A ground plane antenna for the VHF aeronautical band

Last month I built my first antenna. It was a ground plane antenna constructed out of a UHF female jack panel mount connector (sometimes referred to as a SO-239, like this one) and approximately 10-gauge clothes hanger wire.

A ground plane antenna made from a panel mount UHF connector and 5 segments of clothes hanger wire.

A ground plane antenna made from a panel mount UHF connector and 5 segments of clothes hanger wire.

I also used a male UHF to female BNC adapter and built my own 10 ft coaxial cable with BNC connectors. I used RG-58 and these BNC connectors because they don’t require an expensive crimp tool.

The reason I built this antenna is that for years the Saskatoon Soaring Club has struggled establishing reliable radio communications between their winch and their gliders. It’s important to mention that at Cudworth (CJD2), we use 123.2 MHz for communications. What was happening is that while the glider is on the ground, the winch can “hear” the glider, but the glider can’t “hear” the winch. A hand held radio is used in the winch and a more powerful radio permanently mounted radio is used in the glider. Testing revealed that it was likely that the radiation pattern of the antenna on the winch poorly suited for the needs of the club (more details below). The solution was to build this ground plane antenna, since they have the radiation pattern required by the club.

The old quarter wave antenna on the winch relied on using the metal roof of the cab as the “ground plane”. So long as the ground plane is sufficiently large (at least 1/4 wavelength in radius around the feed point of the antenna) the radiation pattern will be toroidal (donut shaped) and able to cover low elevation angles.Screenshot of spectrum analyzer At 123.2 MHz, a quarter-wavelength is approximately 60.9 cm (24 inches). Unfortunately, the amount of metal on the roof of the cab was insufficient (only about 20 cm). Theoretically this means that the resulting radiation pattern will be conically-tapered in elevation resulting in very little radiation being emitted at low elevation angles (low to the ground).  This document describes the problem in greater detail and shows that for real-world situations the radiation pattern is in fact more complicated than the naive description just given. In contrast, the ground plane antenna is built to include it’s own ground plane (see the 4 radials in the picture above). While searching for antenna solutions, I also stumbled upon the inverted L antenna, very cool.

Cutting the vertical wireAfter building the antenna, I needed to tune it. Thankfully, I was allowed to use a network spectrum analyzer at work to do this. The antenna started at a resonant frequency of 98.995 MHz (with the vertical wire at a length of about 30 inches). Of course, tuning the antenna to a higher frequency required me to shorten the vertical wire and the radials. It was interesting to shorten the antenna wire by 2 inches at a time and observe the resulting change to the resonant frequency measured by the spectrum analyzer. By carefully trimming the wires of the antenna, I was able to tune it to a resonate frequency of 123.7 MHz with a bandwidth of approximately 20 MHz or so.

Finally, the antenna was tested in by mounting it to the winch. This was accomplished using U-bolts, tape, and 5 feet of PVC pipe.

The ground plane antenna temporarily mounted on the SSC's winch for real-world testing.

The ground plane antenna temporarily mounted on the SSC’s winch for real-world testing.

Testing was performed by first attempting to communicate with a grounded glider using the old antenna on the winch and then mounting the new antenna and attempting communication. As before, using the old antenna, 2-way communication could not be established, but when using the new antenna the problem was resolved!

The antenna works well and club members are very happy to be able to now communicate more easily with the winch. This has made the winch launching operation safer too.


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