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A Horizontal Loop for 80-Meter DX - davekalahar.com

30 August 2002By John S. Belrose, VE2 CVIntroductionIn 1997 the author published an articleon vertical full (and ground plane typehalf) wave loops for 80 meter In thatarticle it was noted that perpendicular( Horizontal ) polarization is the preferredpolarization, particularly at low elevationangles, since horizontally polarized wavesare hardly affected by the finite conduc-tivity of the ground in front of the exception when vertical antennas comeinto their own is a vertically polarized an-tenna over very good ground, near the sea-shore or over alkaline salt practical 80 meter Horizontal dipoleis, however, not an ideal antenna for optimum communications with distantstations the antenna s radiation patternshould have a null overhead, to minimizenear vertical incidence sky-wave signalsfrom atmospheric noise and interference.

30 August 2002 By John S. Belrose, VE2CV Introduction In 1997 the author published an article on vertical full (and ground plane type half) wave loops for 80 meter DX. 1 In that article it was noted that perpendicular

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Transcription of A Horizontal Loop for 80-Meter DX - davekalahar.com

1 30 August 2002By John S. Belrose, VE2 CVIntroductionIn 1997 the author published an articleon vertical full (and ground plane typehalf) wave loops for 80 meter In thatarticle it was noted that perpendicular( Horizontal ) polarization is the preferredpolarization, particularly at low elevationangles, since horizontally polarized wavesare hardly affected by the finite conduc-tivity of the ground in front of the exception when vertical antennas comeinto their own is a vertically polarized an-tenna over very good ground, near the sea-shore or over alkaline salt practical 80 meter Horizontal dipoleis, however, not an ideal antenna for optimum communications with distantstations the antenna s radiation patternshould have a null overhead, to minimizenear vertical incidence sky-wave signalsfrom atmospheric noise and interference.

2 And a low angle lobe to maximize recep-tion/transmission over paths to distantstations. To achieve such a pattern with ahalf-wave dipole it would be necessary toinstall the dipole a half wavelength abovethe ground, that is to say, at a height of 40meters for the 80 meter band. This is im-practical in many full-wave Horizontal loop for the 80meter band at a practical height of 15meters is a popular antenna nicknamed a Loop Skywire that has been in TheARRL Antenna Book for years. In theauthor s view this antenna does not havethe desired radiation pattern for 80 meterDX. Aside from the fact that the directand ground-reflected waves reinforce atan elevation angle of 90 , the loop itselfhas some directivity in this broadside di-rection.

3 Doug DeMaw has referred tosuch an antenna as a cloud warmer. A Horizontal Loop for80-Meter DXWorking DX on 80 meters doesn t necessarily require big towers ortrees. This 80-Meter quad loop system requires only supports ofmodest height a better single-element antenna may be hard to appear on page 1 (A) Top view of the DJ4 VMloop; (B) Top view of a quad loopshowing the transposed phasinglines; and (C), showing the systemfeed line and the coordinate system(note the antenna is shifted in the +Ydirection for clarity). Note: Feed linesare not to scale in A and Carr, N4PC proposed a solutionfor this He fed diagonally op-posite corners of a square loop with equalbut oppositely phased currents.

4 For a full-wave loop this produces a null in theoverhead radiation pattern, akin to thetime-honored W8JK array a pair ofclosely spaced dipoles fed out of 80 meters N4PC used a 3/4-wavelengthloop, which had the desired elevation pat-tern, but with corner feed the azimuthalpattern is skewed compared with the loopmodeled by the to the author s simulationusing W7EL s EZNEC Pro version of thenumerical electromagnetic code NEC-4D, the antenna s impedance at the inputto a transmission line feeder of practicallength, is not a particularly convenientvalue to tune and match. N4PC did notcomment on this. In fact, he did not in-clude the phasing and the feeder trans-mission lines in his model, and so hecould not comment on the input imped-ance of his antenna system although hereported no problem with tuning andmatching his antenna on all bands 80through 10 meters.

