BASIC ELECTRONICS

Part 4

A Course of Training Developed for

THE UNITED STATES NAVYby the New York firm of

Management Consultants and Graphiological Engineers

VAN VALKENBURGH, NOOGER & NEVILLE, INC.

W1GANICENTRAL]JJBRARY

Adapted to British and Commonwealth Usageby a Special Electronics Training Investigation Team of

the Royal Electrical & Mechanical Engineers

LONDON

THE TECHNICAL PRESS, LTDNEW YORK

THE BROLET PRESS

British and Commonwealth Edition first published 1959

©Copyright 1959 by

VAN VALKENBURGH, NOOGER & NEVILLE, INC.

New York, U.S.A.

All rights reserved

American Edition first published 1955

©Copyright 1955 by

VAN VALKENBURGH, NOOGER & NEVILLE, INC.

New York, U.S.A.

U S. Library of Congress Catalog Card No. 55-6984

All rights reserved

W1GANPUBLIC

LIBRARIES

7a 13

1

AT ub^i 3*1

Made and printed by Offset in Great Britain by

William Clowes and Sons, Limited, London and Beccles

PREFACE

IN THESE six Manuals on BASIC ELECTRONICS and the five which have pre-

ceded them on BASIC ELECTRICITY, there lies the core of an illustrated

Course of Technician Training—carefully planned, brilliantly simplified, and radi-

cally new—which was developed some years ago at the request of the United States

Navy by a distinguished New York firm of management consultants and graphio-logical engineers, Messrs. VAN VALKEN BURGH, NOOGER & NEVILLE, INC.The Course has since become standard in U.S. Navy Training Schools. More than

50,000 men have taken it as ah essential part of their training to technician level in

14 different Navy trades; their average training time has been cut by half; andsupplies of Course materials are now held as part of the Navy's official War Mobiliza-tion Stores.

The text of the Course was subsequently released in a condensed form to the

general public in the United States, where it has proved an outstanding success. In

addition to large sales to individuals, to schools and to technical institutions of all

kinds, more than a score of world-famous companies have taken the publishedManuals for use in their Apprentice Training Schemes, and have found that theyenable them to turn out qualified technicians both faster and at less cost than didthe old methods of text-book and lecture. Several American trade unions (who takea keen interest in the "up-grading" of their members to more skilled and better-paid

jobs) have chosen the Manuals as the best available training materials for their

purpose.

This notable Series is now being made available, in a revised, reset, and suitablyre-worded edition, to users in Britain and the Commonwealth.

While negotiations with the American authors were still in progress, word reachedthe British publishers that there had recently been set up, under command of Train-ing Headquarters, Royal Electrical and Mechanical Engineers, at Arborfield in

Berkshire, a special "Electronics Training Investigation Team" whose task was to

devise solutions for some of the training problems which would face the British

Army when National Service ended, and when the Army's increasingly elaborateelectrical and electronics gear would have to be manned and serviced by recruits

entering the Army with none of the technical knowledge which many NationalServicemen had hitherto brought with them into the Forces.

It seemed possible that most of the REME requirements for a new-style, yettechnically sound, instructional approach could be met by a suitably edited British

version of the VVN&N Manuals. A visit to Arborfield was accordingly arranged,where the reception given to the Manuals, with their attractive appearance andproved record of success, was enthusiastic; and after a careful evaluation of. their

merits and potential suitability had been made, War Office consent was secured toa proposal that the work of adapting text and illustrations to British notation andterminology should be undertaken by the Electronics Team at Arborfield.

Later, while this work was still proceeding, a decision was reached to adopt therevised Manuals as basic texts for the training of future REME technicians, and anorder for large numbers of complete sets of the Manuals was placed. Early interest

was also shown by several other branches of the Armed Forces, notably the Royal

Corps of Signals and the Royal Air Force. Military Advisers to the High Com-missioners of at least six leading Member Nations of the Commonwealth submitted

early proofs of the English edition to their respective Ministries of Defence.

The original U.S. Navy Course was based on a novel technique of teaching

developed by the Authors after extensive research and practical experience with

thousands of students. Immense pains were taken to identify and present only the

essential facts about each new concept or piece of equipment. These facts were

then explained in the simplest possible language, one at a time; and each was illus-

trated by a cartoon-type drawing. Nearly every page in every one of the Manuals

carries one or more of these brilliantly simple "visualizations" of the concept

described.

The approach throughout is non-mathematical. Only the simplest equations

needed for working with the fundamental laws of electricity are employed. Yet

there has been no shirking of essentials, even when they are difficult; and students

with higher qualifications and educational background find nothing in the Manuals

to irritate or slow them down. They merely pass on to the next subject quicker

than the rest.

Despite their Services background, the Manuals have been proved suitable for

civilian use. Their purpose, however, is limited to the training of technicians, not

of engineers. They aim to turn out men capable of operating, maintaining, and

carrying out routine repairs to the equipment described—not men capable of invent-

ing or improving it.

They present a unique simplification of an ordinarily complex set of subjects—so

planned, written and illustrated as to become the best and quickest way to teach or

learn BASIC ELECTRICITY and BASIC ELECTRONICS that has ever been

devised.

In these Manuals, first things come first—and only the essentials come anywhere.

Their accuracy and thoroughness, combined with their extreme lucidity, will make

their publication a landmark in technical education in Britain and the Common-

wealth.

Page

TABLE OF CONTENTS

Section

1 The Role of the Transmitter 4.3

2 Class C Amplifiers 4.9

3 The Basic Three-stage Transmitter 4,17

4 Frequency Multipliers 4.35

5 Transmission Lines 4.40

6 Aerials 4.59

7 Continuous-wave (CW) Transmission 4.72

8 Amplitude Modulation 4.77

9 General Review of Transmitters 4.92

10 Introduction to Receivers 4.95

Index 496

This Course in

BASIC ELECTRONICS

comprises 6 Parts

This is PART 4

It is preceded by a Course in

BASIC ELECTRICITY

comprising 5 Parts

all uniform with this volume.

Part 1 explained the General Principles of Electricity.

Part 2 described and discussed D.C. and D.C. Circuits.

Parts 3 and 4 described and discussed A.C. and A.C. Circuits.

Part 5 described and discussed A.C. and D.C. Machines.

an&/uwibm4lfei&

§1: THE ROLE OF THE TRANSMITTER 4.3

Introduction

Probably few of you have had any direct experience with transmitters. The word

itself may even be unfamiliar. But you have undoubtedly talked often enough about

one particular type of transmitter—a radio station

!

When you listen to a radio, the sounds you hear are the result of signals travelling

to the radio receiver through the air. If someone were to ask you how those signals

happened to be in the air, you would probably say, "A radio station broadcasts

them."

Different transmittersoperate on different

FREQUENCIES

There are other things you already know about transmitters from your experience

with radio sets. You know that "changing stations" is also called "tuning." Fromthis you realize that different transmitters operate at different frequencies. Youselect the station you want to listen to by tuning your radio to the frequency of that

station.

TyH >ADifferent transmitters

have different

POWER OUTPUTS

You have also noticed that some stations come in stronger than others. If differ-

ent transmitters at equal distances away have different power outputs, the station

whose transmitter has the largest power output will be heard the loudest.

If there are two stations whose transmitters have the same power output, you will

hear more loudly the station which is the closer to your radio set.

You see you really knew a few things about transmitters all the time—even if the

word itself came as a new one to you

!

4.4 [§ I

A Simple Transmitter

The simplest possible form of transmitter would consist of an oscillator generating

a high-frequency signal—the type of oscillator doesn't matter—connected to an

aerial. This aerial would then radiate a signal constant in amplitude, and of the

same frequency as the oscillator.

A simple transmitterconsists of •

If your home radio set picked up the constant-amplitude signal from such a trans-

mitter, you would hear nothing at all. And though there are some special com-

munications receivers used by the G.P.O., the Army, etc., which would produce a

constant audio tone if they received it, in neither case could any message be "read"

from the incoming signal.

Such a signal is said to contain "no intelligence."

To get intelligence into the signal, the oscillator would have to be turned on and

off with a key, so as to produce dots and dashes such as are used in the morse code.

Putting the transmitter to work —

dit"

A signal of this type contains intelligence, since a message can be obtained from

it. Special communications receivers would produce a sound somewhat like "dit-

dah-dit"—which a radio operator who knew the morse code would interpret as the

letter "R."

§ I] 4.5

A Simple Transmitter (continued)

Almost every transmitter, however, contains a good deal more than just an oscil-

lator and an aerial.

There are two main drawbacks to connecting the oscillator directly to the aerial.

The first is that the power output would be limited, because there are no stages of

r.f. amplification between the oscillator and the aerial to build up the strength of the

r.f. signal. And power output is important because it determines the distance over

which the transmitted signal can be picked up by a receiver.

The second consideration is frequency stability. An oscillator from which a large

amount of power is drawn has a tendency to drift in frequency. And any drift in

the frequency of a transmitted signal means that a portion of the message will belost by an operator trying to receive it.

For these reasons—low power output and poor frequency stability—oscillators are

not usually connected directly to an aerial.

result in poor reception ....

4.6 [§A Simple Transmitter (continued)

To overcome the drawbacks of connecting an oscillator directly to the transmitting

aerial, one or more stages of amplification are connected between the oscillator and

the aerial.

The stage which is connected to the aerial itself is usually called the "final poweramplifier." The other stages of amplification are known by several names. Some-

times they are referred to as the "first and second power amplifiers"; sometimes as

"intermediate power amplifiers."

The first power amplifier, since it serves to isolate the oscillator from variations

of load, is also called a "buffer" amplifier.

Oscillator

Signal

c=> c=> =>

1st Power2nd power

Amplifier . A ,.,. . 1-*,^*

(Buffer)AmP llfier Amplifier

AmplifiedSignal

The r.f. signal is generated in the oscillator circuit and is amplified by the first and

second power amplifiers, which drive the final power amplifier. The powerful

signal from the final power amplifier is fed to the aerial, which in turn radiates the

signal into space.

As has been said, the r.f. signal by itself does not contain any intelligence, though

several things can be done to it which will make it able to contain or carry a message.

For this reason, the r.f. signal is commonly referred to as "the carrier wave." It

is not, of itself, the message; but it can carry a message to some distant point.

§1] 4 -7

Modulation, and Keyed Transmission

The process by which the carrier wave is changed so that it can carry a message is

called "modulation." Every communications transmitter needs modulation, because

the carrier by itself (i.e., unmodulated) cannot convey intelligence.

In most transmitters the message is transmitted either in code (radio telegraphy)

or by voice (radio telephony).

The most common types of code transmission are "continuous wave" (CW) and

"modulated continuous wave" (MCW). In CW transmission the r.f. to the aerial is

interrupted, or turned on and off, with a hand key; so that the carrier is radiated as

dots and dashes. CW is used primarily for long-distance communication. Aspecial receiver is needed to receive it.

^CW\TRANSMISSION

Phones

In MCW transmission, a constant-amplitude audio frequency is superimposed on

the carrier. The carrier is then turned on and off with a key, just as in CW trans-

mission. Any receiver with the proper frequency range can receive MCW.

MCW Keyed

TRANSMISSION ,M°<"^tedRF

^

v

>.RF

Transmitter

AudioOscillator

DIT -Drr.

Receiver

Phones

M#

4.8[§ |

Radio Telephony

Radio telephony, or voice transmission, is also of two types. In the most commontype used, the amplitude of the carrier is varied at a rate dependent on the frequency

of the voice signal, and to an extent dependent on the amplitude of the voice signal.

This is called "amplitude modulation" (AM). It is the type of transmission used in

the standard radio broadcast.

AMVoice modulated RF

UnmodulatedRF Carrier

-A*.

vVoicewaves

v Phones

V

RFTransmitter

1Amplitudemodulator

AMTRANSMISSION

Receiver

Microphone **?

The other type of radio telephony, which is being used more and more, is called

"frequency modulation" (FM). Here the frequency of the carrier is shifted back and

forth at a rate equal to the frequency of the voice signal. FM transmission is com-

paratively free from "static" interference, and is used in place of AM when the latter

may be difficult to receive. It will be fully explained in Part 6.

FMVoice modulated RF

UnmodulatedRF Carrier

A.

V

RFTransmitter

Voicewaves

Microphone ^

Frequencymodulator

Note that there are also some other types of modulation which are used in special

communications equipment. They will not be covered by this basic course, since

they are not widely used.

§2: CLASS C AMPLIFIERS 4.9

The Three Classes of Operation—A Reminder

The type of amplifier most commonly used in transmitter circuits is the tuned

Class C amplifier.

You remember from your study of amplifiers that there are three main bias con-

ditions for amplifiers—Class A, Class B, and Class C.

In Class A operation, the grid is biased near the midpoint of the linear portion of

the anode current-grid voltage curve. The a.c. signal on the grid causes the grid

voltage to vary above and below the bias value. The current variations are propor-

tional to the grid voltage, since the grid voltage swing does not go beyond the linear

portion of the curve. Anode current flows throughout the entire a.c. cycle, since

the grid voltage does not drive the valve into cut-off.

In Class B operation, the grid is biased at or near its cut-off value. The a.c. signal

drives the valve into cut-off for approximately half the cycle. Thus the valve con-

ducts for about 180 degrees of the cycle and is cut off during the other 180 degrees of

the cycle.

In Class C operation—the type of operation with which you will be most con-

cerned in your study of transmitters—the grid is biased considerably beyond cut-off.

The valve remains cut off for most of each a.c. cycle, and current flows in the valve

only when the a.c. signal increases the grid voltage above cut-off. The anode current

therefore flows in the kind of pulses shown below.

OPERATION OF

CLASS

?4tHfeU£ten6

-8 -6 -4

No Anodecurrent flowing

Cut-off

TwiceCut-off

Current flowingall the time

Class B Current flowshalf the time

Class CCurrent flows

less thanhalf the time

^

4.10

Tuned Class C Amplifiers

IS 2

OHT+

The operation of a Class C amplifier will become clear when you analyse whathappens in a tuned amplifier such as the one shown in the schematic diagram above.

An a.c. signal is developed across the tuned-circuit in the anode of the previous stage.

This voltage also appears across the r.f . choke (RFC) in the grid circuit of the tuned

Class C amplifier stage. The d.c. bias provided by the bias battery causes the valve

to operate in Class C.

The pulses of anode current

which flow as a result of this type

of operation deliver a "kick" to the

tuned-circuit in the anode. This

"kick" makes the tuned-circuit

oscillate, so that the part of the

cycle during which anode current

has stopped is filled in. (For a

review of how oscillations are kept

going in a tuned-circuit, refer to

the Section on oscillators in Part 3.)

The anode voltage is the differ-

ence between the H.T. voltage and

the a.c. voltage across the tuned-

circuit. When the pulse of anode

current flows, the voltage at the

anode end of the tuned-circuit goes

negative, and therefore subtracts

from the H.T. voltage. When the

voltage across the tuned-circuit

reverses and goes positive at the

anode end, it adds to the H.T.

voltage.

As a result, the anode voltage

waveform varies above and below

the H.T. voltage level, as shown in

diagram 4 opposite.

HT+

GridVoltage

AnodeCurrent

Voltage

acrosstuned

circuit

Total

Plate

Voltage

§2]

Tuned Class C Amplifiers (continued)

The reason why tuned Class Camplifiers are universally used in

high-powered transmitters is be-

cause of their high efficiency of

operation, which results in a maxi-

mum of radiated power.

The power supplied to an ampli-

fier is always greater than the

power which can be got out of it.

The reason is that some of the

power put into an amplifier is used

up by the valve; only the re-

mainder can appear as useful out-

put in the load. The power used

up by the valve equals its anode

voltage times its anode current.

