[Lefty]

The users hand and body act as the ground plane. you just have to accept that it is not very efficient.

You n=might often see the "old hands" - place the handheld radio on or near a car - as that acts as a brilliant ground plane / reflector.

[SteveX]

The solution is to hand straighten the aerial. Then tie a copper wire counterpoise to a metal plate underneath it. Go flying with an SWR meter and try it. Land. Cut 1cm off said wire. Go flying and check SWR again. Repeat until best SWR obtained. Then refit new length of wire (since it will have become too short passing through that peak).

That should get some debate going on here................

[Kemble Pitts]

Flying John

Ref the co-ax connector and braiding. With the cork gasket in place there is no electrical path between the antenna and the ground pain.

The ground plain acts as a reflector. It does no harm if there is a connexion between the antenna and ground plain (maybe via the fixing screws) but this is incidental and it is not actually required.

It's also worth noting that most antennae bases, including inside the mounting holes, are anodized aluminium which renders them non-conductive. Unless you undo all of the good anti-corrosion work of this anodizing and the nice paint job on the fuselage (not a good idea) you're not going to achieve a low resistance connexion anyhow.

I have approved (as a DOA) several aircraft modifications with this 'characteristic' and it works every time.

As I said, not my original theory but provided to me by antenna specialists at HR Smith, who know a thing or two.

... and I've not met an avionics LAE who believes it!

[Cantabrigan]

The significant bit about the connection to the ground plane is the impedance - which at VHF is not the same as the d.c. resistance that you measure with a multimeter.

If the ground plane and the base (or connector) of the antenna have overlapping surfaces separated by a thin insulator then it acts as a capacitor. If the area is big enough and the insulator thin enough then the capacitor's impedance is low and it acts like a short circuit at VHF

So, subject to any quirks of the particular radio design, it doesn't need a d.c. connection from the antenna shield to the ground plane. But it does need a low impedance.

Note - this about a typical monopole antenna, not a dipole.

[Kemble Pitts]

Cantabrigan

Thanks for that explanation, explains why you don't need metal to metal contact - which eliminates the need for an antenna-shaped corrosion hole in the aircraft!

[flyingeeza]

So I guess all the radio designers who stipulate in their manuals that you need a good electrical connection to a ground plane at the antenna base are all ignorant.

[Rob L]

Ground plane:

If the Foxbat is a derivative of the Kitfox, then it has a steel-tube fuselage. Use that as the ground plane (it works on Cubs and similar-era types).
Rob

[Ian Melville]

Ground plane:

If the Foxbat is a derivative of the Kitfox, then it has a steel-tube fuselage. Use that as the ground plane (it works on Cubs and similar-era types).
Rob

IIRC the area in question is a 6061 sheet aluminum and as far as I know not a derivative of the kitfox.

[Rob L]

Ah, but put the monopole aerial on top where the wing attaches to the fuselage and use the wing/fuselage bolt to earth a rather large ground plane et voila!

(I use an old-style ELT whip aerial for radios on the three Taylorcraft I have owned...ne'er a problem!)

[Flying_john]

I realise that this is now anecdotal as I no longer have the test records, but years ago at the advent of Cellular radio in the U.K , I was deeply involved with a company called Panorama Antennaes in South London. They had a test site where there was a complete car body mounted on the roof of a building several hundred metres from their lab which was mounted on a turntable, for all intents and purposes it was in "free space".

We were able to mount different aerials and different configurations of aerial on various parts of the vehicle body ; transmit through them and using a spectrum analyser and RSSI measuring equipment measure the radiated field, both at hi band VHF, Lo band VHF and 900Mhz cellular frequencies. Now the best bit was that you could then rotate the whole thing, car and aerials, in sync with a plotter in the lab and and produce a polar plane diagrams.

We tested loads of different designs, 1/4 wave, half wave, 5/8, co-linears, you name it we tested it. We also tested the effect of electrical bonded ground planes v non bonded, to study the effect of not connecting the braid of the co-ax to the conducting surface of the car. In all tests the effective radiated field was reduced without an electrically bonded ground plane. Thats not to say it did not work when it was just an ungrounded plane, but it was less efficient.

I respect other aerial manufacturers opinions but would like to see their evidence that it is better to decouple the braid of the feeder from the metal ground plane.

[jamespearce]

Ground plane:

If the Foxbat is a derivative of the Kitfox, then it has a steel-tube fuselage. Use that as the ground plane (it works on Cubs and similar-era types).
Rob

Different animal Rob. The Aeroprakt A22L Foxbat (from the Ukraine) is structured very differently, most of it using aluminium, with the exception that the flat wing and elevator surfaces are fabric covered.

[Kemble Pitts]

flyingeeza - it would seem so...

Flying_john - as I noted in my original post 'at these frequencies...'. Cellular phones, depending upon which Generation, are up in the UHF bands where you will need an electrical connexion between antenna and ground plain.

Rob L - Foxbat is no relation of a Kitfox.

[Flying_john]

both at hi band VHF, Lo band VHF and 900Mhz

Yes we tested at Lo Band (70Mhz) Hi Band VHF (153 - 172Mhz) and Celluar (900Mhz)

[PaulSS]

I enjoy the technical side of these discussions because I seem to learn quite a lot from those experienced in varying fields. I nodded along with the theory that the Foxbat's antenna should be vertical and even agreed with the reasoning but then realised that I fly with a very similar aerial set up to the OP's and do not have the problems he has. Yesterday I experimented with all sorts of directions for transmitting and receiving and nobody noticed any difference.

I considered that my antenna may be actually more horizontal than the OPs but I don't actually think this is the case. When I'm in the flying attitude I reckon the long bit of the antenna points up at about the same angle as the Foxbat's.

This is in an RV4 (so it's metal, as per the Foxbat) and the antenna looks very similar. Also, coincidentally, the transponder aerial looks to be in a similar location to the Foxbat's and I have no problems on that score either.

I am not going to argue that making his antenna vertical will not solve the problem but my experience, albeit in a different aircraft, is that there may be other gremlins afoot.

[Paul_Sengupta]

I think the problem isn't so much that the antenna is bent back like that, but that it's bent back like that plus it's on the sloping bit at the back so that it's effectively shielded by the rest of the fuselage and engine in the forward direction.