1. 2 The aim of this Course is to Give a) Basic notions in Radio Propagation at microwave...

2

The aim of this Course is to Give

a) Basic notions in Radio Propagation at microwave frequencies,

b) application to Radio Link Design in the frequency range from about 450 MHz up to 60 GHz.

• Means :

- Course notes

- Lab simulatorss

3

• Prerequisites: - basic notions in:

- Modulation techniques,

- Radio equipment and systems

- Elementary electromagnetic physics.

• Conclusion :

• Course objective : actively involving the reader in navigating through the text and in practicing with exercises in the field of microwave link design.

4

In telecommunications, information can be analog or digital.

since the 1970’s , MW Analog systems have been almost completely replacedreplaced by digitaldigital systems.

Now evenNow even analog traffic, such as voice calls, are converted to digital signals ( sampling), to facilitate long distance transmissionlong distance transmission and switching.

5

Terrestrial MW systems have been used since the 1950’s( wartime radar technology).

Today, modern digital microwave radio is world widely deployed to transport information over distances of up to 60 kilometers ( sometimes farther).

Microwave radio is totally transparenttransparent to the information carriedinformation carried : which can be voice voice, datadata, videovideo, or a combination of all three.

6

• Transport can be in a variety of Transport can be in a variety of formatsformats : : circuit-switched Time Division Multiplex (TDMTDM) packetpacket-based data protocols such as ATM, Frame Relay or IPIP, Ethernet, Ethernet.

In some cases, packetized data can be overlaidoverlaid on a TDM frame structure such as:

- PDH, - SDH or SONET.

7

• Microwave radio advantages over cable/fiber-Microwave radio advantages over cable/fiber-based transmissionbased transmission:

Rapid Deployment

No right-of-way issues – avoid all obstacles

Any requirement to seek permissions :cost & time delays.

FlexibilityFlexibility: simple redeployment & capacity adjustment.

8

Losing customers ≠ Losing assets as in Cables & fibers

• Easily crosses city terrainEasily crosses city terrain (extremely restricted,& very expensive, to install fiber in city terrains and street crosses).

• Operator-owned infrastructure - no reliance on competitors.

• Low start-up capital costsLow start-up capital costs : independent of the link distance.

9

Minimal operational costs.Radio infrastructure already existsRadio infrastructure already exists (rooftops,

masts and towers). Microwave radio is not susceptible to not susceptible to

catastrophic failurecatastrophic failure ( cable cuts,) and can be repaired in minutes instead of hours or days.

Better resistance to natural disasters (flood, flood, earthquakes).earthquakes).

where the fiber was not always available (the radio is only choice)

10

Fiber is very cost effective where extremely high bandwidths are required.

However, in the access portion of the network,

where the maximum capacity requirements are less than STM-4,STM-4, radio has an obvious radio has an obvious advantageadvantage.

Note STM1 = 155 Mbits ; STMn = n*155Mbits

11

The 3 basic components of the radio terminalThe 3 basic components of the radio terminal

Two radio terminals are required to establish a MW “hop”.

1- digital modem interfaces with digital digital terminal equipmentterminal equipment, converting customer traffic to a modulated radio signal;

2- a radio frequency (RF) unit : Frequency converter + RF amplifier up to around 1 watt.

3- a passive parabolic antenna to transmit and receive the signal.

12

2-Basic configurations for MW terminals

1-Non-protected, ( 1+0) :Non-protected, ( 1+0) :

- Any major failure component will result in

a loss of customer trafficloss of customer traffic.

- cost-effective when traffic is non-critical, or where

alternate traffic routing is available.

2-2-protected (1+1) : Main + hot standby (protected (1+1) : Main + hot standby (Monitored Monitored Hot Standby (MHSB)Hot Standby (MHSB)))

- twice expensive, but No loss of customer traffic.

14

•

15

33- Space DiversitySpace Diversity 4- Frequency Diversity4- Frequency Diversity 5- Polarization diversity5- Polarization diversity 6-Angle diversity6-Angle diversity

In addition, Some radios are fitted to use In addition, Some radios are fitted to use ODU attached directlyODU attached directly to the back of the to the back of the antenna, eliminating antenna feed lines and antenna, eliminating antenna feed lines and attendant feed line losses).attendant feed line losses).

16

• Used to reinforce the radio dispersive fade margin . The new technology of Mw radio don’t need this type of diversity

17

•

18

• Two very important characteristicimportant characteristic of digital MW transmission is:

A- immunity to noiseA- immunity to noise

B- the ability of the radio to operate in the B- the ability of the radio to operate in the presence of presence of adverse adverse environmental environmental conditions.conditions.

19

A- A- immunity to noiseimmunity to noise

• NoiseNoise refers to the effects caused by unwanted electromagnetic signals that interfere and corrupt the received signal.

20

• Microwave systems operate in so-called

“licensed” frequency bands“licensed” frequency bands between 2 and 38 GHz (tightlytightly regulation the use of these frequencies ensure that each operator will not cause interference to other links operating in the same area).

