GPS loss of signal explanation

I can tell you with 100% certainty that when I put the suction cup antenna
in the center of the windshield above the Narco stack in our 172 and
transmit on 119.4, I lose all satellites. This has happened to me with my
old III Pilot and also my Garmin 196. If I put the antenna in the very
lower left corner away from the radio stack it seems ok. It is just that
plane.

In my specific instance it happens not departing, but when I switch to tower
frequency and transmit when landing. Usually right after I call the field
in site and they switch me to tower. Two separate Garmin units, two
different antenna's. It requires a power on and off.

I can also count on losing all satellites when flying right over the
approach control radar antenna on the KLN90B in the panel on another
airplane and it requires a power cycle. It only has happened at my home
airport.



I'm not an EE so I can't comment on this guy's conclusions, other than
to say that the Garmin installation manuals for panel-mount GPS units
specifically warn against running the GPS antenna cable near nav/com
radios. My avionics shop told me that modern radios are sufficiently
shielded, but obviously, some older radios are not. Garmin uses the
same active antenna system for panel-mount as they do for portable.

I can also say that I have KX170B nav/coms and have never had a problem
with interference on the 396.



JKG