G8JNJ

Block diagram of Ground Station

Which will take in 432MHz and output 2400MHz (plus images, so it needs a filter).

The conversion gain at 2400MHz is right on the edge of the mixer performance, but the output level can be increased by diverting the signal via the LO output amplifier stage.

Then a 5 turn helix which gave approx 4.5dB better performance.

The Helix was copied from G6LVB's AO40 S-Band design Modify an Anologue Sky TV Dish for AO

Helical feeds - read the notes on optimisation inc. axial ratio, beamwidth and feepoint adjustment  http://www.g6lvb.com/60cm.htm

I think it would be better to use a larger diameter outer support tube around the helix acting as a radome.

The helix reflector plate could probably be made a bit smaller, but it didn't seem to degrade the RX S/N by any appreciable amount, so I didn't bother chopping it down. I used PCB because I had it to hand, but aluminium, brass or whatever would be OK. 

The underside of the 1/4 wave impedance matching line one the first turn of the helix has got self adhesive Mylar foil on it so that it doesn't short against the body of the N connector, when it is bent close to the reflector in order to achieve the best Return Loss.