• The frequency band characteristics are also tightlytightly specified on a worldwide basisworldwide basis by ITU. ITU.

21

• Equipment are controlled, to meet stringentstringent

specifications ( ITU standards, National as FCC and ETSI).

• This is in contrast to the “unlicensed”“unlicensed” frequency bands of 2.4 and 5.8 GHz2.4 and 5.8 GHz :

No control, Unlicensed systems themselves incorporate countermeasures to avoid noise and countermeasures to avoid noise and interferenceinterference, such as spread-spectrum spread-spectrum transmissiontransmission

22

B- B- the ability of the radio to operate in the the ability of the radio to operate in the presence of adverse environmental presence of adverse environmental conditions.conditions.

a perception that microwave is still a perception that microwave is still unreliable unreliable due to due to “fading”“fading” . .

This is largely a This is largely a remnantremnant of the analog days. of the analog days.

However, However, digital digital radio systems today are able radio systems today are able to to counteract fadingcounteract fading effects in a number of effects in a number of waysways

23

Fading is known to occur as a result of Fading is known to occur as a result of primarily primarily two phenomena.two phenomena.

1- Firstly, 1- Firstly, multipathmultipath interference affects mainly interference affects mainly lower frequencies lower frequencies below 18 GHzbelow 18 GHz..

This happens when the reflected signal arrive This happens when the reflected signal arrive slightly later than the direct signal path .it slightly later than the direct signal path .it reduces the reduces the ability of the receiver to ability of the receiver to correctly distinguish the data carried on the correctly distinguish the data carried on the signal. signal.

24

• Fortunately, modern radio systems Fortunately, modern radio systems can can compensatecompensate for this form of interference for this form of interference through countermeasures such as: through countermeasures such as:

signal equalization [using DSP-filtering to signal equalization [using DSP-filtering to cancel the echoes (pre-echoes & post-cancel the echoes (pre-echoes & post-echoes) due to Multipath].echoes) due to Multipath].

Forward Error Correction,Forward Error Correction, diversity receiver configurations.diversity receiver configurations.

Multipath fade measurement parameter is often called the reliabilityreliability of the link ) ( ( ثقة -(ذا

25

2- Secondly, precipitationprecipitation, mostly in the form of

rain, can severely affect microwave radio systems in the higher frequenciesthe higher frequencies above 18 above 18 GHzGHz.

Microwaves cannot penetrate rainMicrowaves cannot penetrate rain, so : the heavier the downpour, and the higher the

frequency, the greater the signal attenuation. Rain fade measurement parameter is often

called the availability of the link

26

Although there is noway to counteract rain fadecounteract rain fade other than higher transmission power.

The mechanisms of rain fade are very well understood:

models models have been developed by the ITU to enable links to be planned within extremely extremely accurate tolerancesaccurate tolerances based upon particular rainfall profiles.

27

• Conclusion : As a result, modern microwave systems can be

designed for extremely high link total availabilities in excess of 99.999%,99.999%, translating to link downtimes of literally seconds annuallyseconds annually, which is easily comparable to that provided by supposed “error-free“error-free” optical fiber systems.

• Note : total availability concerns the 2 types of the fade

28

Microwave applicationsMobile Cellular NetworksMobile Cellular Networks

• to provide serviceservice for customers and to generate immediate revenueimmediate revenue, cellular carriers need to connectconnect their cell sites to switching stations, and have chosen microwave due to:

• its reliabilityits reliability

• speed of deploymentspeed of deployment

• cost benefits over fiber or leased-line alternativescost benefits over fiber or leased-line alternatives.

29

• Microwave radio will be heavily deployedheavily deployed in the emerging 2.5 and 3G 3G mobile infrastructures: More data usage

• greater numbers of cell sites.

30

Last Mile AccessLast Mile Access A significant proportion of business premises lack

broadband connectivitybroadband connectivity : Wireless provides the perfect medium for connecting new customers to overcome the last mile bottleneckthe last mile bottleneck.

Even if an operator chooses to use unlicensed or multi-pointmulti-point wireless technologies to connect customers, high capacity microwave provides the high capacity microwave provides the ideal solution for ideal solution for backhaul backhaul of customer trafficof customer traffic from access hubs to the nearest fiberaccess hubs to the nearest fiber

31

• Private Networks:Private Networks: Companies now have high speed LAN / high speed LAN /

WANWAN network requirements and need to connect parts of their business in the same campuscampus, city or country. city or country.

Microwave radio is able to provide rapid, high rapid, high capacity connectionscapacity connections that are compatible with Fast and Gigabit Ethernet data networks, enabling LANs to be extended without reliance on fiber build-out.

