"Dave" wrote in message
...
I assume that you mean the GPS receiver side of the unit (rather than
the FRS/GMRS 2-way radio). I'd guess that the performance would leave
something to be desired. But then that is
dependent on the type of airplane. I fly a Cessna high wing, and would
never be witthout an external antenna for my GPS. No doubt the built-
in antenna would work fine under a bubble
canopy. While on the subject, I'd also comment that trying to make a
non-aviation GPS serve any useful purpose in the cockpit is difficult.
Certainly it is possible to do so - but it's a lot
of work.

Some models work better than others. I had a yoke mounted Lowrance (in a
high wing) that would simply not work at all without using the external
antenna, however the 496 mounted in the same spot works reasonably well.
The problem you run into is if you make a turn, the signal may be lost
because the tracked satellites are now being blocked by the airplane, but
the 496 seems to recover pretty quickly.

Mounting location is also important. I know folks who have their Garmins
mounted above the radio stack and others who use the suction cup mount on
the front window where it wraps around the side, all on high wings and have
no problems using the attached antenna. I suspect the Garmins may acquire
new satellites a bit quicker than most.