Posted for Bob <rbelshe@attbi.com>:

I had my Garmin antenna mounted on top of the stack for quite a while
and it worked just fine, until I discovered that tuning my COM radio to
certain frequencies (119.9 was one of them) would completely blank out
the satellite signals.  The bar graph showing signal strength would drop
like a rock as soon as the COM was tuned to this freq.    Fixed by
putting the GPS antenna inside the fuselage behind the baggage bulkhead
at the rudder cable fairlead.  Bob Belshe
Lancair 235/320  500hrs.

[ what comm were you using? -Rob@Logan.com ]