Since the anode current of a

Class C amplifier flows during less

than half the cycle, the average

anode current is less than it is in

Class A or B operation. There-

fore less power is used up by the

valve, and more power can get to

the output.

This makes the Class C ampli-

fier more efficient, and therefore

more desirable for use in a trans-

mitter.

4.11

ANODE VOLTAGE

If the tuned-circuit in the anode is not tuned to the frequency of the input signal,

then the voltage across it will be lower—in proportion to the extent to which it is

mistuned. The further "off" it is tuned, the less power will appear across it, and the

more power will be dissipated by the valve. Then the efficiency of the amplifier is

lower, the valve heats up more, and the power output is lower.

VARIATION OF ANODE VOLTAGE

AS TUNING VARIES • ••

Well belowsignal

frequency

Approachingsignal

frequency

Slightly lessthan signal

frequency

At signal

frequency3

4.12 [§ 2

Fixed Bias

The term "fixed bias" describes any method of obtaining bias in which the bias

remains fixed as the strength of the input signal varies.

Fixed bias may be obtained from a negative power supply, from a motor-generator

set with a negative d.c. output, or from a battery. Each of these methods will keep

the grid at a constant negative d.c. voltage which will not vary whatever the strength

of the signal input.

Fixed Bias may be obtained from...

12 H>| 1 7J « 4J *•

One of the advantages of fixed bias is that the valve remains cut off under no-signal

conditions.

The disadvantage of fixed bias is that the gain of the amplifier remains constant;

so that if the grid signal varies in amplitude, the output will similarly vary. This is

not desirable in a transmitter, because the output to the aerial must remain constant

in amplitude if the radiated signal strength is to remain constant.

If the bias could be made to vary as the signal input to the amplifier varies, the

amplifier output could be maintained practically constant.

§ 2] 4.13

Self-bias

The term "self-bias" describes any grid bias which results from the current flow

in the valve which is being biased. You are already familiar with the two methodscommonly used to provide it.

In the first method, a resistor placed in the cathode circuit makes the cathodepositive with respect to earth, and therefore makes the grid more negative than the

cathode. The bias voltage developed across this resistor is equal to the average

current multiplied by the size of the resistor.

If a large cathode resistor is used, the bias voltage will thus also be large; and the

resistor can in fact be made sufficiently large to cause the bias to approach cut-off

when there is no signal on the grid.

GRID VOLTAGE CATHODE CURRENT

NoSignal

No signal

current

Cut-off

When a signal is applied to the grid, the cathode current will increase on the posi-tive half-cycles; and will become zero (cut-off) on the negative half-cycles. Theaverage current will thus be increased; and the bias will also increase.

cut-off -/- J

\J \J

Small/

• Bias SiKna]

Averagecurrent

If a larger signal is applied to the grid, the current will be larger during the positive

half-cycles of voltage, but will remain zero during the negative halves. Thus, theaverage valve current increases as the grid signal becomes larger, resulting in in-

creased bias for larger signals.

A ACut-off J-

„. LargeBias signal

Averagecurrent

This effect of bias varying with signal strength tends to stabilize the amplitude ofthat portion of the grid signal above the cut-off level. As a result, the amplitude ofthe current pulses in the anode will not vary as much as their corresponding gridsignals vary.

Because of the above-mentioned effect, self-bias tends to produce amplitudestability of the anode signal, and is therefore sometimes called "automatic bias."

The cathode bias method just described, however, is not common in high-poweredtransmitter circuits.

4.14 [§2

Self-bias (continued)

The other, and very common, type of self-bias arrangement found in transmitters

makes use of the current which flows from the cathode to the grid at the positive

peaks of the signal output. This is called "grid-leak bias."

Class C Amplifier with

Grid-leak Bias

HT+

To Anode OfPrevious Stage

Whenever the signal drives the grid positive, the grid draws current; and in doing

so charges up capacitor C-l to make the grid negative again. Resistor R-l provides

a path for C-l to discharge slightly between the pulses of grid current flow.

The main advantage of this type of bias is that it develops a voltage whose ampli-

tude depends on the strength of the input signal. If this input signal increases, the

grid will draw more current, and the bias will become more negative. After the new

value of bias has become established, the peaks of the larger input signal will not

drive the grid very much more positive than did the weaker signal.

Thus, the peaks of the larger signal will only cause about the same amount of

anode current to flow as did the peaks of a smaller signal. In this way, grid-leak

bias provides for amplitude stability.

The main disadvantage of grid-leak bias is that it depends entirely on the presence

of a signal in order to develop any bias voltage at all; it does not therefore protect

the valve when there is no signal on the grid.

If the oscillator of a transmitter stopped oscillating for any reason, the grid-leak

arrangement in the amplifiers would not develop any bias; since the grid would not,

under these conditions, be driven positive. The transmitting valve would then draw

a very large current with zero bias, and might therefore be seriously damaged.

§ 2] 4.15

Combination Bias

The most common bias arrangement in transmitters is a combination of fixed bias

and grid-leak bias.

The fixed bias is sufficient to limit the current to a low value, or even to cut-off in

the absence of a signal. And when a large enough signal is present to drive the grid

positive, then the grid-leak bias which is developed stabilizes the amplitude of the

output.

In this way, combination bias both protects the valve and stabilizes the output.

{^dutaiam Bias

4.16

REVIEW

[§2

Class C Operation. The grid of the

valve is biased well below cut-off, so

that anode current flows only in pulses.

1 -8

6 A A

•1c

r\•4 1/

1

-

No anoda 9}current flowing

*<*

i -6 - i 1-2 : o

> ^0cr

Tuned Class C Amplifiers. Used in

transmitters because they are very

efficient when tuned to the frequency

of the input signal.

Grid-leak Bias. Depends on grid

current, and varies as the strength of

the input signal changes.

Combination Bias. A combination

of fixed and grid-leak bias most

commonly used in transmitters.

TOANOOEOFPREVIOUSSTAGE

§3: THE BASIC THREE-STAGE TRANSMITTER 4.17

The Three Basic Circuits

A block diagram of a basic three-stage transmitter is shown below. All three

stages are operated in Class C for high efficiency. The ECO master oscillator (MO)generates the r.f. signal, which can be varied (for example) between 2 and 4 mega-

cycles.

The intermediate power amplifier (IPA) both amplifies the r.f. signal and isolates

the master oscillator from the final power amplifier, so as to improve frequency

stability. The IPA is therefore called a "buffer amplifier."

The IPA may also act as a frequency doubler to double the oscillator frequency.

(The way in which it works in this role will be explained later.) The output

frequency of the IPA can therefore vary between 2 and 4, or between 4 and 8 mega-

cycles.

The final power amplifier (PA) generates a large amount of power output and

delivers it to the aerial, usually at the same frequency as its grid signal.

Basic

THRU STAGE

\Transmitter

-GeneratesRF Signal

IPA

(Class C)2 to 4 Mc/s

4 to 8 Mc/s

Intermediate PowerAmplifier

Buffer Amplifier

Frequency Doubler

v

PA

(Class C)2 to 4 Mc/s4 to 8 Mc/s

Final PowerAmplifier

4.18 [§3

The Oscillator

The purpose of the electron-coupled master oscillator is to generate a stable r.f.

signal, the frequency of which can be varied over a given range.

The ECO operates as follows:

—

The oscillator section of the ECO is composed of the grid and screen circuits,

and is a Colpitts oscillator. The oscillator frequency is determined by the grid

tuned-circuit, consisting of L-l, C-l, C-2 and C-3. The screen, which acts as the

anode of the oscillator section, is coupled to the tuned-circuit through the r.f. de-

coupling capacitor, C-5. Grid-leak bias is developed across R-l by the discharge of

C-4.

The r.f. choke in the cathode circuit provides a low resistance d.c. path to earth

for the cathode. But because the high reactance of the choke does not allow r.f. to

flow through it, the r.f. must flow through C-3 (the feedback capacitor) to the

cathode. The screen-dropping resistor, R-2, drops the screen voltage to the correct

value. The r.f. oscillations generated in the oscillator section of the ECO are

electron-coupled to the anode through the flow of anode current.

The r.f. choke in the anode lead acts as a high impedance for the r.f. signal, and

serves the same purpose as the anode load resistor in an audio amplifier. The r.f.

coupling capacitor, C-6, passes the signal to the grid of the IPA.

^^^g Master Oscillator(ECO) 9|5Ki. . . Generates RF Signal

RFC

MO(ECO)

2 to 4 Mc/s

C-4ll-^IPA

1 I 1"1-i:£ c-2i iR-i

L-l«

RFC=:c-3

^HT+

§ 3]41»

The Intermediate Power Amplifier

Two functions of the intermediate power amplifier are to isolate the oscillator for

improved frequency stability, and to amplify the r.f. signal in order to drive the

power amplifier more efficiently.

The IPA also serves to increase the tuning range, if desired, by doubling or tripling

the generated frequency in its anode tuned-circuit.

The operation of the IPA is as follows:

—

A combination of grid-leak and cathode bias is provided by R-3 : C-6, and by

R-4 : C-7 respectively. Resistor R-5 drops the screen voltage to the correct value.

The screen decoupling capacitor, C-8, is returned directly to the cathode, rather

than to earth. This provides a more direct path back to the cathode for any r.f.

variations on the screen.

The r.f. choke in the anode lead acts as a high impedance for the r.f. signal, and

serves the same purpose as does the anode load resistor in an audio amplifier. C-9

is a coupling capacitor which passes the r.f. to the tuned-circuit, and at the same

time blocks the d.c.

The anode tuned-circuit, C-10 and L-2, can either be tuned to the IPA grid signal,

or it can be tuned to twice the grid signal frequency—in which case the IPA is called

a "doubler." When the IPA doubles, the isolation between the grid and anode

circuits is improved, and there is less chance of the IPA breaking into oscillation.

Doubling has another advantage in that it raises the carrier frequency while still

permitting the oscillator to operate at a lower frequency, at which it will be more

stable.

Capacitor C-ll couples the r.f. to the grid of the power amplifier.

§f|§ Intermediate Power Amplifier (IPA) liH. . . Intermediate Power Amplifier

. . . Buffer Amplifier. . . Frequency Doubler

IPA(Class C)2 to 8 Me/s

HT+

4.20B ,

The Power Amplifier

The purpose of the power amplifier is to increase the power of the r.f. signal sothat it can be radiated by the aerial.

The PA usually operates at the same frequency as does its preceding stage; andonly in unusual cases does the PA act as a doubler.

The PA operates as follows:

—

Capacitor C-ll couples the r.f. from the output of the IPA to the grid of the PA.Here, as in the IPA, there is a combination of grid-leak and cathode bias providedby R-6 and C-ll, and by R-7 and C-12, respectively.

The r.f. choke, while providing a d.c. path from anode to H.T.(+), also acts as ahigh impedance anode load for the r.f. signal.

C-13 couples the r.f. to the tuned-circuit, and blocks the d.c.

The anode tuned-circuit C-15 : L-3 (note, by the way, that the anode tuned-circuitis sometimes called the "tank circuit" in a final power amplifier circuit) is tuned tothe grid signal frequency; and a high r.f. voltage is developed across it. The high-powered r.f. signal in the anode tuned-circuit is coupled by coil L-5 to the aerial for

radiation.

Coil L-4 couples some energy back to the grid through capacitor C-14, which is

called a "neutralizing capacitor." The purpose of this neutralizing circuit will

become apparent to you a little later on.

§i§ltl The final Power Amplifier(PA) WMm,

PA(Class C)2 to 8M</s

L-5 TOAER1

§3]

Diagram of a Complete Three-stage Transmitter

4.21

Q X

8'8

4.22

The Transmitting Valve Filament Circuit

In the transmitter circuit shown on the

previous page, the valves (other than

the rectifier) were indirectly heated. In

many transmitters, however, the trans-

mitting valves used have directly heated

cathodes, which are capable of supplying

the large current requirements. Tungsten

cathodes are commonly used because of

their relatively long life.

The use of directly heated valves,

however, complicates the wiring of the

cathode circuit slightly, as shownopposite.

The filament is connected across a

secondary winding of a filament trans-

former. This secondary winding is

centre-tapped in order to prevent the

50 c/s filament current from affecting

the anode current of the valve.

The centre tap of the transformer is

connected to earth through the r.f. chokein order to prevent the r.f. current fromflowing in the transformer winding.

The r.f. current gets to the filament

through C-l and C-2.

The d.c. valve current flows throughthe r.f. choke, divides in going through

the filament transformer winding, andarrives at the filament.

Because the d.c. current divides, both

ends of the filament are at the same d.c.

potential. If one side were less positive

than the other, more anode current

would be drawn from that side. Butsince the two sides of the filament are

at the same potential, equal currents are

drawn from each, with the result that

the valve has a longer life.

[§3

Typical cathodecircuit of

transmitting valve

II t IIC-l ^ C-2

r TflRP—

I

RFC

pWOOOOOOOtn Filament* A Transformer

Path for

50-cycleheater current

,1.

Path for

RF current

/"5W555S55W

Path for

DC valve

current

.0 RFC

''655559S5705';

§ 3] 4.23

The Purpose of Tuning

If a Class C amplifier is to operate efficiently, the anode tuned-circuit must

resonate at the same frequency as the grid signal.

If the tuning capacitor is variable, the anode circuit will be either on or off

resonance, depending on the setting of the variable capacitor.

Adjustment of the variable capacitor so as to make the anode tuned-circuit

resonate to the grid signal is called "tuning."

When a transmitter is de-tuned, a weak signal will be radiated—so weak that even

receivers tuned to the transmitter frequency may not pick up the signal.

When a transmitter is tuned to a given frequency, all the tuned-circuits in the

transmitter are tuned to resonate at this given frequency. The transmitter then

radiates a stable signal at maximum efficiency and with maximum power output.

The tuning of a transmitter is therefore the most important procedure in its opera-

tion.

OriginalSignal

Detuned. ••

transmitter.

0««llll««Oi"«0

I 1st Power I , ~Oscillator Amplifier

2"d ^°f

wer

(Buffer)AmPWl"

Final

Power

F>W TransmittedvSignal

Original

Signal III<=> IIIc>

111

1 1 xst Power n , „~. . ,, . I . ..... 2nd PowerOscillator . Amplifier . , ,

1 1 Ir, tc AmplifierI

(Buffer) 1

K

oFinal

Poweramplifier

f^>

TransmittedSignal

4.24 [§3

Tuning Methods

A tuned-circuit in series with the anode of a Class C amplifier can be compared to

a rheostat in series with the anode.

When the anode circuit is completely de-tuned, it acts just as a very small resistance

in the anode. As a result, the anode voltage will always be nearly equal to H.T.,

and the pulses of current (when grid is driven above cut-off) will be large. The d.c.

meter (M-l) which measures the average of the current pulses will therefore read

high.

Tuned Class CAMPLIFIER

CIRCUIT

\ ' HT+M-l

As the tuning is varied so that the resonant frequency of the anode tuned-circuit

comes closer to the grid signal frequency, so the impedance of the anode tuned-

circuit rises.

Now a signal voltage appears across this impedance. Just as in an ordinary

amplifier, when the grid signal is positive, the anode voltage drops because of the

voltage drop across the anode load. Since the anode voltage is now lower than it

was before, during the time the grid is driven above cut-off, the pulses of anode

current will be lower in amplitude; and therefore their average value will be less.

When the anode tuned-circuit is tuned to the grid signal, the anode impedance

is at its highest point; and the voltage drop across this impedance is therefore at its

highest point also. In consequence, the anode voltage (i.e., the difference between

H.T. and the voltage drop across the load) is at its lowest, point. The anode cur-

rent pulses (and therefore the average anode current) will also be at their lowest

value.

