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A Portable 2-Element Triband Yagi

November 200135By Markus Hansen, VE7 CAeveral years ago I entered the ARRLN ovember Sweepstakes CW contestin the QRP category, operating froma Portable location. It turned out to be avery frustrating experience with only3 W of output power and dipole anten-nas. After the contest I decided that thenext time I entered a QRP contest it hadto be with gain philosophy has always been to tryto keep life as simple as possible. In otherA Portable 2-ElementTriband YagiHave you everdreamed about a por-table beam you coulduse at your summercottage, while camp-ing or on Field Day?Dream no longer.

36 November 2001 trying different design parameters to de-velop a triband 2-element portable Yagi using wire elements. The basic concept comprises three in-

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Transcription of A Portable 2-Element Triband Yagi

1 November 200135By Markus Hansen, VE7 CAeveral years ago I entered the ARRLN ovember Sweepstakes CW contestin the QRP category, operating froma Portable location. It turned out to be avery frustrating experience with only3 W of output power and dipole anten-nas. After the contest I decided that thenext time I entered a QRP contest it hadto be with gain philosophy has always been to tryto keep life as simple as possible. In otherA Portable 2-ElementTriband YagiHave you everdreamed about a por-table beam you coulduse at your summercottage, while camp-ing or on Field Day?Dream no longer.

2 Thisportable beam can be rolled up and stashed in your car s ski boot!words, I look for the easiest way to ac-complish a goal that guarantees t get me wrong: Dipoles work par-ticularly well considering the time and ef-fort put into making them. But adding areflector to a dipole antenna increases theoverall gain about 5 dB, depending on thespacing between the elements . This ex-tra gain makes a significant difference,especially when you are dealing withQRP power levels. My 3-W transmittedsignal would sound like a power-house just by adding another piece ofwire! And it would be inexpensive Solar Cycle 23 in full swing,having an antenna with gain on 15 and10 meters also became a parameter was the sale of thefamily van, which meant the new antennahad to fit into the ski boot of our these constraints in mind, I useda computer antenna-modeling program,SFigure 1 Dimensions for VE7CA s 2-Element wire Triband 2001trying different design parameters to de-velop a Triband 2-Element Portable Yagiusing wire basic concept comprises three in-dividual dipole driven elements , one eachfor 10.

3 15 and for 20 meters tied to a com-mon feed point, plus three separate re-flector elements . The elements are strungbetween two (7-foot) long,2 2-inch wood spreaders, each just longenough to fit into the ski boot of the the lightest wood possible, such ascedar, pine or spruce to keep the totalweight of the antenna as light as poles would also work, or PVCpipe reinforced with maple doweling toensure they don t bend. (Wood has thebenefit of being easy-to-find and veryaffordable).Adding a reflector element relativelyclose to the driven elements lowers thefeed-point impedance of the driven ele-ment, so a simple hairpin match was em-ployed to match the driven elements to a50- feed line.

4 Figure 1 shows the lay-out and dimensions of the Hairpin MatchThe matching system is very simple andfoolproof. You should be able to copy thedimensions shown in Figure 2 and not needto retune the hairpin match, unless youplan to use the antenna in the top portionsof the phone bands. The dimensions inFigure 2 produced a very low SWR un-der :1 over the CW portions of all threebands. However, even in the lower por-tions of the SSB bands, the SWR doesn trise above 2:1. SWR measurements weremade at the end of a 25-meter (82-foot)length of RG-58 coax feed may wonder why I used such along feed line.

5 First, when operating froma Portable location it is better to be longthan short. Nothing is more frustratingthan finding that the coax you took alongwith you is too short. Further, when Ichange beam direction I walk the antennaaround the antenna support, thus requir-ing a longer length than if I went directlyfrom the antenna to the operating you are concerned about line lossyou can run RG-58 down to the groundand larger-diameter RG-8 or RG-213 tothe operating position. You may also findthat in your particular situation a shorterlength of coax will do. An 18-meter (59-foot) long piece of RG-58 has a loss ofabout 1 dB at 14 MHz, which is entirelyacceptable considering the convenienceof using coax the Hairpin MatchIf after raising the antenna the SWRis not as low as you want in the portionof the bands you plan to operate, firstdouble-check to make sure that all theelements are cut to the correct length andthat the spacings between the driven ele-ments and reflectors are correct.

6 Next youcan adjust the hairpin match. Connecteither an antenna SWR analyzer or atransmitter and SWR meter to the end ofthe feed line and pull the antenna up toFigure 2 Close-up view of the 3 At A, comparison of elevationpatterns for VE7CA yagi as a horizontalflat top (solid line) and tilted 45 fromvertical (dashed line). At B, comparisonsof azimuth patterns for a 10 height. Determine where thelowest SWR is on 15 meters. By movingthe shorting bar on the hairpin match upor down you can adjust the lowest SWRpoint to the middle of the portion of the15 meter band you prefer.

7 If your prefer-ence is near the top end of 15 meters youmay have to shorten the 15-meter drivenelement slightly. After adjusting the 15-meter element and hairpin match, adjustthe 10 and 20 driven- elements lengthsseparately, without changing the positionof the shorting bar on the hairpin hairpin match is very rugged. Youcan attach the feed line to it with tape,roll it up, pack the antenna away and evenwith the matching wires bent out of shapeit just seems to want to SupportAdhering to my constraint to keepthings as simple as possible, I only use onesupport for the antenna, typically a the antenna is raised to its operat-ing position it is a sloping Triband achieve this, attach a rope to each endof the 2 2 s to form a V-shaped sling, asshown in the Figure 1.

8 Attach a length ofrope to one sling and pull the antenna up atree branch, tower or whatever verticalsupport is available. Tie a second lengthof rope to the bottom sling and anchor theantenna to a stake in the ground. By put-ting in two or three stakes in the groundaround the antenna support, you can walkthe antenna around to favor a particulardirection. To change direction 180 , givethe feed line a pull and the array will flipover. So simple but very effective!Local or DXOne of the features of a sloping an-tenna is that you can adjust the take-off(elevation) angle.

9 For example, if you areinterested in North American contacts(whether for casual QSOs or the ARRLSS contest), then sloping the antennaaway sideways from the support structureat 45 with the feed point approximately8 meters (26 feet) above the ground, willyield a 20-meter pattern similar toFigure 3A. Here, the maximum lobe isbetween 10 and 60 in elevation. Thepattern of the antenna in a flat-top hori-zontal configuration at meters (30feet) is overlaid for comparison. You cansee that the tilted beam has better low-angle performance, but at higher angleshas less gain than its horizontal counter-part.

10 Figure 3B shows an overlay of theazimuth patterns for these two configu-rations at a 10 takeoff DX is your main interest, then youwant to position the antenna even closerto vertical to emphasize the lower eleva-tion angles. Figure 4 shows the patternNovember 200137on 20 meters when the antenna is tiltedsideward 10 away from vertical, againcompared with the other orientations inFigure 3A. The feed point is 6 metersabove ground and the model assumesfresh water in the far field, which is thecase at my Portable that the radiation patternis quite dependent on ground conductiv-ity and dielectric constant for a verticallypolarized antenna.


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