Garmin 396 Reception Problems - Latest Observations

In my previous posts, I described what appeared to be random signal lock
losses with my new 396. Changing everything did not help. I got the
dealer to swap the unit with a new one and I upgraded the software to
the latest level.

It appeared to work well for over 2 months, however, it started fritzing
out again the other day.

This time, moving the GPS antenna DID make dramatic differences. So, I
was content on knowing that shifting the antenna back a few inches when
it went out to lunch would make it spring back to life.

However, I had read that certain nav/com freqs (even just receiving)
would give the thing fits. I already knew that constantly transmitting
on 123.0 would hack the GPS signal to zero after 6-8 seconds of
transmitting. But today, I discovered that simply tuning to 109.5 to
view one of the the local runway's ILS would almost immediately lose all
sats. Zero. As soon as I switched to some other freq, bingo. Immediate
solid reception. My #1 Nav/Com is a Narco Mk12D. Others have reported
these radios giving certain GPS receivers fits. It tried my #2 Narco
(Nav 122) and it did not appear to affect the GPS that much if at all.
Both navs receive from the same antenna with a splitter on it.

So now what? Some suggested that the Narco nav/coms were not properly
shielded. While others speculated that the interference is coming
straight from the antennas for the nav/com. How do I shield the box in
the stack or the GPS antenna to see if it makes a difference? What
material(s) will work?

Thanks,
Mike

Make SURE your antennae connections are good. Wiggle them and see if
you can make them fail. Then tape them TIGHT. Put the antennae
somewhere where it has clear view of the sky (skylight is ideal). Move
the antennae around and see if it makes a difference in your strength
bars.

Try using an antenna mounted on top of the airplane if you are trying
to use some type of internal antenna you ARE at going to have
problems.

With the antenna located INSIDE the airframe you have 'wave guide'
conditions allowing all types of radio RFI that extends well into the
Microwave region exceeding the GPS signals

Don't be cheap and Stop messing around ! have an external antenna
mounted they cost about $ 187 ( RAMI AV -GPS [aircraft spruce] )
takes about an hour to install. Route the External GPS Antenna cable
to a BNC bulk head antenna connector located on the instrument panel
then a short patch cord to the '396

If the airplane is a rental tell the owner to have one installed. It is
NOT unreasonable in this day and age of GPS portable navigation
devices that Rental aircraft provide the external antenna.

On 19-Nov-2006, Mike Spera wrote:

So now what? Some suggested that the Narco nav/coms were not properly
shielded. While others speculated that the interference is coming
straight from the antennas for the nav/com.


There is little chance that interference from a NAV radio would emanate from
the nav antenna -- it's more likely coming from the radio itself. Any
receiver has a lot of internal oscillators that can mix with one another to
create interference in all sorts of frequency bands. If designed well, all
of this RF "crap" should be retained in the receiver through proper
shielding, but not all radios are created equal (que the Narco bashers).

Short of an external GPS antenna, what I would try is to shield the antenna
as much as possible. If you place it on the glare shield, try putting a
shield, in the form of several layers of aluminum foil, between the antenna
and the glare shield. Play around with the shield a bit with the "worst
case" radio nav/com tuning you have found.

-Elliott Drucker

As other posters have noted, an external antenna will probably help. But
GPS operates at normal signal levels that are below the noise threshold.
Fancy software and hardware extracts the GPS signal.

Your Nav or Com is a superhet radio using a local oscillator (small
transmitter) to beat with the incoming nav or com signal and create an
intermediate frequency. The local oscillator (LO) has harmonics that extend
into the Ghz region, raising havoc with your GPS receiver. So even with an
external antenna you might still have problems. You might try an amplified
antenna, mounted away from your nav-coms. (most remote GPS antennae contain
an amplifier that will get its power from the 396). It will boost the GPS
signal.....and if mounted far away from the navcoms, the amplified antenna
might raise the S/N ratio high enough to allow GPS operation. Go for a unit
with at least 15db of gain, but remember the longer antenna coax will drop
the signal a few db (depending on quality of the coax cable).

Regards,
John Severyn
@KLVK

Mike Spera wrote:
It appeared to work well for over 2 months, however, it started
fritzing out again the other day.

What else changed?

Some suggested that the Narco nav/coms were not properly shielded.

This might be true for the case where you just tune something in (the
ILS or a particular comm frequency) but don't transmit. As others have
mentioned, there is a low-power local oscillator running in the receiver
all the time.

While others speculated that the interference is coming straight from
the antennas for the nav/com.

This is more likely to be true in the case where the GPS doesn't drop
out until you transmit on a particular frequency for a while.

How do I shield the box in the stack or the GPS antenna to see if it
makes a difference?

Just moving the antennae further away from one another is one way. It's
probably easier to move the GPS antenna further away, since it's a new
addition and its cable isn't buried in a harness somewhere. As far as I
know, most GPS antennae have built-in amplifiers, so moving it another
few feet from the GPS receiver isn't that big of a deal. If you're
buying a new GPS antenna, look for one with a thicker coaxial cable -
this is _usually_ an indication of a better-shielded cable.

For shielding the box in the stack, sometimes it helps if you can narrow
down where the interfering signal is coming from. It could be radiating
right out of the box itself, or it could be sneaking out along the
antenna, power, or mic/headphone leads and radiating from them. If
tuning in the ILS will kill the GPS on the ramp, troubleshooting will be
a lot easier. Assuming that you can reproduce it on the ramp, you might
try it with all the headsets and PTT switches unplugged. If the GPS
comes back, start looking at the audio wiring. If all the audio wiring
is tip-top, you _might_ be able to install a filter on the audio inputs
and outputs on the radio.

