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The Field Day Special Antenna

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The Field Day Special Antenna NOrcN'9 AELIEVE WE'RE Trythe CARRYIK A HAM STATION "FD Special" AN9 ilO.EL ARCAf] 4!l Antenna l- l> .. Lookingfor an antennathat's simple, inexpensive,lightweight and easy to install?Here's one that fits the description. By RoyW. Lewallen,*WTEL "juices" hile the Field Day were want the nuisanceof establishinga decent thelower curve of Fig. l. It showsthe effect - flowing last year, my Field Day ground system which most vertical of losseson thegain Qosses don't affectthe partnersand I decidedto become arraysrequire. front-to-backratio, and changeonly the more competitivewithout compromising The theoreticalgain and front-to-back scalingof thepattern). Fig. 3 showsthe pat- our generalphilosophy: Use all homemade ratio of 2-element arrays with terns of arrays with 135, 160 and gear,pack it in to the site,and don't let l/8-wavelength spacing between the 18O-degreerelative spacing. All aredrawn operating interfere with watching the elementsare shown in Figs. I and 2. Note to the samescale. The 1,/8-waveleneth- scenery.Since most of our gearalready ran the acceptedQRP powerlimit of l0-W dc input and had been designedto provide high outputefficiency, the antennaseemed like the reasonablepoint of attack(our high qualitysuperheterodyne receivers have not o beenthe limiting item). The constraintsdic- -4 tatedthat an antennabe lightweight,por- = tableand easy to put up. It shouldalso have o5 substantialgain. <2 5 TL LOSS/ EL€MENT Becauseof our WestCoast location, the 3 o front-to-backratio wasnot a concern.But havinga reasonablywide lobe toward the Eastwas. We desireda low SWR because r30 r40 150 160 i70 of the relativelyhigh lossof our RG-58/U ANGLEBETWEEN ELEMENT CURRENTS (OEGRE€S) and/or RG-174/U feed line. We werein- terestedonly in the CW portion of the band,but thisantenna works well over all Fig. 1 - Curvesthat show gain versus phase angle for two-elementarrays with 1/8-wavelength of theband. It seemedthat 20meters would spacing. be our main "money-maker," so we designedthe antenna for that band.It can be scaledfor otherbands. too. The Research Although a number of antennatypes 30 might havedone the job, I settledquickly on a horizontal,close-spaced, driven array. Experiencehas shown that driven arrays are generallymore tolerant of imperfect t5 constructionand erectionthan are parasitic F arrays.Experience and much measurement io haveconvinced me that horizontalarrays 5 outperform vertical ones in the high- i frequencybands, except perhaps from an o rJo 140 150 t50 170 exceptionallocation. In addition,we didn't -5 ANGLE BETWEEN ELEMENT CURRENTS( D€6REES) '5470 S.W. 152ndAve., Beaverton,OR 97007 Fig.2 - TheoreticalF/B ratio for a two.elementarray with 1/S.wavelengthspacing June1984 21 r_ry** is that, again with spaced, 135-degree-fedarray is frequently overlooked) difficulty "ZL phasedelay of calied the Special."' The close-spaced, only a few exceptions,the terminated 1SO-degree-fed array is known as an current in an imperfectly '(8JK."' From 135to 160degrees, phasing transmissionline doesn't equal the elec- line. This effect isn't was chosen becauseof the combination of trical length of the phasing designed relatively high insensitivity to Ioss, minor: The of a casually of degrees. reasonable gain and wide forward lobes' array can easily be off by tens 8O-degree Note that the gain stays about the same in In one design I investigated, an produced of phase shift. this range, ensuring good performance if line 139 degrees the phasing isn't exactly as predicted. Ac- The Solution tually, it's much easier to generate and maintain precise 180-degreephasing than There are a number of approaches the angles I've chosen - particularly over toward correct feeding of an array. My a wide frequency range. choicewas to investigatesome simple feed There's one major flaw (usually fatal) in systemsto seeif any would yield resultsthat a simple analysis like the one presented came closeto the desiredcharacteristics' I here: It assumesthat equal-magnitude cur- wrote a computer program that would rents are flowing in the elements. This is solve, iteratively, for element-current Fig.3 - Dipole array patternsfor 135, 160 to realize, for even in arrays with magnitude and phase angle, plus feed-point not easy 180-degreerelative phasing at )J8 spacing and for this one tlpe of array, given elements spaced % wavelength or greater, CurvesA, B and C, respectively,represent impedances mutual coupling has a profound effect on these conditions.Add 5 dB for dBd. These the array specifics.Several configurations element impedances. This changes them curves are based on the arraybeing fed with looked promising, and one of the simplest eoual currenls. dramatically and unequally, as a rule. This proved adequate.This wasan array oftwo impedancechange