32

• Disaster RecoveryDisaster Recovery Natural (earthquakes, floods, hurricanes ) and

man-made (terrorist attack and wars) disasters can wreak havocwreak havoc on a communications network:

Microwave is often used to restore communications when transmission equipment has been damaged by or other natural disasters, or man-made conflicts such as ( Kuwait, Serbia and Kosovo)

33

The Digital DivideThe Digital Divide

Microwave radio plays a key role in bridging the digital divide :

quickly establishquickly establish a network of access hubs and high-speed backhaul network to bring advanced communications services to areas that would normally have to waithave to wait..

34

Developing NationsDeveloping Nations

Microwave has traditionally allowed developing nations the means of establishing state-of-the-art telecommunications quickly over often undeveloped and impractical terrain ( deserts, jungle or frozen terrain where laying cable would be all but impossible.

35

Control and MonitoringControl and Monitoring Public transport organizations, railroads, and

other public utilities are major users of microwave.

These companies use microwave to carry controlcarry control and monitoring information to and from power substations, pumping stations, and switching stations.

37

• A- Understanding db & db units :

A- db : The ratio of 2 signals may be expressed in db by :

in case of voltages : V1/V2

( V2/V1)db = 20 log10 ( V2/V1)

in case of Powers : P1/P2

( P2/P1)db = 10 log10 ( P2/P1)

38

• ExampleExample :

a signal if 10 w is applied to long transmission line . The power measured at the load end is 7 W. What is the loss in db

• Solution :Solution :

39

Table of some common ratiosTable of some common ratios Ratio ® Factor Power ratio (db)

10 log10 R

Voltage ratio (db)

20 log10 R

1:1 1 0.00 0.00

2:1 2 3.01 6

10:1 10 10.00 20

100:1 100 20.00 40

1000:1 1000 30.00 60

1/10 0.1 -10 -20

1/100 0.01 -20 -40

1/1000 0.001 -30 -60

40

30 dB is an increase of 1000X in power





2 dB is an increase of 1.6X in power

1 dB is an increase of 1.25X in power

0 dB is no increase or decrease in power

-1 dB is a decrease of 20% in power

-2 dB is a decrease of 37% in power( roughly a decrease of 1/3)





-30 dB is a decrease of 99.9% in power

41

• B) : - db-power units - dbw : is the unit of power expressed relatively to 1W P(dbw) = 10 log10 P(w)

- dbm : is the unit of power expressed relatively to 1mW

P(dbm) = 10 log10 P(mw)

Attention : 0dbw = 30dbm= 1W 0 dbm = -30dbw = 1mw

42

P (dbm) = P(dbW) + 30 P(dbW) = P (dbm) -30

43

• Example ProblemExample Problem• If the two antennas in the drawing are "welded"

together, how much power in dbm will be measured at point A? (Line loss L1 = L2 = 0.5) –suppose no ideal antenna coupling

• Multiple choice:a. 16 dBmb. 28 dBmc. 4 dBmd. 10 dBme. < 4 dBm

44

• Answer:• The antennas do not act as they normally

would since the antennas are operating in the near field. They act as inefficient coupling devices resulting in some loss of signal. In addition, since there are no active components, you cannot end up with more power than you started with. The correct answer is "e. < 4 dBm."

• 10 dBm - 3 dB - small loss -3 dB = 4 dBm - small loss

45

Example :

Convert 10dbm in dbw ; -2dbw in dbm

Solution : given P(dbW) = P (dbm) -30 = 10-30 = -20 dbW

P (dbm) = P(dbW) + 30 = -2 + 30 = 28 dbm

Example : consider the 2 following configurations

10dbm ?

Gain 3db Gain 10db

46

C) - db-voltage units

- dbmv : is used in RF receiver in which the system

impedance is 50 Ω.

It is the unit of voltage expressed relatively to 1mv

v(dbmv) = 20 log10 v(mv)

47

- dbµv : is used in RF receiver in which the system impedance is 50 Ω. It is the unit of voltage expressed relatively to 1µv :

v(dbµv) = 20 log10 v(µv)

Example : The received RF effective voltage at the input of radio receiver is 0.5mv

. Find the input voltage in dbµv & the input power in dbm

SolutionSolution

48

-Field db units : Electromagnetic field

a- Electric field E in dbµv /m

E (dbµv/m) = 20 log10 E (µv/ m)

b- Magnetic field H = E/377 where H in A/m and E in v/ m

49

•

50

Power and Field Db-units

•

51

c) received power in dBm at the RX-antenna

where Gr is the RX-antenna gain.

Pdbm = Edbµv/m + Gr - 20 log (FMHZ) – 77.2

In case of isotropic Rx-antenna

Pdbm = Edbµv/m - 20 log (FMHZ) – 77.2

Received voltage into 50 input receiver: P (dBm) = U (dBµV) - 107

52

• Example:Example: The received RF effective voltage at the input of

radio receiver is 0.5mv

a) Find the input voltage in dbµv & the input

power in dbm.

b) knowing that the receiver’s antenna has 20db Gain and the transmitted frequency is 10GHZ,

Find the Electric field at the antenna location In dbµv/m and V/m .

