a microwave generator, receiver, and reﬂectorl-mit-ab3

8/3/2019 A Microwave Generator, Receiver, and Reflectorl-MIT-ab3

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AMicrowaveGenerator,Receiver,andReflectorMay11,2004

Introduction1Hertzfirstgeneratedelectromagneticwavesin1888,andwereplicateHertzsoriginalexperimenthere. Themethodheusedwastochargeanddischargea capacitor connected to a spark gap and an antenna. When the sparkjumpsacrossthegap(onceper0.13milliseconds inourexperiment),theantenna isexcitedbythisdischargecurrent,andchargesoscillatebackandforth intheantennaattheantennasnaturalresonance frequency. Forourexperiment this natural resonance frequency of the antenna is very high,about 2.4

109 cycles per second or 2.4 GHz. For a brief period around

thebreakdown(spark)theantennaradiateselectromagneticwavesatthishigh frequency. Wewilldetectandfindthewavelengthoftheseburstsofradiation. Usingtherelationf=c = 31010cm/s ,wewillthendeducethenaturalresonance frequencyoftheantenna,around2.4GHz,andshowthatitiswhatweexpectonthebasisoftheverysimpleconsiderationsgivenbelow.TheExperimentalMethodHere in broad outline is how the experiment works. The 25 pF capacitorshowninFig.1ischargedbyahighvoltagepowersupplyonyourelectronics

1WearegratefultoDr.PeterDourmashkin,ProfessorJohnKing,andProfessorPhillipMorrisonforelementsofthisdescriptionandforthedesignofthemicrowaveexperimentitself,whichisfrom8.02X.

1


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2board. Thisvoltageistypically700Volts,butthisisaverysafelevelbecausethe

current

is

limited

to

avery

small

value

across

the

capacitor.

When

the

voltage is high enough and the distance between the conductors in yoursparkgap issmall enough, thecapacitor discharges acrossthegap. Thishappens when the electric field in the gap exceeds the breakdown field ofair (about a 10kV/cm). The radiation we are seeking is generated in thisdischarge(seeexplanationbelow). Afterdischarging, thecapacitorchargesupagainthrougha4.5resistor. Thetimeconstantis:

=RC=(4.5106)(251012F)=1.3104secSothechargingandbreakdownwillgenerateasparkdischargeaboutevery0.13

msec.

TheResonantFrequencyOfTheAntennaDuringthedischargeacrossthesparkgap,theantennaactsasanLCoscillator inwhichenergysloshesbackand forthbetweenmagneticandelectricenergy. Initially,whenthetwohalvesoftheantennaarechargedupwithoppositessignsofcharge,thetwohalvesoftheantennaactascapacitorplateswithenergystoredintheelectricfieldbetweenthem. Whenbreakdownstartsandcurrentflowsacrossthegaptodischargetheantennahalves, energy istransferred

from

electric

to

magnetic

energy

associated

with

the

flowing

current. The antenna halves discharge, but the current continues on in the

same direction (overshoots), charging up the two halves of the antennato the opposite polarity from the original configuration. The current thengoestozero,buttherechargedantenna(withoppositepolarity)nowdrivescurrent intheoppositedirection, dischargingtheantennahalvesagainandthenrechargingthembacktotheoriginalpolarity.

This process continues on and on for many cycles at the resonance frequency = 1/LC. How fast do these oscillations take place? Here is acrude estimate that turns out to be right. If l is the length of one of thehalvesoftheantenna(about31mminourcase),thenthedistancethatthecharge oscillation travels going from the (+-) polarity to the (-+) polarityandbackagaintotheoriginal(+-)polarityis4l (fromonetipoftheantennatotheothertipandbackagain). ThetimeT ittakestodothis,assumingthatinformationisflowingatthespeedoflight,isT=4l/c,wherec isthespeedoflight. Ifthechargesarebeingshakenbackandforthatafrequency


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3

Figure 1: Microwave transmitting antenna. In the newer version of thetransmitter, the thumb tack conductors have been replaced by newer halfcylindricalconductors.


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4

Figure2: Potentialbetweentheconductorsasafunctionoftime. Theslowrisecorrespondstothechargingfromofthecapacitorandtherapiddropisthedischargethroughtheionizedairbetweentheconductors.

of 1/T, theywill radiate electromagnetic radiationatthis frequency. For l=31mm,thisestimateofthefrequencyradiatedis

frad =1/T =c/4l= 3108m/s =2.4109Hz=2.4GHz.0.124m

Electromagneticwaveswiththisfrequencywillhaveawavelengthofc/f,or4l.

Thus our simple picture predicts that we will generate electromagneticradiationwiththisantennawithwavelengthsofabout12.4cm,andthat issomethingthatwewillexperimentallyconfirminthisexperiment.

The oscillationsofthechargebackand forthbetween the twohalvesofthe antenna damp out as energy is dissipated, and the antenna is finallydischarged. Then the 25 pF-capacitor starts charging up again, ultimatelyheaded toward breakdown in another 0.13 millisecond. The antenna willthusemitsburstsofdampedradiationatfrequenciesaround2.4GHzevery1.3104 sec. TheoveralltimeserieslookslikeFig.2onalongtimescale:

And, if we blow up the region around the spark discharge, is shown inFig.3:

Thecurvesjustaboverepresentthecurrentflowinginyourreceiverasaresultoftheoscillatingelectricfieldsofthemicrowaveradiation. Wehaveadiode in the receiver that allows current to flow in only one direction, andwedetectthatcurrentusingtheamplifierandmultimeter.


