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Antenna mounting on a wood or composite aircraft

Am I right that any such antenna e.g. the usual type

needs to have a ground plane under it, and the base of the antenna needs to connect to the ground plane?

What are the suggested ground plane dimensions?

If the ground plane is missing, what would happen? Would it just have a bad range?

This post doesn’t relate to a TB20.

Administrator
Shoreham EGKA, United Kingdom

Hello Peter

I have made my own antenna for my experimental plane wood.

A double dipole with balum, made with copper foil
The advantage is that I can put it inside the vertical stabilizer
While adjusting SWR (reflected power) <3 (I have 1.2 SWR)
Use a good cable
You have to put a ground plane, or dipole with balum, to avoid problems

antenna with balum similar…

calculate ground plane.
http://www.csgnetwork.com/antennagpcalc.html

Last Edited by celtico at 01 Apr 14:37
pasion por volar
LEVX CERVAL

needs to have a ground plane under it, and the base of the antenna needs to connect to the ground plane?

Yes.

If the ground plane is missing, what would happen? Would it just have a bad range?

Then the cable shield would most likely act as a counterpoise.

You’d probably end up with these effects:

  • higher VSWR, leading to less output power from the transmitter
  • skewed radiation pattern
  • possibly more radiation in the cabin and in the avionics panel, potentially leading to EMI issues with avionics equipment. A sheath current choke might help against that.
LSZK, Switzerland

I use copper tape as the ground plane and it works well. My advice was “the bigger the better” so I have one around 18 inches by 18 inches.

Forever learning
EGTB

Yes, a ground plane is required and it should have a good connection to the braid of the coax feeding the antenna. Minimum sizes I heard mentioned are some 10-15 inches squared. However most composite builders prefer a dipole made of two strips of adhesive copper, on the fuselage inside. One Mr. Jim Weir was a particular advocate of this technology, but it was also recommended in the construction manual of the Europa kitplane.

EBZH Kiewit, Belgium

A good rule of thumb is to have a ground plane with a radius, which is at least the antenna’s length. Larger ground plane is no problem, smaller can cause issues as indicated by tomjnx.

Or build / install a dipole antenna inside as celtico suggest.

JP-Avionics
EHMZ

The TT21 transponder manual says the following, the quoted 120 mm at 1090 MHz is on par with the calculator linked in a post above. For VHF aeronautic radio frequencies such rectangular ground plane would be of no practical size, if the plane is not already metal – that’s why the dipole is used more.

The smallest
practical ground plane is a square around 120mm per side; as the size
increases the performance may actually get worse, but will be better by the
time the ground plane is 700mm 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.
LRSV, Romania

While checking for an intermittent radio problem, which had appeared after 14 years without problems, it seemed the ground plane had never been connected. After re-assembling, radio is now OK, but I don’t find it any better than before.
Our Trig TXPDR dipole is inside the wood fuselage.
PS I was just the tool-fetcher during the investigation.

Maoraigh
EGPE, United Kingdom

I dipole is certainly a good idea in a plane with a nonconducting fuselage. I wonder however how one is going to comply with nonionising radiation regulations.

LSZK, Switzerland

I dipole is certainly a good idea in a plane with a nonconducting fuselage. I wonder however how one is going to comply with nonionising radiation regulations.

That’s what I was thinking. The stuff will go right into the cockpit, and it’s not exactly low power.

It’s also going to play havoc with avionics. GPS is already easy to wipe out – 11th and 13th harmonic of VHF, etc.

Mesh would be better than tape because it can be glued on with resin and won’t come off, whereas tape could come off and jam something. Especially if there is oil / ACF50 around the inside surface.

Administrator
Shoreham EGKA, United Kingdom
60 Posts
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