is a function of not only folded dipoles that were self-resonant, "mutual impedance," but also the relative spacedl,/8 wavelengthapart and connected by a taut pieceof 300-ohm TV ribbon with magnitudes and phases of the currents o r.zv flowing in the elements. In an array as F one half twist. The feed impedance was spaced as these, coupling is so in- closeto 50 ohms resistive.There was some closely z timate that it could be argued that the term inductive reactancethat could be corrected "driven artay" is a misnomer. For exam- 5 r.ro by adding two small-valuecapacitors at the point. The elementcurrent ratio was ple, the feed-point impedances of the 2 feed elementsin a l,/8-wavelength-spacedarray, = I . l3: I , with elementphasing that was 154 can be made to degrees.This was not the 124degrees one assuming equal currents 1.OO flow. are 14.3 r4.35 might expect from the 56 electrical degrees FREQ, ( MHz ) - assuminga velocityfactor of 0.8 (A) of line Phase Angle Loading Lagging - minus the 180degrees caused by the half Between Currents Element Element twist. (Degrees) (Ohms) (Ohms) It was this array that we used for Field 150 Day, with very good results(see section on 135 28 - i46 28 + j46 160 L3 - i22 13 + i22 performance). However, when the array 180 9+j0 9+j0 z was reconstructed at the home QTH, a was noticed when a t45 dramatic rise in SWR I than the low This showsquite a changefrom the 74 + operating the antennaat other end of the band. Computer analYsis j0 ohms value that eachelement exhibits zs UC frequency, whenit is not coupledto anotherelement' showedthat, abovethe design parts 33 tco the phase angle increased. This caused a The fact that the resistive of the two- F z feed- elementimpedances are equal, and the substantial lowering of the element E plus of the reactancesare equal in magnitude,is a point impedance, narrowing peculiarityof the particularelement spac- forward lobe. The analysisalso showed the the ing chosen.For otherspacings they will be antenna to be well-behaved Delow a similar unequal, and the reactancescan be dif- design frequency. Consequently, (figuratively speaking) ferent in magnitude,as well as in sign. array was designed gavegood resultsover the This mutualcoupling isn't undesirable; ior 14.5MHz. It Fig.4 - Elementcurrent ratio (A) and phase phasing varies in fact, it's essentialfor obtaininggain in 20-meter band. Element angle(B) as a functionof frequency. 14.0 MHz to 148 the presenceof rather severe pattern from 135 degreesat with current ratios cancellationthat is commonin theseclosely degreesat 14.35MHz, (seeFig. 4). The gain spacedarrays. The lowerimpedances cause from 1.04to 1.13:l from more elementcurrent to flow for a given high reactance makes them tricky to feed can be calculated as fairly constant power input, therebyincreasing the fields properly. 4.5 to 4.6 dBd acrossthe band. Again, the impedance can be cor- from the elements.In thesearrays, the in- Why do these different and reactive feed array feed-point provide low SWR. The creasedfield strengthis sufficientto com- impedances make feeding the arrays so dif- rected easily to a the pensatefor the fact that the fields from the ficult? The first problem is that, with few calculated patterns for the antenna at 2o-meter band elementsdon't add in phasein any direc- exceptions, the magnitude of current out top and bottom endsof the take into tion. They partially or completelycancel of a line not terminated in its characteristic are shown in Fig. 5. These phasing, instead. But, the lower feed-point im- impedance won't be equal to the current account the changes in element "If pedancesmake them more sensitiveto into the line. In classic you can't fix it, spacing, current magnitude and element iosses,and the low resistancewith relatively feature it!" fashion, this impedance- self-impedancewith frequency. transforming property is put to good use Construction in the form of the %-wavelength Q quality 'Notes appearon page 24. section.s The second (and almost always The antenna is made from 22 osr- .qnf||t F- 32'r' l- I I 300-oHM TV a'5-3/4" TWIN LEAD I t. t- vI OIR€CTION OF MAXIMUM RAOIATION ?p 50-fL COAXIAL FEEOLINE SIMILAR TO FIG,7 {A) Fig. 5 - Calculatedantenna patterns for the wITHOUT 8NC CONN. AN0 33O pF high and low ends of the 2o-meterband. Add 5 dB for dBd. 300-ohm TV line to the dimensionsgiven in Fig. 6. Sketchesof the insulatorsare pro- vided in Fig. 7. They are made from scrap piecesof epoxy-glassPC-board material. This results in ruggednessand minimum FEEo LINE (8) weight. The spreadersare readily available \ lGfoot lengthsof "l inch" (l-5/16 in OD)' - schedule40 PVC pipe. The capacitors are ,"",\0J". used only to provide a good match to 50-ohm feed line: They don't otherwise affect the performance of the array. Small - 500-V mica or monolithic ceramic units Fig.6 Electrlcaldimenslons for the WTEL array(A).
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