No. 30: The V-Yagi

L. B. Cebik, W4RNL

In May, 1998, QST, NW3Z and WA3FET published an interesting 3-element beam for 40 meters, using one tubular element and two wire elements. The result was a light weight (for that band) beam with excellent SWR and F-B characteristics. It superficially resembled an old design by Dick bird, G4ZU, but had been optimized for performance resembling that of a Moxon rectangle: great F-B ratio, wide pattern forward, and direct coax feed. However, with an extra element, it had more gain. Why not adapt the design to 10 meter?

As Fig. 1 shows, the adapted antenna for 10 meters has decent gain and good front-to-back ratio across the first MHz of the band. The patterns are taken with the antenna modeled at 35' (1wl) up.

The SWR curve strongly suggests that direct coax feed is certainly in order. As always, I recommend a choke balun at the feedpoint.

Now that we have seen what the antenna can do, all we need to know is what the antenna looks like and how hard it might be to build. Like all Yagis, it will have significant side-to-side and front-to-back dimension. However, unlike the usual Yagis, only one of the elements will be made from aluminum tubing, while the other two will made from #14 AWG copper wire.

As the sketch shows, the driven element is the longest part of the antenna. Overall, it is 197" long, with a insulated mounting plate to permit direct connection of the feedline to the element. The inner parts of the element are 48" lengths of 1" diameter hardware store aluminum, while the outer ends are made from 7/8" diameter tubing from the same source.

The dashed lines crossing the driven element are booms to support the wire elements. They can be made from fiberglass or aluminum, with the center boom stronger, since it is about 12' long. The end support booms can be lighter, but need to support the wire ends. If the end booms are made from aluminum, they should be insulated from the driven element and set farther outward, so that the wire elements can terminate at an insulator. Simple UV resistant rope (3/16" diameter) connects the wire and end-support boom to the driven element ends, which helps prevent the wires from loading the end supports too much.

The wires are each 94.4" long from the center boom to the end (at an insulator or at the support boom). At the ends, they should be between 30 and 31 inches from the driver. Since both wires are the same length, we need to load one of then to electrically lengthen it to become a reflector. A shorted transmission line stub about 65.55ø long will do the job, although you may want to adjust the exact length when tuning up the antenna. The length in degrees translates into 50.125" of RG- 58/RG-8 (velocity factor 0.66) line. You can use standard stub equations to calculate the length of 300-Ohm or 450-Ohm line as a substitute.

The V-Yagi will not give all of the gain that a 12' boom is capable of giving if the elements were linear and fatter. In fact, this antenna is about 1 dB shy of maximum gain for the boom length. On the plus side of the ledger, the antenna is quite light, study, and has a smaller turn radius, since the corners are shortened by the slope of the director and reflector wires. A TV rotator should turn the beam with ease. However, you may not have to turn this beam as often as you might have to turn a standard Yagi, since the beam width is quite a bit wider.

The V-wire for 10 meters is not for everyone, not even for every addict of home brew antennas. However, I have learned over the years that different folks have different needs, different skills, and access to different materials. So I never try to prejudge what mechanical designs are acceptable and which are not. That would limit folks to only my own level of construction ability. Instead, I pass along ideas for designs, and let those who can make good use of them have at it. Others can pass up this design, hoping for a more suitable one in the next column.

Over the past 7 years or so, we have looked at a lot of antenna idea. And yet, we have only scratched the surface.

Updated 12-1-2000. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.

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