviouslythe degree to which equipment is susceptible to interference will depend very largely on the design of thereceiver and the quality of components used. (This will of course be reflected in the price paid for hardwareand the cheapest products will often not give satisfactory performance in difficult environments).

UHF equipment has been used very successfully for automated warehousing systems, where the radiotransceivers are frequently mounted on the electrically-driven vehicles themselves and in close proximityto high power solid-state switching circuits with no ill effects. Poor performance can usually be attributedto (i) poor siting of antennas, (ii) excessive range or (iii) antennas too close to the ground thereby providinginsufficient 'clearance' for a successful transmission path. (See section 4.1 for more information regardingtransmission paths).

8.3 INTERNAL SOURCES

8.3.1 REFERENCE CLOCKS IN ASSOCIATED EQUIPMENT

Today’s modern electronic equipment will almost certainly contain a clock for timing purposes. Quite oftenthese clocks are derived from a high frequency crystal that is divided down to the lower frequency of thecircuitry in use. It is possible for higher multiples or harmonics of the crystal used as the clock source tofall on the exact frequency that the radio link is trying to operate on.

This is obviously a highly unfortunate and unlucky set of circumstances and is more likely to affect a VHF

INTERFERENCE Wood & Douglas Ltd Applications Manual

42

radio system rather than UHF as each multiple of the crystal will decrease in energy. However, ascomputer clock speed increases, this situation is changing and UHF is becoming more affected.

It should not be forgotten that a modern high quality radio will use a heterodyne or mixing technique todown convert the high frequency UHF or VHF signals to low frequency where it can be more readily filteredand processed. Typically this will be a conversion to 45MHz, 21.4MHz or 10.7MHz, followed by a secondconversion to 450kHz, 455kHz, 470kHz or similar. A clock sub-multiple at any of these frequencies willcause similar problems.

8.3.2 RADIO RELATED SOURCES

A radio receiver in its conversion processes will need a locally generated frequency source to mix with theincoming signal. The difference of the incoming signal (RF) and the local oscillator (LO) is called theintermediate frequency (IF) and as detailed above, is usually a commonly accepted frequency. The secondconversion process is likewise.

For each conversion the LO signal could be a source of self interference whereby a multiple of perhaps thesecond LO signal falls on the frequency being listened to. For example, 20.945MHz is often used inreceivers to convert 21.4MHz to 455kHz. The 21st harmonic of 20.945MHz is 439.845MHz and couldtherefore be a problem if this frequency is being used.

The fortunate fact on this type of interference is that the W&D test procedures will bring these to light priorto issue and appropriate steps can be taken to minimise or solve the problem.

8.3.3 POWER SUPPLY RIPPLE

Quite often modern equipment has a switch mode PSU as part of its circuitry to ensure maximum powerefficiency. SMPU’s are electronic chopper circuits which create an a.c. waveform from a d.c. input to allowit to be transformed to a new potential where it can be rectified and smoothed back to d.c.

This process causes switching noise at the chopper frequency. A great deal of care needs to be employedto ensure that this noise is fully filtered and suppressed from the power supply lines fed to the radio moduleand careful choice of switching frequency is needed.

If, for instance, the switching frequency were to be 25kHz and this were to pass through into thetransmitter it could cause spurious sideband signals at 25kHz intervals around the main carrier output. Anyreceiver listening to adjacent channels would hear these as interfering signals. Likewise, a 25kHz ripple onthe receiver local oscillator signal (LO) would create a whole array of mini LO’s on the adjacent channelfrequencies causing the receiver to effectively listen on these channels at a reduced sensitivity.

Care and attention in filtering of the supply to the modules is therefore essential. All W&D modules acceptan unstabilised supply and then undertake further stabilisation and filtering. However, it is difficult to ensuretotal immunity to all possible frequencies that could be carried on the PSU wiring.

8.4 IN BAND INTERFERENCE

In-band interference can be defined as problems due to signals which are within the RF passband of themodule in question. This will be the switching bandwidth as detailed on a data sheet.

8.4.1 ON CHANNEL INTERFERENCE


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This is the worst case and most obvious form of interference. It is when another user has chosen exactlythe same frequency as the one you wish to use. It can also be most frustrating to verify if the other signalis intermittent or has a long duty cycle. This leads to data loss on the required link for no apparent reasonat odd times of the day and of course never when a site visit is arranged. If this is the case, a full site radiosurvey will be needed to identify the existence of the problem and to locate its source and, if necessary,its legality. W&D have developed a range of software controlled radios that can be used as a unattendedlogging devices for on site surveys. Please contact the Sales Office for details.

If logging of data corruptions can be undertaken, a plot can be made to try and correlate when and howoften the problems occur. Is it only during working hours? Is it at the same time each day or hour etc?Having traced the source and its legality there may be potential to negotiate with the other user to modifytheir power output levels or antenna configuration to reduce the impact on your service. It may also bepossible to modify your own receiving site’s immunity to the interference by use of a directional antenna,or changing from vertically polarised aerials to horizontal or vice versa. Remember, however, that this typeof interfering signal is always going to be on the frequency and is not a characteristic of the equipment youare using. In some ways however, the technical specification of the radio may be too good, i.e., toosensitive and this may be adding to your problem. Perhaps try fixed attenuation in the aerial lead. This willreduce sensitivity AND transmitted power but may help if you have adequate performance to throw someaway. It will make the interfering signal that much less dominant.

Remember that your Wood & Douglas radio receiver module is capable of letting you monitor what is onthe frequency by simply connecting to the recovered audio output with a high impedance ear piece or smallamplifier driving a speaker. Much can be learned by this simple monitoring of the frequency.

8.4.2 ADJACENT CHANNEL INTERFERENCE

Each of the telemetry bands are divided into channels in either 12.5kHz or 25kHz steps. The channel eitherside of the one you have chosen to use is the adjacent channel. The ability of your receiver to only listento the signal on the required frequency while ignoring any signals on the adjacent channels and beyond iscalled the selectivity of the receiver or adjacent channel rejection.

The ability of the receiver to discern only the channel that you wish to receive is a reflection of the filteringof the first and second intermediate conversion frequencies. More filtering will ensure better rejection ofsignals on adjacent channels but also a more expensive radio. Typically figures of 60dB should be soughtto ensure relative trouble-free operation from this effect.

What does this mean in practice? Simply that a signal on the adjacent channel has to be 1,000,000 timeslarger in amplitude to be heard at the same level as the signal trying to be received. This sounds impressive,but in the radio world such variations in levels are possible.

A more complex cause of adjacent channel interference can come about due to an effect called reciprocalmixing. This is more common in poorly designed synthesised receivers.

The mechanism for this effect is that a poorly designed synthesised local oscillator in a receiver will notinject a pure signal into the conversion mixer. Imagine it as a signal with a whole host of noise sidebandsgradually diminishing in amplitude away from the centre frequency being injected. Mathematically, if anyof this energy spreads over into the adjacent channels and beyond, it will provide a ‘local oscillator’constituted of noise, for any signals on the adjacent channels. This will mix and create a down conversionof the adjacent channel signal to the intermediate frequency in use, just as the wanted signal does.

