Hello Paul,
OK being in Oz - we are suffering the depths of winter at the moment and the temperature here in south Lincolnshire is forecast to drop to about -5 degrees Celsius tonight!
The aerial certainly looks like a colinear-type and the magnetic-base electrically couples the earthy-side to the groundplane by capacitance. In this instance there is no need for a physical electrical connection, so just attaching it to a steel sheet is sufficient. The effect of the groundplane is exactly the same whether the aerial is for transmitting or receiving. The only real difference is that if it's used for transmitting, the electrical matching of the aerial (high VSWR) to the transmitter may be bad enough to damage the transmitter output stages. That's why you've seen comments that differ for transmitting and receiving aerials. For a receive-only setup, although the effect on radiation pattern and electrical matching is exactly the same, the receiver will still work but not optimally. It's surprising what you can use for an aerial to receive radio signals, but when you compare what you are receiving on a piece of "damp string" to a properly configured and tuned aerial there is usually a vast difference.
As for the GAV-868 aerial, I think it must be a shortened 1/4 wavelength - the website doesn't give much away other than the height of 70mm which is far too short for a 1/4 wavelength at 868MHz. A 1/4 wave at 868MHz is about 86mm uncorrected. That would mean it needs a groundplane, so yes, mounting it on an alloy groundplane is perfect. It appears to be designed to mount directly onto a typically metal airframe.
The GAV-868 seems expensive for what it is. A standard 1/4 wave aerial cut to the correct length (about 82mm corrected) would work just as well if not better than the GAV-868 (larger aperture), but then the GAV-868 can be fitted with no configuring and it's guaranteed to work out of the box.
You mention mounting the aerials within a couple of feet of each other, but I'm not sure about the minimum separation required between the transponder aerial and the P3I one. A transponder transmits typically 100 or 200 watts or so depending on model and class and I'm not sure if the P3I RF input stages could handle this sort of power close by repeatedly, so does anyone else reading this (Maybe Keith?) have any thoughts about aerial location/separation?
