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ECE123
Midterm
Spring2017
Besuretoincludeunitswhereappropriate.
1. Adipoleis0.5mlong,and2mmindiameter,isoperatingat30MHz,andismadeof
copperwithaconductivityof6*107S/m.Intheshortdipoleapproximation,
a. Whatistheradiusofthereactivenearfieldinmeters?
b. Whatistheradiationresistance?
c. Whatistheohmiclossresistance?
d. Whatistheradiationefficiency?
e. Whatistheeffectivecapacitance?
2. Alosslessquarter-wavelengthmonopoleantennaisattachedverticallytoaninfinite
groundplane.Itisfedbya75Wcablewithnomatchingnetwork.
a. Sketchtheimagecurrentsrelativetothecurrentsintheantenna.
b. SketchtheradiationpatternintheE-planeandH-plane,labelingeach.
c. WhatisthereturnlossindB?
d. Whatisthemaximumgain,andthemaximumrealizedgainindBi?
e. Whatistheeffectiveapertureinsquarewavelengthsbasedonthegain?
3. AnelectricallysmallloopantennahasitsaxisorientedalongX.Itislyingveryclose(<<l)
toaninfinitegroundintheXYplane.Theantennaismatchedand100%efficient.
a. SketchtheradiationpatternintheE-planeandHplane,labelingeach.
b. WhatisthemaximumdirectivityofthisantennaindBi?
NowassumethisloopantennaisanAMradioreceivingantennaoperatingat1MHz,
andconsistingofathinwirethatiswound100timesaroundaferritecorewitha
diameterof1cmandarelativepermeabilityof1000.Assumetheantennais100%
efficient.Neglectingtheeffectsoftheground,
c. Whatisitsradiationresistance?
d. Whatisitseffectiveaperture?
e. Whatisitseffectiveheight?
4. AnFMradiotransmitterbroadcasts10kWfromasmalldipoleantennaatafrequency
of100MHz.Thereceiverisanothersmalldipoleantennathatis20kmaway.Both
antennasarealigned,perfectlymatched,100%efficient,andorientedformaximum
gain.Thereceiverhasabandwidthof150kHzanditsnoisetemperatureis500K.
a. WhatistheEIRPofthetransmitter?
b. Whatisthereceivedpower?
c. Whatisthenoisepoweratthereceiver?
d. WhatistheCNRindB?
e. WhatisthemaximumdistancebetweentheantennastomaintainCNR>20dB?
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