Because this is an ‘accepted’ signal to the receiver due to its mathematical relationship of RF to LO to IFconversion, no amount of IF filtering will make any difference. It’s a real signal as far as the receiver IFis concerned. It will be reduced in its dominance only in direct relationship to the amplitude of the localoscillator signal to the amplitude of its adjacent channel noise in the adjacent channels.

This is an important factor to check with the receiver supplier. Ask what is the adjacent power of the


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(6) Lower third-order intermod

(7) Upper third order intermod

Figure 19 Interference caused by third order intermodulation

receiver oscillator. It should be specified for a transmitter but rarely is for a receiver. A good figure is 70dBand ideally 80dB. Figures below this will falsify the adjacent channel rejection figure quoted if it is basedpurely on poles of IF filtering rather than the real dynamic effect of these.

Incidently the accurate measurement of adjacent channel rejection is also subject to how noisy the signalgenerator is which is being used for the measurement.

Like many such related problems, paying a little more for a quality receiver will help avoid this type ofproblem. Very low cost receivers having little adjacent channel filtering could let the whole band be receivedat once if sufficiently strong signals were on every channel!

8.4.3 INTERMODULATION

This can be an in-band or out of band effect but it is easier to explain as an in-band one.

When two frequencies mix, as could happen for example within a transmitter output stage, or perhaps inan overloaded receiver input, they generate further frequencies (plus additional sub-multiple frequencies).This is the phenomenon known as intermodulation. By far the most troublesome intermodulationfrequencies are 'third order' products which can be calculated thus:

where: f1 = lower transmitter frequencyf2 = upper transmitter frequencyintermod = unwanted third order intermodulation frequency


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EXAMPLE

Taking a practical example where our transmitter is working on 458.575MHz and there are two nearbytransmitter sites operating on f1:458.525MHz and f2:458.550MHz, the third order intermodulationfrequencies produced are:-

(2f1-f2) = 458.500MHz (int1)(2f2-f1) = 458.575MHz (int2)

As Figure 21 illustrates, the third order intermodulation frequency produced by the second calculation (int2)lies exactly on top of our main transmitter frequency of 458.575MHz and this signal, if powerful enough,will obliterate our transmissions completely. In order to minimise the risk of any interference being createdwhen planning co-sited radio equipment (especially transmitters), always strive to use the lowest RF poweroutput required to accomplish the task and avoid placing transmitters in close proximity. Also where anumber of transmitters are co-sited, care must be exercised when allocating frequencies if these problemsare to be avoided.

Intermodulation problems also affect receivers in a similar manner, i.e., where unwanted signals are mixingto produce other spurious frequencies. The most common problem is where a receiver is 'in range' of otherlocal transmissions which produce strong signals at the receiver's antenna. If the wanted signal is greaterin strength than the intermodulation products, then it is unlikely to cause a problem. However, if thereceived signal is at a lower level than the intermodulation products then the user will have seriousproblems. (Even if this is not the case there will still appear to be a signal present when the user’stransmitter is switched off).

Such a situation can arise on a multi transmitter site where one transmitter is very close to the receivercausing the overload to occur. Once the receiver is in overload then the problem can occur.

The effect gets much worse if there are lots of overloading signals. The effect also occurs in transmitteroutput stages.

How can intermod be avoided ?

- Careful choice of frequencies will help. An optimum intermod bandplan should be considered.

- Careful choice of transmitter/receiver siting and use of the minimum power necessary for error freeoperation will help to avoid overload situations.

- Careful choice of the receiver module is a major factor.

For a receiver to be able to cope with very large signals in a relatively intermod/overload free manner, thefront end design needs care. It is impractical to have a filter on the receiver front end that is only onechannel wide so it must be accepted that the first RF amplifier stage will see lots of signals as well as theone you require. In order to avoid going into overload by the array of inputs, the front end device willgenerally need a significant amount of current passed through it. This will also apply to subsequent RFstages. Once passed the first mixer selective filtering takes place. This is at the IF frequency however asalready mentioned, where such selective filtering is more practical to achieve.

Therefore a radio receiver which is designed for low current operation will by definition be compromisedon its strong signal handling and intermodulation performance. Typically 5-10mA will be needed in eachRF stage to ensure relatively trouble free operation. Some receivers offered on the market consume only5mA in total.

If you have a crowded radio spectrum on site, this factor must be considered. Check the manufacturer’stotal current consumption and ask what the intermodulation specification is. A figure of 50dB minimumis ideal and for a really good product, a figure of 65dB is typical. As ever this has an impact on costs.

8.4.4 DE-SENSITISATION


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De-sense is a similar dynamic effect caused by strong signals in- band but not on the operating frequency.These totally overpower the front end of the receiver causing it to partially or fully shut down. This leadsto a loss in sensitivity and a reduction in the strength of the signals that the receiver is trying to hear. Itcan be heard as sudden ‘silences’ on the receiver audio. This effect has sometimes been observed on verylow cost radio links where the instructions indicate that the transmitter should not be used ‘too close tothe receiver’ as this causes a problem leading to loss of signal. The reader is left to judge the merits ofsuch a marketing virtue.

An example of where de-sense could occur is when MPT1329 products are used by customers on primesites which also have co-sited MPT1411 equipment. MPT1411 frequencies straddle 458MHz at 457MHzand 463MHz. As up to 10 Watts could be radiated from an MPT1411 device, a local MPT1329 receiverwould struggle unless well designed.

The solution, once again, is care and attention in design of the receiver ensuring that the larger signalhandling parameter such as blocking has been addressed. A very low current receiver will not perform aswell as one with a robust front end. Ask your supplier what the blocking specification is and then decideif it is appropriate for the site being considered. A typical figure should be 80-100dB.

8.5 OUT OF BAND EFFECTS

Out of band effects are due to signals outside the switching bandwidth of the modules in use. Some of thein-band effects already described can also be caused by out of band effects but in general the signalscausing the problem will have to be significantly larger in amplitude due to the filtering effect of the receiverfront end circuitry. This circuitry will not have an infinitely sharp response however and out of band signalscould by definition only be 10MHz or so spaced from the operating frequency. All the previously mentionedinterference effects should therefore be considered when assessing a problem.

8.5.1 IMAGE REJECTION

The mixing process in a superhet receiver takes the incoming radio signal and mixes this with a locallygenerated signal (LO) derived from a crystal or synthesised source within the receiver module. Thedifference frequency between the two will then be taken and processed to recover the information beingcarried by the signal. This difference signal, as already mentioned, is usually chosen from a long list ofstandard frequencies of which 45MHz, 21.4MHz and 10.7MHz are the most common.