A minimum d.c. anode current reading is therefore an indication that the anode

tuned-circuit is tuned to the grid signal frequency.

When an anode tuned-circuit is tuned for a minimum reading on the anode current

meter, it is called "tuning for a dip."

Variations of Anode Voltages and

Currents as tuning varies

HT+-

Anode Voltage

Anode Currenti

i

l ; • • »

I II III »

i »

AVERAGE ': I

Well belowsignal

frequency

Approachingsignal

frequency

V\7

Slightly less

than signal

frequency

Atsignal

frequency

|Slightly more

|than signal

!frequency

§ 3] 4.25

Tuning Methods (continued)

The first step in tuning a transmitter is to set its oscillator to the desired frequency.

This may be done by adjusting the frequency of its output to be the same as the

frequency of an oscillator which is accurately calibrated, and which has been set to

the desired frequency. This comparison is made with an instrument called a wave-

meter or frequency meter.

TUNING THE TRANSMITTER

The next stage to be tuned is the stage

which follows the master oscillator. This is

done by rotating its tuning control until the

milliammeter indicating the anode current

of this stage gives a minimum reading—at

which point the anode circuit will be tuned

to the frequency of the master oscillator.

Initially, this stage is likely to be de-tuned,

and the anode current will be at a fairly high

value. For a time, as the tuning control is

rotated, no change in the milliammeter read-

ing will be noticed—until the tuned-circuit

frequency gets near the oscillator frequency.

At this point the reading on the milliammeter

will start to "dip," and the control should be

rotated slowly.

The .current will continue to decrease as

the tuning control is rotated, until a mini-

mum value occurs. This is the dip reading;

and rotation of the control should now stop.

For if rotation be continued in the samedirection, the circuit will be de-tuned once

more, and the current will again rise.

When this is seen to be happening, the control should be turned in the opposite

direction until the milliammeter again registers minimum current. At this point,

the tuned-circuit is at the same frequency as the signal frequency, and the output of

the stage is maximum.The anode tuned-circuits of the other stages can be tuned in exactly the same way.

4.26 [§3

Tuning Methods {continued)

In addition to the anode current meter, there is another meter which indicates

correct tuning of the anode circuit. This meter is in the grid circuit of the following

stage, and is labelled M-2 in the diagram below.

^ » HT+When the anode circuit is tuned to

the frequency of the input signal, the

voltage developed across the circuit

is at its highest—and so is the out-

put from that amplifier stage. Thelarger the output from that stage,

the greater is the signal to the grid

of the following stage.

The grid of the following stage will draw current whenever the input signal drives

the grid positive. The larger the signal input, the greater will be the flow of current

from the cathode to the grid. Since the signal input to the grid will be greatest

when the anode tuned-circuit of the previous stage is accurately tuned, the grid will

at that moment be drawing maximum current, and milliammeter M-2 (which

measures the average grid current) will be giving a maximum reading.

Thus when the anode tuned-circuit is accurately tuned, the anode current meter

indicates a dip, and the grid current meter of the following stage simultaneously

registers a rise known as a "peak reading."

• •Anode Current • •Grid Current

If the grid circuit has fixed bias or combination bias, no grid current will be

drawn until the signal is fairly large. This will happen some time after the anode

current meter has started to dip. For this reason, the rise in grid current indication

is sharper than is the decrease in anode current indication.

The normal procedure for tuning a stage which has an anode current meter and

which is followed by a stage having a grid current meter, is to tune first for a mini-

mum anode current. * This indication is broader, and less likely to be overlooked

as you vary the tuning. Once you have seen the anode current start to decrease,

watch the grid current meter for a rise. Tune to the point of maximum grid current.

Watching for maximum grid current is the preferred method of tuning.

Jjp 4.27

Tuning Methods {continued)

When an anode tuned-circuit is tuned to the same frequency as the grid signal, the

voltage across the tuned-circuit is at its maximum. And if another coil is trans-

former-coupled to the coil of the tuned-circuit, the voltage induced in this coil will

also be a maximum.This second coil can be connected to a pilot lamp, which will glow if the induced

voltage is large enough. If the anode tuned-circuit is de-tuned from the grid signal,

the induced voltage in the lamp circuit will drop, and the lamp will go out.

The transformer-coupled lamp is therefore a convenient means of tuning a circuit;

for the lamp glows at its brightest when the circuit is tuned to the signal frequency.

This method of tuning is not as accurate as the current meter indications, how-ever, because the lamp circuit loads down the tuned-circuit and de-tunes it slightly.

When using this method for tuning indication, therefore, the coupling must be kept

as loose as possible in order to minimize the de-tuning effects on the tuned-circuit.

The lamp method of tuning can be conveniently used on experimental transmitters

in which the anode coils are accessible. But in many transmitters it cannot be used,

since the tuning coils are out of sight; and tuning is therefore done exclusively bycurrent meter indications.

In high-power transmitters, it is essential to follow the maker's instructions onsetting the "tuning controls before switching on. Failure to do so may result in

insufficient r.f. being fed to the power amplifier stage, causing reduced bias and the

possibility of damage to a valve.

Using a lamp for tuning

At resonance

4.28 [§ 3

Neutralization

Sometimes a tuned Class C amplifier will act as a "tuned-anode tuned-grid"

(TATG) oscillator at the resonant frequency of the tuned-circuits. When it does,

the inter-electrode capacitance between anode and grid will be large enough to pro-

vide an amount of feedback sufficient to cause sustained oscillation.

This type of unwanted oscillation is most often encountered with triodes, because

of their large inter-electrode capacitance. It is seldom encountered with tetrodes

and pentodes, because their inter-electrode capacitance is very low.

When triodes are used as r.f. amplifiers, however, it is possible to eliminate the

oscillations by a process called "neutralization." In this process, a circuit is in-

cluded in the amplifier which counteracts the feedback effect of the inter-electrode

grid to anode capacitance.

There are two circuits which can be used to neutralize the grid-to-anode capaci-

tance, and so to reduce the possibility of oscillations. Both achieve neutralization

by feeding back a signal from the anode to the grid through a neutralizing capacitor.

This signal is opposite in phase, and equal in magnitude, to the signal fed back

through the grid-to-anode capacitance.

These circuits are called "anode neutralization" and "grid neutralization" respec-

tively. They get their names from the part of the circuit in which the feedback

voltage is developed.

Above is shown the circuit for anode neutralization. Cga is the grid-to-anode

capacitance, represented in the schematic as a capacitor external to the valve. Cn is

the neutralizing capacitor—that is, the capacitor through which the neutralizing

signal is brought to the grid.

The tuning coil, L-l, is centre-tapped at point C, which is placed at r.f. earth by

the r.f. decoupling capacitor CB . Since points A and B are at opposite ends of

coil L-l, they must be 180 degrees out of phase. Therefore the r.f. voltages measured

at points A and B with respect to earth are 180 degrees out of phase, and equal in

amplitude if the centre-tap at C be assumed to be in the exact centre of the tuning

coil.

The neutralizing capacitor, Cn , is connected between point A and the grid, while

the inter-electrode ca'pacitance, Cga , is between point B and the grid. Therefore the

phase of the voltage fed from the anode to the grid through Cn is opposite to the

phase of the voltage fed through the grid-to-anode capacitance; and the voltages

therefore cancel each other out.

Cn is made variable, so that the amplitude of the signal fed back through it can be

made to balance out exactly that fed back through Cga .

§ 3] 4.29

Neutralization (continued)

In the particular anode neutralization circuit just considered, both plates of the

tuning capacitor and one plate of the neutralizing capacitor are at a high d.c. poten-

tial with respect to earth. Therefore, the rotor of the tuning capacitor must be in-

sulated from earth.

But in many common types of tuning capacitor, the rotor is common to the

capacitor frame; an insulated mounting must therefore be provided to keep the

capacitor frame insulated from the chassis.

The anode neutralization circuit can, however, be modified in such a way that nod.c. voltage will be present on the rotor plate. Insulated mountings need not beused in such circuits.

In the schematic on the left (below), the rotor of the tuning capacitor is earthed.

The tap on the coil is earthed for r.f. through the 0-05 fxF r.f. decoupling capacitor.

The tap is also connected to H.T. through a radio frequency choke.

Observe that only part of the coil from A to B is in the tuned-circuit. The re-

mainder of the coil from B to C is transformer-coupled to the A-B portion of the

coil, and thus picks up r.f. for the neutralizing circuit.

In the other schematic, the tuned-circuit is capacity-coupled to the anode, so that

the d.c. anode current flows only through the radio frequency choke. One side of

the tuning coil and tuning capacitor connect directly to earth; and the tuning andneutralizing circuits are thus completely isolated from d.c.

HT+

ANODE NEUTRALIZATIONRFC

HT+

4.30

Neutralization {continued)

[§3

Grid Neutralization Circuit

•:&tJt:o*+:*ltH!*Ji!iSj&

The other type of circuit which provides a means of neutralizing the grid-to-anode

capacitance is the grid neutralization circuit.

In this circuit, the neutralizing voltage is applied to end B of the centre-tapped

coil L-l, while the anode-to-grid feedback voltage appears at end A of coil L-l.

If these two voltages are equal and of the same polarity, they will cause currents to

flow in the balanced grid tuned-circuit whose effects will caftcel each other out.

If, therefore, steps be taken to adjust Cn so that it is equal to Cga , the voltages

coupled through these capacitors will cancel each other, and the stage will not

oscillate.

Once a neutralizing capacitor is adjusted for a particular valve, it will require

only an occasional check. But if the valve be changed for a new one, the neutralizing

capacitor will need adjustment; for the new valve will have a slightly different value

of Cga .

The neutralizing capacitor may also need adjustment when the operating fre-

quency of a transmitter is changed by a large amount.

§3] 4.31

Neutralization Procedures

The procedures for neutralization are almost independent of the type of neutral-

izing circuit used.

The first step is to remove the anode voltage from the stage to be neutralized, the

object being to ensure that any signal present in the anode circuit is due to the inter-

electrode capacitance coupling between the grid and anode.

Then the master oscillator, and those amplifier stages which precede the un-

neutralized stage, are tuned. This will provide a strong signal to the grid of the

un-neutralized stage.

The next step depends on the indicator used; but it always involves the adjust-

ment of the neutralizing capacitor until there is a minimum amount of energy trans-

ferred to the anode circuit.

If there be a grid current meter, the grid current can be used to indicate the correct

adjustment of the neutralizing capacitor. When this capacitor is not properly

adjusted, the grid current will dip as the anode circuit is tuned through resonance.

When the circuit is properly neutralized, there will be no dip in the grid current whenthe anode circuit is tuned to resonance.

INPUT

Gridmtcurrent vwn

meter

Effect of varying anode tuning on grid current

LARGE DIPwhen stage is

poorly neutralized

JHK^^'p^»

SMALwhen neut

is imp

LDIPralization

roved

/

NO DIPwhen properlyneutralized

4.3215 *

Neutralization Procedures (continued)

Other methods used to adjust the neutralizing capacitor make use of devices

which can indicate the presence of r.f. energy in the de-energized anode circuit.

Devices which can be used for this purpose include the oscilloscope, a neon lamp, a

small flashlight bulb, or a sensitive d.c. milliammeter.

The device chosen affects the accuracy of neutralization, but not the method of

adjusting the neutralizing capacitor.

indicators used in neutralizing

As before, the circuits in the transmitter which precede the un-neutralized stage

are tuned to provide a strong signal to that stage. The anode supply voltage is dis-

connected from the anode of the stage; and when the anode is tuned to resonance,

the indicator will show either a maximum current flowing in the tuned-circuit, or a

maximum voltage across the tuned-circuit.

The anode circuit remains tuned to resonance; and the neutralizing capacitor is

adjusted until the voltage across (or the current in) the tuned-circuit appears on the

indicating device as a minimum.

§ 3] 4.33

Parasitic Oscillations

In a transmitter which is operating correctly, the tuned Class C amplifiers serve

only to amplify the ri. generated by the master oscillator. Sometimes, however,

the inductance of wires in the circuit combine with stray capacitance to form tuned-

circuits which are resonant to frequencies much higher than the desired transmitted

frequency.

These stray tuned-circuits will often cause the amplifiers to oscillate at very high

frequencies; and these oscillations (called "parasitic oscillations") are then trans-

mitted together with the desired frequency.

Parasitic oscillations are undesirable because they cause undue power losses and

so reduce the efficiency of the transmitter; and also because they cause interference

with other transmitters.

One way to eliminate parasitic oscillations is to improve the wiring by shortening

leads and by re-locating any components which may be in the parasitic oscillatory

circuit.

If this does not help, low-value resistors, or chokes of a few turns of wire, should

be connected directly to the grid and anode leads. These added components will

have very little effect on the amplification of the desired frequency; but they will

isolate the grid from the stray tuned-circuits sufficiently for the parasitic oscillations

to be eliminated.

Components which are placed in a circuit to eliminate parasitic oscillations are

called "parasitic suppressors."

Sometimes parasitic oscillations can be eliminated only by completely re-wiring

the circuit.

4.34

REVIEW

The Three Stages. The

master oscillator, the inter-

mediate power amplifier, and

the final power amplifier make

up the basic three-stage trans-

mitter.

PA

MO

IPA

I Sk>

Tuning. For efficient opera-

tion, the anode tuned-circuit of

the amplifier must resonate at

oscillator frequency. Adjust-

ment of the variable capacitor

to bring this about is called

"tuning."

P>

Tuning Methods. The anode

circuit of each transmitter stage

may be tuned by adjusting the

variable capacitor either for

minimum d.c. anode current, or

for maximum grid current in the

following stage.

Original [-^ |||j|j|r—N Hi r-N

signal '

—

yI llillli

1—y DHIII!

—

v

TransmittedSignal

Neutralization. Anode or

grid neutralization circuits may

be used to counteract the feed-

back effect of the grid-to-anode

capacitance in amplifiers using

triodes.

§4: FREQUENCY MULTIPLIERS 4.35

The Purpose of Frequency Multiplication

Until now, it has always been assumed that the anode tuned-circuit of an amplifier

stage in a transmitter can be tuned only to the grid signal frequency, whatever that

may be. For example, if the grid signal frequency is 1 Mc/s, the anode circuit is

also tuned to 1 Mc/s.If the grid signal is a pure sine wave, the anode circuit can indeed be tuned only

to the frequency of this sine wave (called the fundamental), and to no other. But it

so happens that generated frequencies are very seldom pure; they usually contain

harmonics of the fundamental frequency.

This is especially true in transmitters, where Class C amplifiers introduce manyharmonics into the generated signal. For example, if the master oscillator (oper-

ating in Class C) generates a 1 Mc/s sine wave, that sine wave is rich in harmonics—it contains not only the fundamental (1 Mc/s), but also the second harmonic(2 Mc/s), the third harmonic (3 Mc/s), etc.

If a signal rich in harmonics is applied to the grid of a tuned amplifier, the anodecircuit can be tuned to any one of the harmonics present in the original grid signal

—

with a consequent stepping-up of the frequency of the output signal. In this way, if

the output of an oscillator be only 1,000 kc/s, the output of the buffer amplifier maybe made 2,000 kc/s by tuning its anode to the second harmonic of the signal putinto it; the output of the next amplifier may be made 4,000 kc/s in the same way;and so on.

