I found the problem! Those with 396s might be interested in the
solution.

I setup the 396 w/ external antenna in the car and proceeded to drive
to the airport. Satellite signal was great for about 15 miles until I
arrived at an intersection in the city. All of the sudden, signal
strengths quickly dropped to zero and I got the "signal lost"
message. A half mile or so later, all was fine. Strange...

Continuing towards the airport, I arrived at another spot where the
signal dropped to zero. It was in an especially open area, so
obstructions were not the problem. I quickly pulled over. The signal
stayed at zero, and I looked around to see what would be causing it.
Sure enough, about a quarter mile away were two cell towers with a
clear line of sight to my vehicle. If I moved such that a building was
between me and the cell towers, signal came back. If I switched to the
"stick" antenna, signal came back as well.

So, the problem is the external antenna has some kind of reaction when
in close proximity and line of sight to certain cell towers. Just like
the kind of situation right after takeoff at CHD, and also in the
pattern. At high altitudes, it's not an issue, just in the pattern
near this area. This also explains why sat lock is rock solid in
cruise and out in the boonies.

I did further testing which confirms my theory. Seems I'll be
requesting a new external antenna from Garmin, or purchasing a 3rd
party external antenna. For now, I'll just use the "stick"
antenna. This was certainly unexpected.

--Dan


Dan wrote:
As a followup to my previous post on 396 GPS reception problems, has
anyone noticed GPS reception issues around the Chandler, AZ airport?
As soon as I leave the ground, I seem to always lose satellite lock on
my Garmin 396. I lose satellite lock like crazy in the pattern. Leave
the area, and everything is fine. Throughout 2 cross countries, not one
problem with the satellite. Get near CHD (and once on the ground at
FFZ) and problems are guaranteed.

It's not the on-board radios, since this has happend in 2 different
aircraft, and in one of them, the only radio in use was a GNS 430.
(430 did not lose sat lock).

Maybe there is some kind of interferance, although this pattern of
behaviour is very strange!

--Dan