ANTENNAS PETERDODD,G3LDO. E-MAIL:G3LDO@UKONLlNE.CO.UK

AntennasMore on the Grid Dip Oscillator (GDO)

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PHOTO1: Q-MaxGDO-IA usesa 12AT7double.triode,onehalfusedasanoscillatorandtheotherhalfasa half-waverectifier.

GRIDOIPOSCILLATOR.Averyusefulitemof testequipmentwhenworkingwith antennasandtunedcircuitsisthe GridOipOscillator,generallyknownasa GOO.Fornewcomerswho maynotknowofthis device,it isusedformeasuringthe resonanceof tunedcircuitsandantennaelementswithoutactuallyconnectingthe instrumentto thecircuit undertest.

Whena tuneableoscillatorisplacedclosetoa nearbyresonantcircuit,someoftheoscillatorpowerisabsorbedwhentheoscillatoristunedto thesamefrequency.Someindicationofoscillatorenergylossiscan beusedto indicatethis condition.Theoriginalinstrumentsusedavalvewith a microammeterconnectedbetweenthegrid andearth,asshowninFigure1. Thismeasuredgridcurrent,whichis proportionalto the peak-to-peaklevelof theoscillatorwaveform.In practiceadip in thegrid currentlevelisobservedastheoscillatoris tunedpastthe resonantfrequencyof circuit

undertest, hencethetitle of this instrument:GridOipOscillator(GOO).

Thecircuitshownin Figure1 isfromtheARRLRadioHandbook,1947 [1]. Thisinstrumentisdesignedasa portableworkshopitemofequipmentformeasuringtheresonanceof tunedcircuits. It usesa probemadefroma short lengthof twin transmissionlineanda loopL2to accesstunedcircuitswithintransmittersor receivers.ThisGOOwas builtinto anenclosure150mm (6in) squaresowasquitedifferentfromthe shapeofGOOsto which we arenow accustomed.

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Itwas alsousedfordeterminingthevaluesofcapacitorsand inductorsusing

. the arrangementsshown in Figure1aand b.Thelink-coupledprobeL2 isusedto measurethe resonancesof tunedcircuitsin confinedspaces.It can alsobeusedto measuretheresonantfrequencyof a tunedcircuitwherethe inductanceisa ferritering bycouplingL2 aroundthe ring.Therewas no mentionin[1Jofusingthis GOOto measuretransmissionlineor antennaelementresonances.

An exampleofa laterGOOisshown inPhoto1. It isan old Q-MaxGOO-1Athatoperatesverywell rightup intotheVHFband.It comprisesa 12ATl doubletriode,onehalfusedasanoscillatorandthe otherhalfasahalf-waverectifierfor the 330V HTsupply.

A FETGOO.Thesedaysmostof theseinstrumentsusesolidstatedevices,usuallya FET.In this casewe areinterestedin thevariationsingatecurrentsowe canstill callit a GOO(GateDipOscillator).Thesubjectof GOOswas promptedbya questionfromPaulBadley,MOPIB,who was buildingapublishedversionaGOO[2]. Hewantedtoknowwherehecouldobtaina dual-ganged365pF variablecapacitorshownin thiscircuitandthediameterof the coils.Thecircuit Idescribedin [2] was basedon twoverysimilardesignspublishedinQST,TechnicalCorrespondence[3] [4]. Thecircuitof myvariationof thesetwo designsisshowninFigure2.

Thiscircuitdoesnot measureFETgatecurrentdirectly;insteadit measuresthetotalsourcecurrent,which isaffectedbythevoltageat thegateof the FET.However,thevariationofcurrentthroughresonanceisonlyasmall partof thetotal sourcecurrent.Thedip isenhancedbyoffsettingthe meterreadingusinga potentiometerin a bleedernetwork.This issetsothat the meterreadsabout75%FSOwhenthe instrumentis notcoupledtothe resonantcircuitundertest.

MAY 2010 . RADCOM

I built oneversionof the FETGOOintoa defunctJapanesenuvistorLDM-810 GOOby usingthe chassis,case,calibratedtuningmechanismandcoils.Thevalueof the twin-gangedcapacitorturnedoutto be 120pFasshownin Photo2.

At a laterdate,G3Z0M usedthis circuitasa basisfora GOOkit and hespecifiedatwo-ganged365pF polyvariconvariablecapacitortogetherwith the speakerDINplugsascoil formers,shownin the circuitthat appearsin [2].

A GOOshouldpossessgoodsensitivity,which can becheckedbysqueezingthe coilof the GOObetweenthe thumb andforefingerand notingthe meterdeflection;this shoulddip to at least50% of the maximumreading.TheGOOshouldalsobecapableof measuringresonanceof a high-Qtunedcircuitatadistanceof 6 to 7cm (2-3in). I usea paralleltunedcircuit,comprisinga 5mH inductorwith a 100pFcapacitance,asa 7MHzstandard.Thecircuitshownin Figure2possessesthis sensitivityin the HFand lowerVHFbandsbut is unsuitableforVHFwithoutmodificationto somecomponentvalues.