5 In this article the author uses numeri-cal simulation to address the radiationcharacteristics and the tuning and match-ing details of a symmetrical full wavequad loop designed specifically for 80meter 200231A Horizontal Square Loop with aW8JK-Like Radiation PatternW. Bolt, DJ4VM, described a multi-band vertical quad loop with both of thevertical sides fed in-phase by means of a phasing line. 4 This symmetrical feedarrangement (Figure 1A) has the advan-tage of ensuring a symmetrical currentdistribution on the loop and hence a clean radiation pattern over severalbands (40 meters to 10 meters).Our interest here, however, is a hori-zontally polarized loop operating in W8JK mode.

6 Rotating the plane of hisloop 90 , we now have the horizontalloop, in which the system feed line con-necting at the center of the phasing linescan drop vertically at right angles to theplane of the loop (see Figure 1C). Thesymmetry of this loop arrangement is author decided to carry out a de-tailed numerical modeling study for thisloop fed with a balanced, transposedphasing transmission line (opposite sidesof the loop fed out of phase). The hori-zontal full wave loop, /4 or 20 meterson a side for MHz, at a height of 15meters over average ground, is numeri-cally modeled as three separate cases:Case 1) Out-of-phase sources placedat the centers of the sides of the loop (thesides parallel to the Y-Z plane);Case 2) Wires are added for the con-ductors of a 600- phasing line betweenthe opposite sides with the source placedat the middle.

7 (Note: The conductors forthe transmission line feeding the side ofFigure 2 Principal plane elevation (A) and azimuthal (B)patterns for the antenna of Figure 1B at a height of 15meters over average 3 Horizontal quad loops showing currents on the loop wires for the casewhere the phasing lines provide in-phase feed (A); and out-of-phase feed (B). Note theleft phasing line is the transposed feeder. The frequency is loop in the X direction are trans-posed to provide the out-of-phase feed;see Figure 1B.)Case 3) Case 3 differs from Case 2only in that a system feed transmissionline is added, with the source on a jumperwire between the transmission line con-ductors at its bottom or transmitter end(Figure 1C).

8 All wires are no. 12 copper. The spac-ing for the conductors of the transmis-sion lines is 150 mm (for an impedanceZo = 600 ). The length of the transmis-sion line feeder is 14 reason for the three-case modelingsequence is to be sure that the transmis-sion lines included in the model are per-forming correctly. We find that they areby computing the feedpoint impedances ofthe two models with different NEC-4D input impedance for the Case3 antenna at MHz is + j462 .For the Case 2 antenna, fed by a 600- open wire transmission line 14 meterslong, the input impedance is + j464 according to TL, the transmission line pro-gram by N6BV and published by the source impedances are, ofcourse, different for all three configura-tions, radiation patterns are substantiallyidentical.

9 The maximum gain is decreasedslightly with the addition of the transmis-sion lines as would be expected becauseof the high SWR on the transmission lines(see below). At MHz, the gains in theprincipal plane (the Y-Z plane) dBi, dBi and dBi, for cases1, 2 and 3, respectively. From this point,all discussion will be of the antenna sys-32 August 2002 Table 1 Impedance at the Junction of the Phasing Lines for Out of Phase Feed(W8JK-like Mode)FrequencyImpedance ( )GainTake-off(MHz)(dBi)Angle ( ) j + j + j of Case 3 and system impedance will refer to the impedance at the trans-mitter end of the common feed principal plane radiation patternis shown in Figure 2 at a frequency MHz.

10 For interest, the pattern is alsoshown at MHz, and MHz. Thecalculated antenna system impedancesand radiation characteristics (gain andtake-off angle, , are given in Table 1).We are concerned here with the radia-tion pattern for 80 meter DX. Clearly theradiation patterns shown in Figure 2 arealmost ideal: a deep overhead null, and abidirectional pattern with a take-off angle(41 ) that is low for the practical height(15 m) of the loop. The azimuthal patternis also good (for an 80 meter antenna) witha front-to-side ratio about 10 , at MHz, conductor loss inthe system feeder transmission line at-tached to the junction of the phasing lineswill be an important consideration becauseof the high SWR.


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