If the GPS stays dead with the audio stuff unplugged, if you can reach
the connector, and if this is something a mortal in your situation is
allowed to do, you might reach up behind the nav/com radio and unscrew
the antenna connector. (Don't try transmitting with the antenna
disconnected, but receiving is OK.) If that brings the GPS back, start
looking at the nav/com antenna cable and especially the connector. Some
of these connectors are very massive and the shield conductor should be
soldered with a 1.21 gigawatt soldering iron, but it instead gets
soldered with a generic 30 watt pencil, leading to bad connections. Or,
the shield on the coaxial cable can simply get damaged over the years.

If that doesn't help, it might be coming from the power leads, but that
is somewhat harder to rule out. You can make sure the power connections
are clean and tight, but beyond that, it's "try it and see" - like
rigging a small battery right next to the radio for testing, or adding
a filter across the power and ground lines.

For the "box itself" case, (again, if mortals are allowed to do this),
you can take the box out of the panel and look it over. Make sure all
the cover screws are present and tight and make sure all of the covers
are making good contact with one another. The physicists will tell you
that it is possible to add fingerstock and jumpers to make sure the case
parts are really solidly connected to one another, but the lawyers may
object. There is probably a ground connection in the multi-pin
connector, the antenna shield is probably also grounded, and the radio
may get a bit of a ground through its mounting. You can at least make
sure all the fasteners that hold the radio into the panel are present
and tight, and it may be possible and helpful to add a braided ground
strap from the radio case to the airframe.

Disclaimer: This is based on experience with two-way radios, GPS
receivers, and audio equipment in ground vehicles and in fixed
installations. I don't have an A&P; I don't even have a TG&Y. Some of
this may not be allowable owner maintenance. Your mileage may vary.

Matt Roberds

"Mike Spera" wrote:

So now what?

Make sure all the radio antenna connections are sound. If that doesn't fix
it, replace the radio. I had the same problem with two Cessna ARC-385s at
different times, first the top one, then a few months later the bottom one.
Swapping them for units off the avionics shop's shelf fixed the problem each
time.


--
Dan
C172RG at BFM

Mike Spera wrote:
In my previous posts, I described what appeared to be random signal lock
losses with my new 396....

Been covered lots of times: Here is a link to the earier thread on this
topic:

http://makeashorterlink.com/?Z16D35513

Mike Spera wrote:
In my previous posts, I described what appeared to be random signal lock
losses with my new 396...

You have three solutions:

1. Get rid of the 12D

2. wire the "keep-alive" pin on the 12D direct to the fused always-on
"clock" circuit in your airplane. (see #3, below)

3. Always program a non-offending frequency into your NAV receiver
immediately after turning on the avionics master. (By default, the 12D
defaults to 115.5MHz if the keep-alive is not connected. This is ONE of
the Nav frequencies that clobbers the GPS)

Mike Spera wrote:
In my previous posts, I described what appeared to be random signal lock
losses with my new 396. Changing everything did not help. I got the
dealer to swap the unit with a new one and I upgraded the software to
the latest level.

It appeared to work well for over 2 months, however, it started fritzing
out again the other day.

Mike,

I took my 396 through another hard reset procedure as Garmin requested,
and that had some effect. Not necessarily more satellites, but those it
did receive were much stronger. Probably coincidence, because if the
almanac is corrupt (as Garmin suggests), it would look for the wrong
satellites (which it wouldn't find) and the signals for those sats would
be at or near 0.

This time, moving the GPS antenna DID make dramatic differences. So, I
was content on knowing that shifting the antenna back a few inches when
it went out to lunch would make it spring back to life.

Never noticed this before. In my case, once the receiver goes deaf,
repositioning of the antenna makes no difference.

However, I had read that certain nav/com freqs (even just receiving)
would give the thing fits.
snip

I was doing some instrument currency yesterday. My safety pilot was
playing with the 396 while I was under the hood and noted:

1) Transmitting on almost any frequency (122.8 most common that day)
caused the 396 to go briefly deaf. It took a few seconds to
reinitialize but the signals eventually came back. I'm usually too busy
transmitting to notice this.

2) We both verified that 115.5 dialed into a Narco slimline nav or (for
example) 119.32 into the comm is like hitting a light switch when it
comes to satellite reception on the 396. I've been hitting that more
lately because it is indeed the frequency to which the Narco nav
defaults if the keep-alive circuit is non-functional, and the keep alive
was accidentally disconnected recently during a wiring overhaul.

The impact of the local oscillator interference is far greater than I
would have expected, but it's not entirely unexpected. Needless to say,
the G430 installed in the panel not 2" away from it is happy 100% of the
time.

3) However, in spite of the Narco interference issue, my 396 still goes
on the fritz once it's been on 1+ hours. It will stay deaf (no sats)
for periods of 10+ minutes and then come back only to hear maybe 3-4
sats at 50% signal or worse, and get stuck in 2D positioning mode. And
here's the kicker -- turning off the offending panel-mount radios
(indeed the master switch) has no effect. There is something else at
work here.

-Doug

--------------------
Doug Vetter, ATP/CFI

http://www.dvatp.com
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