If an incoming radio signal is on 455MHz and a 45MHz intermediate frequency is being used, then thereceiver must locally generate 400MHz to create a 45MHz difference. Alternatively a locally generated490MHz would still create 45MHz difference frequency. These two possible injection frequencies from theLO both give the same result. One is a ‘low side mix’ and one is a ‘high side mix’. Equally however, forany one injection frequency there will also be a second RF frequency which will give a 45MHz IF. In theexample above, whilst 455MHz mixed with 400MHz gave 45MHz, so does 355MHz mixed with 400MHz.This second frequency which is acceptable to the IF, is called the image frequency. Remembering it issimple. Think of it as a frequency that is a ‘mirror’ frequency, as it is equally spaced or ‘reflected’ at the‘other’ side of the LO signal, whether as a high or low side mix.

If the receiver front end filtering is not adequate to reject any signals on this image frequency, they willpass through and be just as acceptable to the IF processing electronics, causing a serious problem.

This image rejection parameter is specified on radio data sheets and a figure of 60dB or more should betargeted. A manufacturer can readily achieve this figure by using a high first IF frequency since the imageis always separated by twice the IF frequency from the wanted signal.

Incidently, the mixing process will create a sum of the two frequencies but this is totally ignored and easilyfiltered out.

8.6 SUMMARY


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This is a brief outline of all of the more obvious mechanisms for interference to occur within modernsuperheterodyne receiver modules and transmitters. The problems of spurious responses in receivers ismade worse with crystal derived local oscillators as there is more potential for other frequencies to bepresent at the mixer of the receiver. This is because the LO signal is usually derived by multiplying a lowfrequency crystal to a higher frequency. This is a dirty process with potential for generation of manyspurious outputs that will be injected into the mixing process. Modern synthesised receivers have a localoscillator generated at the final injection frequency and therefore are cleaner.

As an OEM there will be confusion when choosing between radio receivers all of which are apparently toETSI 300 220 or MPT1329, but at widely ranging prices. It is hoped that the above details will help clearthe confusion as to which is a good device and which not. It will also hopefully have illustrated some ofthe problems that a poorly specified receiver could create when operated in today’s crowded spectrum.

If you want a robust, reliable device to withstand today’s demanding telemetry band then you must askand pay attention, to those confusing spec sheet parameters such as intermodulation, blocking, de-sense,image rejection and adjacent channel rejection. The success and reliability of your radio link may dependon it. But so will the price.

Wood & Douglas have always adopted the policy that a radio receiver must have these parameters wellspecified as standard. Use has always been made of high quality components and techniques to ensurethe success of a radio link utilising W&D products.

Faced with a problem on a site it is possible by the use of additional components (bandpass filters, hybridcombiners, circulators, etc,) to reduce the effects of some interference effects. Like all remedial action thiscan prove expensive and may require a great deal of space in which to mount the necessary items of extrahardware. In all cases it is therefore more prudent to plan any multiple radio-based installation in advance so that, withcareful consideration of frequencies and positioning of sites, the user can expect the best possible results.Wood & Douglas can provide the necessary resources required in order to assist with this planning; pleasecontact the Sales Office for more information.


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Wood & Douglas Ltd Applications Manual MODULATION AND MODEMS

9 RS232 serial interfaces also provide handshaking but with numerous differing conventions adopted by manufacturers.

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Section 9Modulation and Modems

9.1 WHAT TYPE OF SIGNALS CAN I TRANSMIT?

Almost any signal can be transmitted over a radio link provided the necessary signal conditioning isperformed between the user’s equipment and the radio circuits themselves. In other words, radiotransmitters and receivers only handle analogue signals, so provided we convert our original informationinto an analogue form (i.e., an audio frequency), we can use a radio for communication purposes. (Seesection 7.4 for more information regarding conversion from digital to analogue format).

Wood & Douglas have a wide range of RF designs which cater for most requirements so far identified bytheir user groups. These range from basic OEM radio modules, which can be incorporated into otherequipment, to stand-alone radio modems supplied in weatherproof housings, or completebroadcast/telemetry network systems.

All Wood & Douglas radio transmitter modules accept either analogue or digital modulation inputs, and withminimal external circuitry may be adapted to cater for almost any signal type required. The radio receivermodules are normally supplied without data recovery circuitry, allowing users to implement the requiredlevel of sophistication and style of output thought necessary. In practice, therefore, this means that theoutput will be an analogue signal (sometimes referred to as the audio output) of around 200mV typicallyinto 10K ohms. This is a deliberate policy decision with regard to RF module designs which allowsmaximum flexibility for all potential users.

9.1.1 ANALOGUE DATA

Of the many varieties of analogue signals that may be considered, the following types are commonlyhandled using radio telemetry equipment:

1) Current loops2) 4/20mA instrumentation3) Voltage4) Frequency/tone

Wood & Douglas transmitter modules accept audio tones between 10Hz and 3kHz at voltages between300mV and 3V peak to peak. A number of integrated circuits are available which provide signal conversionfor the other forms of analogue signals, e.g., from 4/20mA current inputs to a voltage suitable for directconnection to a Wood & Douglas transmitter module. By interposing these devices between the user’ssource signal and the radio transmitter we have the ability to handle almost any type of analogue signal,irrespective of amplitude.

9.1.2 DIGITAL DATA

Digital data comes in various forms which may be confusing to the unwary. The terms serial or parallelfor example describe the way in which the information is presented, i.e., serial information is sent downa single wire (or radio channel), whereas parallel data is transmitted down a number of wiressimultaneously. This may comprise eight or more wires and will include a number of control signals(otherwise known as "handshakes"),in addition to the eight data lines.9

MODULATION AND MODEMS Wood & Douglas Ltd Application Manual

10 Various encoding techniques are becoming available which allow higher data rates to be used whilst maintaining thelimited RF bandwidths permitted. Consult the sales office for information regarding the current level of development.

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Figure 20 Simple illustration of serial vs. parallel data

Figure 21 The most commonly used form of RS232 interface

Obviously, where we use more than one wire toconvey our data, we can pass much moreinformation in a given time than is possible usingjust a single wire. The use of handshaking alsogreatly enhances the performance of parallel orserial data links allowing intelligent decisions to bemade concerning the flow of data, where, forexample, a receiving terminal requires a re-transmission following corruption of a data block.Remember that a radio link only handles a serialdata stream and does not provide the means tocontrol data flow (in real time) using handshakes.

All digital data intended for transmission over radio links therefore will be in a serial format which canloosely be described as RS232. Technically speaking, the term RS232 defines a single-ended bipolarasynchronous serial communications protocol; in plain English this means that the data travels down asingle wire, has voltage peaks both positive and negative and has built-in timing within each data byte sent

RS232 is a serial data communications standarddevised in 1969 by the EIA (Electronic IndustriesAssociation) in response to the (then) growing needfor some form of control over the use of telephonelines for the transmission of digital information.(RS232C was simply a subsequent revision of theoriginal standard). At roughly the same time theConsultative Committee on International Telegraphyand Telephony devised their own serial standardknown as CCITT V.24 which is almost identical toRS232 in style and operation.

