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.
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.
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.
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