This process of converting the input frequency to the grid to a higher frequencyat the anode by tuning to a harmonic of the fundamental is called "frequency multi-plication."

tynequencif MuttifJicatfonM/ 4 Mc/s

Masteroscillator

Buffer(Tnpler

2nd I. P. A.Tripler)

F P. A,

The reason why frequency multiplier circuits are useful in transmitters is becauseoscillators operate more satisfactorily at low frequencies. By making use of fre-

quency multiplication when a high frequency is required, the oscillator can be left tooperate at a low frequency, while the multiplier circuits step up the oscillatorfrequency to the one desired.

For very high frequencies, crystal oscillators are often used in order to obtain goodfrequency stability. But it is impractical to manufacture a crystal to vibrate atsuch high frequencies. Therefore, the crystal oscillator is operated at a much lowerfrequency, and the desired output frequency is obtained by frequency multiplication.

4.36 [§4

The Final Power Amplifier

The maximum power which can be radiated from a transmitting aerial depends

on the power output of the final power amplifier (FPA). If the final power ampli-

fier has a power output of 100 watts, the aerial can radiate 100 watts, and no more.

A frequency multiplier has a lower output than has the same stage used as an

amplifier at the fundamental frequency.

If a final power amplifier which is capable of an output of 100 watts at the funda-

mental frequency is used as a doubler, its power output will be only about 65 watts

—if as a tripler, 40 watts—as a quadrupler, 30 watts—and so on. As the multi-

plication of the frequency increases, so the power output decreases.

And so, because the power output of a transmitter depends to a great extent on

the output of the final power amplifier, the FPA is not usually operated as a fre-

quency multiplier. All the multiplication of. the oscillator frequency has to take

place in the intermediate power amplifiers.

If you had this circuit.

and wanted this output

. 12Mc/s

_\/_ 100 W

__ __ __

F.P.A.MasterOscillator

1st

I. P. A.2nd

I. P. A.

you could

use this.

4McM Jl2Mc/fe\ Jl2Mc/A

X4 ~" X3 "F.P.A."

12 Me/a100 W

12 Mc/j

— X4h

12Mc/<

-F.P.A.

12 Mc/s

100 W

lMc/s>

|M.O.

but. .Not this12 Mc/s65 W

becausedoubling in the F.P.A.would result in

lower power output.

§4 4 -37

Frequency Doubling

Let us now examine a typical doubler circuit—that is, a circuit in which the out-

put frequency is twice the input frequency—and see how it works.

The circuit of a frequency doubler is not very different from that of an amplifier

which operates at the input frequency. The only differences are that the anode

circuit will be tuned to twice the input frequency; and that no neutralization is

required, since the input and output operate at different frequencies, so reducing

the possibility of self-excited oscillations.

WAVE FORMS IN A TYPICAL DOUBLER CIRCUIT

The doubler circuit is operated in

Class C, with the anode tuned-circuit

resonant at twice the grid signal fre-

quency. The pulses of current at

the same frequency as the input

signal flow from the cathode to the

anode, energizing the anode tuned-

circuit and causing it to oscillate at

twice the grid signal frequency.

Between pulses of anode current,

the tuned-circuit continues to oscil-

late. This is because the pulses of

"current always arrive at the same

time during alternate cycles of the

Cut-off

Bias

IWWHT+

doubled frequency, thus energizing the tuned-circuit at the right moment.

When accurately tuned, the voltage across the doubler tuned-circuit is at a maxi-

mum, and the voltage at the anode at a minimum, when current flows. The indica-

tions for tuning to twice the frequency are therefore the same as they were for tuning

to the input frequency.

The anode current meter will indicate a dip as the anode circuit is tuned to twice

the input frequency. At the same time, the grid current meter will indicate a rise.

4.38 [§4

Frequency Tripling

A frequency-tripling circuit (more briefly known as a tripler) has an output fre-

quency three times the input frequency. The appearance of the circuit is the sameas that of a doubler, or of an ordinary amplifier. Frequency tripling is accomplishedby tuning the anode circuit of the tripler to the third harmonic of its input frequency.

Input

voltageCut-off

Bias

HT+

Pulses of current flow from cathode

to anode—one pulse per cycle of

applied signal. These pulses arrive at

the tuned-circuit during every third

cycle of output voltage, and deliver

enough energy to the tuned-circuit to

sustain oscillations during those cycles ___________________«»_«.when no pulses occur.

The same tuning indications hold for frequency doubling and tripling as for funda-

mental frequency amplification. When the circuit is tuned accurately to the third

harmonic of the applied frequency, the voltage across the tuned-circuit will be larger

than if the circuit were poorly tuned. This will cause the voltage fed to the next

stage to be larger, which results in more grid current.

The larger voltage across the accurately tuned -circuit causes the anode voltage to

be at a low value when the valve conducts. This results in decreased anode current.

Therefore the correct tuning of the anode circuit—whether it be tuned to the input

frequency or to the second or third harmonic of the input frequency—is always in-

dicated as a dip on the anode current meter, or as a rise on the grid current meter.

The grid current meter in the following stage will also show a rise.

INPUT

GRD ,

CURRENT(MAMETER

FREQUENCY

CIRCUIT

§ 4]4.39

Tuning Indications

At this point the question arises, "How can you tell to which frequency—the

fundamental, or one of its harmonics—the anode tuned-circuit is tuned when the

anode current meter indicates a dip reading?"

The only way is to use a frequency indicator such as a wavemeter, or a calibrated

dial if the tuned-circuit has been previously tuned.

If you are working with an uncalibrated transmitter, the thing to do is to tune a

stage, starting with the tuning capacitor fully meshed. The first dip indicates that

the tuned-circuit is tuned to the fundamental. This can be checked with the wave-

meter.

As you continue decreasing the capacitance, you come to a second dip (not as

pronounced as the first), which is the second harmonic. Again you can check the

frequency with the wavemeter.

Continue decreasing capacitance, and you may come to a third dip (provided the

circuit constants are correct), which in turn is not as pronounced as were either the

first or the second dips. This dip indicates that the anode tuned-circuit is tuned to

the third harmonic. Here, too, you can check the resonant frequency by using the

wavemeter. :.„....„..............................._

'

=

-lllliliilAnode current ijlj.

'illmeieT dip II.

Third ®Harmonic(Triple r)

4.40 §5: TRANSMISSION LINES

Introduction

The function of a transmitter is the radiation of r.f. energy into space, so that

signals may be picked up by receiving aerials situated at various distances from the

transmitter.

You have studied oscillator and Class C amplifier circuits, whose function it is to

generate and amplify r.f. energy. Now other circuits are needed, in addition to the

ones just mentioned, to transfer the amplified r.f. from the anode circuit of the final

power amplifier into surrounding space.

These additional circuits are coupling circuits, transmission lines and aerials.

Just as a speaker in audio work transfers audio energy from electronic circuits into

the air, so the aerial is the means of transferring r.f. energy from the electronic cir-

cuits into space. The transmission line is the conveyor, or link, between the trans-

mitter and the aerial; and the coupling circuit connects the final power amplifier

tank circuit to the transmission line.

HOW RF IS DELIVERED FROM TRANSMITTER TO SPACE

AERIAL \ TRANSMISSION LINERadiates RF Supplies aerial with RF

COUPLING CIRCUITCouples RF from tank circuit

to transmission line

Totransmission

line

Final PA

In this Section you will learn about coupling circuits and transmission lines

—

what they are like and how they do their job. Aerials will be discussed separately

in Section 6.

§ 5]4.41

Coupling Circuits

A coupling circuit is used to transfer energy from the output of the transmitter to

the transmission line which feeds the aerial.

In addition to doing its job of transferring energy, the coupling circuit isolates the

aerial system from the high d.c. potentials present in the anode of the final power

amplifier. The coupling circuit also determines the amount of power transferred

from the tank circuit of the power amplifier to the transmission line input.

The simplest coupling circuit is direct coupling from the tank circuit to a single-

wire transmission line. A small capacitor is always placed at the input to the line

to block the d.c. from the aerial. The coupling is adjusted by varying the tap on

the anode tank coil.

DIRECT COUPLING4HT +

HI-*Blockingcapacitor

To single

wire i:n*

Another simple coupling circuit is inductive coupling to the tank circuit with an

untuned coil of a few turns. This type of coupling is used principally with un-

tuned transmission lines (to be discussed later).

INDUCTIVE COUPLING

A system of untuned coupling called "Link Coupling" is used when the aerial

coupling is remote from the tank circuit. The link consists of two pick-up coils of

about two or three turns, connected by wires and coupled to the tank and the aerial

coupling circuit respectively.

LINK COUPLING

IS 54.42

Tuned Coupling Circuits

A more commonly-used type of coupling is tuned coupling, in which the couplingcircuit is tuned to the operating frequency.

The advantage of tuned coupling is that it is frequency-selective, and so mini-mizes the possibility of undesired frequencies being radiated. In addition, since thetuned coupler is almost always variable-tuned, it can compensate for changes in theimpedance of the transmission line, and thus ensure maximum power transfer fromthe final power amplifier to the line at all times.

When the transmission line has a low input impedance, a series-tuned couplingcircuit is used. Series tuning is called "current feed," and can match the final PAto the low line impedance.

Se^,iHT+

PUHGTo line

with UflViinput

impedance

When a transmission line has a high input impedance, parallel tuning, called

"voltage feed," is used. Here the high impedance of the parallel tuned coupling

circuit matches the high input impedance of the line, and maximum power transfer

is effected.

To line

with lMriil

input

impedance

If the input impedance of the line is other than purely resistive, either of the above

two tuned coupling circuits can be adjusted so that the reactance of the line is can-

celled by the reactance of the tank circuit. This results in a pure resistive load andgives maximum power transfer.

§5] 4.43

Transmission Lines

A transmission line provides a means of transferring electrical energy from one

point to another. You know of at least one use of a transmission line in carrying

50 c/s power from the generator to a point of application.

In transmitters, transmission lines are similarly used to convey r.f. power fromone point to another. For example, a transmission line is used to carry r.f. powerfrom the transmitter to the aerial when the latter is some distance from the trans-

mitter.

7%cuuw£^iatt ^,irte&

50%flO€U&l

4.44 [§5

Frequency and Wavelength

Before you learn the theory of transmission lines, you should understand some-

thing about the properties of a radiated wave—its velocity of propagation (that is

to say, how fast it travels), its frequency, and its wavelength.

For purposes of simplicity, consider an a.c. generator sending 50 c/s energy

along a transmission line. Assume that the rate of travel of the a.c. is the same

as the velocity of electro-magnetic radiation in free space, which is constant at

186,000 miles per second (or 300,000,000 metres per second) regardless of the

frequency.

DISTANCE TRAVELLED

IN VlOO Of A SECOND

50cps

&t*<a

Hi

I860'miles

.ilk

DISTANCE TRAVELLED

IN 1/50 OF A SECOND

If the generator starts its generating action at the zero voltage point on the sine

wave, after a half-cycle has elapsed (1/ 100th of a second in time), the zero voltage

point will have travelled a distance which can be determined by 'multiplying the

velocity of the wave by the time duration for a half-cycle. This distance is about

1860 miles (186,000 x 1/100) which is approximately the distance from London to

Tobruk.

When another half-cycle (making a total of one full cycle) has elapsed (l/50th

of a second), the zero voltage point will have travelled a distance of 3720 miles

(1 86,000 X 1 /50), which is the approximate distance from London to Nairobi. This

distance of 3720 miles is the wavelength of the 50-cycle a.c, or the distance which

the wave travels during the time interval for one complete cycle.

§5] 4.45

Frequency and Wavelength (continued)

The wavelength of any radiated wave can be determined by multiplying the velocity

of the wave by the time taken for one full cycle of the wave. It is usual to express

wavelengths in metres ; so the units used in such calculations are : for velocity, metres

per second ; and for time, fractions of a second.

Now the time taken for one full cycle is 1 divided by the frequency (1//), so the

wavelength of a radiated wave is the constant velocity of propagation (v) divided by

the frequency (/). This is expressed algebraically as X=v/f, the Greek letter

"lambda" being the symbol generally used for wavelength.

This formula can also be written in the form v=f\; and when you look at it ex-

pressed thus, you can see that (since v is constant) the higher the frequency (/) the

shorter is the wavelength (A), and vice versa.

The wavelength of a signal whose frequency is 10 Mc/s can be found from the

formula A=v//as follows:

, 300,000,000 „ ,A=10,000,000

= 30metres

So the wavelength of the 10 Mc/s signal is 30 metres.

Similarly, the wavelength of a signal whose frequency is 15 Mc/s is:

, 300,000,00 1Q .

Q ,A=15,400,000

=1948metres

And the wavelength of a signal whose frequency is 1439 kc/s is:

. 300,000,000 ____tA=

1,439,000=208>5metres

You will find later that aerials and transmission lines are always defined in terms ofthe wavelength of the signals they radiate, rather than in frequencies.

In the table below are listed a few familiar transmitters, and their broadcasting

frequencies and wavelengths.

Transmitter Frequency Wavelengthin metres

BBC Light ProgrammeLong Waveband 200 kc/s 1500

Wrotham (VHF) 891 Mc/s 3-36

BBC Home Service

Medium Waveband 908 kc/s 330-4

Wrotham (VHF) 93-5 Mc/s 3-208

BBC Third ProgrammeMedium Waveband 647 kc/s 463-7

Wrotham (VHF) 91-3 Mc/s 3-285

Radio Luxembourg 1439 kc/s 208-5

BBC Television Channel 1

Vision 45 Mc/s 6-66

Sound 41-5 Mc/s 7-23

ITA Channel 9

Vision 194-75 Mc/s 1-54

Sound 191-25 Mc/s 1-57

4.46 [§5

The "Equivalent Circuit" of a Transmission Line

A typical transmission line used to convey r.f. energy from one point to another

may consist of two parallel lengths of wire held apart by insulating spacers, as illus-

trated below.

An r.f. transmission line will have a certain amount of resistance, capacitance,

and inductance along its length. The resistance is simply the resistance of the wire;

the inductive effect is caused by the expansion and collapse of the magnetic field

(generated by current flow) along the entire length of the line; and the capacitance

exists because the two conductors of the line act as plates of a capacitor separated

by a dielectric (which in this case is air).

Since the line illustrated can be theoretically "broken up" into a number of small

segments, each having equal amounts of inductance, capacitance, and resistance, the

entire line can be represented as consisting of a series of L, C, R networks connected

as shown below.

Transmission line and its equivalent circuit.

§ 5] 4.47

Characteristic Impedance

Suppose an r.f. generator is connected across a transmission line. The generator

will set up a voltage across the line, which forces a current to flow. The amplitude

of this current will be determined by the resistance, the inductance, and the capaci-

tance of the line, which together make up the line's impedance.

If the magnitude of the input current be now measured, and then divided into the

input voltage, the input impedance (Zin) of the line will be found.

If the line be now assumed to have infinite length, the input impedance of such a

line is termed the "characteristic impedance" of the line.

The symbol for characteristic impedance is Z .

You know that, when a pure resistance loads a generator, all the power generated

will be dissipated by this resistance. In the same way, if a generator could be madeto send electrical energy down an infinitely long transmission line, the energy wouldtravel down the line indefinitely, and would all be dissipated by the infinitely long

line.

In other words, the infinite line would act as a resistance equal in value to its

characteristic impedance, Z .

The infinite line can therefore be replaced by a resistance equal to its characteristic

impedance; and the generator will send the same amount of power into the resistance

as it did (theoretically) into the infinite line.

*t$?

*'l.l.

4.48 [§5

Line Termination in Characteristic Impedance

If a transmission line is terminated in a resistive load equal to its characteristic im-

pedance, this load will absorb all the energy from the line which has been applied

to the input by the generator. This is the ideal condition for maximum powertransfer.