In either case the data is sent in 'serial' format i.e., one character after the other down a single telephonewire (or in our case radio channel). The 'speed' (data rate) of RS232 is effectively limited due to therelatively large voltage excursions employed (between ±12 volts) which coupled with the severerestrictions in cable length (a maximum of 50 feet) place limitations on the practical use of the standardin computer systems of the 90's. RS232 has therefore been largely superseded by other modern high-speed methods in order to serve the current generation of telecommunications equipment, although youwill almost certainly still find at least one RS232 port on the rear panel of your PC.

RS232 is, however, ideally suited for the purposes of radio transmission for two reasons; (i) we only havea single channel as our communications medium and (ii) because the bandwidth of the transmission isseverely limited by the authorities, we are unable to transmit at high speed10 and therefore thecomparatively low data rates of RS232 are acceptable.


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Strictly speaking, when specifying the data input and output to radio-based communications equipment (assupplied by Wood & Douglas), our use of the term RS232 is somewhat misleading because only 2 physicalconnections (data and ground) are implemented of the possible 21 connections to be found on a fully-implemented RS232 interface. There are, however, additional inputs and outputs provided on the modulesavailable from Wood & Douglas which enhance the overall operation of the equipment. (Please consult therelevant operating manual for more information).

9.2 USE OF MODEMS

A modem is simply a device (usually a single integrated circuit) for converting digital information toanalogue information (MOdulator/DEModulator), or vice versa. Modems were originally designed to permitdigital information to be carried over the public telephone network and were sold as third party add-on’sto PC users. With the widespread use of digital communications today, modems are often built into PCsas standard, complete with a user program to automatically dial numbers and send data.

An RF modem has many advantages over conventional line modems. The most obvious is that there isno longer a physical connection between the transmitting modem and the receiving modem. Thus one orboth ends can move easily without the necessity of moving any cables. This may also have a cost benefitas there will be no prohibitive line installation and rental costs. There may also be physical constraintswhere a cable is impractical (the North Sea for example).

An RF modem has another advantage over line modems in that a transmitter can communicate with allreceivers on the same radio channel simultaneously. In emergency situations, the time taken to get amessage to a large number of outstations may be important.

In operation therefore a modem simply responds to a series of digital one's and zero's presented to it bya computer, by generating two distinct audio frequency tones which it then sends down the telephone line(radio link). The receiving modem only needs to distinguish between the two tones whose frequency isalready established and can therefore apply very selective filtering to remove all other unwanted signalspresented to it. As mentioned elsewhere in this manual, the advantage of an audio tone representing adigital signal is that the averaged d.c. level of such a tone is zero so any integrating effects due to meandata content are eliminated. The radio can then be used and regarded as a capacitively coupled circuit.

Wood & Douglas use modems to provide similar facilities for their customers using radio modules in placeof telephone lines. A range of standard modules complete with integral modems offering RS232 datainput/output are available ranging from simplex transmitter/receiver pairs to half duplex transceiversoperating at 9600 Baud. Digital information presented to Wood & Douglas' standard 1200 Baud radiotransmitter (SurTel) can be in any format the user wishes, provided the data rate is the same as that of themodem. i.e., Data can be synchronous or asynchronous, word lengths are entirely decided by the user,the use of start and stop bits plus parity checking is also in the hands of the user. Consider this type ofradio data link as a 'transparent' communications medium. Serial Computer data is transmitted andreceived at widely differing speeds, which are quantified as bits per second or Baud rate. Simple telephonemodems tend to operate at fairly low speeds 300 to 1200 Baud max. Computers on the other hand cancommunicate with other computers at very high speed (many thousands of bits per second or more), whenconnected via a cable.

When using radio as the transmission medium however, the data rate is restricted because of the narrowbandwidths available within the frequencies allocated. Currently Wood & Douglas can provide asynchronous radio link using GMSK techniques operating at 9600 Baud (maximum); other developmentsare expected in the near future. It should be stressed that not all radio modems will necessarily work witheach other, as their compatibility will depend upon which tone set is employed by the manufacturer of themodem itself. For example American modems use a 'Bell' standard, whereas British networks operate withthe standard CCITT tone set.


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1E-07

1E-06

1E-05

0.0001

0.001

0.01

0.1

Bit

Erro

r Rat

e

10 15 20 25 30 S/N Ratio (dB)

1200 baud

2400 baud

4800 baud

9600 baud

Modem Bit Error Rates

9.2.1 MODEM TYPES

Wood and Douglas have a range of RF modems to suit different applications. Which is most suitabledepends on the following factors:

Single or two way communicationReliability and speed of data transmissionIntelligence of user interfaceType of data

9.2.2 SINGLE OR TWO WAY COMMUNICATIONS

Modems can be supplied as one way links, half duplex links (both way, but only in one direction at onetime) and full duplex (in both directions simultaneously). If only a single frequency is available, only a singledirection link or a half duplex link can be provided. This is because the receiver is disabled while thetransmitter is active. For full duplex operation, two frequencies must be used. These frequencies mustbe separated in frequency far enough apart such that the transmitter cannot effect the receiver. In the UKthere are no license exempt bands which are wide enough for this as typically a 5 MHz minimum split isneeded between the two services. Instead for a full duplex link to be provided, one path (A to B) will needto operate at VHF and the other (B to A) will need to operate at UHF.

9.2.3 RELIABILITY AND SPEED OF TRANSMISSION As with most modem systems, the faster the transmission speed, the more susceptible the data is toexternal interference and noise. RF modems are further restricted by the rules governing the licensing ofthe radios. To ensure transmitters do not interfere with other transmitters on adjacent frequencies, thereare strict limits on the levels of deviation (how much the transmitter can be modulated) and on how higha modulating frequency can be. This severely limits the upper frequency response of the RF link which inturn limits the maximum rate of the modulating signal.

A further consideration is the channel spacing (how far apart adjacent channels are) of the frequencyallocation being used. As the RF spectrum becomes more crowded, narrower channels are being used.In the UK and Europe the three common spacings are 12.5, 20 and 25 kHz. The narrower the channelspacing, the lower the maximum modulation bandwidth.


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One of the most effective modulation systems for use with radios is Gaussian Minimum Shift Keying(GMSK). The maximum baud rate using GMSK is 4800 baud at 12.5 kHz and 9600 baud at 20 and 25kHz channel spacings.

This data rate of 9600 baud is the most realistic maximum baud rate that can be used at the moment.Wood & Douglas have found that the use of faster modulation systems results in a higher Bit Error Rate(BER). A consequence of such errors is the increased need to resend the information which means that theeffective throughput baud rate begins to fall with no real speed advantage to the user.

Several modems are available at various maximum baud rates, using different modulation schemes. Therelative performances of these modulation schemes in the presence of noise is shown in the graph on theprevious page.

