UpdateSo do you put the rubber duck antenna on the connector for the Quantum antenna, or disconnect the A6 from the aircraft wiring and fit it direct?
Is the radio connected to the intercom, and does the radio still splat if on the aircraft antenna but not connected to the intercom?
Update based on JCurtis pointers:
Inner/screen on coax measured, open with AVO.
Radio set to Headcorn, 122.210
Radio with rubber duck on its own battery, unconnected to aircraft in any way = perfect
Radio with aircraft antenna on its own battery, unconnected to aircraft power or intercom = badly splatting
Looks like antenna.
Second unattached radio with rubber duck tested as receiving Headcorn, radio with aircraft antenna just splatting at the same time, none or minimal RX.
Antenna cable inner and screen measured for continuity, now with two pairs of hands we can see the screen to the ground plane is intermittent sometimes, continuity at best.
Remove far end of cable, at termination with ground plane. Clean up and re-attach. Re-measure. Not intermittent.
Problem solved.
Nothing to do with the PAW. Unfortunately, a fault that appeared at the same time as the install, most likely because of disturbance.
Everything put back to intended config as per destructions.
Now working. SkyDemon integration tested.
Final test soon after everything is 100% tidied up and ready to fly. Ferrite beads on USB cable to PAW, and A6 DC power entry.
I have printed out the page with the location of L1 and L2, fault finding LEDs, and also the two pages of trouble shooting help in the manual. Both pages have been laminated and stored with the aircraft.
Suggest anyone with a Quantum do this with the main wiring diagrams too.
Big thanks to JCurtis for help.