Pole mount versus flush mount GPS and XM antennae

[Mark Redmon]

Hello,
I'm researching a Gamin GPS 17x antenna and GXM 51 antenna installation on a fiberglass hardtop on an Aquasport 275. Both pucks can be pole mounted or flush mounted. My girls like to jump off the hardtop, so I'd like a flush install. Is there a measurable signal degredation if I flush mount the GPS and XM pucks instead of using a short pole? I don't have a radar now, but will add an open array (taller than a dome) later, so there would not be interference between a pole-mounted puck and the radar beam.
Thanks,
Mark

[RickK]

The GPS signal from above is a very weak signal so it doesn't take much to disturb it beyond what an antenna can pick up.  I think all GPS Mfrs recommend putting it above the radar sweep.

[John Jones]

If you bottom fish and it bothers you that at low speeds when you are trying to find your spot and the gps starts jumping around and makes it hard to hit that spot then you want the antenna mounted LOW.  That jumping around is caused by the rolling of the boat and the higher the antenna is the more pronounced the movement will be.  The gps is sensitive enough that it thinks the boat is moving side to side when it is just rolling.

As far as radar, as long as the puck is not directly in the plane of radiation from the radar it will be fine.