Comms ground plane design.

[blue up]

I've read through AC43 (?) and various bits of internet info on mounting comms antennas but have a quick question on ground planes. Since I have a curved wood/fabric fuselage, I need to mount the comm aerial onto a curved section of skin and wondered if the ground plane can be likewise curved? I'm using a 1/2 wave ant which is about 3 feet long rather than the normal 1/4 wave.
Any thoughts, websites or tips would be most welcomed.

http://i82.photobucket.com/albums/j279/foggythomas/fredinclose.jpg

[dixi188]

Many years ago I did a VHF install on a Wooden A/C. I think it was a Jodel.
We stuck some Kitchen foil around the area of the antenna and bonded it to the base, that worked as far as I remember.
Hope this helps.

[blue up]

Was that a 1/4 or 1/2 wave? I'm not even sure if a 1/2 wave NEEDS a ground plane.
I'm about 10 miles from the nearest twr frequency so I can't even test the aircraft from here, unfortunately.

[jxk]

As a suggestion you may want to consider the way Jodel/Robin type aircraft mount their aerials through the fuselage onto a flat surface beneath.

[dixi188]

Blue up,

IIRC it would have been 1/4 wave antenna. We put a piece of metal about 4" square above and below the plywood to attach the antenna to and stuck about 4 sq. ft. of kitchen foil on the inside.

It was back in about 1975 and I haven't been involved in radio installations since about 1977.

I've also seen the the installations referred to by "jxk"

[Fark'n'ell]

Blue up

1/4 Wavelength is all you need. At the middle aircraft band (127.00mhz) this is 590mm give or take a couple of mm.The ground plane needs to be twice this plus 2.5% (minimum) It does not matter if the ground plane is larger.There are other issues such as matching impedance due to the angle of the ground plane to the antenna but I wouldn't worry to much about that.
To calculate the length of your antenna use the following formula.

For half wavelength antenna 150 divided by frequency in Mzh
For quarter wavelength antenna 75 divided by frequency in Mzh

If you use a quarter wavelength antenna the curvature of the ground plane will not affect the impedance as much as a half wave antenna will.

Hope this helps, I could write several pages on this subject but if you would like some more information PM me.

[blue up]

The 1/2 wave antenna is already fitted to an ali plate that is only about 12 inches square and bends through about 120 degrees so it is late in the day to be changing the aerial length. Is a 1/2 wave able to work without a ground plane, as one site suggests (but not convincingly)?

[cone zone]

The ground plane is the other half of the antenna and radio performance would be severely effected without it.

I would supplement the existing plane you have with a sheet of thin copper which can easily be shaped. Do a VSWR check if you can, this will give you a good idea of the efficiency of your installation.

[blue up]

Any extra ground plane would be no bigger than the existing unless it was curver right down the sides of the fuselage since the fuse is only about 8 inches wide at that point.

[blue up]

Farrk'n'ell. I've sent a PM. Thanks! :ok: