Garmin 396 Reception Problems Solved!

On Tue, 18 Sep 2007 18:06:25 -0500, Mike Spera
wrote:

Well, as many of you can remember, I have posted my saga about
intermittent reception problems with my Garmin GPSMAP 396 from day one.

The Narco Nav/Coms or the splitters that are attached to them appear to
be the culprit (both the MK-12D AND Nav 122). I could always select the
harmonic freqs that would send the Garmin out to lunch, but it was not a
constant problem. Sometimes a freq that would tank the Garmin on one day
would be fine the next. This had me doubting the radio stack.

I found that minute changes in the antenna position on the glareshield
would alter the problem dramatically. Sometimes, moving it 1/4 inch
would do it. 8 sats up around 80% to nothing. Zip. Just by moving the
antenna a little bit. Even if the thing was only receiving it would
tank. Local area ILS freqs would bomb the unit (109.5). Turns out that
the default nav freq when you power up the Garmin will lose sat lock. On
the ground, it is fine. Take off and climb and the antenna will move a
wee bit due to the full throttle and high deck angle. Lost sat
reception, but not every time. If the antenna moves just the right way,
it will receive although at a MUCH lower signal strength and only 3 or 4
sats. Move it just a touch and it will degrade enough to lose lock,
maybe down to 0 strength on all sats.

I first thought that the problem of lost sat lock was due to LOW signal
strength and switched to a higher gain antenna. Switching to the higher
gain antenna actually made the problems worse. With it, many more radio
freqs tank the Garmin. Both on COM transmit and NAV receive.

It appears that mounting the antenna up high on the windshield may be
the permanent cure. I cannot justify the ridiculous cost of an external
antenna. $300- $400 for a twenty buck antenna is nuts.

I'll report back if this does the trick. Anyone know of a nifty little
"shelf" I can mount in the Cherokee up high? Maybe something sliding
into the windshield trim plastic?
Mike

We had a similar problem with a Skyforce II GPS which failed when some
NAV frequencies were selected. Checking the satellite signal strength,
on the GPS, confirmed it dropped to zero when the NAV receiver was
tuned to some frequencies. The GPS receiver was being blocked.

Whilst we do have Narco MK12D+ that was not the problem. The DME
(Narco IDME 891) is slaved to the NAV so the fault would clear when
the DME was turned off.

The fault was cleared by fitting an external GPS antenna but before
blaming the DME it should be noted that the Skyforce II GPS was the
problem. It is an old design and the receiver is not as good as the
replacement Skyforce IIIc which works very well.