• • . FOR MAXIMUM POWER

Tload I

Take as an example the case of a transmission line feeding an aerial. If a certain

type of aerial, called a "half-wave dipole," is used, the impedance at its centre feed-

point is 73 ohms. It follows that, in order to get maximum power transfer from

the transmission line to the aerial, the characteristic impedance of the line should

be 73 ohms, or close to it.

When this is the case, the line is said to be "matched" to the aerial.

• . . MATCHING LINE TO AERIAL

lf...ZAE

=730. .

for Maximum Power Output

Z should equal 73

A

§5] 4.49

Non-resonant and Resonant Lines

When a transmission line is matched to a load (Zioad = Z ), the a.c. voltage

measured across the line at any point will be the same, if you discount the slight

voltage drops in the line caused by its resistance. The current measured at any

point in the line will also be the same. This condition is shown in the illustration

by equal readings on the r.f. voltmeters and ammeters placed along the length of the

line.

The effective voltage and current distribution along the line can be shown graphi-

cally by two straight lines, indicating that the effective r.f. voltages and currents are

equal all along the length of line.

Such a line is called an untuned, or non-resonant, line. A transmission line will

always be non-resonant if it is terminated in its characteristic impedance—which

you will remember is the condition required for maximum power transfer.

distribution along an Untuned Line

^-^a$-^&-r-Qi

© @ E R = Zq

If a line is not terminated in its characteristic impedance, it is said to be "mis-

matched"; and not all the r.f. energy travelling down the line will be absorbed at

the load end. The amount of energy that will be absorbed depends on how close

the value of the load impedance is to the characteristic impedance of the line.

Since the load of a mis-matched line does not absorb all the energy coming downthe line, that part of the energy which is not absorbed must be reflected back up the

line. This energy is called the "reflected wave."

A mis-matched line therefore has two waves flowing through it—the forward waveand the reflected wave. These two waves combine all along the line (now called a

"resonant line") to form a resultant wave called a "standing wave."

4.50 [§5

"Standing Waves" on a Rope

It will help you to understand better how energy travels down a transmission line,

and how reflected waves generate standing waves on the line, to consider a rope, oneof whose ends is fastened to a wall while the other end is held in the hand.

When the hand flicks the rope once, a vibration starts to travel down the rope.

If the rope were infinitely long, the vibration would continue down the rope for ever.

(This is what would happen with an infinite length of transmission line, or an un-

tuned line, in that the energy put into the line is completely absorbed.)

The Long Transmission Line

and the Long Rope

Toinfinity

*•

But when the vibration travelling down the rope reaches the end attached to the

wall, it is in fact reflected back towards the hand. Similarly, when a transmission

line is mis-matched, the electrical energy is reflected back towards the generator.

If the hand vibrates the rope at a constant rate, the reflected vibrations combine

with the oncoming vibrations to produce standing waves along the rope.

At some points along the rope, the forward and reflected vibrations will be in

phase, reinforcing each other to produce vibration of large amplitude. At other

points they will be out of phase, thereby cancelling each other; and the rope will

appear to be motionless at these points.

In a similar manner, standing waves of voltage and current are formed on a trans-

mission line when it is mis-matched.

STANDING WAVES on a rope

§ 5]4.51

Open and Shorted Transmission Lines

When a transmission line is open at its end, the forward and reflected waves com-

bine along the line to form points of varying effective voltage and current.

At the open end, the effective voltage is a maximum, and the effective current is

zero. (It is easy to see that the current must be zero at all times at the open end,

because it is an open circuit. And, since charges build up on the open ends, a large

voltage difference will always exist there.)

At every half-wavelength distance from the open end, these conditions of voltage

and current will repeat themselves; and between these half-wave points the effective

voltage and current readings will vary as a sine wave varies.

The meter reading in the illustration below shows the variations in the effective

voltage and current along the length of the line at quarter-wavelength distances from

the open end to the input. The waveforms shown are actually a plot of these voltage

and current readings at different points along the line.

Observe that the waveforms (called, you will remember, "standing waves") cause

the voltage and current to be zero at certain definite points along the line. Notice,

too, that when the current is zero, the voltage is maximum; and when the voltage

is zero, the current is maximum.

Standing waves on

OPEN CIRCUITEDLINE

When the transmission line is shorted at its terminating end, however, the voltage

at that end must be zero; because no voltage can exist across a short. But the

current at the short will be a maximum; because the short provides a zero resistance

path through which current can flow.

Just as in the open-circuited line, these voltage and current conditions at the

terminating end will repeat themselves at half-wavelength intervals back from the

short circuit.

Observe that the standing waves on the short-circuited line are displaced a distance

equivalent to a quarter of a wavelength (90 degrees) compared to waves on the open-

circuited line.

Standing waves onSHORT CIRCUITED

LINE SHORTED

[§54.52

The Input Impedance of a Line

In a transmission line terminated in its characteristic impedance, the voltage andcurrent readings are the same all along the line. Therefore, the impedance anywherealong the line is constant, and equal to its characteristic impedance.

In other words, if you were to break off the line anywhere along its length, andmeasure the impedance (Zin) looking in towards the load end, the impedance value

measured would always be the same, and would be equal to the characteristic

impedance, Z —which is resistive.

When a transmission line is terminated in any other than its characteristic im-

pedance, it becomes resonant, and develops standing waves. The input impedance

then varies with the length of the line, because the effective values of the current and

voltage vary along the length of the line.

The reactance of the input impedance also varies—being sometimes resistive,

sometimes capacitive, and sometimes inductive. A resonant line has, therefore, the

characteristics of a resonant circuit, which presents a resistive load at the resonant

frequency, and inductive or capacitive reactance on either side of the resonant fre-

quency.

resohnMl\HE

§5] 4.53

The Input Impedance of a Short-circuited Line

A short-circuited line offers very low resistance at its shorted end, since the voltage

is minimum and the current is maximum. This low resistance is repeated every

half-wavelength back from the shorted end.

Since the line is called "resonant," it is convenient to think of the low resistance

points along the line as series-resonant circuits. For example, the input impedance

at a half-wavelength section of shorted line is that of a series-resonant circuit.

A quarter-wavelength back from the shorted end, however, the current is minimumand the voltage is maximum. This is therefore a point of high resistance.

These points of high resistance are repeated every half-wavelength back from the

first high resistance point. They can be considered to be parallel-resonant circuits,

just as the low resistance points can be considered series-resonant circuits.

Between the high and the low resistance points, the input impedance is either a

capacitive reactance or an inductive reactance.

From the shorted-end to a quarter-wavelength back from the terminating short

circuit, the input impedance is inductive. The inductive reactance is low in the

vicinity of the short circuit, but increases in magnitude as you approach the quarter-

wave point. Exactly at the quarter-wave point, the impedance is a pure high

resistance.

Between a quarter-wavelength and a half-wavelength, the input impedance is

capacitive reactance. This capacitive reactance decreases as the half-wavelength

point is approached; until, at the half-wavelength point, the impedance is a pure

low resistance.

The type and magnitude of the input impedance, as seen at different points along

the short-circuited line, is illustrated below.

INPUT IMPEDANCE ALONG A SHORT CIRCUITED LINE

4.54 [§5

The Input Impedance of an Open-circuited Line

In an open-circuited line, the terminating impedance (open circuit) is a high

resistance, and therefore acts like a parallel circuit.

A quarter-wavelength back, the input impedance is a low resistance; it therefore

has the characteristics of a series-resonant circuit. Between the open-circuited endand a quarter-wavelength back from the open circuit, the input impedance is capaci-

tive; and between a quarter and a half-Wavelength, the input impedance is inductive.

If you compare the open- and short-circuited lines, you will observe that, for a

given wavelength back from the end, the reactances are opposite to one another.

Where one is capacitive, the other is inductive, and vice versa.

INPUT IMPEDANCE ALONG AN OPEN-CIRCUITED LINE

*hMil? 1 TT TU^

ToGenerator

The following diagrams illustrate different lengths of open and shorted lines, and

the input impedance they present to a generator.

It is obvious from the above diagrams that the terminating conditions at the end

of the line determine the type and magnitude of the input impedance at any point

along the line.

§5] ".55

Some Applications of Transmission Line Principles

Suppose you now learn about a few of the many applications of transmission lines

in electronic equipment.

A shorted quarter-wave transmission line, known as a "stub," will, for instance,

offer a very high impedance at its input. It can therefore be used as a metallic

insulator to support a two-wire transmission line without shorting the line.

A/4 STUB USED AS A METALLIC INSULATOR

Two-wiretransmission

line

Supportingmetal base

The shorted quarter-wave stub also makes a very effective filter for harmonic

frequencies of the fundamental which it is not desired to transmit.

For the fundamental frequency, the stub is a high impedance, as was shown above.

For the second harmonic, the stub becomes a half-wavelength in length. It will

therefore act as a short circuit across the transmission line, shorting out the un-

desirable harmonic and preventing it from getting to the aerial.

/ 2 STUB ASHARMONIC FILTER

LOWIMPEDANCE

4.56 B s

Some Applications of Transmission Line Principles (continued)

An important application of a short transmission line—or "tuned line section,"as it is called—is to tune out the reactance of a load on a transmission line, thusleaving the load resistive.

Suppose, for example, that a 300-ohm line is feeding a load which looks like a300-ohm resistance in parallel with a capacitance. Since the load is not completelyresistive, standing waves will exist on the line, and maximum power transfer to theload will not be realized.

But if an inductance could be placed in parallel with the capacitance, so as to

effect a parallel-resonant circuit, the transmission line would look into the 300-ohmresistive component in parallel with the high resistance of the parallel-resonant cir-

cuit. And since the high resistance of the parallel-resonant circuit is so much greater

than 300 ohms, the transmission line would effectively "see" only the 300-ohmresistance.

The effect of the capacitance in preventing full power transfer to the load wouldthus be cancelled out.

The way to introduce an inductance across the load is to place a quarter-wave

shorted stub, with a movable shorting arm, across the load terminals. Then, bymoving the short so that the stub becomes less than a quarter-wavelength long, the

input reactance of the stub becomes inductive.

The value of this inductance can be varied by means of the movable short until it

exactly cancels the capacitance of the load, thus leaving the load resistive.

K A/4H ___=A 300 i i High

MATCHED LINE...No standing wavesMaximum power transfer

Quarter-wave sections of line are also used as transformers, or as matching devices

to connect circuits of unequal impedances. If a low-impedance input circuit is to

be connected to a high-impedance grid circuit, the input circuit may be tapped downon the coil of a tuned-circuit, as shown below.

If a tuned line is used, the input circuit can similarly be tapped down on the

tuned line. This is an example of a tuned line used as a step-up transformer.

Lowimpedance

input

circuit

A quarter-wave stub can be used as a step-down transformer to match a high-

impedance line to a low-impedance dipole aerial. The line is connected to the high-

impedance input of the stub, while the aerial is connected near the low-impedance

shorted end of the same stub.

High impedance I

line -r-

Stub used as 1 High imp. 1

a step-downtransformer

line -•_ ^4 -»

1 l

§5] 4.57

Types of Transmission Lines

Many different types of transmission lines are used in electronic applications.

Each has a certain characteristic impedance, current-carrying capacity, insulation,

or physical shape fitting it to meet a particular requirement.

Shown below are some of the most frequently used transmission lines.

SingleA simple method of feeding an aerial /Wire

from a transmitter is to use a single-wire

transmission line, with an earth return com-

pleting the circuit.

Another type of transmission line consists of two parallel wires, maintained at

a fixed distance from one another by insulated spacers. Since this type of line is

not screened, losses of power occur in it by reason of radiation, and of absorption

by metallic objects. The use of the line is therefore restricted to comparatively low-

frequency transmission; and it should be strung only in places where it will be away

from metallic objects and out in the open.

TWO WIREOPEN LINE Insulated spacer^ Parallel wires'

Some of the disadvantages of the two-wire open line are overcome in the concen-

tric line, which is made of a cylindrical copper tube with a thin conductor running

full length through the centre. The inner conductor is kept centred by spacers, and

the outer conductor is earthed in order to screen the inner conductor.

Since the concentric line is mechanically rigid, however, it can be used only for

permanent installations.

CONCENTRIC

LINE Outer conducto Inner conductor

The inflexibility of the concentric line is overcome in the coaxial cable, which

consists of an inner conductor embedded in an insulating material, which is in turn

covered with copper braid. The coaxial cable incurs, however, much higher powerlosses than does the concentric line.

COAXIAL

CABLE

Insulation Rubber covering-.

Innerconductor Copper braiding

At very high frequencies, the losses in any of the above-mentioned lines becomeexcessive; and wave guides must be used instead. Wave guides are made of roundor rectangular hollow tubes.

WAVEGUIDE Rectangular hollow tube

[§54.58

REVIEW

Transmission Lines. The purpose of a

transmission line in a transmitter is to con-

vey r.f. energy from the transmitter to the

aerial. The characteristic impedance of

the transmission line must match the input

impedance of the aerial, if maximum power

transfer to the aerial, and therefore maxi-

mum radiated power, is to be realized.

Characteristic Impedance. A transmission line has a characteristic impedance (Z ).

If it is terminated in a load equal to its characteristic impedance, maximum power is

transferred to the load, and no standing waves will be created on the line.

. . . MATCHING LINE TO AERIAL

lf...ZAE

= 73 n...

for Maximum Power Output

Z should equal 73H

Standing Waves. When a transmission line is terminated in a load other than its

characteristic impedance, some of the energy is reflected from the end of the line back

towards the generator. The forward and reflected waves combine along the line to form

standing waves.

The voltage and current distribution along an open and a shorted line respectively are

shown again below.

OPEN SHORTED

§6: AERIALS 4.59

The Function of an Aerial

The purpose of a transmitting aerial is to convert the power delivered by the

transmission line into a wave called an "electro-magnetic wave." This electro-

magnetic wave is then radiated through space.

All aerials work on the same principle—the aerial current generates an electro-

magnetic field, which leaves the aerial and radiates outwards as an electro-magnetic

wave.

The aerials you will be concerned with now are those which have been designed

as transmitting aerials. But before you begin on them, you may be interested to

hear of an example of aerial action in which planning and design has played no part

at all.

THE AERIALCONVERTS ELECTRICALPOWER INTO ELECTRO-MAGNETIC WAVES

Electro-Magneticwaves

Try touching your finger to the Y-plate terminal of an oscilloscope. You will

see a 50-cycle waveform on the 'scope screen, which obviously must come from your

body.

What is actually happening is that your body is picking up 50-cycle electro-

magnetic waves which are being radiated from the mains wires which carry 50-cycle

current. The wires are in fact acting as transmitting aerials, although they were

obviously not designed for that purpose.

Body picking up50 cvcle

radiation

tiom mams

[§64.60

How an Aerial Works

If the wires of an open-ended transmission line are bent back at right angles tothe line, at a point one quarter-wavelength back from the open end, a simple aerial is

formed which is called either, a "half-wave dipole," a "doublet," or a "Hertz aerial."

The voltage and current distribution on this aerial are the same as on the originaltransmission line.

Bend transmission line here

If at any point on a transmission line, the voltage on one wire is (+) 10 volts with

respect to the other wire, then you could also say that the voltage on the second wire

is (— ) 10 volts with respect to the first wire. So also on the half-wave dipole aerial

wires, the voltages at any two corresponding points on the two wires are equal, but

opposite in polarity. The same thing is true of the currents.

Therefore, to indicate polarity as well as amplitude on the wires Which comprise

the transmission line and aerial, the waveforms must be re-drawn as shown.

