Approach "Not recieving Transponder recycle..."

[gemiller]

Non-circular ground planes should work just fine. Circular will work just fine as well. The lobes in the elevation plane that Trig worries about are near zenith (straight up) if the antenna is mounted on top and near nadir (straight down) if the antenna is mounted on the bottom. The gain on the horizon is good regardless. This effect was evident in the radiation patterns I provided. The technical issues that I was having problems with dealt with "ground plane resonance" and avoidance of circular shapes. The fact remains that antenna locations for both experimental and production GA aircraft are not chosen to optimize RF link performance and the builders and manufacturers don't seem to care. I can live with that. But the pilot needs to be aware that because of the lack of antenna siting considerations he can get burned with poor reception at a most inopportune time -as has happened to me.

Thanks for following up with me.

 

[Canadian_JOY]

Gary - thank you for taking the time to provide such an excellent discourse on ground planes. In a former life as a vendor to Boeing and Airbus I was fortunate to have spent a bit of time with the engineering gurus and was able to absorb some valuable pieces of information. This post of yours is one of those which I will file away as a gold nugget.

I do have a question or two with respect to ground planes - I'm hoping you won't mind indulging me. I'm building Glasair Sportsman which is a composite aircraft which also has an internal 4130 steel tube "cage" to carry loads. The composite skin is a little over 1/4" thick and in the standard Sportsman is laid up using fiberglass and foam (not carbon fiber).

When installing an external transponder antenna and using an internal aluminum sheet ground plane, the antenna base is elevated from the ground plane. What is the impact on radiation pattern?

Since the internal cage structure is metallic, some builders have opted to use a cage mounting tab as the pickup point to mount a VHF comm antenna. The mounting tabs are about an inch in length. Again, what impact would this have on radiation pattern?

When using smaller ground planes such as the 120mm transponder ground plane as recommended by Dynon/Trig, would the data you have provided support the conclusion that such a ground plane results in energy being radiated within the cabin? This would seem an important consideration from a safety of exposure point of view.

Also, would you be able to provide guidance on that fancy conductive paint that has been used by Boeing to provide an external ground plane? I'd love to find a reasonable source of such a paint as it would make antenna installations on composite airframes much, much easier. There are some commercial sources of conductive paints... What would be a recommended maximum resistance value that would yield a useable ground plane?

Sorry if this is proving to be a torrent of questions. I've lost access to the cadre of antenna gurus with whom I used to work so any input you might be able to provide would be much appreciated.

 

[gemiller]

For the transponder antenna, just make sure that the base of the antenna has a good low resistance connection - less than 1 ohm - to the ground plane and that the resistance across the antenna connector is infinite.

For the VHF antenna the wavelength is 92 inches or so. I don't see that the mounting tab approach is doing much since it is so small compared to a wavelength. Thus, the geometry of the metallic cage is going to effect the radiation pattern significantly. One could, in principle, calculate this effect with FEKO or some other tool. But perhaps the better approach is to just use a large internal aluminum sheet ground plane and mount the antenna near the center of it.

The finite size ground plane will result in electromagnetic radiation getting into the cabin. Because the danger posed by exposure to RF radiation is from heating (the energy is non-ionizing), it is the average power that one worries about. The pulse width of the transponder transmit pulse is very narrow and the repetition rate is low such that the average power is very low. With the shielding of the small ground plane providing additional attenuation, I don't think it is something to worry about.

We used electrodag for the conductive paint. It is not cheap. You can google it and I think you will find it costs hundreds of dollars/gallon (it has silver in it). Using a simple ohm meter and separating the probes by a small distance - say 6 inches - one should read essentially zero ohms. If it is much higher, that would indicate the need to locally increase the thickness of the paint.

I hope this helps. Good luck with your project.

 

[Canadian_JOY]

Thank you for taking the time to respond to my questions, Gary. Your sharing of your knowledge is very much appreciated!

 

[RVDan]

There are also materials by Spraylat that we have used to provide groundplanes on composites.

 

[shenweas]

Hi all, Just thought I would give an updated and hopefully it can be useful information to someone here.

First off it looks like Gary is doing a good job educating us on antenna characteristics. I got my ham licence a while back but MAN! that antenna theory is hard for me to get my head wrapped around!

I included a drawing showing the location of my transponder antenna. The first location was the one that provided a dead spot in ATC seeing my reply. As best I can tell it was about 10 degrees either side of the nose.

This is an all metal airplane but it does have a nose wheel and I am assuming the nose gear (~10 inches ahead of the antenna) was obstructing the transmissions.

I moved the antenna to the location #2 and there seems to be not problems now.