9.2.4 INTELLIGENCE OF USER INTERFACE

As the speed of the modulation system increases, the setting up of the communications channel becomesmore complex. The faster the modem, the more important it is to synchronise the receiving modem withthe transmitter. The data may also have to be put into packets for transmission to enable CyclicRedundancy Checking (CRC) to be carried out. The receiving modem may also need to be told how manydata bytes are being sent so that it can check how many are received. There are also the inherent delaysthrough the radio system. A transmitter needs a period of time for it to come up to power and for thereceiver to detect the presence of RF power. This can take from 1 to 20ms, depending on the type of radioequipment used.

The slower Wood & Douglas modems are un-intelligent in that they have no means of storing data orproviding synchronisation sequences. These must be supplied by the user. The GMSK modems that Wood& Douglas have are intelligent and handle all the data protocols for the user. All the user has to do is inputdata at the transmitter and this data will appear at the receiver after a certain delay. The data is bufferedat the transmitter while the transmitter is turned on and the data packet prepared for transmission. At thereceiver the data is output after the packet is received. This has the disadvantage that there is a delaythrough the modem system which is larger than for the slower baud-rate modems. This is usually offsetby the faster data for larger packet sizes. The GMSK modem also carries out a form of data compressionas the transmitted data is sent as 8 bit data only, the Start and Stop bits being removed.

9.2.5 TYPE OF DATA

So far we have discussed simple serial data. This will be predominantly at RS232 levels. The slower Wood& Douglas modems have a link selectable choice of either TTL or RS232 levels. The GMSK modem hasonly an RS232 interface. For communicating at RS422, RS485 Wood & Douglas recommend the use ofexternal converters.

Other forms of data can be sent over an RF modem. These can be from simple on/off commands toanalogue and digital data. Wood & Douglas have a wide range of system building blocks with the followingcharacteristics:

ON/OFF Control:

Simple continuous transmission of one or many inputs to one or many relay outputsFail-safe operation with a closure on non-receipt of valid dataLatched commands only transmitted on a change of state of an inputAddressable modes with one transmitter and many addressable receivers


54

Analogue Links:

Multiple analogue inputs transmitted over radio link to analogue outputsDigital inputs can be multiplexed with analogue data


55

9.2.6 INTELLIGENT MODEM FUNCTIONS

The GMSK modems have many features available due to their in-built intelligence. These are listed below:

Configurable RS232 baud rates from 300 - 38,400 baudOdd, even or no parity9600 or 4800 RF baud rate (to allow for different channel spacings)Configurable source and destination addresses for multiple users on the same channelConfigurable transmitter/receiver start and stop delaysConfigurable packet size for optimum data transmissionConfigurable preamble lengthSquelch over-ride facilityCRC check on or offChannel change of the transceiverData scrambling as standard (used to inhibit any d.c. offset of the data field)

These parameters are set up using a small PC program supplied with the modems. This program has aneasy to use menu structure and auto-detects which COM port the modem is connected to. The softwarehas several useful test features:

a) Transmitter test program which outputs a continuous bit pattern.

b) Link test program which outputs a continuous data pattern which can be monitored on a built-interminal program.

The configuration data is stored in non-volatile memory within the modem.

The configuration data can also be modified using simple ESCAPE [ESC] sequences. This is especiallyuseful when changing the channel of the transceiver as only one serial port is required. If the user softwareor data cannot be modified a separate serial port is supplied through which the transceiver channel changecan be carried out.

The modem also has a STANDBY input which enables the modem and transceiver to be turned on and offwith a logic input. This is especially useful in low power applications when the unit can be powered uponly when required for data transmission.

Details of the configurable parameters are given below:

RS232 Interface

The baud rate and parity of the RS232 interface can be changed from 300 to 38,400 baud. Theparity can be set for Odd, Even and None. For 7 bit data plus parity, use None.

RF Baud rate

As has been described earlier, the maximum baud rate in a 12.5 kHz channel is only 4800 baud.The software within the modem must be set to the baud rate hardware selection on the PCB.The RF baud rate must not be changed without the corresponding change to the hardwaresettings.

Addressing

The units are sent from the factory with the default addresses of 10 and 10. One of up to 255addresses can be set for the source or destination address. Within a unit the addresses can bedifferent. This is most useful where a central station is communicating with a series of out-


56

stations. The destination address of the out-stations is set to the source address of the base-station to inhibit the out-stations receiving data from another out-station.

Transmitter/Receiver Delays

A transmitter takes a finite time to come up to full power. Any data transmitted during this timewill not be received reliably at the receiver. To ensure no data loss the transmitter start time isfactory pre-set for the transceiver fitted to the modem. There is also a certain time for thereceiver to switch from transmit to receive which is also factory pre-set.

Pre-amble Length

To ensure that the receiver modem system synchronises to the transmitter before any data issent, a pre-amble string (defined by the modem manufacturer) is sent. It is normally left at thedefault setting of 20 bytes. In certain circumstances where the received signal strength can beguaranteed, this can be reduced. There is no simple formula for this, the best way is by trial anderror. If the delay through the modem link is not critical then it is advisable to leave it at thedefault setting.

Packet Size

There are two packet sizes allowable of 64 or 128 bytes. If each transmitted packet length isless than 64 bytes then this should be set. With variable or greater than 64 bytes it is advisableto leave it at the default setting of 128. The GMSK system requires an overhead of 7 bytes perpacket as well as the pre-amble. The more packets that are transmitted the higher the overhead.

Squelch Over-ride

With the increased amount of radio traffic, there is now a higher chance that there may be somelow levels of interference which could cause the receiver to think that it is receiving a validsignal. This is used by the processor to determine whether to transmit data or not. In a systemwhere the user knows that there is nobody transmitting, it is best to enable this option. Theprocessor will then ignore the squelch input and transmit the data immediately. If theinterference is low level it will not interfere with the received signal at the other end of the link.

CRC Check On/Off

The receiver checks the Cyclic Redundancy Check of each received packet. If the user does notwish to receive corrupt data then the CRC should be enabled. Any received packet with a CRCerror will be discarded. If all data is to be received then the CRC check should be disabled.

Channel Change

When a synthesised transceiver is fitted to the modem the serial channel change input can beconnected either to the modem processor or to a pin on the modem connector. When connectedto the modem the channel can be changed either through the configuration PC program or usingan ESCAPE sequence embedded in the transmit data.

Data Scrambling

To remove any d.c. offsets (caused by multiple 1s or 0s in the data stream) in the transmitteddata which can effect the performance of the FM radio link, the data is scrambled to ensure thatthis d.c. offset cannot occur. Any person with the relevant receiver and modem system will onlysee a scrambled form of the input data.


57

9.2.7 INTERFACING REQUIREMENTS

The simple modems need more intelligence within the user interface to ensure that data is successfully sentacross the link. An input must be provided for turning on the transmitter. A delay (or extra pre-amblecharacters) must be sent to ensure that the beginning of the message is not lost. At the receiver the usermust be aware that data is being received and strip off the pre-amble. As there is no flow control the usercannot put data into the modem quicker than the modem can transmit it.