Deduce the Magnetic field value.

54

Exercises on Db • A cable has 6 dB signal loss . Find the signal at the

output of this cable ,knowing that the input signal is 1mW.

• an amplifier has 15 dB of gain. Find the signal at the output of this amplifier ,knowing that the input signal is 1mW.

• Complete the following sentences: – a)Every time you double (or halve) the power level, you add

(or subtract) ……. dB to the power level. This corresponds to a ……. percent gain or reduction.

– b) ……dB gain/loss corresponds to a tenfold increase/decrease in signal level.

• A 20 dB gain/loss corresponds to a ……….-fold increase/decrease in signal level.

55

• Exercises on Dbm (dB milliWatt) A signal strength or power level 0 dBm is defined as …. mW (milliWatt) of power into a terminating load such as an antenna or power meter.

• Small signals are negative numbers. For example, typical 802.11b WLAN cards have +15 dBm (….mW) of output power. They also specify a -83 dBm (………pW.) RX sensitivity (minimum RX signal level required for 11Mbps reception). Additionally, a) 125 mW is ….. dBm, and

b) ….mW is 24 dBm.

56

CH2- Antenna and space CH2- Antenna and space

propagationpropagation

Recommended Software AndrewRecommended Software Andrew

57

Antenna Basic questionsAntenna Basic questions

which cause some antennas to acceptaccept one wave and rejectreject others?:

The physical size of an antenna : defines the efficiently radiated or received frequencyfrequency

The shape of the antenna determine the directivity of an antenna

The property of polarization describes the angular pointingangular pointing of the EM field vector

58

Antenna Electromagnetic field radiation : General discussion An antenna serves two basic functionstwo basic functions: 1- it matches matches the characteristic impedance of

the transmission line to the intrinsic impedance of free space (To avoid any reflections back to the source or load)

2 - Second, the antenna is designed to directdirect the electromagnetic radiation in the desired direction.

59

• Isotropic Point Radiator It is a fictitiousfictitious ideal isotropic point radiatorpoint radiator. it would

radiate power equally well in all directions in a volume sense. it would have an omni directional pattern in all planes. All real real antennas have some directivity.

Isotropic antenna practically doesn’t exist Omni-directional fictive

Hertz Isotropic point

antenna G=1 fold or

G = 0 Dbi

60

• Radiation PatternThe radiation pattern is a plot of the relative

strength (more often power densitypower density in db )of the antenna radiation as a function of the orientationorientation inin a given plane.

• Example :radiation pattern of

ANTMAN :ANDREW CORPORATION

MODNUM:FP10-34MODNUM:FP10-34

• LOWFRQ:3400; HGHFRQ:3900

61

ANTMAN: ANDREW CORPORATIO

N

MODNUM: FP10-34

PATNUM: 6605

DTDATA: 19790309

LOWFRQ: 3400

HGHFRQ: 3900

GUNITS: DBI/DBR

LWGAIN: 37

MDGAIN: 38.3

HGGAIN: 38.8

AZWIDT: 1.9

ELWIDT: 1.9

ATVSWR: 1.06

FRTOBA: 60

ELTILT: 0

POLARI: H/H & V/V

NUPOIN: 37

-180 -59

-125 -59

-125 -56

-110 -56

-85 -38

-35 -38

-30 -33

-25 -33

-15 -29

-15 -25

-8 -19

-4 -17

-2.8 -17

-2.5 -11.3

-2 -5.4

-1.5 -2.5

-1 -1.1

-0.5 -0.3

0 0

62

Continuous H/H

&V/V

0.5 -0.3

1 -1.1

1.5 -2.5

2 -5.4

2.5 -11.3

2.8 -17

4 -17

8 -19

15 -25

15 -29

25 -33

30 -33

35 -38

85 -38

110 -56

125 -56

125 -59

180 -59

POLARI: H/V & V/H

NUPOIN: 13

-180 -60

-105 -60

-15 -42

-10 -41

-4 -37

-2 -28

0 -28

2 -28

4 -37

10 -41

15 -42

105 -60

180 -60

63

Expanded Scale

0

10

20

30

40

50

60

70

800 5 10 15 20 40 60 80 100 120 140 160 180

64

• Antenna Gain

• Ratio of the power density at a particular location from an antenna with directivity to the power density from an ideal isotropicideal isotropic antenna radiating the same powerthe same power:

The power is taken away from some directions and added to the power in other directions, and the result is :the result is :

65

• The antenna referenceThe antenna reference

- most often used is :

the hypothetical

(Gain units dbi).

- Sometimes a "real-life""real-life" antenna such as the (Gain units dbd).

66

Figure 1: Half-wave dipole vs. isotropic antenna

• antenna referenceantenna reference

67

• ReciprocityReciprocity• Basically, it states that the properties of the antenna

used for transmission will be samebe same as when used for reception.

• In realistic terms, the transmitting antennathe transmitting antenna must be constructed to handle a much larger powerlarger power level than at the receiver

• The best interpretation is to assume similar field similar field patterns and impedance propertiespatterns and impedance properties of a given antenna used in the TX or RX.

68

•

• Antenna ReciprocityAntenna Reciprocity

69

c- Power density of electromagnetic Power density of electromagnetic

energyenergy in W/m2 :

An ideal isotropic point radiator transmitting power PT. The power density pd upon the surface of the sphere of radius r will be equal at all points and will be

In free space propagation ( far field ) E in v/m, Pd = E2/ 377

• Sometimes we define the radiation intensity as

• the unit of U is watts / steradian.steradian. Know that

70

• Power density of Power density of electromagnetic electromagnetic

energyenergy in W/m2

71

• the radiation intensity in

watts / steradian

72

• Solid AnglesSolid Angles

• solid angle spanned by a cone is measured by the area of intersection of the cone with a sphere: differential solid angle can be assigned a direction.

• Unit: steradian

• (full sphere = 4)

73

• Example :• 1-The power density 10 km from a transmitting antenna is 0.06

ìW/m2. Determine the radiation intensity.• 2-The radiation intensity from a transmitting antenna is 50 W/sr.

Determine the power density of a receiving antenna located 25 km from the transmitting station.

Solution

• 1-

• 2-

74

• 2- E.M Radiation From an Antenna

• Time-varying Time-varying voltages and currents in an antenna produce time-varying electric and magnetic fields that travel radially awayradially away from the antenna at a velocity determined by the medium in which the electromagnetic fields are propagating.

• There are two distinct regions of electric and magnetic fields surrounding an antenna: near near field and far field.field and far field.

75

• They are defined by the distance from the antenna as a function of the wavelength of the electromagnetic radiation and size of the antenna D.

•

• The fields in the far field are transverse fields; i. e., the electric and magnetic field intensities are transverse to the direction of propagation: This condition is referred to as plane wave propagation.– Both transverse and radial electric and magnetic

field intensities exist in the near field region.

76

• The radiation patterns that describe the radiation intensity of the antenna as a function of angle are usually patterns for the far field.patterns for the far field.

• Example 1: Determine the distance from a 100-MHz half-wavelength dipole to the boundary between the near field and the far field.

• Solution: = 3 m. Therefore, the length of the dipole is 1.5m. The distance to the far field can then be determined

77

Example 2• A parabolic reflector antenna with a diameter D operates

at 2.3 GHz . Determine the far field distance for this antenna.

• Case1 : D= 1m = 0.13 m. Rff = 2D2/ = 15m

• Case2 : D= 20m = 0.13 m. Rff = 2D2/ = 6Km

D=20m then Rff = 6Km :The preceding analysis shows that the far-field distance for a high-gain antenna can be far-field distance for a high-gain antenna can be very largevery large. The measurement of the far field radiation pattern for a large antenna operating at a high frequency can be a very difficult task.

78

• 15-4 Radiation Patterns

• In general, the radiation pattern of an antenna is a three-dimensional plotthree-dimensional plot of the relative strength or radiation intensity of an antenna as a function of the coordinate systems.

• Since it is difficult to present 3-D3-D information, typically the radiation patterns are shown as a pair of two-dimensional plots:pair of two-dimensional plots:

79

3-D Amplitude

• Open-ended waveguide sectionsOpen-ended waveguide sections

80

•

Open-ended waveguide sectionsOpen-ended waveguide sections

E-plane H-plane

81

Figure - Comparison of rectangular- and polar-

coordinate graphs for an isotropic sourceisotropic source.

82

• Figure - Anisotropic radiator Anisotropic radiator : Rect. & Polar Coordinates

83

• - Polar-coordinate graph for anisotropic

(directive) radiator.

84

• The first plot shows the radiation intensity as a function of the angle in the plane of the electric field intensity vector - E plane pattern and the second plot shows the radiation intensity as a function of the angle in the plane of the magnetic field - H plane pattern.

• The E plane and H plane are orthogonal to each other and are referred to as the principal plane patterns.

85

• Typical E and H plane plots:• Consider a simple half-wave dipole antennahalf-wave dipole antenna

aligned with the y-axis with the center at the origin. A typical

• E plane radiation pattern for a half-wave dipole antenna is shown in Figure (a). This plane is the –y z plane in this case. A typical H plane radiation pattern for a half-wave dipole is shown in Figure (b).

• This plane is the –x z plane for the orientation given. Note that the pattern is omnidirectional in the H plane.

86

•

87

• Normalized Gain FunctionsNormalized Gain Functions

• Radiation patterns are often normalized to the maximum gain by dividing the gain as a function of the two angles by the maximum gain to obtain the normalized gain. The normalized gain will be represented as

• This means that the normalized maximum gain is 1, and the gain at other angles is less than 1.

