6+Microwave+Comm+System.pdf

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Microwave CommunicationsSystem

Maria Leonora Guico

Tcom 126 2nd

Sem Lecture 6

Introduction Advantages/Disadvantages

Microwave Devices:- Waveguides


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Why Use Microwaves?Frequency spectrum used for radio communication isgetting crowdedMore frequency spectrum is required to carry wider- bandwidth video and digital informationTechnological advances have overcome the high cost ofthe special equipment required to generate, transmit andreceive microwavesThis has opened the microwave spectrum for cellphones, wireless LANs, digital satellite radio andwireless broadband


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IntroductionMicrowaves are Ultrahigh (UHF), Superhigh (SHF) andExtremely high (EHF) frequenciesThe practical microwave region is 1 40 GHz

Microwave signals have wavelengths between 1 cm to 60 cm.Full-duplex operation is generally required of microwavecommunications systems, each freq band is divided in half(lower half low band; upper half high band)


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Microwave Frequency Bands


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Advantages of Microwaves

Greater bandwidth (carry large quantities of info) available athigher frequenciesHigher frequencies mean short wavelengths, require relativelysmall antennas (with very high gain)Underground facilities are minimized. No need for physicaltransmission media such as coaxial cables or optical fibers(hence, no right of way acquisitions)

Radio signals more easily propagated around physical obstaclesIncreased reliability, less maintenance


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Disadvantages of Microwaves

For frequencies below 30 MHz, standard circuit analysisapplies (current-voltage relationship)This relationship is not usable at microwave frequencies.

Most components and circuits are analyzed in terms ofelectric and magnetic fieldsMeasuring techniques are more difficult to perfect andimplement at microwave frequencies


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Disadvantages of MicrowavesTransit time of charge carriers becomes a problem atmicrowave frequencies

At low frequencies, this is not a problemAt microwave frequencies, transit time becomes a high percentageof actual signal period (transit time determines maximum bit ratepossible)

Necessary to use specialized components

Microwaves limited to line-of-sight


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Simple Components ecome Complex

Added Characteristics at Microwave Frequencies

Resistor Capacitor Inductor

Effects of short leads on components


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Skin AffectSkin Affectis the concept that high frequency energy travelsonly on the outside skin of a conductor and does notpenetrate into it any great distance.Skin Affectdetermines theproperties of microwave signals.


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Free Space & Atmospheric

Attenuation

Free space & atmospheric attenuationis defined by the loss the

signal undergoes traveling through the atmosphere.

Caused by changes in air density and absorption by

atmospheric particles.


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Diffraction

Diffractionis the result of variations in the terrain the signal

crosses


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Reflection

Reflectionscan occur as the microwave signal traverses a body

of water or fog bank; cause multipath conditions


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Intro to Waveguides

Long parallel transmission lines radiate electromagneticenergy while transporting itIf used at microwave frequencies, virtually all energy is

radiated and very little arrives at the antennaCable losses increase at high frequencies, above 6 GHz awaveguide must be used


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Waveguides

Waveguides are hollow metal conducting pipes designedto carry and constrain the electro-magnetic waves; usedto direct the signal from the RF unit to the antenna. Pipe through which EM wave travels; reflects from thewalls Rectangular waveguides (brass or aluminum) are mostcommon

Can be rigid or flexible

Rectangular waveguide


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WaveguidesOperate essentially as high-pass filtersHave no radiation losses; dielectric loss very smallInside is often coated with silver to reduce resistance and

minimize transmission loss


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Signal Injection and ExtractionSignal is introduced into the waveguide by an antenna-likeprobeProbe creates an electromagnetic wave that propagatesthrough the waveguide

The position of the probe determines whether the signal ishorizontally or vertically polarizedSimilar probe can also be used to extract the signal from thewaveguide


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Signal Injection and ExtractionSignal is reflected (introduces 180 phase shift)

and amplifies original signal

Vertically polarized


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ModesWaves can propagate in various waysTime taken to move down the guide varies with the modeEach mode has a cutoff frequency below which it wontpropagateMode with lowest cutoff frequency isdominant mode

Low-order mode: Faster propagation


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Mode DesignationsTE: transverse electric

Electric field is at right angles to direction of travelTM: transverse magnetic

Magnetic field is at right angles to direction of travel

TEM: transverse electromagneticWaves in free space are TEM


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Rectangular WaveguidesDominant mode is TE10

1 half cycle along long dimension (a)No half cycles along short dimension (b)Cutoff for a = c/22:1 frequency range in its dominant mode

Modes with next higher cutoff frequency are TE01 and TE20Both have cutoff frequency twice that for TE10


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First number following the TE designation represents the number of half-cycles of thewave along the dimension (a) of the rectangular waveguide, the second represents the no.of variations along the short dimension (b)Multimode propagation causes dispersion (interference between waves)

Modes in Rectangular Waveguides


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Cutoff FrequencyFor TE10 mode in rectangular waveguide witha = 2 b

Waveguide will not transmit energy below this frequencyf c is in MHz and a is in meters

ac

f c 2

A waveguide is essentially

a high-pass filter

Height, b, is normallyhalf the width


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Answers to Example 1a. Find the cutoff frequency for the TE 10 mode in an air-dielectric waveguide with an inside section of 2cm by 4 cm.b. Over what frequency range is the dominant mode theonly one that will propagate?

f c = c/2a = 300x10 6 m/s/2x 4 x 10 -2m) = 3.75 x 10 9 Hz or 3.75GHz

The dominant mode is the only mode of propagation over a 2:1frequency range, so the waveguide will be usable to amaximum frequency of 3.75 x 2 = 7.5 GHZ


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Usable Frequency RangeSingle mode propagation is highly desirable to reducedispersionThis occurs between cutoff frequency for TE10 mode andtwice that frequencyIts not good to use guide at the extremes of this range


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Example WaveguideRG-52/UInternal dimensions 22.9 by 10.2 mmCutoff at 6.56 GHz

Use from 8.2-12.5 GHz


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Group Velocity

Waves propagate at speed of lightc in guideWaves dont travel straight down guideSpeed at which signal moves down guide is the groupvelocity and is always less thanc

2

12 g

v ca

2

1 c g

f v c f


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Examples1. Find the group velocity for the waveguide whose larger

dimension is 4 cm., at a frequency of 5 GHz.

2. A waveguide has a cutoff frequency for the dominantmode of 10 Ghz. Two signals with frequencies of 12 and

17 Ghz propagate down a 50 m length of the guide.Calculate the group velocity for each and the difference inarrival time for the two.


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Answers to Examples

1. v g = 198 x 106

m/s2. For 12 GHz signal: v g = 165.8 x 10 6 m/s; t 1 = 301.6 ns

For the 17 GHz signal: v g = 242.6 x 10 6 m/s; t 2 = 206.1 ns

t1-t

2= 95.5 ns


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Phase VelocityNot a real velocity (>c)Apparent velocity of wave along wallUsed for calculating wavelength in guide

For impedance matching, etc.

2

1

f f

cv

c

p


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Characteristic ImpedanceZ 0 varies with frequency

2

0

1

377

f f

Z c


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Guide WavelengthLonger than free-space wavelength at same frequency

2

1

f f c

g


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Coupling Power to Guides

How power can be put into and taken out of the guideThree common methods to launch a wave down a guide:

Probe: resembling quarter-wave monopole antennaCouples to the electric field; located at an E-field maximum

Loop: couples with magnetic field; located at an H-fieldmaximumHole: at an E-field maximum


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(b) Loop


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Directional Coupler

Launches or receives power in only 1 directionUsed to split some of power into a second guideCan use probes or holes


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