The transponder fin antenna must be 3? from the COM antenna on the belly at the main spar. Under the baggage on the RV4 satisfies this criteria. Garmin says you need 3? minimum RG400 to the fin antanna. Rather than run all the RG 400 from the avionics shelf, why not put the transponder in the baggage bay with 3? of antenna cable ? Not trying to save cable $ but rather minimize cable exposure and corruption of other low power conducters. Comments appreciated.