Thanks Ian and Ritchard for your input it’s much appreciated, having looked at the Blade type transponder antennas they must be having a laugh with prices north of a complete rosetta,
being aware of the blanking effect of the engine and crew to radio transmissions i was trying to find a suitable place that had the least blanking, Unfortunately Ritchard my underbelly panel is very busy electrically so not suitable, so was looking at the lower fuselage towards the tail, I have a VHF antenna
on the stern post so that probably rules out the fin due to its proximity ?
will have to revaluate my choices with a possible internal antenna instead,