With the faster modems flow control becomes more important as data can be presented to the modemat a higher baud rate than it can be transmitted at. When the transmit buffer (-400 bytes) is nearly full,the CTS output is applied to inhibit further data input until the buffer is empty. If the modem is workingin a system with other modems which could be transmitting on the same channel, the modem is configuredto set the CTS output to indicate that the system is Busy. If the modem is working in a system where itis known that other modems will not be transmitting, then the TX OVERRIDE facility can be enabled. Thisallows the modem to transmit when there is interference on the channel.

9.2.8 HAYES© COMPATIBLE INTERFACE

Wood & Douglas have an external interface system which enables the modems to be controlled withHayes© compatible commands. These enable the user to ‘dial-up’ radio modems on a single channel systemin the same manner as if they were normal telephone modems. As a further addition the interface can tryand set up the call on more than one channel. This enables multiple frequencies networks to be set up.

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Section 10Operating modes

10.1 SIMPLEX OR DUPLEX

These two terms are often confused and misused and are therefore defined below. (These definitions arebased on conventional modem usage as opposed to voice radio traffic conventions).

SIMPLEX The passing of information in one direction(transmitter to receiver).

DUPLEX The passing of informationi n t w o d i r e c t i o n ssimultaneously. (Points A-B/B-A). This requires theuse of two separate radiofrequencies (channels).

SEMI-DUPLEX The conveyance of data in (HALF-DUPLEX) both directions alternately (point A to

B followed by point B to A).

Wood & Douglas Ltd Applications Manual OPERATING MODES

59

It is important for the reader to understand the difference between duplex and semi-duplex. Duplex(sometimes also called full duplex), permits the passage of information in two directions at once; this isvery similar to the telephone where two people are able to speak over each other (should they so wish!)Semi-duplex (or half-duplex) involves alternate transmissions, i.e., transmission from A to B is allowed tofinish before transmission from B to A is initiated and so on. (Either one or two frequencies may be used).

When the term semi-duplex is used in RF engineering, it implies the use of some method of switching thevarious transmitters and receivers on and off at the relevant times. This creates the need for either humanintervention or some artificial intelligence (i.e., computer etc) as a means of controlling this switchingaction.

10.1.1 DUPLEX RADIO LINKS

Where duplex radio channels are implemented, there must be a large separation between the twofrequencies used to transmit and receive; this will normally be in excess of 5MHz at UHF. This frequencysplit is necessary to prevent the transmitter 'blocking' the receiver associated with it and additional filteringwill sometimes be required in order to ensure satisfactory performance.

The use of full duplex telemetry systems is not therefore generally permitted due to the large amount offrequency spectrum which would be taken up. Certain radio telemetry bands, such as those covered byMPT1411 for example, are given duplex status, but these would normally only be used by water authorities(or similar bodies) for the supervision and control of their widely-dispersed plant. Unlike the de-regulatedtelemetry bands which Wood & Douglas is concerned with, the use of MPT1411 approval equipmentrequires individual licences to be obtained for each installation; such licences specify the frequencies andpower levels permitted.

Whilst this method is also permitted for use by the broadcasting authorities, the DTI does not sanctionsimilar techniques for industrial or commercial users seeking a duplex facility in the low power telemetrybands.

10.1.2 SEMI-DUPLEX RADIO LINKS

Given the choice, most users would opt for full duplex radio communications. However, because we arefaced with a shortage of available frequencies (and for the reasons stated above) economies in spectrummust be made. For low power telemetry purposes therefore, customers needing two-way communicationswill be required to use simplex or semi-duplex (as opposed to duplex) radio links.

The extensive range of telemetry modules available from Wood & Douglas may be configured for eithersimplex or semi-duplex working as required. In addition simplex/semi-duplex transceiver modules providinga complete transmitter and receiver within a single package are available, including built-in switching fora single antenna connection.

10.1.3 SIMPLEX RADIO LINKS

Simplex radio links make up the majority of sales within the company's telemetry activities; often a simpleone-way link is all that is required to replace an existing cable or to gather information from a remote site.No doubt as technology advances together with the demand for radio and telecommunication services ingeneral, we will see a whole new generation of radio based products offering improved features andperformance.

11 available from the Radiocommunications Agency, New Kings Beam House, 22 Upper Ground, London, SE1 9SA . 0171211 0211

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Section 11Licencing information

Until fairly recently in the U.K. all users of radio-based communication systems were obliged to obtain anoperating licence from the DTI.

All radio equipment manufactured and/or used in this country must be tested in accordance with theregulations specified by the DTI depending on the application and intended use and must have been properlycertified before sale or use. (This is common practice amongst most other countries, each of which willhave an appointed authority exercising control over the use of the radio spectrum).

The regulations published by the DTI11 are given the prefix MPT with a four digit code according to theactual service concerned. Under this practice, manufacturers of radio apparatus must submit one or moreworking production models to one of the approved testing houses for full type testing and approval beforeoffering the product for sale.

The aim of the MPT approval system is to ensure that equipment so validated meets basic performancecriteria thereby minimising the possibility of interference being caused by the use of inferior equipment toradio services, e.g., emergency services, pagers or other accredited users.

In the past all users of radio telemetry in the U.K. were obliged to apply for a licence to operate theirequipment. This licence was renewable and is still required for non-exempt devices. One of the morerecent developments is the provision for licence-exempt devices operating in both the VHF and UHFtelemetry bands (in the U.K.). Users of devices approved under these regulations are not required to obtainan operating licence.

The current de-regulated services in the VHF bands are controlled under MPT1328 (173MHz) and MPT1329for the UHF telemetry & telecontrol frequencies (458MHz). Any equipment manufactured and subsequentlygaining DTI approval under this provision must bear the legend ̀ WT licence exempt' (and state the relevantspecification e.g., MPT1329) somewhere on the external housing of the equipment.

Internationally there are similar regulations. European PTT bodies (equivalent to the United Kingdom's DTI)have specifications and standards broadly in line with the U.K. These are prefixed with the letters ‘ETSI’.Consequently many Wood & Douglas products are, if not already approved, readily approvable to Europeanstandards. In the USA the regulatory body is the FCC. In North America the specifications are in generalnot as severe as they are in Europe.

The new ETSI regulations and specifications are slowly becoming the rationalised standards across Europewith the majority of the Low Power Devices (LPD’s) now being tested against ETSI 300 220. This is ageneric specification which then has country specific modifications. In the UK these remain for the momentas MPT1329 etc. These will be updated to MPT16XX versions in the coming years to reflect moreprecisely the pan European regulations. The higher level mobile radio specification MPT1326 is nowknown as ETSI 300 086 and the data applications equivalent is ETSI 300 113.

EMC approvals in Europe for radio modules is undertaken against ETSI 300 339 or the later ETSI 300 683.

Customers interested in obtaining equipment meeting European or worldwide standards are asked tocontact the Sales Office or our Internet page (http://www.woodanddouglas.co.uk) as this list is frequentlyupdated as new approvals are obtained.

