So I finally got round to installing a fixed PA into my Foxbat. The foxbat battery is under the P1 seat and with my proposed aerial locations under each seat it made sense to keep the whole installation local to this.
Wired the power supply direct into the battery via the aircraft master battery cutout and then it's own fused switch to a 12v to 5v convertor with a micro usb output to feed 3A to the Pi. (It's an old classic which actually fits better as it's smaller than a rosetta). Switch mounted between pilots legs giving excellent access to turn it off in flight if needed. Heat shrink crimped terminals and heat shrink solder joints were used. Negative cable from convertor taken straight to battery.
I did buy a cheap GPS mouse off ebay but it didn't seem to want to work and in the end found that the standard GPS dongle on a short usb extension lead stuck on the the door post low down by the pilots seat seems to work perfectly getting excellent satellite lock even in the hanger .
A standard transponder antenna - I used this one from Mendelssohn; A SMA Female to MCX Male Right Angle Pigtail 50cm RG316 Cable and a SMA Male To BNC Male Convertor connects the pigtail to the antenna and the other end is the correct fitting for the dongle supplied with the PA (Rosetta users this may be different).
I drilled a hole in the floor being careful to keep a good distance from anything structural and having talked the location through with my inspector beforehand. Clean off the paint around the inside of the hole to give a good connection for the groundplane and add a large washer I found at home with the smaller serrated washer supplied with the antenna against the metal plane body.
PA 968 Mhz Aerial:
From information on this forum it seems a standard flarm antenna is close enough to the PA frequency to work well so I bought this which comes with cable and the correct connector for the PA Fitted into a hole in the exact mirror location under the P2 seat with another large washer added.