88

• Since normalized antenna patterns cover

a significant dynamic rangesignificant dynamic range, typically from 1 down to 101 down to 10-4-4 or less or less, antenna radiation patterns are normally plotted in decibels on a linear scale, usually on a polar plot.

G (db) = 10 log G (folds)G (db) = 10 log G (folds)

89

• Antenna Beamwidths and Sidelobes

An ideal antenna would have a radiation pattern whose normalized gain is 1 over the desired 1 over the desired angular beamwidthangular beamwidth and 0 at all other angles0 at all other angles.

• Beamwidth

The antenna beamwidth is defined as the included angle between the -3 dB-3 dB (Half power gain) points on the normalized gain pattern.

90

•

91

Lobes

• The main lobemain lobe is the antenna beambeam defined between the first null on either side of the maximum gain angle.

• Typically for high-gain antennas,

the null-to-null beamwidth is 2.5 times

the 3-dB beamwidth.

92

• An antenna will usually radiate some power in undesired directions. The radiation pattern of the Figure has several sidelobessidelobes.

• The levels of the sidelobes determine how much power is radiated in these undesired directions.

• If the antenna is a receiving antenna, the If the antenna is a receiving antenna, the sidelobes will determine the levels of sidelobes will determine the levels of undesired signalsundesired signals that could be received. that could be received.

93

• BacklobeBacklobe

• Another undesiredundesired part of the radiation pattern when single direction transmission is desired is the backlobe.

• A quality factor called the front-to-backfront-to-back ratio is important in these cases. As shown in the Figure , the absolute value of the front-to-back ratio of a dipole is 1, which in decibel form would be 0dB.

• A dipole cannot tell if the signal is coming from the front or back of the antenna.

94

• 15-6 Directivity and Antenna GainDirectivity and Antenna Gain• There are two commonly employed terms used to

describe the radiation characteristics of an antenna:

directivity and antenna gain.directivity and antenna gain.

DirectivityDirectivity is a characteristic of the radiation pattern of an ideal lossless antennalossless antenna while the antenna gain antenna gain includes the ohmic lossesincludes the ohmic losses of the antenna physical structure.

• Directivity : The directivity D of an antenna is defined from the radiation pattern as

96

• Antenna Gain Antenna gain is defined as the ratio of the maximum radiation

intensity Umax to the maximum radiation intensity Uref from a reference antenna with same power input to the antenna.

The difference between directivity and gain is that directivity is referenced to the power radiated by the antenna, while gain is referenced to the power delivered by the transmission line to the antenna. Therefore, gain is always less than or equal to directivity, the difference being the power dissipated in the antenna ohmic losses.

• Normally, antenna gain is expressed as a power ratio and is usually specified in decibels as

97

• The value of gain depends on the gain of the reference antenna. It is important to know what reference has been used for the antenna gain. Two of the common references are as follows:

• 1. a lossless isotropic antenna, in which the radiation intensity is uniform over the sphere surrounding the antenna, i. e., all 4 steradians

• 2. a reference dipole.

• The lossless isotropic antenna is a theoretical concept and has never been realized in practice, while the dipole is readily available.

98

• Antenna measurements of gain are usually referenced to a standard dipole for low-gain antennas or to a standard-gain horn for higher-gain antennas.

• Accurate theoretical calculations of the gain

referenced to a lossless isotropic antenna are possible for both the standard dipole and the standard-gain horn.

• The absolute gain of a standard half-wave dipole with respect to an isotropic radiator is 1.643 or 2.16 dB

Dbi = Dbd +2,16Dbi = Dbd +2,16

100

Exercise in dbd &dbiExercise in dbd &dbi• dBd (dB dipole) dBd (dB dipole)

The gain an antenna has over a dipole antenna at the same frequency. A dipole antenna is the ….. ., least gain practical antenna that can be made.

• The term dBd generally is used to describe antenna gain for antennas that operate under 1GHz (1000Mhz), ( manufacturers calibrate their equipment using a simple dipole antenna as the standard. Then they replace it with the antenna they are testing. The difference in gain (in dB) is reference to the signal from the dipole).

101

• dBi (dB isotropic)dBi (dB isotropic) Unfortunately, an isotropic antenna cannot be made in the real world, but it is useful for calculating theoretical fade and System Operating Margins. The gain of Microwave antennas (above 1 GHz) is generally given in dBi. A dipole antenna has 2.14 dB gain over a 0 dBi isotropic antenna.

• So if an antenna gain is given in dBd, not dBi, …… 2.15 to it to get the dBi rating.

• For example, if an antenna has 5 dBd gain, it would have ………. dBi gain.

102

• Antenna EfficiencyAntenna Efficiency

• It depends on the ohmic losses of the antenna. It is the ratio of the total power radiated from the antenna / the power delivered to the antenna from the transmission line.

• It is also equal

• where D and G are the absolute values of directivity and gain.

