Author Topic: Ground plane for External Antenna in a Jabiru  (Read 325 times)

Etheav8r

Ground plane for External Antenna in a Jabiru
« on: February 02, 2021, 09:36:43 pm »
I am about to embark on the installation of a new PA Rosetta kit with external antennas.  I was thinking of using self adhesive copper sheet 0.1 mm thick for the ground planes for the two PA antennas.

My first question is does the Ground Plane have/need to be flat or is curved OK?  There are no flat surfaces of any size in a Jabiru SK 2 seater that are usefully accessible.

My second question is does it matter if the antennas point at an angle rather than vertically down as it is impossible to install them in the centre of the underside of the fuselage, so the have to be offset to the side, which is curved.

My third question is if I use separate ground planes for each antenna, is there any benefit in linking them with a soldered wire connection (and then possibly also linking the Trig Transponder antenna ground plane as well)?

Many thanks

exfirepro

Re: Ground plane for External Antenna in a Jabiru
« Reply #1 on: February 02, 2021, 11:43:02 pm »
Hi E,

I am about to embark on the installation of a new PA Rosetta kit with external antennas.  I was thinking of using self adhesive copper sheet 0.1 mm thick for the ground planes for the two PA antennas.

The first thing you need to appreciate about antennas is that a basic transmission antenna consists of two matching halves - each 1/4 wavelength long and mounted in a straight line in opposing directions from the centre point, where the (co-ax) feed line attaches. This type of antenna is known as a ‘dipole’ and radiates effectively in a donut pattern through 360 degrees all round the antenna.

This isn’t, however, very practical in an aircraft, (though we can and do use dipoles as ‘internal’ antennas in non-metal bodied aircraft). It is, however, generally better to use a 1/4 wavelength ‘radiator’ outside the body of the aircraft and replace the other ‘half’ of the antenna by a ‘Ground Plane’, which acts as a ‘mirror’ and ensures that the maximum transmission from the antenna is at a low angle off the side of the antenna (360 degrees all round). Without this ‘mirror’, transmission would (at least in theory) be straight off the end of the antenna. In a metal-bodied aircraft, the outer skin acts as the ground plane. In non-metallic aircraft we have to provide an artificial ground plane.

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My first question is does the Ground Plane have/need to be flat or is curved OK?  There are no flat surfaces of any size in a Jabiru SK 2 seater that are usefully accessible.

In an ideal world, the ground plane should be flat and of sufficient size and with the antenna mounted perpendicular right at its centre, The further we deviate from this ‘ideal’ the greater the potential for the transmission pattern to be skewed from the perfect 360 degree horizontal plane, though in practice this isn’t usually critical - especially as at the frequencies we are using, the ground plane only needs to be about 10cm across as a minimum.

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My second question is does it matter if the antennas point at an angle rather than vertically down as it is impossible to install them in the centre of the underside of the fuselage, so the have to be offset to the side, which is curved.

Most aviation antennas should in practice be as near vertical as possible, as if they are 90 degrees out from those on ‘the other aircraft’ each will lose 3dB (i.e. effectively half the signal). But remember your aircraft and the other aircraft will not stay level at all times in flight, so in practice the relative orientation will vary in any case. Just try to mount the antennas in a position where they will be as near to vertical as possible in straight and level flight and, more importantly, where they will have as unobstructed view as possible through 360 degrees around your aircraft i.e. away from things like chassis legs. If you can’t get the ground planes flat, make them a bit larger, say 30cm square - (square is better than round, though again not critical) and centred on the optimal position for your antennas derived as above.

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My third question is if I use separate ground planes for each antenna, is there any benefit in linking them with a soldered wire connection (and then possibly also linking the Trig Transponder antenna ground plane as well)?

Many thanks

Probably not worth the bother and could introduce other issues due to ‘Ground Loops’. In practice it’s more important to ensure that there is a good connection between the ground plane and the outer braid of the coax, which will be the case as long as the shaker washer on the antenna mount is in good contact with the metal ground plane inside the aircraft. Each ground plane will then be connected independently back to the transmitter / receiver via the coax. Of more importance is keeping as much distance as is reasonably practicable between the antennas, especially between the PAW ones and your transponder antenna, to minimise potential interference to the PAW 1090MHz Receiver from the high power transponder signals.

Hope this helps. If not, or if you have any further questions, feel free to come back to me.

Regards

Peter
« Last Edit: February 03, 2021, 08:45:51 am by exfirepro »

Etheav8r

Re: Ground plane for External Antenna in a Jabiru
« Reply #2 on: February 03, 2021, 02:21:48 pm »
Peter

Thank you so very much for such a clear and detailed response. 

I could of course use aluminium flat sheet for the ground plane but understand copper is better and copper sheet of 1mm thick or more is rather expensive, but if it makes a difference and is a better bet than flexible copper foil, then I am sure that I can devise something..

Edmund

exfirepro

Re: Ground plane for External Antenna in a Jabiru
« Reply #3 on: February 03, 2021, 03:07:37 pm »
Hi Edmund,

The thickness of the ground plane isn't critical - neither should the material be as long as it's conductive. If the self-adhesive copper foil you mention is easy to obtain, I'd go with it. It should certainly be easy to fix neatly to the inside of your aircraft bodywork (presumably GRP). If need be, don't be afraid to cut 'slits' into the sheet from the outside edge(s) and allow the 'flaps' to overlap to help it lie flat. As long as each 'flap' remains connected electrically to the centre where the antenna is mounted all will be well. Adhesive between the layers should have no effect as long as the whole piece is electrically connected.

Quick tip, if the bodywork is GRP (which I'm pretty certain it is), it can be beneficial to 'prime' the surface first with a thin coat of 'Evo-Stik' type adhesive and let that almost dry. It etches itself into the plastic and then makes a much firmer bond with the adhesive on the metal foil, making it much less likely that it will come away later. (I have used this tip extensively for permanently attaching 'self-adhesive' Velcro inside Flexwing Pods, where you don't want it to let go later.)

If in any doubt about what you are doing, please speak to your inspector.

Best Regards

Peter

Etheav8r

Re: Ground plane for External Antenna in a Jabiru
« Reply #4 on: February 03, 2021, 07:05:09 pm »
Thank you again Peter.  All clear and I feel confident to proceed.  Yes it is GRP (Epoxy).