A Portable Yagi for 2M SSB/CW
Special to The ARS Sojourner
Even the mildest exposure to mountain VHFing reveals a need for a simple, durable, lightweight, and easily packed and assembled antenna, one that will fit in a rucksack. Cheap would also be a good feature.
While the amateur literature is filled with "designs" for 2 Meter beams that claim to fit these goals, few really qualify. But a recent column in CQ-VHF provided a design that was well researched and documented by a known experimenter. The groundwork for the antenna was presented at the 1994 Microwave Update (ref.1) with more information in the CQ-VHF "Antennas, etc." column titled "More Really Cheap Antennas." (ref.2)
An entire family of Yagis is described in this and the preceding article (ref.3) covering most of the bands from 144 MHz and up. He used wood booms (for good reason) with lightweight wire elements. Aluminum was used for the parasitic elements while the innovative driven element used 1/8 inch copper wire. The Yagi I built used only 3 elements, but Kent also described 4 and 6 element designs.
The unique feature of this antenna is the driven element, shown schematically in Fig. 1. Part A shows a traditional folded dipole driven element. The folded element transforms the 75 Ohm feedpoint impedance of a dipole to 300 Ohms in the absence of any parasitic elements. Britain reasoned that a transformation would also occur if only half of the fold was used, shown in Fig. 1B. The impedance with the half-fold would be unbalanced 150 Ohms. Parasitic elements depress the impedance to 50 Ohms. The result is a structure that can be fed directly with 50 Ohm coaxial cable. Merely connect the center conductor to the end of the folded element and solder the coax braid to the center of the main element at the "ground" point of Fig. 1B. No balun is required, which was confirmed in measurements that Britain performed.
The reader should refer to the original paper for dimensions and construction details. Presumably the original paper can be ordered from CQ-VHF.(ref 4)
Figure 2 shows my first Yagi of this type. This photo shows the antenna in use during the VHF-Sweepstakes, January, 1999, from Ghost Ridge in Oregon's Cascade Mountains. That's Roger, KA7EXM, holding forth on 144.2 MHz CW and SSB. The QTH was reached with a two hour trek on snowshoes. Fig. 3 shows the collapsed antenna, prior to being crammed into a rucksack.
Although the VHF-SS was a great success, the antenna presented a problem, apparent in Fig. 4 . When the copper wire is folded to fit in a pack, there is a tendency for the wire to twist where it goes through the wooden boom. But this is the feedline attachment point. This caused no problems for us, but may with further use. The antenna was rebuilt prior to the June VHF contest with the result shown in Fig. 5 The element and the feedline are now constrained by blocks of Delrin, an industrial plastic. I had some pieces in the junk box that seemed to cry out for such an application. (I have no more -- sorry.) Owing to the increased weight, this insulator became the support that included a U-bolt attaching to the mast. The Yagi boom attaches to the Delrin with a single bolt. The single wire half of the element is inserted through a hole in the boom. (The rubber band prevents the wing nuts from coming loose in the pack.) The complete antenna is shown in Fig. 6
A 10 ft mast is built from a stack of two poles from an old mountain tent. Each pole consists of three sections of 5/8 inch OD aluminum; a short 3/4 inch section joins the units. Fig. 7 shows a wood base built from 1/4 inch plywood, vital on the snows of winter! A PVC pipe-cap holds the mast. One of three plastic tent stakes is also shown, with the rope needed for guying. Also shown is a small triangle of aluminum that serves as a bearing and attachment point for guy ropes. A close-up is shown in Fig. 8
A complete view of the antenna and mast is shown in Fig. 9 This design differs slightly from the original; the folded element is now in the plane of the Yagi while it was at right angles to it in the WA5VJB antenna. Slight tuning changes were observed, but were not significant. The antenna was built and assembled on the mast and the impedance match was measured with a VHF spectrum analyzer, return-loss bridge, and signal generator. The driven element length was trimmed slightly to produce resonance at 144.2 MHz. (ref.5)
The total weight of this antenna is 3.5 pounds, including the mast, guy-lines, stakes, and hardware. We will probably replace this antenna with a 6 element version, which will increase the weight by another pound or so. Further refinement of the feedpoint is anticipated. The 3-element Yagi is aimed by rotating the entire mast, which works well when there is little wind. An extra rope determines the direction in a breeze, shown in the earlier winter photo with the extra line tied to a ski pole.
There are numerous opportunities available for the outdoor VHF enthusiast and we certainly urge you to include them in your own adventures with radio. See the web (ref. 6) for additional photos of portable VHF contesting from Ghost Ridge.
References:
Kent Britain, WA5VJB, "Proceedings of Microwave Update, 1994," available from ARRL.
Kent Britain, WA5VJB, CQ-VHF, October, 1998.
Kent Britain, WA5VJB, CQ-VHF, August, 1998.
The measurement method is presented in Hayward and White, A Spectrum Analyzer for the Radio Amateur, Part 2, QST, September, 1998, page 37.
Some of my photos are shown in http://www.teleport.com/~w7zoi/Hikes.html while some photos of the June VHF contest (99) are shown on Roger's site, http://users.easystreet.com/rhayward/9906vhf.htm.
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Wes Hayward, W7ZOI, is one of ham radio's most prolific authors and experimenters. He is also a legendary outdoorsman. Wes lives in Beaverton, OR.