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Figure3: Currentintheconductorsasafunctionoftime.

SettinguptheApparatusWARNING:DONOTTOUCHPARTSONYOURELECTRONICS BOARD WHILE OPERATING, SINCE THEY MAY BECOMEQUITEWARM!!!

Plug the power supply into your electronics board at the position indicated. Pluginyourreceivingantenna(whichlookslikethetubetotheleft)totheremaininginputjackontheboard. Yourtransmittingantennaistheclothespinassembly,andisalreadyconnected. Onceyouhaveconnectedupthepowersupply,turnonthetransmitter(usingtheoff-onswitch-theLEDwilllightwhenitison).

Your one remaining task is to adjust the spark gap using the wing nuton the clothespin antenna until you get a spark discharge. Once you getthat discharge, and learn how to make it reasonably steady, you can useyourreceiverto makemeasurementsoftheradiation emitted. Arrangethetransmitting

antenna

(Figure

4above

right)

on

your

desktop

as

far

away

from

metal as possible. Put your electronics boards on the desk and somewhatback so that you can explore the radiation field with the receiver (Figure4 above left). You should be able to move the receiving antenna from afew inches from the transmitter to as far as the shielded wire will let you


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Figure 4: The spark gap, transmitting antenna, and the receiver.(The receiverisshownintwodifferentorientations.


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7

Figure5:

go on the other side for larger distances, move the amplifier. Now, withyour power supply connected, vary the distance between the thumbtacksby adjusting the nut. You will be able to see the spark when breakdownoccurs. Tomeasurethecharacteristicsoftheradiationemitted,youexplorethe radiation generated using signals from your receiver that are amplifiedbyanamplifierandreadoutonyourvoltmeter.Exercise 1: Measuring The PolarizationPrediction 1-1: The radiation we are generating is produced by chargesoscillatingbackandforthbetweenthetwohalvesofyourantenna(seeFigure1above).

If

you

hold

the

receiver

in

the

two

orientations

shown

in

Figure

4

above,whichshouldproducethebiggestsignalonthevoltmeterconnectedtoyourreceiver? (Answerbeforestartingthelab.)

Receiverwirevertical ReceiverwirehorizontalActivity 1-1: Measure the polarization with your receiver and check

thisprediction.Exercise 2: Measuring The Angular DependencePrediction

2-1:

The

radiation

we

are

generating

is

produced

by

charges

oscillating back and forth along the length of your antenna. If you movethe receiver in the two patterns shown above(Fig. 5), which should showthe biggest change in signal on your voltmeter over the range of motion?(Answerbeforestartingthelab.)


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8theleftpattern therightpattern

Activity2-1: Measuretheangulardependenceoftheemittedradiationwithyourreceiverandcheckthisprediction. Recordthevoltageonat=0,30, 45, 60 and 90 degrees. Plot on the the graph paper provided. is thepolarangle,takingtheantennaalongthe z axis. Doesyouplotagreewithwhatyouexpect?Exercise 3: Measuring the radial dependencePrediction3-1Doyouexpectthereadingonthevoltmetertogolike1/ror1/r2? (Answerbeforestartingthelab.)

Activity3-1Measureandrecordthesignalstrengthat=0forr=10,20,30cm, .... incrementsasfaroutasyoucanandplotonthegraphpaperprovided. Doesyourplotagreewithprediction?Exercise 4: Measuring The WavelengthThisisbyfarthemostchallengingpartoftheexperiment. Wewillmeasurethe wavelength of the radiation from your transmitter by using a reflectorto reflect the radiation so that it returns to interfere with itself. By measuring the distance we must move the reflector to go from constructive todestructiveinterferenceandbackagain,wewilldeterminethewavelengthoftheradiation.

Firstadjustyourgapsothatthesparkproducesasteadyreadingonthemeter. Unlessyoucanperformthistask,youwillhaveahardtimelocatingthe adjacent maxima and minima blow. Then set up one of the reflectorsyou are given in the positions shown below, and position the receiver andtransmitterasshown. With thisconfiguration,youwant tovary thedistance between the reflector and transmitter, holding both thetransmitterandreceiverfixed inspace.

Search forthe positions forconstructiveand destructive interferencebymoving the reflector slowly toward or away from the transmitter, watchingthe

voltmeter

scale

as

you

do

so.

The

clearest

signal

is

probably

obtained

in

movingthereflectorintheregionfromabout10cmtoabout24cmawayfromthetransmitter. Youhopetoseethevoltageriseanddroprhythmicallywithyour motion as you move along this distance. Adjacent maxima should beseparatedby1

2 wavelength. Amaximumandaminimumshouldbeseparated


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101by4 wavelength. Make a number of different measurements to arrive at a

reasonableaverage

for

the

wavelength

of

your

wave

using

this

method.

Problems In Making The Measurement: It is not easy to get a

convincingresulthereunlessthesparkiscooperating. Cleaningthetackfaceswithascrapofpaper willusually calm thingsdown, at least fora fractionof a minute. Mark the positions of the antenna that give meter maxima(minimawoulddoaswell),performingthesweepseveraltimestocounteractthe sparksgeneraljumpiness. Useyour rulertoget thewavelengths. It isbestforonepersontobothmovethereceiverandjudgethemeterreading,asubtlefeedbackmechanism. Thepeaksareahalf- wavelengthapart.

a microwave generator, receiver, and reﬂectorl-mit-ab3

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