WAVE FORMS SHOWING POLARITY AND AMPLITUDE

Transmission Line Aerial

Observe that the standing waves of voltage and current indicate that the aerial

ends are points of maximum voltage and minimum current; whereas the centre of

the aerial is a point of maximum current and minimum voltage.

§6] 4.61

How an Aerial Works (continued)

You know that, whenever there is a difference in voltage between two points, an

electrical field is set up between these points. You also learnt in Basic Electricity

that when a capacitor is charged, one plate will be positive and the other negative.

The consequence of these two facts is that an electrical field, having a direction

towards the positively-charged plate, is built up between the capacitor plates—as

shown in the first diagram below.

In the same way, the voltage difference between the two wires of an aerial also

generates an electrical field, which has a pattern and direction that you can see in

the second diagram.

Electric Field of aCharged Condenser Electric Field Surrounding

an Amort'h^^x^M^MM^M^M^WM^Mm -̂

Besides this electrical field, there is also a magnetic field, which is generated by

the aerial current. The plane of this magnetic field is at right angles to the direction

of the current flow; and therefore is at right angles to the aerial (see below).

The electrical and magnetic fields are therefore at right angles to each other.

Magnetic Field Surrounding

<ut/hria£

These electrical and magnetic fields alternate about the aerial—building up, reach-

ing a peak, collapsing, and building up again in the opposite direction—at the same

frequency as the aerial current.

In the process of building up and collapsing, a portion of these fields escape from

the aerial, and become the electro-magnetic waves which radiate through space,

conveying the transmitted intelligence to distant receivers.

IS 64.62

Basic Aerials

The half-wave dipole, or Hertz, aerial is one type of basic aerial which finds wideapplication in many kinds of transmitting and receiving equipment.

Another basic aerial is a vertical quarter-wave grounded aerial sometimes called a

"Marconi aerial." If one of the elements of a Hertz aerial be removed, and the wire

which went to that element earthed or grounded, the result will be a Marconiaerial. The earth actually takes the place of one of the quarter-wave elements, so

that the earth and the remaining quarter-wave element form an effective half-wave

dipole.

The current maximum and voltage minimum points are at the base of the aerial,

as you can see below.

8ASf€A ILS

VERTICAL A/4 GROUNDEDAERIAL (Marconi)

When a Marconi aerial is used, the earth directly beneath the aerial must be a

good electrical conductor. Sometimes, in order to improve the ground conductivity

at this point, a length of copper tubing is driven into the ground at the base of the

aerial.

An earth can be simulated by placing metal rods or wire mesh at the base of an

aerial. This arrangement is called a "counterpoise earth."

Since a quarter-wavelength dipole aerial is only half as long, physically, as is a

half-wave dipole, it is often preferred at low frequencies (large wavelength), especi-

ally when there are space restrictions on aerial mountings.

At high frequencies the half-wavelength dipole is extensively used; because, even

though it is longer than the quarter-wave aerial, its overall length is still small—and

it can be made of metal tubing, which makes it self-supporting.

§t]4 -63

Radiation Resistance

In a half-wave dipole aerial, the voltage at the centre is minimum (in fact it is

practically zero), whereas the current is maximum. If you recall the characteristics

of a series-resonant circuit, you will remember that the voltage across it is also

minimum, and the current through it also maximum. At its centre, therefore, a

half-wave dipole is equivalent to a series-resonant circuit when it is operated at the

frequency for which it is designed.

A generator supplying power to a series-resonant circuit works into a pure resistance

(since XL and Xc cancel each other out)—the resistance into which it works being

principally the resistance of the coil. In the same way, a transmission line works

into a pure resistance when a half-wave dipole is connected to it.

This resistance is made up of the resistance of the wire, and another resistance

called the "radiation resistance." Since the resistance of the wire is in practice

negligible, only the radiation resistance need be taken into consideration at this point.

THE INPUT IMPEDANCE OF A DOUBLET LOOKSUKL.

series resonant circuU

The radiation resistance, however, is not a physical resistance. It is, rather, an

"equivalent" resistance—a resistance which, if it were connected in place of the

aerial, would dissipate the same amount of power as the aerial radiates into space.

The value of this radiation resistance can be determined from the power formula,

R = P/I2, where P is the power radiated from the aerial, and / is equal to the aerial

current at the centre of the aerial.

For a half-wave dipole the radiation resistance is about 73 ohms, measured at the

centre of the aerial. This value is fairly constant for half-wave dipoles working at

any frequency.

73A

M^#k-V4-j e/gpfl*A '" -Wr-

.the radiation resistance

(S 64.64

Aerial Impedance

Since a half-wave dipole acts like a series-resonant circuit, it will exhibit eitherinductive or capacitive properties as the r.f. frequency applied to the aerial is varied.When the frequency of the r.f. is just right, the dipole is exactly a half-wavelength

long, and is series-resonant—with its impedance resistive, and equal to the radiationresistance. It is always desirable that a transmitting aerial shall present a resistiveload to the transmission line; for in this way a maximum amount of power will beabsorbed by the aerial, and so radiated.

Optimum conditionm^^^SSESfeexactly x/2 730

[-^gggMh-VWri = -m—The radiationresistance

If the frequency of the transmitter goes up, the aerial will be greater in length thanis the new half-wave. The series circuit will then be operating at a frequency whichis above its resonant frequency. At this applied frequency, the inductive reactancewill be larger than the capacitive resistance, and the aerial will appear inductive

to the transmitter.

Dipole LONGER than ty2 appears INDUCTIVE

= —nppp--*VWV—

/2

If the frequency of the transmitter goes down, however, the aerial will now beslightly shorter than a half-wavelength. The series circuit will be operating at a

frequency which is below its resonant frequency. The capacitive reactance will belarger than the inductive reactance, and the aerial will appear capacitive to the

transmitter.

Dipole SHORTER than A/2 appears CAPACITIVE

B" — —1|—WW—A/2

Tuning the Aerial

You have seen that, when the frequency of the transmitter is varied, the electrical

length of the aerial varies as does the impedance at its input. You also know that

it is desirable to have the aerial impedance resistive for all transmitter frequencies,

in order to obtain maximum radiated power.

The electrical length of an aerial can be effectively increased or diminished (and

the aerial can thus be resonated) by the insertion into the circuit of appropriate in-

ductors or capacitors.

Suppose, for example, that a vertical quarter-wave grounded aerial be less than a

quarter-wavelength long. The input impedance at its base will be both resistive and

capacitive. This aerial can be electrically lengthened (and so resonated) by adding

an inductor of the correct size to cancel the capacitance, thus leaving the aerial

resistive.

The inductor must be placed in series with the aerial at its base, as shown below.

RESONATING A MARCONI AERIAL

O -

looks like looks like —WWW—If a vertical quarter-wave grounded aerial be more than a quarter-wavelength long,

the input impedance at its base will be both resistive and inductive. The aerial can

be electrically shortened by adding a capacitor of the right size to cancel the induc-

tance, thus leaving the aerial resistive.

looks like o-W/VWr—/0OT^-o looks like —WUk-

[§64.66

Radiation Pattern

When an aerial radiates electro-magnetic waves, the radiation will be stronger in

some directions than in others. The aerial is said to be "directional" along its line

of strongest radiation.

In the case of the half-wave dipole, maximum radiation occurs in the plane at

right-angles to the aerial conductors which passes through the point of maximumcurrent in the conductors.

The amount of radiation from a dipole is shown graphically in the diagramsbelow. Maximum radiation takes place along the lines OP, OP', OP", etc.

Dipole\^ t t

Point of maximum*" ~

'

—current

Radiation pattern asviewed in direction of

arrow A'

A radiation tester, called a "field strength meter," can be used to measure the field

strength at all points around the aerial. The radiation patterns for a half-wavedipole mounted both horizontally and vertically are illustrated below.

r —-«.

rf>5$*£«d»

i

MaximumRadiationin this plane

Horizontal

aerial MinimumRadiation

MaximumRadiationin this plane

All of the above diagrams assume that the aerial is isolated in space. In actual

practice, however, the aerial will certainly be located near ground surfaces, so that

the radiation patterns will be appreciably different from those shown above.

4.67

§6]

Wave Propagation

You know that the function of an aerial is to radiate electro-magnetic energy into

space. Once this energy is released from the aerial, it will travel through space until

it is picked up by a receiving aerial-or until it strikes an object and is reflected off

it as is the case with radar transmission.

'it is therefore important to you to know what happens to a radiated wave in space

-what its path is, if it is absorbed by the earth, if it is reflected by the sky, and so

on-in order that you may be able to tell how far the wave will travel before it can

be picked up. . .

The subject of what happens to a radiated electro-magnetic wave once it leaves

the aerial is called the theory of "wave propagation."

When a radiated wave leaves the aerial, part of its energy travels along the earth,

following the curvature of the earth. This is called the "ground wave."

Other waves which strike the ground between the transmitter and the horizon are

called "space waves"; and those which leave the aerial at an angle greater than that

between the aerial and the horizon are called "sky waves."

The ground wave, the space waves, and the sky waves all carry the transmitted

intelligence. But at certain frequencies one of the waves will be much more effective

in transmitting the intelligence than will the others.

At comparatively low transmitted frequencies, most of the radiated energy is in the

ground wave. Since the earth is a poor conductor, the ground wave is rapidly

reduced, or "attenuated," by absorption and is therefore not effective for transmission

over great distances unless large amounts of transmitted power are used.

The medium and long wave-band broadcast frequencies are examples of trans-

missions using ground waves. At these frequencies the effective radiating area

usually lies within 200 miles radius from the transmitter. Stations more than

400 miles away from each other can therefore transmit on the same low frequencies,

and yet not interfere with each other.

(S 64.68

Sky Waves and Ground WavesAt first sight, one would think that sky waves can serve no useful purposes, since

they will only travel straight out into space and get lost.For very high frequencies, this actually happens, and the sky wave is useless But

below a certain critical frequency the sky wave does not travel straight our it isbent back to earth in the upper layers of the atmosphere.

This returning wave is not sharply reflected, as is light from a mirror. It is bentback slowly, as if it were going round a curve; it is therefore called a "refractedwave.

This refracted wave, once it returns to earth, is reflected back to the sky again—where it is once again refracted back to earth. This process of refraction from thesky and reflection from the earth continues until the wave is completely attenuated—the energy of a radiated wave dropping as its distance from the transmitting aerialincreases. A receiving aerial will be able to pick up a signal at any point where therefracted wave hits the earth.

If the sky wave were radiated to the sky at one angle only, of course, no signalwould arrive at any point between those where the refracted wave hit the earth.Sky waves, however, are radiated at all angles to the sky; the earth's surface (beyonda certain minimum distance from the aerial) is therefore completely covered withradio signals.

As the angle of radiation of the sky wave gets steeper, a point is eventually reachedat which the wave is no longer refracted back to earth, but continues travelling oninto space. As a result, there is a zone around the aerial in which no refracted skywave hits the earth.

Since the ground wave itself is only effective over a short distance, there exists azone between the maximum effective radiating distance of the ground wave and thepoint where the first sky wave is refracted back to earth, which is an area of "radiosilence" in which no signals from this particular transmitter are received. This zoneis called the "skip distance" of the transmitter.

Lost SkyWaves

Limit of

Space Waves

First RefractedSky Wave

^'^^f^PMk-^

Limit of

Ground Wave

'::'#:?

rw.m

The critical frequency, which is the frequency above which no sky wave (whateverits angle of radiation) can return to earth, varies—depending on numerous factors

such as the time of day, the time of year, the weather, and others.

§4]

The Space Wave and Fading

At frequencies above the critical frequency, neither the ground wave nor the sky

wave can be used for transmission. At these high frequencies, the ground wave is

rapidly attenuated, and the sky wave is not refracted back to earth.

The only radiated wave which can be used for transmission at these frequencies

is one that travels in a direct line from the transmitting aerial to the receiving aerial.

This type of transmission is called "line-of-sight transmission"; and the radiated

wave is called a "space wave."

Line-of-sight transmission is used in radar for detecting enemy aircraft, and (for

instance) in ship-to-plane communication. The frequencies used are usually above

30 megacycles.

LINE OF SIGHT TRANSMISSION

..tuedtinXadax/

• • • • ^»}i2^BfJtiSi,l-ii;,'!-

Sometimes a receiving aerial picks up two signals which have travelled along dif-

ferent paths from the same transmitting aerial. For example, one signal may travel

direct from the aerial, and the other signal may have been reflected off an object.

Since the signal paths are constantly changing, the two signals will sometimes be

in phase, and at other times out of phase with each other—thus tending either to

cancel or to reinforce each other.

The result is a variation in signal strength at the receiver end which is called

"fading."

^mx^^i

FADING

ignals received

*3»

Frequency Spectrum

There follows now an outline of the components of a radiated wave which areused for transmission at various frequencies:

From 30 to 300 kilocycles (low-frequency band), the ground wave is largely usedfor medium-range communication, since its stability is not affected by seasonal orweather changes. For very long distance communication, the sky wave is used.

From 300 to 3000 kilocycles (medium-frequency band), the range of the groundwave varies from 15 to 400 miles. Sky wave transmission is excellent at night forranges up to 8000 miles. In the daytime, however, sky wave transmission becomeserratic, especially at the high end of the band.

From 3 to 30 megacycles (high-frequency band), the range of the ground wavedecreases rapidly, and sky wave transmission is highly erratic on account of theseasonal factors previously mentioned. Space wave transmission begins to becomeimportant.

From 30 to 300 megacycles (very high-frequency band—or VHF), neither theground wave nor the sky wave are usable; and space wave (Iine-of-sight) transmissionfinds major application.

From 300 to 3000 megacycles (ultra-high-frequency band—UHF), space wavetransmission is used exclusively.

TRANSMITTING FREQUENCIES.

.

.

^ppw^wv

illlilKi,,

$;:&&;;£I&x-Svl|\;X-.\vX;X;X*X\'X*X%P*j-'.X-Xv.*vXvX*XvX*X€*

r -x-x ::'•:•:'.£88$$30 Ke/8 300 Kc/s

Used for Communication3Mc/s

Radar.

5 6]

REVIEW—Aerials

Half-wave Dipole. A half-wave dipole aerial can be considered as a parallel-wire

transmission line whose wires have been bent at 90 degrees to the line a quarter wave-

length from the open end.

Bend transmission line here

Radiation. The voltage and current distributions along the aerial generate electric

and magnetic fields at right angles to each other, which are radiated into space as

electro-magnetic waves. These waves carry the intelligence of the modulating signal,

and can be detected by distant receivers.

Magnetic Field Surrounding

4* Aenai

Wave Propagation. The energy radiated from an aerial consists of sky waves,

space waves and ground waves. Each of these is used for transmission at those

frequencies for which it is best suited.

4 72 § 7: CONTINUOUS-WAVE (CW) TRANSMISSIONThe Advantages of CW Transmission

You will remember from the Section on "The Role of the Transmitter" that amessage can be transmitted either by code or by radio telephony. Code transmissionis either CW (continuous wave) or MCW (modulated continuous wave). In bothcases the r.f. radiated by the aerial is turned on and off, either by a hand-operatedkey, or by an automatic device, in sequences of dots and dashes.CW transmission is very widely used. When a transmitter is modulated by voice

or by MCW, it sends out not only the carrier frequency, but also the sum anddifference (beat) frequencies of the carrier and of the modulation signal. Theseadditional frequencies are called "sideband frequencies." Now in order to pick upa voice or MCW signal, a receiver must be sufficiently broadly tuned to pick up boththe carrier and the sidebands. As a result, it may also pick up a nearby signal inaddition to the one desired; and this interference may make it impossible to under-stand the desired signal.

