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Antenna Grounding Plane Sizing and Method Question

PhatRV

Well Known Member
I am going to bench test my avionics and will test the transceiver and transponder operation outside of the airplane before permanently mount the avionics inside the airplane. I need to mount the various antennae on an aluminum sheet which will serve as grounding plane. What is acceptable size of an aluminum sheet for the transceiver antenna, and a transponder antenna?

If I don't have a large enough aluminum sheet, what is the acceptable alternative method such as cutting up strips of aluminum strips? And how long should each of these strips be? I read the aeroelectric book and saw a picture of these strips but it doesn't list the length.

Thanks
 
Well, in general, the larger the better. But you should try for one wavelength in radius. Now, for com, 120 MHz = 8 feet. On a real airplane, you won't get that in every direction. For the transponder, 1 GHz= 1 foot, that's easier. Now the good news: it doesn't have to be solid, as long as the "holes" are small compared to a wavelength. So you can mount either antenna on, say, a 6" square of aluminum. For the transponder attach a 2 foot square or round piece of metal window screen or similar, to the aluminum plate. For the com, run 8 or so pieces of wire radially out from the plate 8' or so if you can. Make sure the wire and or screen is electrically attached to the edges of the aluminum plate (good clamps should do). And of course, the coax shield must attach electrically (usually thru the connector) to the aluminum plate as well.
 
For your comm antenna, four wires, of whatever gauge you have in your junk box, at the base of the antenna. Each wire ~1/4 wavelength (so 23? - 24? long).

There is no advantage to use more.

The ground plane antenna is a 1/4 wavelength antenna, against a ground plane of at least the same length. Two wires for this ground plane is perfectly adequate for your testing.

For the transponder, a piece of aluminum perhaps 6? in diameter is fine.
Carl
 
Thanks for all the replies. I am glad building a ground plane is so simple without resorting a huge piece of aluminum sheet.
 
The dollar store sells two metal cookie sheets for 4 dollars. Drill a hole in the sheet and it works pretty well. Also doesn?t fall over.
 
Continuing with the cooking theme, aluminum foil is as good as a sheet or plate of aluminum electrically at 100 MHz. It can be quite handy for tests. Needs support, though.
Ron
 
The dollar store sells two metal cookie sheets for 4 dollars. Drill a hole in the sheet and it works pretty well. Also doesn?t fall over.

Just don't use your wife's (or mom's) cookie sheets -- they'll react poorly when they find a 1/2" hole in the middle...

:):):)
 
My ARRL Antenna Book says the size of the radials for a 1/4 wave antenna should be 5% longer than the radiator which, for 1090 MHz, would work out to 2.7". Doubling that for a metal disc would give you roughly 5.5" diameter. So you don't need a really large ground plane at transponder frequencies.
 
But don't confuse antenna radial with a ground plane, apples and oranges. Radials are used when you don't have a ground plane, typical with amateur radio operations. Otherwise you could not use a metal airframe, which is much bigger than 1/4 wave, for good radio transmissions. Just another situation where you shouldn't use amateur radio practices for aviation applications.

In this application, the cookie tray idea is fine.

:cool:
 
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