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Two Small Helical Antennas for 2 Meters - w6nbc.com · PDF fileand turn spacing for VHF are measured in inches ... Two Small Helical Antennas for 2 Meters ... Monopole — Single vertical

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  • June 2011 1

    Helical antennas have long beenknown for their excellent proper-ties.Theyhavenotfoundmuchfavorin the ham community, however, perhapsbecausetheyaremoredifficult tobuild thanstraightdipoles,whipsorYagis.Fortunatelya less well-known, but equally high perfor-mance version of the helical antenna, thenormalmodehelix,isquitepracticalforhamconstruction. Best of all, it is impressivelysmall in size and low in profile. Figure 1showsa2meterbasestation/2normalmodehelix,only6incheshigh.InFigure2,weshowa/4mobilehelix,only3incheshighbothmadefromcommonhardwarestorematerials.It is also quite feasible to apply these prin-ciplestosmallhelicalantennasforHF.

    There is not just one kind of helicalantenna.Helicesexhibit severalmodes.Thetwomostcommonarenormaland axialmode.Manyhamsarefamiliarwithaxialmodeheli-calantennas.Thesearebigcorkscrewshapeddirectionbeams,oftenon longbooms.Theyuselargecircumferenceturns,wideturnspac-ingandalargegroundplaneatthefeedend.Gainisaxialalongtheboom,as themodesnameimplies.

    Thehelical antennas shownhereoperateinthenormalordipolemode.Circumferenceand turn spacing forVHFaremeasured ininchesinsteadoffeet.Iuseoneofthemobileson the roofofmy recreationalvehicle. It ismountedunderaradomemadefromaplasticfoodcontainer(seeFigure3).Beforeinstall-ing it, I kept knocking full sized 2 meterantennasoffofthetopofmyvehicle.

    AClassicalHamExperimentThese antennas are the result of a series

    ofham levelcut-and-pruneexperiments thatbeganonedaywhenIsimplywonderedwhatwouldhappenifIweretowindafullsized2meter/2dipole(38incheslong)intoasmallhelix.Andwhataboutdoingthesamewitha/4,19inchspike?Iknewthattheywouldbesmaller,butwouldtheyworkalmostaswellas

    theirfullsizedstraightcousins?To find out, I made five helices (1 to

    5 turns), each from roughly 38 inches, of14 inchaluminumtubingafull/2.Theseheliceswereallselfresonant.Themobilever-sion ishalfof this,/4of tubing. Imatchedall toa1:1SWRwithsmall inductive loopsasshown.

    According to John Kraus, W8JK (SK),normal mode helices (muchless than/2 in circumfer-enceandturnspacing)havegain perpendicular to thehelix.1Inotherwords,theybehave like dipoles. Youcan visualize the patternby imagining a full sizedstraight dipole lyingalong the center axis ofthehelix.

    To begin investigatingtheirperformanceasreplacementsforfullsizeddipolesandmonopoles,I usedEZNEC tomodel the radiationpatterns.2 Iwasamazedhow littlediffer-encetherewas.Allfivehadmuchthesamegainandradiationpattern.Figure4showstheelevationpatternsforthetotalfieldofthe2meter,twoturn/2helixcomparedtoafullsized lineardipole.Figure5showsasimilarcomparisonofthe/4version.

    So to answer my first question, a small/2helixhasonlyminimallylessgain(about0.4dBless)andslightlyshallowernullsthanafullsizeddipole.Thesameistruefora/4mobile monopole. Yet they have much thesameradiationpattern.Azimuthpatternsarenot shownsince theyareessentiallyomnidi-rectionalfortheverticalorientation.

    Thereisonemajordifference,howeverpolarization.Theelectric(E)andmagnetic(H)fieldsinasmallhelixtradeplacescomparedtoadipole.Thesameisalsotrueforasingleturncompact transmitting loop. Polarization is

    Two Small Helical Antennas for 2 MetersWith performance close to that of full sized antennas, these small helical antennas are low in profile but high in efficiency.

    JohnE.Portune,W6NBC

    1Notes appear on page 00.

    Figure2The114turnlowprofile2metergrounded/4mobilehelixonamodifiedNMOmount.Heightis3inches,diameter6inches.

    Figure1Thetwoturn2meterbase-station/2helix.Heightis6inches,diameteris6inches.NotethecoaxbalunmadefromseveralturnsofRG-58coax.Themastis12inchPVCpipe.

    rotated90.Ahelix,though,hasanadvantagehere.Itisalsosomewhatcircularlypolarized.Thedegreedependsontheheighttodiameterratio.Hencehowonemounts ahelix isnotascriticalasforalinearlypolarizedantenna.ThisisusefulonbandssuchasourVHF,UHFand10meterbands,inwhichbothhorizontalpolarization (forSSB) andvertical polariza-tion(forFM)areinuse.

    Thisisatwoedgedsword.Thegoodnewsis that itprovides the flexibility towork sta-tionsusing either verticallypolarized (FM)

  • 2 June 2011

    Figure6FreespaceEZNEC azimuthpatternofa2meter,twoturn/2helixatthehorizon.Thetotalfieldisinblue,verticalcomponentinred,horizontalgreen.Thefieldofthe/4versionisalmostentirelyverticalgoodformobilework.

    Figure3Groundedmobile/4helixontopofmyRV,mountedunderaplasticfoodcontainerradome.

    Figure4FreespaceEZNEC elevationradiationpatternsofa2meter/2verticaldipole(blue)andthetotalfield(red)oftwoturn/2helix.Notehowsimilartheyare.Thenullsareslightlyshallowerforthehelix.Gainsarenormalized,butonlydifferbylessthan0.5dB.

    Figure5Elevationpatternsofa/4verticalwhip(blue)andthetotalfieldofthe2meter/4mobilehelixaboveperfectground(red)typicalofthebodyofavehicle.

    or horizontally polarized (SSB, CW andAM)antennasequallywell.Thebadnewsisthateachisdownabout3dBcomparedtoalinearlypolarizedantennaof a single sense.The circularly polarized component is alsosuitable for the receptionof satellite signals.Figure 6 shows the free space azimuth pat-ternsofthe/2helixatthehorizonincludinghorizontalandverticalcomponentsaswellasthe total field. Interestingly, thedimensionsof themobile/2version result in the field

    beingalmostentirelycircularsplitevenlybetweenrightandlefthandcircular.

    LossesMysecondquestionwasefficiency.Will

    oneexperiencehighlosswithasmallhelix?As many hams know, small antennas oftenexhibitconsiderablelackofefficiency.Thisisduetolowradiationresistance.Forexample,this shortcoming is common forHFmobilewhips and shortened HF beams, in whichradiation resistance maybeonly a fractionofanohm. Ina full sizedantenna,however,where radiation resistance is closer to50W,efficiencyismuchhigher.

    Lowefficiencyiscausedbythefactthatanantennasradiationresistanceisinserieswithitsconductorandlossresistance.Theportionthatgoes to radiation resistancecreates theradio wave. That which goes to conductorresistanceiswastedasheat.Thereisnoexcep-tion.Togivesomenumbers, if the radiationresistanceofanantennaisexactlyequaltoitsconductorresistance,theoverallefficiencyisonly50%.Halfofthepoweriswasted.Intherealworldofsmallantennas,evengreaterlossisveryoftenthecase.Forexample,atypicalcoilloaded40metermobilewhipantennaisseldommorethan5%efficient.

    Fortunately, loss in small helices canbeminimizedbyobservingtwoprecautions:Use largediameter conductors of low

    resistancemetalscopperoraluminum,notsteel, brass or stainless steel. These metalshavetoomuchresistance.Keeptheturnsofahelixwidelyspaced

    comparedtoitsconductordiameter.Kraus suggests a turn spacingof at least

    fourtimesconductordiameter.Closerspacing

    increases skin effect loss.RFcant flowonthesidesofconductorsfacingcloselyspacedadjacent turns. This makes the conductorsseemsmallerandmoreresistive.Hence,bothantennas here have wide turn spacing, 12times the conductor diameter. In contrast,noticemyfiveturntesthelixshowninFigure7.Ithasminimumturnspacing.

    So the answer tomy secondquestion isthatsmallnormalmodehelicescancomparefavorably to full sized/2 dipoles and/4mobilewhipsinefficiency.

    HPdv9000Highlight

    HPdv9000Sticky NoteIs this correct? I suggest omitting it.

  • June 2011 3

    HamspeakAntenna analyzer Test instrument designed to measure the impedance and standing wave ratio (SWR) of an antenna or an antenna and feed line combination as a function of frequency. See www.arrl.org/reviews-listed-by-issue, look for May 2005.Azimuth pattern Plot of antenna radiation level as a function of azimuth angle around the antenna. Generally provided at a particular elevation angle.Bandwidth The difference between the highest and lowest frequency component of a signal waveform or highest and lowest frequencies passed in a system. In an antenna, the term band-width often refers to the difference between the highest and lowest frequency for which the SWR is below an application suitable value, often 2:1. Boom Structural portion of many antennas. Used to support the active elements and maintain their relative spacing.Coax Coaxial cable. Kind of unbalanced transmission line in which one conductor is a wire in the center of a dielectric with a circular cross section. The dielectric is surrounded by a tubular conductor, often made of flexible braid. In same cable types, the outer conductor is covered by a protective insulating jacket.Dipole An antenna often, but not always, center fed with two halves along the same line. Often refers to an antenna with a length equal to half an electrical wavelength. Often a reference antenna and also used as an element of multielement arrays. EZNEC Proprietary software program that performs analysis of antenna systems based on the Numerical Electromagnetic Code (NEC) antenna modeling engine. There are a number of ver-sions available from www.eznec.com, including a reduced size free trial version. Many of the antenna pattern plots used in QST articles are generated using EZNEC.FM (frequency modulation) An operating mode commonly used on ham radio repeaters.HF High frequency. That portion of the radio spectrum between 3 and 30 MHz. Often called short waves, these frequencies are characterized by long range propagation via ionospheric refraction.Monopole Single vertical antenna element, typically a quarter or more wavelengths long. Often used as a transmit and receive antenna, singly or in combination with other similar antennas.Radiation resistance Portion of antennas input impedance that results in transfer of electrical signal into a radiated electromagnetic wave. In general, the higher the radiation resistance, the higher the efficiency will be.RG-58/U coaxial cable Coaxial cable type with typically 50 W (some variants at 52 or 53 W) characteristic impedance and 0.195 inch outer diameter. Compatible with a PL-259 coaxial plug with the use of a sizing adapter.SWR Standing wave ratio. Measure of how w