CW transmission, on the other hand, has negligible sidebands. A receiver, there-fore, does not need to cover as wide a range of frequencies for a CW signal as it doesfor a voice signal. The receiver may be made highly selective, and there is thusless likelihood of interference when receiving a CW signal.

Carrier Wave Frequency

SGf 0F FHf WU

f^ptSOVTRANSMlW-

CW\

MCW

Carrier Wave Frequency

\ Sideband

L^V FrequenciesmCarrier Wave Frequency i

\Sidebands I Sidebands

There are many different circuits used to obtain CW transmission. They lookdifferent, and they operate differently; but each has the same purpose—namely, toturn the r.f . of the transmitter on and off.

You will learn about some of these circuits in the next few pages.

4.73§7]

Cathode Keying

Regardless of the circuit used, the CW output of a transmitter looks like a series

of pulses of r.f. separated by gaps of no r.f. The gaps between the ri. pulses occur

when the key is up; while the length of each ri. pulse is determined by the length of

time the operator holds the key down.

The simplest method of obtaining CW transmission is by "cathode keying." In

this type of circuit, the key may be connected as illustrated below. When the key

is opened, no current can flow, and no r.f. can be radiated from the aerial. When

the key is closed, the circuit operates normally.

The stage which is usually keyed in this manner is either the master oscillator

itself, or the master oscillator plus one or more of the following amplifier stages.

Seated,

\\v-tr\y\ ladiiectltf

Seated- -

l^M^H^MjB 1 1 *

j

^ Kevi

r^ 1

4.74[§7

Cathode Keying (continued)

The disadvantage of using direct cathode keying is that the operator will get ashock if he puts his fingers across the key contacts while the key is open. For whenthe key is up, the series circuit of key, valve, and H.T. is open at the key; and nocurrent can flow. But with 'the operator's fingers across the contacts, the circuit iscompleted; and current flows—through the fingers!

What happens is that the anode resistance of the valve and the resistance of theoperator's fingers across the key contacts form a voltage divider circuit across H.T.The resistance of the operator's fingers will usually be large compared to the anoderesistance—with the result that most of the H.T. voltage (of about 300 volts) will beacross the key, and therefore across the fingers.

*/%& & efutvateHt ta t&U

HOW A VOLTAGE DEVELOPS ACROSSTHE OPEN KEY CONTACTS

To safeguard the operator, a slight variation can be made to this basic circuit.

The key is connected to a low voltage circuit containing the coil of a relay. Whenthe coil of the relay is energized, the contacts of the relay (which are in series withthe cathode circuit) close, and permit the stage to operate normally.

KEYING CIRCUITSUSING RELAY

Armature ContactsTo

circuit

The relay consists of a soft-iron core,

a coil and an armature. The armatureis attracted to the core when the coil is

energized, thereby closing the contacts.

A spring re-opens the contacts when the

coil is de-energized.

Electro-magnet

To voltage •*

source

To circuit

Insulation

Armature

RELAY Returnspring

Blocked-grid Keying

Keying can also be accomplished by changing the grid voltage of the stage being

keved.

When the key is open, the grid bias is well beyond cut-off; so that the r.f.grid

signal can never bring the valve into conduction, and no r.f. signal appears at the

anode. When the key is closed, the bias is the normal value for Class C operation,

and the stage operates normally. This type of keying is known as "blocked-grid

keying."

In the circuit shown on the left below, the key (or relay) controls the d.c. bias on

the grid of an intermediate power amplifier. With the key open, the voltage on the

grid is equal to the bias voltage, which is many times cut-off value. With the key

closed, the grid is connected to a voltage divider which provides normal operating

bias to the valve.

With the key down, therefore, the transmitter is sending out an r.f. signal. This

signal is interrupted each time the key is opened.

The same idea can be applied to the screen grid (see the circuit on the right

below).

In this circuit, the voltage varies from a positive operating voltage with the key

closed, to a negative blocking voltage with the key open. When the key is opened,

the screen is connected through resistor R to the bias voltage, which is sufficient to

cut off the stage completely. When the key is closed, the screen is connected

directly to H.T. The purpose of R is to limit the current flowing from the bias

terminal to H.T.(+) when the key is closed.

In this circuit, as in the last, a keying relay may be used to protect the radio

operator from high d.c. voltages.

BLOCKED GRID KEYING CIRCUITS

CONTROL GRID

RFInput

o *

SCREEN GRID

HT>

US

RFInput

o—

Bias Bias Bias

[§74.76

Keying Valve Circuits

Relay or key contacts cannot close or open circuits as quickly as a valve canstart or stop conducting. Therefore some applications use one or more valves tokey the r.f. circuits. These valves are called "keying valves."

There are several variations of keying valve circuits; but they all turn the trans-mitter on when the hand key is closed, and off when the key is opened.

In the circuit below, the keying valve is connected in series with the cathodeof the power amplifier valve. The transmitter will be on when the keying valve con-ducts, and will be off when the keying valve is cut off.

The keying valve can be keyed by either of the blocked-grid keying methodsdescribed previously.

' ' HT+

INPUTo©OUTPUT

A simplified schematic of another type of keying valve circuit is shown below.With the key open, current flows through R-l and R-2, producing a large voltagedrop across these resistors. Resistor R-l is the PA screen dropping resistor, andresistor R-2 is the PA anode dropping resistor. The keying valve current flowsthrough R-l and R-2, causing the power amplifier's screen and anode voltages todrop, thereby cutting off the power amplifier.

When the key is closed, bias is applied to the grid of the keying valve so that it

will be cut off. As a result, the screen and anode voltages of the power amplifierincrease to their normal values; the power amplifier conducts; and the transmittedpulse is radiated from the aerial.

VWVVfR-2

INPUTO—1|

HT+

O OUTPUT

§8*. AMPLITUDE MODULATION 4.77

What Amplitude Modulation Is

The type of voice transmission most commonly used is one in which the ampli-

tude of the carrier is varied in accordance with the amplitude of the voice signal.

This method of modulating the carrier is called "amplitude modulation.

MCW transmission is amplitude modulation in which a steady audio frequency is

used instead of voice, to vary the amplitude of the r.f.carrier.

In addition to the normal oscillator and power amplifiers, an AM transmitter con-

tains a modulator, whose job is to apply the audio-frequency signal to the PA, where

it is combined with the r.f. carrier wave.

A block diagram of a typical voice AM transmitter is shown below.

AmplitudeModulatedTransmitter

MasterOscillator

IPA 1^V

/sys^^\ M PA

AudioAmplifier

Modulator

#

When you are operating an AM transmitter, it is essential that the modulator

unit be operating during transmission; because the intelligence which is to be trans-

mitted must come through the modulator. If the modulator is either defective or

disconnected, only unmodulated r.f. will be transmitted; and a receiver at some

distant point will not receive any message.

MODULATOR

K8Sidebands

When an r.f. carrier is amplitude-modulated, the effect is to add new frequenciesto the transmitted signal, in addition to the original carrier frequencyFor example, if in MCW transmission a 500-kc/s carrier is modulated with a 2000-

cycle audio note, the frequencies radiated by the aerial will contain, in addition tothe carrier frequency, the sum (502 kc/s) and difference (498 kc/s) frequencies be-tween the carrier and the modulating audio frequency.These new frequencies are called "sidebands"-the higher frequency being known

as the upper sideband" and the lower frequency the "lower sideband." The rangeof frequencies transmitted from the lower sideband to the upper sideband is knownas the "bandwidth" of the transmission.

In the example above, the bandwidth is 4 kc/s—from 498 kc/s to 502 kc/s. Butif the modulating audio signal were reduced in frequency from 2000 to 1000 cyclesthe sidebands would be closer to the carrier frequency, and the bandwidth would beonly 2 kc/s.

It is the sideband frequencies, and not the carrier frequency, which contain theintelligence of the transmission. If, for example, an MCW receiver were to pickup only the carrier and exclude the sidebands, no intelligence would be received

MCW Sidebands— Bandwidth ^

§MA I

Si

..ower

deband"1

^ UpperSideband

W^Ww 498 500 502

2kc Frequency-Kc/s

modulation 1

In a voice transmission, the modulating signal contains many frequencies—someas high as 5000 c/s. As a result, voice transmissions contain many sidebands (onesideband for each frequency), which may be as much as 5 kc/s on either side ofthe carrier frequency.

This type of transmission, therefore, may have a bandwidth of as much as 10kc/s.

VOICE Sidebands

VoiceModulation

Carrier

Lower Sideband

1

Upper Sideband

-liiH mJJiiHlu :l'- n-':i!

|l ^'i|,,|; iIiii! ::l

,i l!;

:

i :,; ill;, , l!n, H I. ;:'),

495 500 505500

BandwidthlOKcM i

§ 8]4.79

How Modulation is Accomplished

In an unmodulated transmitter, the amplitudes of the anode current pulses in the

Class C amplifiers are the same, cycle after cycle.

These anode current pulses flow to an LC circuit, which is tuned to the r.f.fre-

quency or to a multiple of it. The pulses of current deliver a certain amount of

power to the tuned-circuit, and this power remains the same for each cycle. It

follows that the amplitude of r.f. voltage across the tuned-circuit must remain the

same for every cycle.

0—

J

JUUUUUUUL

When the transmitter is modulated, the amplitude of the anode current pulses is

made to vary according to the amplitude of the modulating signal.

Thus the amplitude of the r.f. current varies from one cycle to the next, and the

power delivered to the tuned-circuit also varies. This varying power, in turn, causes

the r f voltage across the tuned-circuit to vary.

These variations will follow the modulating signal in amplitude and frequency—

and in this way modulation is accomplished.

moduloTRANS* MCW VOICE

ModulatingSignal

Anode CurrentPulses

VoltageAcrossTunedCircuit

[§84.80

The Modulator

In MCW and voice amplitude modulation, a modulator is used to impress theaudio on to the r.f.

For voice, the modulator is nothing more than an ordinary audio amplifier whichprovides the voltage or power needed to vary the amplitude of the transmitter's r fFor MCW, the modulator contains an audio oscillator which drives the audio

amplifier. The output is a pure sine wave which varies the amplitude of the rfpulses at a rate equal to the frequency of the a.f. sine wave, and to an extent depen-dent on the amplitude of the a.f. sine wave.

Since the modulator is connected to the stage of the transmitter which is to bemodulated, its output must be of sufficient power to produce the necessary variationsof current m the modulated stage of the transmitter. For this reason, Class B push-pull amplifiers are often used as the final stage in the modulator unitThe following schematic illustrates a push-pull amplifier which can be used as a

modulator It is almost exactly the same as the push-pull amplifier described inPart 2 of Basic Electronics. The only difference lies in the modulation transformerwhich has a different turns ratio and a higher current capacity than had the output'transformer you met earlier on.

PUSH-PULL AMPLIFIER^ M • -***•

-r

—

mw

The modulating voltage may be applied in series with any of the electrodes of thevalve to be modulated. And the name of the type of modulation used depends onthe electrode to which the secondary winding of the modulation transformer is con-nected.

For example, "anode modulation" is achieved by connecting the output of themodulator m series with the anode circuit.

mOther types of modulation used with triode valves are "grid modulation" and

cathode modulation"; while in pentode valves, "screen grid modulation" or "sup-pressor grid modulation" may be used in place of the other methods

14Anode Modulation

In the simplified circuit of the power amplifier shown below, the modulating audio

voltage is applied to the anode of the valve. The audio voltage, since it is in series

with the d c anode supply voltage, will cause the total applied anode voltage to vary

above and below H.T. by an amount equal to the peak audio voltage, and at a rate

equal to the frequency of the audio.

Simplified circuit for

fw#kmM****

While the applied anode voltage

is varying, a constant amplitude of

r.f. is being fed to the grid of the

valve from the output of the pre-

vious stage, the IPA.

During the positive half-cycles

of the audio, the anode voltage of

the PA is higher than H.T., and

more anode current therefore

flows. So on the positive half-

cycles of the a.f. modulating

voltage, a greater r.f. voltage is

developed across the tuned-circuit.

During the negative half-cycles

of the audio, the anode voltage is

lower than H.T., resulting in less

current flow and less voltage

developed.

The amplitude of the r.f. output

voltage thus varies as shown in the

bottom illustration opposite, which

is of an amplitude modulated wave.

The power available at the

anode of the modulated amplifier

is shared between the carrier wave

and its sidebands. The power in

the sidebands is derived from the

modulator, while the power in the

carrier is derived from the H.T.

supply. In a 100-watt transmitter

using anode modulation, the modu-

lator may have to supply about 70

watts.

Cut-off

Bias

300 V

Fixed grid

Signal

Modulating

HT+

- 300 V+800 V

+500V

4200VX

Varyinganode supph

voltage

Resultant

anode

[§84.82

Grid Modulation

If the audio voltage is applied in the grid circuit instead of in the anode circuityou get grid modulation.

The effect of the modulating voltage is to vary the grid bias at an audio rate Theanode current flowing during each r.f. cycle will then vary as the grid bias increasesand decreases.

Tht+

CIRCUIT FOR

INPUT ,j ifadMt***** t

ModulatingVoltage

AnodeCurrent

Grid Voltage_Grid_bigsi. JMODULATOR

In the accompanying waveforms yousee that the total grid voltage is the sumof three voltages—the r.f. input voltage,

the a.f. modulating voltage, and the d.c.

bias voltage.

During the positive half-cycles of themodulating voltage, the bias decreases;and during the negative half-cycles, thebias increases. Since the r.f. will

always vary about the bias level, thepositive half-cycles of r.f. are raisedduring positive modulation peaks, anddepressed during negative modulationpeaks. It follows that the anode cur-rent pulses are larger in amplitudeduring the positive half-cycles of theaudio voltage than they are during thenegative half-cycles.

Since the voltage developed across theanode tank circuit varies with the anodecurrent amplitude, the r.f. output voltagewill also vary according to the modulat-ing signal.

Grid modulation is used in compact or mobile transmitters, because this type ofmodulation does not require a modulator with a large power output.When the weight of the modulator is only a minor consideration, however anode

modulation is used instead, because (despite the larger modulator it requires) itproduces much better results.

Output

§8] 4.83

Other Methods of Modulation

Anode voltage has almost no effect on the anode current in a pentode or in a

tetrode; so in these valves anode modulation is never used. Instead, the audio

voltage is applied to the screen grid—and the results are almost identical to those

of anode modulation with a triode.

•-HT+

MODULATEDOUTPUT©

Screen Grid Modulation

The variations in screen voltage

cause the amplitude of the r.f.

pulses of anode current to vary;

and this causes the output to be

modulated by the audio signal.

Modulation can also take place when the audio output of the modulator is con-

nected in the circuit of the suppressor grid. With a negative voltage on it, the sup-

pressor can control anode current in the same way as a control grid can, except that

the valve is less sensitive to voltage changes on the suppressor. Of course, only

pentode valves which have external connections to the suppressor can use this type

of modulation.

The operation is very similar to control grid modulation, and the modulator does

not need a large power output.

Suppressor Grid ModulationMODULATED

OUTPUTo

Grid bias'

If the audio voltage is applied to the cathode of the valve, the cathode's voltage

will vary with respect to earth. This will have the same effect as applying the audio

voltage to every other electrode in the valve simultaneously; for applying the voltage

to the cathode causes the voltage on every other valve electrode to vary with respect

to the cathode.

Cathode modulation is, therefore, in effect, a combination of the other types of

modulation.HT+

Cathode Modulation MODULATEDOUTPUTc

*° li

4.84 [§ 8

The Modulation Pattern

The oscilloscope can be used to good advantage to indicate the extent to which the

output of a transmitter is modulated. It can also indicate the presence of distortion

in the modulation.