Wood & Douglas Ltd Applications Manual GLOSSARY OF TERMS

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Wood & Douglas Ltd Applications Manual

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Section 12Glossary of terms

AFSK Audio Frequency Shift Keying - The use of audio tones as a modulation signal in orderto represent specific inputs. Logic 0 and 1 data inputs can be represented by two audiotones. Because the transmitter only sends the two tones, very sharp filtering of thereceived signal is possible thereby making this form of data transmission highly immuneto interference.

AM Amplitude Modulation - the impression of an analogue signal upon a carrier wave byvarying the amplitude of the carrier wave in sympathy with the analogue signal.

attenuator A resistive (passive) device used for reducing the amplitude of a signal. Specified in dB.

bandwidth The upper and/or lower range of frequencies over which an electronic circuit willrespond. (Usually quoted at the -3dB points along the performance curve).

BERT Bit Error Rate Tester - used to measure the errors encountered during digital datatransmission.

bit A single item of digital data.

byte A byte is a collection of 'bits', usually assumed to be 8 bits.

carrier The continuous transmission of a particular frequency upon which a modulation signalcan be superimposed to convey information.

channel The specific centre frequency about which a radio signal is transmitted.

channel spacing The frequency separation made between adjacent channels in order to avoidinterference. (Closely related to the bandwidth)

channelling Normally used to express the width of a radio channel in Hertz.

coaxial A type of cable used for the transmission of high frequency signals. Constructed usingan inner conductor wholly surrounded by an outer (circular) braid thus forming a cablewith a 'co-axial' type of construction. This type of cable is said to be unbalanced.

dB (decibel) The dB is a common feature in all aspects of RF engineering. The dB is nota unit of measurement, (such as the Volt or the Amp), but is rather the ratio betweentwo distinct levels of power, voltage or current. If, for example, we were to apply asignal of 1 Watt to an amplifier and we then obtained an output of 2 Watts from theamplifier, we could say that we had a gain of +3dB. (i.e., a doubling of our originalpower level).

The formula for calculating the ratio in dB between two POWER levels is:

10log10 (Pout/Pin).

When calculating decibel ratios for VOLTAGE the formula becomes:

20log10 Vout/Vin.


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dBd Measure of an antenna's gain relative to that of a half-wave dipole.

GLOSSARY OF TERMS Wood & Douglas Ltd Applications Manual

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dBi Term used mostly by antenna designers specifying aerial gain relative to an isotropicradiator. To convert antenna gain given in dBi to dBd subtract 2.15.

dBm As already stated above, the decibel is not a defined unit of measurement but whereengineers wish to imply some known reference level, (in order to clearly define a value),then various suffixes are applied to the dB in order to establish the reference.The dBm is one of the most common uses of the decibel, where a reference is statedthus:

0dBm = 1 milliWatt in 600 ohms. (Normally used in audio engineering)0dBm = 1 milliWatt in 50 ohms. (Normally used in RF engineering)

i.e., where a power level of 1mW is measured in a circuit whose impedance is knownto be 600 ohms (or 50 ohms).

dBW As for dB above but with reference to 1 Watt. (0dBW=30dBm).

de-emphasis The removal (in a receiver) of pre-emphasis previously applied by a transmitter therebyreducing the high-frequency noise generated within the receiver circuitry.

deviation The amount by which the transmitter carrier frequency is varied by the addition of amodulating signal during FM transmission. (Measured in kHz).

demodulation The process within a radio receiver of separating the carrier and the modulation signalsto leave only the modulation signal thereby extracting the transmitted information.

diplexer (Sometimes called duplexer) A passive device for combining signals of differentfrequencies and powers the output of which could be used to feed a common antenna,for example, (will accept signals in either direction).

dummy load A high-power resistor usually connected to the output of a transmitter in place of theantenna during testing to prevent radiation of the transmission.

duplex The simultaneous two-way exchange of information also termed full duplex. (Atelephone conversation is full duplex).

ERP The apparent power level radiated by an antenna when measured in the direction ofmaximum field intensity, i.e., measured as if the antenna was an isotropic radiator.

feeder Usually taken as indicating a coaxial cable used for carrying high frequency signals, e.g.,connecting a transmitter (or receiver) to its antenna.

flag An output signal used to provide confirmation of an event taking place. Usually avoltage level change.

FM Frequency modulation - the impression of an analogue signal upon a carrier wave byvarying the frequency of the carrier wave in sympathy with the analogue signal.

FSK Frequency Shift Keying - Modulation of a carrier wave whereby the carrier frequencyis shifted to pre-determined upper and lower levels which represent specific inputconditions. Typically logic 0 and logic 1 are the most common inputs.

gain A term used to denote the amplification (gain) of an electronic circuit or element.

GHz Thousands of millions of Hz. (109Hz)

high pass


65

filter A device which removes low frequency signals.

hybrid combiner Passive device providing high isolation between two inputs feeding a common output.

Hz Hertz - The unit of measurement of frequency. (1Hz = 1 cycle per second)

impedance The term used to describe the opposition to current flow in an AC circuit (as opposedto resistance in a d.c. circuit).

insertion loss The amount of signal lost after passing through a circuit or specific component. (Theloss is measured in dB).

isolator Isolates one part of an RF circuit from another and prevents unwanted 'reflections' ofother signals affecting a signal source. Typically an isolator is used to present aconstant impedance load to a transmitter and to reduce the level of other signals pickedup by the antenna to which the transmitter is connected.

isotropic radiator A conceptual antenna which radiates power uniformly in all directions, i.e., having aspherical radiation pattern. (Such an antenna cannot be built in practice).

key An external control which activates the RF drive circuits within a transmitter i.e., causesthe carrier wave to be transmitted.

kHz Thousands of Hz. (103 Hz).

low pass filter A device which removes high frequency signals.

MHz Millions of Hz. (106 Hz).

microwave Normally taken as meaning any signal within the SHF band. (As used in satellitecommunications etc., see also SHF).

modem An integrated circuit combining the two separate functions of MOdulator andDEModulator. A device for converting signals from digital to analogue format, or viceversa, for transmission over low quality medium.

modulation The process of mixing (combining) a carrier signal with data, speech etc., therebycreating a composite modulated signal.

narrowband Describes a device which handles only a small or narrow band of frequencies. (See alsowideband).

PA Power amplifier - often a separate external module (usually having its own heatsink)used to provide high-power amplification of the output from a transmitter.

pre-emphasis The boosting of high frequency signals in a transmitter with the object of reducing highfrequency noise in a radio link as a whole.

repeater An intermediate radio station equipped with a receiver and a transmitter locatedbetween a main transmitter and remote receiver. Having received incoming signals therepeater simultaneously re-transmits them thus restoring the original signal level. (Usedto cover long distances without the use of very high power).

resistance The term used to describe the opposition to current flow in a d.c. circuit.

selectivity The ability of a radio receiver to pick out the required signal from two or more signalson different frequencies arriving at its input.

semi-duplex Alternate two-way transmission of information.

