Transponder antenna installations

I am installing a new vane style antenna for my transponder. My
original antenna was located where the cg hook used to be on my
discus. This location often tangled with the trailer ramp when
loading the glider. Next I tried a whip style on top of my instrument
panel cover. All of the self proclaimed experts said that was a
terrible place. Much to much power too close to the rest of my
avionics and electronics. I now intend to place it on the underside
of the fuselage as far aft of the main gear housing as I can reach.
My question is, does it have to be mounted near the centerline, I'd
rather not tangle with the overlap of the two fuselage halves. How
far off the center line can I go before the biannual inspector will
object or I will lose effectiveness? IT