Gaz,
It is actually unusual for us to experience any significant issues with Mode C/S or ADSB reception with PAW. When investigated, most of the reported Mode C/S reception 'issues' turn out to be down to bad configuration, such as range or altitude filter settings set too tight or Mode C/S simply not switched on in PAW or the Nav system, or visible aircraft either not transponder equipped, flying with their transponder turned off, or flying below radar interrogation altitude or in areas not effectively covered by primary radar.
GA Mode C/S generally runs between 70Watts and 250Watts out at best
at the Transponder, with the Trig TT21 (an 'average' example) putting out around 125Watts in an 'ideal' installation and these transponders
only 'transmit' when interrogated by either primary ground radar or CAT / Military aircraft TCAS. ADSB on the other hand transmits its position and altitude automatically without any need for Radar or TCAS interrogation. Whilst GA ADSB operates with similar transmit power to its base transponder, CAT ADSB generally runs about 500Watts out
at the antenna, which is 1000 times the output power of PilotAware's own extremely reliable P3i signal.
Assuming you're fitting it to a metal bodied aircraft, or on a suitable 'ground plane', the PilotAware 1/4 wave monopole works fine for 1090 MHz reception, though you need to waterproof the connection e.g. with a smear of silicone or a suitable rubber washer. They aren't designed for external use in high speed aircraft, however, so if you have one of those and want a good external 1090MHz antenna, look at the ones sold for use with transponders, e.g.
https://www.gps.co.uk/harrys-hm109s-stub-transponder-dme-antenna/p-0-1894/ .
Any of these will work equally well for PilotAware 1090MHz reception. Just remember to keep it a sensible distance away from your own transponder antenna to avoid swamping (overloading) the PAW receiver.
Regards
Peter