Agoodindicationof the resonancedipis important.SomecommercialGOOshavesmallmeters,which area problemforopticallychallengedold-timerslike me.A largemetermakesusingthis instrumentmucheasier.TheG3WPOFETdip meter [5] uses an audioindicationofdip, which isverynice- ratherlike usinga metaldetector.

A QUESTIONOFVALUES.InanswertoMOPIB'squestionregardingthevalueof thevariabletuning capacitor,variouscapacitancevalueshaveadvantagesanddisadvantages.If a largevalueof capacitanceis usedthenfewercoilsarerequiredto coveragivenfrequencyrange.Thedownsideis that theresonancedip will bemoredifficultto locatebecauseof the largeoscillatorfrequencyshiftfor agiveamountofcapacitorrotation.If asmallercapacitanceis usedit givesthefrequencydial a bandspreadeffectandthedip iseasierto locate,howevermorecoilswillbe requiredto coveragivenfrequencyrange.TheQ-maxGOOusesquiteasmalldualganged40pFcapacitorandthefrequencycoverage1.5 to 300MHz requires8 coils.It doesmakefor a readablefrequencydialasshownon Photo3.

Buildingmanyitemsofelectronicequipmentcan belikenedto followinga cookingrecipe.Youcanusethe 'OeliaSmithmethod'whereyoufollowthe recipeexactlyusingthe exact

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PHOTO 2: A FET GDO

using the chassis,case, calibrated tuningmechanism and coilsof a defunct LDM-810nuvistor GDO.

PHOTO 3 : Frequency dial of the Q-Max GDO.

values of specifiedcomponents. Or

you can use themore interesting 'Jamie Oliver approach'where you throw in components whose values

or types you know from experience will do thetrick. In this way the design can be builtaround the components from the junk box.

The most important part of a dip oscillatoris the tuning capacitor, slow motion driveand if possible a read-out dial that can becalibrated in frequency. Sometimes a wholeassembly can be obtained from an oldtransistor radio. Choose a coil plug and

socket arrangement that is practical. Thecircuit shown in Figure2 uses 2-pin coils,which allows the use of crystal holders or

2-pin plugs and sockets for the coil.

JUNK BOXGDO.Ajunk box GOO,usingthe circuit shown in Figure2, is shown inPhoto 4. It uses a two-ganged 60pF variablecapacitor for tuning and a crystal holder forthe plug in coils. The coil socket should belocated as close to the tuning capacitor aspossible so that the coil leads can be keptshort. The rest ofthe circuit can be wiredaround these main components.

The arrangement shown in Photo 4 wasdesigned for a specific application, measuringthe resonant frequency of antenna elements.There are two main departures from thetraditional GOOdesign.

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FIGURE 1: Grid dip oscillator for 3 to 60MHz using a battery. (6)valve type 1A5G. Reproduced from the ARRL Radio Handbook, 1947.

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The first is the coil. Coupling a smalldiameter GOO coil to wire or tubular antenna

elements is almost impossible because thecoupling is very loose. G6XN suggested usinga coat hanger shaped coil where flat section

of the coil can be arranged in close proximityto the antenna element. While this is fine for

VHF this arrangement is very cumbersome

at HF frequencies because of physical siz,eof the coil. A more practical solution is

to wind the coil on a rectangular sectionusing several turns to make up the requiredinductance. This former can be a short

wooden board, say, 4 inches wide and halfan inch thick.

The coil shown in Photo 4 uses 7 turns

of 16SWG (or similar) plastic covered wire,which tunes from 8 to IBM Hz, coveringthe 14MHz band (the band of interest at

the time). The other departure from thetraditional GOO is the use of a frequencycounter instead of a calibrated frequencydial. A frequency counter is a worthwhile

investment and has many other uses inhome construction projects. Ifyou areusing a GOO without a connection for afrequency counter the frequency can besampled using single turn loop round theGOO coil. The coupling of this pick-up coilto the coil winding can be adjusted withrespect to the main winding until just

enough energy is available to operate thefrequency counter.

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FIGURE2: Circuitdiagramof a FETGDO.Operationof thecircuit isdescribedin thetext.

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Thecoilformerboardalsoprovidesaplatformfor the GOO,frequencycounterand evena notepad. The flat sidedcoilcouplesinto anyantennaelement,withthe boardprovidinga stablepointto restthe measuringkitagainstthe elementwhilemeasurementsarebeingmade.Thecouplingbetweenthe GOOcoil andthewire antennaelementis maintainedby placingthe wireantennaelementin the groveformedbyplasticwirefixingclips.This isa usefulfeatureif youarebalancedon top ofa step-laddertryingto measurethe resonanceofa parasiticquadelement.

REFERENCES[1] TheRadioAmateur'sHandbook,1947 Edition,ARRL.[2] TheAntennaexperimenter'sGuide,SecondEdition,

PeterDodd,G3LDO.[3] 'TechnicalCorrespondence',QSTJune 1972,

PeterLumb,G3IRM.[4] 'TechnicalCorrespondence',QST,November1971,

W1CER.[5] 'TheG3WPOFETDipOscillatorMk2',

byA LBailey,G3WPORadComApril 1987.

PHOTO 4: The antenna element resonance

measuring kit. Note the method of maintaining a

constant coupling to the antenna element (red wire)and the GDO coil.

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