103

1- An antenna is transmitting 200 W of power. The maximum power density at a distance of 10 km is 3.184 mW/m2. Determine the directivity of the antenna.

104

• 2- An antenna with a directivity of 16 dB is transmitting a power of 1 kW. Determine the maximum power density at a distance

of 50 km from the antenna.

105

• 3-An antenna has an efficiency of 95% and the directivity is 33 dB. Determine the antenna gain in dB.

106

• 15-7 Effective Area of an RX Antenna Effective Area of an RX Antenna

((capture areacapture area ))

. It is the area by which the power density in watts per unit of area is multiplied to obtain power in watts delivered to the load.

It is close to the physical area of the antenna.

The effective antenna area Ae of the parabolic

reflector is given by

108

• 15-8 Polarization

• By definition, the polarization of an electromagnetic wave propagating in free space is the orientation of the electric field orientation of the electric field intensity vector relative to the surface of the intensity vector relative to the surface of the earth. earth.

• There are two basic types of polarization: linear linear and and ellipticalelliptical..

•

109

• In linearlinear polarization, the electric vector does does

not change orientationnot change orientation as it travels away from an observer : 2 Types : H& V

• In ellipticalelliptical polarization, the electric vector rotates asrotates as it travels away from an observer, and the tip of the electric vector traces an ellipse : sense here mean Clockwise and anticlockwise.

110

• An antenna transmits vertical, horizontal, right-hand (RH) circular, or left-hand (LH) circular polarization depending on the antenna design and orientationantenna design and orientation.

• There is a significant cross-polarization loss of approximately 30 dB30 dB.

• This loss also occurs in case of cross sens rotation in circularly polarized antennas.

• This characteristic is used to provide polarization polarization diversity in communication systems. diversity in communication systems.

111

Polarization Requirements for Various Frequencies

wave type Ground wave Sky wavesDirect waves

(including satellite)

frequency Band Low & Medium Short waves VHF , UHF,SHF

Polarization possible to be used Vertical Vertical or Horizontal Vertical orHorizontal

Polarization practically to be used Vertical Horizontal Vertical orHorizontal

WhyThe earth is

fairly good conductor,short out Eh

a) Less auto and electro ignition, b) Less building absorption c) More simple support antenna structure

No ionosphere entry or reflexion

112

• 15-9 Antenna Impedance and Radiation Resistance

• when it is excited by an appropriate AC source The antenna acts like an a complex complex impedance impedance to the source providing power to it

Z = R + jX

• This impedance can be measured by an appropriate RF bridgeRF bridge .

113

• Antenna ImpedanceAntenna Impedance• Ideally, it should be Ideally, it should be purely resistive Rpurely resistive R at the at the

frequency of operation and equal to the frequency of operation and equal to the characteristic impedance of the line connected characteristic impedance of the line connected to it. to it.

• Self Impedance of the isolated antenna If an antenna is isolated from ground and any

other surrounding objects, this impedance is the self-impedance self-impedance of the antenna & at the resonance it is purely resistive ( Z= R +j0)

114

• Mutual impedance of the antenna.

When other antennas, objects, or ground is other antennas, objects, or ground is nearnear the antenna, the currents flowing in these objects have an influence on the antenna impedance.

The antenna impedance is then determined both by the self-impedance of antenna and by a mutual mutual impedanceimpedance between the antenna and the nearby nearby objects.

115

Radiation Resistance• The radiation resistance is the real part of the

complex antenna impedance of a losslesslossless antenna. It is equal to

• Where : - Prad is the amount of this energy leaving a sphere surrounding the antenna per unit of time is the power radiated by the antenna.

• - Irms is the rms value of the antenna current magnitude at the input terminals of the antenna.

116

• Example : An antenna has an rms current of 3 A flowing into the antenna, and it is transmitting 1 kW of power. Determine the radiation resistance of the antenna.

• Solution

• The radiation resistance is determined from (15-29).

117

- Simple DipolesSimple Dipoles

- Folded Dipoles- Folded Dipoles

- Antennas Above a Ground Plane - Antennas Above a Ground Plane

- Monopole Antenna- Monopole Antenna

- Waveguides and horn antennas- Waveguides and horn antennas

- Parabolic antennas- Parabolic antennas

118

•

• Simple DipolesSimple Dipoles

- Folded Dipoles- Folded Dipoles

- Antennas Above a - Antennas Above a Ground Plane Ground Plane

• - Monopole Antenna- Monopole Antenna

119

• Dipoles• One of the simplest and most commonly used

antennas is the half-wave dipole, formed from a two-wire parallel transmission line as shown in The following figure.

• Starting with an open-ended line, which has a voltage maximum at the open end and a voltage minimum back from the open end, the two conductors are bent 90o from the transmission line as illustrated in the Figure.

120

• The theoretical length of the antenna is the

• Diameter d of the wires is assumed to be much smaller than the length, and the spacing D at the feed point must be small compared with the length.

