Transponder Antenna Ground Plane

[mmarien]

In the June 2011 Kitplanes magazine Jim Weir describes making a transponder antenna ground plane round or octagon of a specific size based on the 1090 MHz frequency wavelength. The SV manual describes the antenna ground plane for the SV-XPNDR-26X in a composite aircraft as being anything but a circle and to avoid multiples of the wave length.

Why is the ground plane for the SV-XPNDR-26X different?

 

[flybuddy]

Didn't read the article but it sounds like he is describing how to make ground plane antenna. I think the ground plane that Dynon is talking about is simply a grounding plate to be used with a normal external antenna as there would be no conventional ground in a composite a/c.

 

[mmarien]

Wouldn't they be the same thing. Both are for composite aircraft.

 

[dynonsupport]

We'll need to ask our RF expert about the ground planes. We'll do so and get back to you, but it may take a few days. As we understand it though, and as our manual states, the issue with small circles that are near to wavelength dimensions is that they become critical. I think if you get them off by just 1mm you can destroy their efficiency. The use of a large, non circular ground plane makes the dimensions non-critical. At least we all agree that some supported aluminum foil would work great!

Also, that article is a bit misleading. Not sure why you would build a $20 antenna when you can buy the other one for $18. Funny that a guy in an experimental magazine used the price of the TSO'd antenna when there are non-TSO'd ones for 1/5 the cost:

http://www.aircraftspruce.com/catalog/avpages/ted_transponder.php

The statement from our manual for posterity:

Antenna Ground Plane
When a conventional aircraft monopole antenna is used it relies on a ground plane for correct behaviour. For ideal performance the ground plane should be very large compared to the wavelength of the transmission, which is 275 mm. In a metal skinned aircraft this is usually easy to accomplish, but is more difficult in a composite or fabric skinned aircraft. In these cases a metallic ground plane should be fabricated and fitted under the antenna.
As the ground plane is made smaller, the actual dimensions of the ground plane become more critical, and small multiples of the wavelength should be avoided, as should circles. Rectangles or squares are much less likely to create a critical dimension that resonates with the transmissions. The smallest practical ground plane is a square around 120 mm per side; as the size increases the performance may actually get worse, but will be better by the time the ground plane is 700 mm on each side. Anything much larger than that size is unlikely to show significant further improvement.
The thickness of the material used to construct the ground plane is not critical, providing it is sufficiently conductive. A variety of proprietary mesh and grid solutions are available. Heavyweight cooking foil meets the technical requirements, but obviously needs to be properly supported.

 

[dynonsupport]

We got word back from our RF expert, and here's the simple version, which isn't much different than we thought:

A perfect circle ground plane is great at one frequency since all edges are the same distance from the antenna. It's awful at all other frequencies. Unfortunately, you need to get this just right, and with a transponder, we transmit at one frequency (1090 MHz) and receive on another (1030 MHz), so there is no perfect circle that meets both frequencies.

We actually like the octagon idea, because it varies the distance between the antenna and the edges, which gives it good performance over a range of frequencies, but so does the rectangle/square that Dynon recommends. We REALLY don't like the idea of a circle, and disagree with the Kitplanes article that an octagon and a circle would likely be equal performers. They are if you cut your foil by hand with a pair of scissors and don't get a lot of accuracy. They aren't if you go break out the laser cutter and make a circle with 1mil accuracy, like some people would inevitably do under the false assumption that accuracy was a good thing here.

Next comes the idea of the size. You really do want a ground plane that is either huge, or specifically sized to the frequency you are using. If 120mm is the right size, 240mm is really, really the wrong size. One of the issues here is that the structure of the plane can interfere and/or change the proper size. So unless you can test and refine the install, there's no easy way to say what the right size is, but "huge" is always a good bet. So we recommend 120mm (which actually varies a bunch because it's a rectangle) or greater than 700mm (which is just "huge").

Thus our recommendation that you not make a circle, and you make it either a specific size or big. It provides most of the theoretical efficiency of a perfect ground plane without the caveats of needing to get a perfect ground plane, well perfect. One of the worries is that you actually want a rough, "inaccurate" ground plane, which can be hard to convince a homebuilder of We really like Mr. Weir's hand cut, visually non-symmetrical, foil ground plane. That's what we'd call perfect for the application at hand.

Now of to the kitchen to get some aluminum foil, scissors, cardboard and double sided tape!

 

[mmarien]

Thanks for the great explanation. I'm having trouble with my transponder (GTX327) but now I think I narrowed it down to the harness. It works great in another aircraft and I've tried a couple of antennas and built a ground plane with aluminum tape that's rectangular but neither helped. I was going to use Jim's ground plane but I think mine matches the description in the SV manual so it's probably not the antenna. That leaves the mount or harness to investigate.

I know the SV-XPNDR-26X would solve my problems but would leave a hole in my panel  and my pocketbook.