The signal reception on my Garmin 420 is really dodgy.  We've tested the unit on a bench and it worked great.  We've swapped the antenna, no change. We swapped the antenna cable, it actually got quite a bit worse (the old one had intermittent issues, the new one is dodgy, nearly full time.)

If I route the cable 'just so', I can make it work fairly well on the ground with the canopy open, but it tends to go to hell when I close the canopy. 

If I unscrew and lift the glare shield about 1/2" - 1", the reception is great.  The GPS antenna is attached to the top of the glare shield on the co-pilot side.

I'm going to get some help from a shop in debugging this...but it seems to me that part of the canopy is either causing interference within the antenna cable, or the antenna itself.

Has anyone experienced anything like this before? This is in a 360.

Keith