microwave techniques

8/7/2019 microwave techniques

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An electromagnetic waves withfrequencies that ranges fromapproximately 1 GHz to 300 GHz or more. And its wavelengths fall between

1cm and 60 cm.

Offers tremendous bandwidth for communications and at leasttemporarily resolve the problem of

spectrum crowding.(available for new radio services as well asfor wide-bandwidth signals such as TV,multiplexed signals, or computer data.)


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U sed primarily for telephonecommunications, radar, and satellitecommunications.

Other microwave applications includecable TV, space communications,radio astronomy, and heating.

The RF spectrum below U HF is mostlyalready fully occupied leaving little or no room for the growth of new radioservices. The allocation of the RFspectrum is handled by the Federal Communications Commission (FCC)in the U nited States.


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U se of SSB signals only one sideband is used whichcuts the spectrum usage in half

Limiting the deviation of FM signalshelps to control the bandwidth

Improved receiver selectivity phase-shift keying, datacommunications new modulationtechnique

Multiplexing help put more signals in into asingle channel


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More difficult to analyze

Different measurement techniques

Resistors, capacitors, and inductors

act like LCR circuitsConventional semiconductors donot work owing to internalcapacitances and long transit time

Special, expensive vacuum tubesare used for power amplification

Line-of-sight transmission differences

Excessive signal reflection and absorption


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It is a means of carrying electromagneticenergy from one place to another.

Balanced transmission line is not used

for microwaves because of radiationlosses. Coaxial cable is not used becauseof its high attenuation.

Can be used as tuned circuits, filters and

even reactive components. In microwavefrequencies, it is constructed by usingprinted circuit boards (PCBs) or ICmanufacturing techniques.


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Microstrip is a flat conductor separatedfrom a large conducting ground plane byan insulating dielectric.


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Stripline is a flat conductor sandwichedbetween two ground planes. It is moredifficult to make, but it will not radiate asmicrostrip does.

Both stripline and microstrip are widelyused to form the tuned circuits used inmicrowave receiver front ends and inthe low-power amplifier sections of transmitters


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A hollow metal pipe with a circular or rectangular cross section used for carrying microwave signals from oneplace to another.

Acts like a high-pass filter, passing allfrequencies above its cut-of frequency andrejecting those below it.

The preferred transmission line for microwaves.


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3 common methods:

Probe: at an E-field maximum

Loop: at an H-field maximum

Hole: at an E-field maximum


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af co 2300

!

The cutoff frequency fco of a waveguidedepends upon its physical size. iFor arectangular waveguide, it is 300/2a.where a is the wide dimension of thewaveguide in meters.


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High Frequency

Medium Frequency

Low Frequency

Cut-off Frequency

As the operating frequency gets lower,the angles decrease and the pathbetween the sides shortens.


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The microwave signal carried by awaveguide is made up of electric (E)and magnetic (H) fields that bounceoff the walls of the waveguide as they

propagate along its length.


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The modes of a waveguide describethe various patterns of electric andmagnetic fields that are possible.

A transverse electric (TE) mode is onewhere the electric field is transverse or perpendicular to the direction of propagation.

A transverse magnetic (TM) mode isone where the magnetic field isperpendicular to the direction of propagation.


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W aveguides are available in standardlengths and sizes, and special piecesare used for right-angle bends and 90

twists.


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Half-wavelength sections of waveguideswith shorted or closed ends are knownas resonant cavities since they "ring" or oscillate at the frequency determined by

their dimensions.

Cavity resonators are metallic chambersof various shapes and sizes that are usedas parallel-tuned circuits and filters. Theyhave a Q of up to 30,000.


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Point-contact diode perhaps the oldest microwavesemiconductor device

Schottky or hot-carrier diodes has a metal semiconductor junction


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Point-contact and Schottky or hot-carrier diodes are widely used asmixers in microwave equipment as theyhave low capacitance and inductance.

Varactor diodes are widely used asmicrowave frequency multipliers.Multiplication factors of 2 and 3 arecommon with power levels up to 20 W andefficiencies up to 80 percent.


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Step-recovery or snap-off diodes are alsowidely used as frequency multipliers withmultiplication factors up to 10, power ratings up to 50 W . and efficiencies

approaching 80 percent.

A Gunn diode is a microwavesemiconductor device used to generatemicrowave energy. W hen combined witha microstrip, stripline or resonant cavity,simple low power oscillators withfrequencies up to 50 GHz are easilyimplemented.

A Gunn diode is a microwavesemiconductor device used to generatemicrowave energy. W hen combined witha microstrip, stripline or resonant cavity,simple low power oscillators withfrequencies up to 50 GHz are easilyimplemented.


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Both IMPATT and TRAPATT diodes areGaAs devices operated with high reversebias to produce avalanche breakdown.Both are used in microwave oscillators.

Tunnel diode is another negativeresistance diode. It is used to producelow-power microwave oscillators.


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A klystron is a microwave vacuum tubeusing cavity resonators to producevelocity modulation of the electron beamand produce amplification.


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K lystrons are available which producefrom a few to many thousands of watts.

A single-cavity reflex klystron is used asa microwave oscillator.

K lystrons are being gradually replaced byGunn diodes and traveling-wave tubes.


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A magnetron is a diode vacuum tubeused as a microwave oscillator in radar and microwave ovens to produce powersup to the megawatt range.

In a magnetron, a strong magnetic fieldcreates circular paths of electron flow to

excite cavities into oscillation.


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A traveling-wave tube (T W T) is amicrowave power amplifier with verywide bandwidth.

A microwave signal applied to a helixaround the T W T produces velocity anddensity modulation of the electron beamover a long distance which induces a

higher-power signal in the helix.


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The most commonly used microwaveantenna is the horn , which isessentially a rectangular waveguidewith a flared end.

A pyramidal horn flares in bothwaveguide dimensions. A sectoral hornflares in only one dimension. A conical

horn flares in a circular guide.


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Horn antennas are directional andproduce a beam width in the 10 to 60range with a gain in the 10-to 20-dBrange, depending upon dimensions.

The gain and directivity of a horn is adirect function of its various dimensions.

The length of a typical horn is computed

by the following formula :

f

300!P


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The horizontal beam width of a pyramidalhorn may be computed with the simpleexpression:

P/8 0

wB !

Its gain can be computed by:

2

4

P

T KAG !


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A parabolic or dish-shaped reflector isused with most microwave antennas tofocus the RF energy into a narrow beamand increase gain.

The parabolic reflector usually has adiameter that is no less than 10wavelengths at the operating frequency.


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The gain and directivity of a parabolicreflector antenna is directly proportionalto its diameter.

Given an area A=3.14piR2, its gain andbeam width can be computed by thefollowing formulas:

26

!P

DGP/

58

DB !


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Parabolic reflector antennas are fed byplacing a horn antenna at the focal pointor by placing the horn at the center of thereflector and placing a small reflector atthe focal point. The latter is known asCassegrain feed.

A helical antenna is made up of six toeight turns of heavy wire or tubing toform a coil or helix. It is fed with coax andis backed up with a reflector.


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Helical antennas can receive either vertically or horizontally polarized signals but can only receive acircularly polarized signal of the same direction.

A popular omnidirectional microwave antenna is thebicone.

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microwave techniques