If a pick-up loop, connected to the 'scope input terminals, is brought close to the

anode tuned-circuit coil in the output circuit of a modulated transmitter, the 'scope

can be used to show what is called the "modulation pattern."

stffahd***"VAtUm

If the modulating voltage is a sine wave

(as in MCW), the pattern on the right is

obtained. This pattern is useful in deter-

mining the presence of distortion.

XAA/

A pattern such as this, for instance, would

indicate that the positive peaks of the modu-lating voltage are not causing corresponding

peaks in anode current. This could be due

to incorrect grid bias, to saturation due to

low emission, or to insufficient excitation of

the power amplifier stage.

When breaks appear in the modulation

pattern, the transmitter is said to be "over-

modulated." This is usually caused by

excessive modulating voltage; but it may also

be due to insufficient r.f . signal voltage on the

grid, or to excessive grid bias voltage on the

final power amplifier (or on the modulated

stage).

§8] 4.85

The Trapezoid Figure

The trapezoid figure is another oscilloscope pattern which is often used to deter-

mine the presence of distortion in the modulated signal, and to show how much the

signal is being modulated. It has the advantage of making possible the detection

of certain types of distortion which cannot be detected by means of the pattern

described on the previous page.

To produce the trapezoid figure, the modulating signal is used as an external

horizontal sweep signal, instead of the internal time-base voltage of the 'scope. The

vertical deflection is still the modulated r.f . output of the transmitter.

The advantage of using trapezoid figures over the other modulation pattern to

analyse the operation of a transmitter is that they are easier to interpret.

A typical arrangement for showing trapezoid figures is illustrated below. The

vertical input of the 'scope is coupled to the anode coil of the power amplifier, and

the horizontal input is coupled to the a.f. modulating source.

In order to understand how trapezoid figures are formed, you must first know

something about the action of the vertical and horizontal plates inside the cathode

ray tube.

Arrangement for observing Trapezoid figures

Tomodulator

Pickup loop

on PA coil

4.86 [§ 8

The Trapezoid Figure (continued)

The picture you see on an oscilloscope screen is the path followed by a beam of

electrons striking the inner surface of the cathode ray tube.

In the cathode ray tube there are two pairs of metal plates which deflect the

electron beam from its path. The top and bottom plates, or "Y" plates, move the

electron beam vertically. The "X" plates move it horizontally from left to right.

The "Y" plates are connected to the signal under observation. This signal dis-

places the electron beam in a vertical direction.

Under normal operating conditions, the "X" plates are connected to the output of

an oscillator built into the oscilloscope. This oscillator, called a "time-base gen-

erator," generates a saw-tooth voltage which sweeps the electron beam across the

face of the 'scope screen, from left to right, at a selected constant speed.

If the input signal to the vertical plates is the familiar sine wave of voltage, the

combined action of this signal and of the horizontal sweep acting on the electron

beam produce the sine wave picture.

Sometimes the internal time-base generator is disconnected, and an external signal

is used as the sweep voltage. This is what is done to produce the trapezoid figure.

Y-Input

Terminals I m

L

X-InputTerminals

«//

VerticalAmplifier

v*.s?

HorizontalAmplifier

VerticalDeflection Horizontal

Plates Deflection

Plates

Sweep Voltage

TimeBase

Generator

IfieSetpePeflfecefoit 7%ates

§8]

The Trapezoid Figure (continued)

4.87

ModulatingVoltage

The trapezoid figure is produced in the following manner. When the modulating

voltage is at its most negative value, the 'scope sweep (which is produced by the

modulating voltage) will be at the left of the 'scope screen (Point C).

As the modulating voltage increases to its most positive value, the electron beam

will swing over to the right-hand side of the screen (Point A).

If the modulating voltage were a perfect sine wave, the electron beam would be

midway between the sides of the trapezoid figure (Point B) when the modulating

voltage is zero.

At any instant, therefore, the horizontal position of the electron beam is a measure

of how negative or how positive the modulating voltage is.

At the same time as the electron beam is moving from one side of the screen to

the other under the influence of the modulating voltage, so this modulating voltage

is causing the transmitter output to increase and decrease.

The transmitter output is applied to the

'scope to produce vertical deflections.

When the modulating voltage is at its posi-

tive peak, the transmitter output (and the

height of the 'scope picture) are greatest.

Thus the right-hand side of the trapezoid

figure shows the largest amplitude.

When the modulating voltage is at its

negative peak, the transmitter output (and

the height of the 'scope picture) are at their

minimum. This occurs when the electron

beam is at the left-hand side of the screen.

ModulatingVoltage

TransmitterOutput

Y Input

X Input o

Because of the way in which trapezoid figures are obtained, they represent a

graph of the output voltage as compared to the modulating voltage.

If the output voltage is always proportional to the modulating voltage—as it will

be when the modulation is linear—there will be a straight line along the top and on

the bottom of the trapezoid.

[§8

The Trapezoid Figure (continued)

FIGURE USING INTERNAL TIME-BASE TRAPEZOID FIGURE

The two 'scope presentations shown above are for the same condition of modula-tion. You could determine the maximum height (peak) and the minimum height(trough or valley) of the r.f. from either figure.

You could also determine the linearity of the modulation from either presentation;but it is easier to do so from the trapezoid.

If the modulating voltage is decreased in amplitude, the peak and trough points onthe wave-pattern come closer together. The same effect is displayed in the trapezoidpattern as a change in the horizontal and vertical dimensions.

The following illustrations show, one above the other, both types of waveformpresentation for three different modulating voltage amplitudes.

EFFECT OF VARYING AMPLITUDE OF MODULATING VOLTAGE

Small modulatingvoltage

WWVLarge modulating

voltageLarger modulating

voltage

§8] 4.89

Percentage Modulation

The "percentage modulation" is a measure of the extent to which the carrier is

modulated.

If it is modulated 100%, the maximum height of the modulated wave is twice that

of the unmodulated wave, and the minimum height is zero.

For this modulating condition, the trapezoid figure is a triangle.

For radio telephony, it would seem to be ideal if signals could be transmitted

with 100% modulation, because the modulated r.f. would then be transmitted at

maximum power. In practice, however, over-modulation would always tend to

occur. Moreover, the diode detector of a receiver cannot handle 100% -modulated

signals without introducing some distortion.

Equipments are normally set up, therefore, to give 60% to 80% modulation only.

^SJw**-For maximum power in transmission,

modulate 100%

Unmodulated carrier

If the maximum height of the modulated wave is more than twice that of the un-

modulated wave, and the minimum height is zero for more than an instant during

the cycle, the carrier is over-modulated.

This condition produces gaps in the wave figure, and a line extending from the

left-hand side of the triangle in the trapezoid figure. The more the wave is over-

modulated, the longer are the gaps, and the longer the line.

^^- distorts the signal and interferes

with other carrier frequencies

4.90 §8]

Percentage Modulation {continued)

Since over-modulation is undesirable, both because it distorts the signal andbecause it generates unwanted sidebands which may interfere with adjacent carrier

frequencies, you will have to find out the exact percentage of modulation whichmay be present in any transmitter you are checking.

The exact percentage modulation can be calculated by using the following formula:

modulationH max. —H min.

H max. +H min.xlOO

"H max." is the maximum height of the modulated wave, and "# min." is its

minimum height. These values can be measured from the 'scope pictures—the

trapezoid figure is more convenient for this purpose, but the wave figure itself gives

sufficiently accurate results.

In the figures below, H max. is 8 divisions and H min. is 2 divisions. The per-

centage modulation is therefore 60.

60j£ modulation

%modulation =

If H max. is 9 divisions and H min. is 1 division* the percentage modulation is

therefore 80.

80% modulation

-x 100 = 80%

max.9 divisions

§8]

REVIEW—Amplitude Modulation

Amplitude Modulation. A method

which uses either voice or an audio

signal to vary the amplitude of an r.f.

carrier wave. The modulator is that

part of the AM transmitter which com-

bines the audio and r.f. signals.

RF AUDIO

4.91

MCW

RF

+ AAA "I •

VOICE VOICE

+ WvA**"* mm

Sidebands. Frequencies contained in

the transmitted signal in addition to the

r.f. carrier frequency. Sidebands are

equal to the sum of, and to the difference

between, the carrier and modulating

signals. In MCW there are two side-

band frequencies; in radio telephony

there are many.

—! Bandwidth !—

Lower|

Sideband

UpperSideband

498 500 502

Carrier

Lower Sideband I Upper Sideband

uiu 4iidiyiiiiuiiiLiL

Anode Modulation. The arrange-

ment whereby the modulating signal

varies the PA anode voltage, thus modu-

lating its output in response to the

audio signal.

Grid Modulation. The modulating

signal is applied to the grid of the PAvalve. Varying grid voltage in this

manner controls PA valve anode cur-

rent, and so modulates output voltage.

Trapezoid Figure. An oscilloscope

pattern obtained by using the trans-

mitter output voltage as the Y input of

the 'scope, and the modulating signal

as the X input.

Percentage Modulation. The

measure of the extent to which the r.f.

carrier is modulated. Over-modulation

produces a distorted signal, and intro-

duces unwanted sidebands.

60X modulation

4.92 §9: GENERAL REVIEW OF TRANSMITTERSCW Transmission. An r.f. signal is

generated in the transmitter by an r.f.

oscillator, and radiated into space. In-

telligence is imparted by turning the

transmitter on and off with a telegraph

key. CW is generally used in long-

distance communication. It can only

transmit code (e.g. morse).

KeyedRFSo—r a

Key rf.RF

Transmitter

MCW Transmission. A constant ampli-

tude audio frequency signal is superimposed

on the r.f. carrier wave. Transmitter

is switched on and off by means of a

key as in CW transmission. Also for

code only.

Radio Telephony. In amplitude modu-lation, a voice signal varies the amplitude

of the r.f. carrier. Transmission is con-

tinuous; and is the type used for standard

radio broadcasting.

KeyedModulated RF

S~ ^\

&H Hif- 1 II

RFTransmitter

AudioOscillator

AMVoice modulated RF

UnmodulatedRF Carrier

* MM

RFTransmitter

\ Ampli\ modui

Amplitudemodulator

Microphone ±

Grid-leak Bias. A resistor and capaci-

tor are included in the grid circuit of an

amplifier valve to make the amplifier

operate in Class C. The amount of bias

depends on the grid current, and varies as

the strength of the input signal changes.

Combination Bias. A combination of

fixed and grid-leak bias most commonlyused in transmitters. TO ANODE OF

PREVIOUSSTAGE

§9]4.93

GENERAL REVIEW of Transmitters (continued)

Three-stage Transmitter. The master oscillator (MO), the intermediate power

amplifier (IPA), and the final power amplifier (PA) make up the basic three-stage

transmitter.

Tuning. For efficient operation, the anode tuned-circuits of the amplifiers must

resonate either at oscillator frequency or at one of its harmonics. Adjustment

of the variable capacitor to reach this condition is called "tuning." Anode voltage is

maximum, and current minimum, at the correct frequency.

ANOOENEUTRALIZATION

QROUTT

§ jfotl

Neutralization. Anode or grid

neutralization circuits may be used to

counteract the feedback effect of the

grid-to-anode capacitance of triodes

used in transmitter amplifiers.

4.94

GENERAL REVIEW of Transmitters (continued)

Transmission Line. Used to con-

vey the r.f. signal from the transmitter

to the aerial. For maximum power

output the characteristic impedance of

the line should equal the input

impedance of the aerial. Coupling

circuits are used to match the

transmission line to the transmitter.

AERIALRadiates RFJ

r—

[§*

TRANSMISSION LINESupplies «*ri»l i with RF

* COUPLING CIRCUITCouples RF from tank circuit

to transmission line

Aerial. Radiates the energy

received from the transmission line

into space. Electric and magnetic

fields generated by current and voltage

waves on the aerial expand and col-

lapse as the transmitter signal varies.

Sidebands. Frequencies contained

in the transmitted signal in addition

to the r.f. carrier frequency. Side-

bands are equal to the sum of, and to

the difference between, the carrier and

modulating signals. In MCW there

are two sideband frequencies; in radio

telephony there are many.

— | Bandwidth jo-

LowerSideband Sideband

488 500 502

Lower Sideband

Can ler

Upper Sideband

ililll iililliiilii am .;ili .:!»,;; II. Illl llii^j. iiMfcllfii 1,455 5(K) 505

Anode Modulation. The arrange-

ment whereby the modulating signal

varies the PA anode voltage, thus

modulating its output in response to

the audio signal.

Grid Modulation. The modulating

signal is applied to the grid of the PAvalve. Varying grid voltage in this

manner controls PA valve anode

current, and so modulates output

voltage.

§10: INTRODUCTION TO RECEIVERS 4.95

You have now learnt how a signal is generated in the transmitter by a radio-

frequency oscillator; how it is amplified; and how its energy is then transferred by

a coupling circuit to the transmission line. You have learnt that the aerial converts

the power delivered to it by the transmission line into electro-magnetic waves.

You have found out that these electro-magnetic waves are radiated into space;

and some of the ways in which they can be modulated so as to carry "intelligence"

with them as they speed to their destination.

The next thing, obviously, is to find out what happens to them when they get

there, and how the message which they carry is taken from them and made either

intelligible or usable at the other end.

Ih Part 5 of Basic Electronics, therefore, you will study Receivers. You will see

how the almost inconceivably weak currents generated in the receiving aerial by the

incoming signals are amplified until they become manageable; how the messages

they carry are detected (or, as it were, "taken out of the envelope" in which they

have been travelling); and how these messages, in turn, are amplified until they can

be used to operate a loudspeaker, or otherwise put to useful work.

Now let's go on and see how messages are received

!

^,w\\\\\\\\U4£

INDEX TO PART 4

(Note: A cumulative index covering all six Parts will be found at the end of Part 6)

Aerials, 4.59, 4.71

dipole, 4.60

radiation pattern, 4.66

radiation resistance, 4.63

tuning, 4.65

Amplifiers, 4.9, 4.16

classes of operation, 4.9

class C amplifier, 4.10

Amplitude modulation, 4.77, 4.91

modulation patterns, 4.84

percentage modulation, 4.89

Anode modulation, 4.81

Class C amplifiers, 4.10, 4.16

bias arrangements, 4.12

Coupling circuits, 4.41

tuned, 4.42

CW transmission, 4.72, 4.92

keying circuits, 4.73

Dipole, 4.60

Frequency, 4.44

Frequency multipliers, 4.35

doubling, 4.37

tripling, 4.38

tuning indicators, 4.39

Frequency spectrum, 4.70

Grid modulation, 4.82

Keying, 4.73

Modulation (AM), 4.7, 4.91

anode, 4.81

grid, 4.82

other methods of, 4.83

Neutralization, 4.28, 4.34

Radio telephony, 4.8

Sidebands, 4.78, 4.94

Three-stage transmitter, 4.17, 4.34, 4.93

complete diagram for, 4.21

FPA, 4.20

IPA, 4.19

oscillator, 4.18

tuning methods, 4.24

valve filament circuit, 4.22

Transmission

AM, 4.8

FM, 4.8

keyed, 4.7

voice, 4.8

Transmitter lines, 4.40, 4.94

applications, 4.55

characteristics impedance, 4.47

non-resonant and resonant lines, 4.49

open-circuited line, 4.51, 4.54

short-circuited line, 4.51, 4.53

Transmitter, 4.3

three-stage, 4.17, 4.34

Wavelength, 4.44

Wave propagation, 4.67

ground wave, 4.68

space wave, 4.68, 4.69

WIGANCENTRALLIBRARY<-- i n. 1—WWW8

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