122

Practically Mounted dipole

•

123

• Input Impedance of Dipole

• A voltage minimum and a current maximum occur at the feed point, which means that the impedance is a minimum at that point.

• The actual value of the impedance of the half-wave dipole is 73+ 42.5 j .

124

• The reactive component can be eliminatedeliminated by

tuning the antennatuning the antenna, which is accomplished by shortening the lengthshortening the length by about 5% from 0.5 to 0.475, corresponding to approximately 95% of the theoretical length.

• When properly tuned, the half-wave dipole has an impedance of 73 73 resistive resistive, which for a lossless dipole is the radiation resistance of the antenna.

125

• The dipole is a balanced antennabalanced antenna and must, therefore, be fed by a balanced transmission line.

• Since the most common transmission line providing the best impedance match is coaxial cable, a balunbalun must be used to properly connect a coaxial cable to a dipole.

126

• Radiation Patterns Radiation Patterns • The E plane and H plane radiation patterns of the half-wave

dipole were shown in the following Figure as examples of examples of patterns. patterns.

• The E plane pattern is like a doughnut with two maxima broadside to the dipole and a null at both ends of the dipole.

The 3-dB beamwidth is 78o. The isotropic power gain or directivity for a lossless /2 dipole is 1.643 folds or 2.16 dB.

•

128

• As shown in the Figure , the polarization of the

dipole is parallel parallel to the dipole.

• The H plane pattern is illustrated in the Figure and is a uniform circularuniform circular pattern with a constant gain for an angle of 360o about the dipole.

129

• Effective AreaEffective Area

The effective area of a dipole can be determined from the isotropic gain and is

for f= 98MHz(=3m) ; Ae =1.218m2

130

• Folded DipoleFolded Dipole

A folded dipole is constructed from a /2 length of

300- twin-lead transmission line (see figure).

The combination of the 73-73- self-impedance of the dipole and the mutual impedance from the parallel conductor connected at both ends increases the antenna impedance of the folded dipole to 280 280 ..

• Therefore, the folded dipolefolded dipole is a balanced 280- antenna, which closely matches the 300- twin-lead balanced transmission line.

131

•

132

Practically mounted Folded antenna

•

133

• The folded dipole is the perfect antenna for stations

that require a truly professional antenna for their broadcasting. One powerful advantage of a folded dipole antenna is that is has a wide bandwidth, in fact a one octave bandwidth. This is the reason it was often used as a TV antenna for multi channel use. Folded dipole antennas were mainly used in conjunction with Yagi antennas.

• No tuning or adjustment is needed for any frequency on the band which makes this antenna the only one to use if you plan to move your transmitters frequency often.

134

Specifications for the folded professional antenna

Max Power Input 300 Watts

Impedance 50 Ohms

Gain 0dBd

VSWR Better than 1:1.5 (88 - 108 MHz)

Frequency Range 88 - 108 MHz (No Tune)

Connector N-Type Female

Dimension 1600mm(height) x 150mm x 35mm

Weight 2kg

136

• 15-11 Antennas Above a Ground Plane

A ground plane is a uniform good ground plane surface beneath an antenna- constructed from good conductors, or at some frequencies, the earth acts as a good ground plane.

• Electromagnetic fields cannot exist in a perfect conductor, and any wave incident upon a perfect conductor will be reflected.

137

• Figure illustrates this situation. To satisfy the

boundary condition that no tangential component of electric field can exist there, the reflected wave will be shifted in phase by 180o.

• A concept known as image theory is used to determine the characteristics of an antenna above a ground plane.

138

• The reflected wave is like a direct wave from an is like a direct wave from an identical antenna located within the ground identical antenna located within the ground planeplane the same distance from the boundary as the real antenna is above the ground plane.

• This situation is illustrated in the following Figure.

• The image antenna is similar to an The image antenna is similar to an image image formed in a mirror at optical frequencies.formed in a mirror at optical frequencies.

139

•

140

•

141

• Monopole AntennaMonopole Antenna• An important and commonly used antenna is

the /4 monopole antenna/4 monopole antenna on a ground plane as shown in the Figure.

• This is an unbalanced antennaunbalanced antenna since the feed point is between the monopole and ground and it has vertical polarizationvertical polarization.

• The radiation resistance is 36.5 , and the antenna impedance has a reactive component of 21 j 21 j . When the monopole is located close to the ground plane, the image antenna forms a dipole as shown in the previous Figure .

142

• The E plane radiation pattern is that of a /2

dipole with only one-halfonly one-half of the pattern above the ground plane.

• The ground plane can be achieved either by a grounded metal disc or by radial wires as shown in the following Figure.

•

• The roof of a vehicleroof of a vehicle such as a car or truck can form a ground plane for a /4 monopole.

143

•

1. 2 The aim of this Course is to Give a) Basic notions in Radio Propagation at microwave...

Documents

Transcript of 1. 2 The aim of this Course is to Give a) Basic notions in Radio Propagation at microwave...