GLOSSARY OF TERMS Wood & Douglas Ltd Applications Manual

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sensitivity The measure of the ability of a radio receiver to recover a signal at low level withacceptable results.

SHF Super High Frequency - 3GHz to 30GHz.

signal-to-noiseratio The ratio in decibels (dB) between wanted signals and unwanted noise.

simplex The transmission of data etc., in one direction only or alternate two-way transmissionusing a single radio frequency (channel).

splitter Very similar to the hybrid combiner, but working effectively in reverse this devicedivides a signal into two (or more) outputs while maintaining a degree of isolationbetween the outputs.

spurious emissionsUnwanted radio frequency signals generated (and therefore transmitted) by radiotransmitters or receivers during normal operation.

squelch The term squelch is applied to that part of a radio receiver’s circuitry which preventsunwanted noise from reaching the audio output during periods when the wanted signalis weak or absent. ( Sometimes referred to as the 'mute' circuit.)

SWR Standing Wave Ratio - a means of defining how well two RF circuits are matched.Circuits which are not properly matched will waste power.

UHF Ultra High Frequency - 300MHz to 3GHz.

VHF Very High Frequency - 30MHz to 300MHz.

wavelength Instantaneous distance between crest (i.e., amplitude peaks) of an unmodulatedelectromagnetic wave (i.e., radio transmission) as it propagates through a given medium(usually air).

wavelength = speed of propagation (m/s) divided by frequency (Hz)

At 458MHz, for example,:

wavelength = (3 x 108) divided by (458 x 106)

• 0.655m or 65.5cms

wideband Describes a device which handles a broad band of frequencies (see also narrowband).

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INDEX

Adjacent channel rejection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9, 10, 35AF bandwidth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Amplitude modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Analogue signal input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3, 37-39Antenna

Co-linear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16, 25description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23directional . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23dish . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30End fed dipole . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27front to back ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27gain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16, 23half wave dipole . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16height . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14helical . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16, 25mixed working . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29non-directional . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23other . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29plate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29polarisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24, 28portable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26quarter wave whip . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25Yagi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27, 29, 31

Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Approval certificate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34Approvals

definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

Asynchronous . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37, 39Attenuation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32Audio output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37Automated warehousing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35Bandpass filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36Baud . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34, 38, 39Calculating range

example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21Carrier wave . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Channel spacing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9, 13Crystal filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10, 35Data recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37Decibel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31Decibel dB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

ratios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Deviation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7, 10Digital data input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3, 37Directional antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27DTI

licence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43regulations & approval . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34telemetry frequencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Duplex operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41ERP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28Error correction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11


68

Feedercable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15, 32types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9, 30, 35Filters

bandpass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36crystal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Frequencychoice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13planning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Frequency modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Front to back ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27Ground plane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16, 25-27Half power relationships . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31Handshaking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37Hybrid combiners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36Interference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27, 28, 34-36, 43

DTI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34electrical . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35immunity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Intermodulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35Licence exemption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43Licencing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43Line of sight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12, 14Low-loss coaxial cable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33Modem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

use of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39MPT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43Non-directional antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23Parity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39Propagation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12, 15, 24

described . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12how far? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14line of sight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Radiohorizon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14, 16modems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

Radio paging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Range

calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16horizon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16maximum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16radio horizon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14UHF signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

RF bandwidth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8RF power

maximum (UK) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28RS 232 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39RTD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34Security alarms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Selectivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10, 35Semi duplex operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

Semiduplex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22Signal type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37Simplex operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41Spot frequencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Spurious outputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11


69

SurTel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26, 38Synchronous . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39Telecommand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Telecontrol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Telemetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

bands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12Terrain correction factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20Third order intermodulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35, 36Transceivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35, 42Transmitter enable time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Transmitter power

factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18maximum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Unity gain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24Wavelength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12, 24Yagi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

gain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31mounting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28


70

APPENDIX AThe 868 to 870MHz band

In July 1998, the U.K. Radiocommunications Agency (RA) allocated the frequency band from 868 -870MHz for use by Short Range Devices exempt from licencing. This band is a harmonised pan-Europeanband which means that in principle it can be allocated by any member country of the EC. It is anticipatedthat in future this will mean that there will be a common low power licence free Telemetry band throughoutEurope. However, it remains to be seen what will happen in practice and at the time of writing, the onlyknown country to have allocated the band for licence free use is the U.K.

The radio equipment used at 868MHz has to be approved to the European standard ETS 300 220. Thereis no specific U.K. approval required.

Whilst 868MHz is a UHF frequency, we will refer to it as the “868MHz” band to avoid confusion with the458MHz UHF band. In order to provide some control over the usage, and potential interference, within thisband the RA have issued a set of Technical Parameters which govern the usage within sub-sections of thisband. These are shown on the Table on the next page which is taken from the RA document RA114 (Rev6) which is the “Short Range Devices - Information Sheet”, copies of which can be obtained from the RA.

As can be seen different sub-bands have different power levels (note these are erp) and are allocated fordifferent uses. It should be noted that there is, for the first time, a duty cycle specified. This must be takeninto account when deciding whether the band is suitable for a particular application.

Propagation at 868MHz is likely to be slightly less than at 458MHz frequencies but it will penetratebuildings and built-up areas marginally better, and certainly better than at VHF frequencies. Aerials will beconsiderably smaller, about half the size of a 458MHz aerial, and will be less obtrusive and easier toaccommodate.

Wood & Douglas have a range of modules complementary to their UHF and VHF ranges which operate at868MHz. The Sales Office will be pleased to advise you regarding any other aspects of this potentiallyexciting new band.


71

Technical parameters for the 868 to 870MHz band in the U.K.

General Telemetry and Telecommand

Sub-band Powererp

Use Channel spacing Duty cycle Comments

868.0 - 868.6 MHz 25mW Narrow bandWideband data

Spread spectrum

25kHz100kHz

<600kHz

<1% It is recommended that equipment whichallows automatic channel selection of a free

channel is to be used, so as to avoidinterference with CT2 equipment.


Spread spectrum

25kHz100kHz

<500kHz

<0.1%

869.3 - 869.4 MHz 10mW Narrow band 25kHz <10% Use restricted to home automation. Anaccess protocol must be used.


25kHz#250kHz

<10%


25kHz50kHz or <300kHz

Up to100%

Alarms

868.6125, 868.6375,868.6625, 868.6875 MHz

10mW Narrow band 25kHz <0.1% General alarms

869.2125, 869.2375 MHz 10mW Narrow band 25kHz <0.1% Social alarms

869.2625, 869.2875 MHz 10mW Narrow band 25kHz <0.1% Security alarms

869.6625, 869.2875 MHz 25mW Narrow band 25kHz <10% General alarms

NOTE: In the frequency bands where channel spacing is defined, the centre of the first channel is at a distance of channel spacing/2 from the lowerfrequency band edge.

RADIO TELEMETRY APPLICATIONS MANUAL

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