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Weird Sidetone Item

CharlieWaffles

CharlieWaffles

Well Known Member
I have a PS Engineering PAR100 EX as my audio panel and Com2. My GTN 750 is my Com1. Everything works fine when I am transmitting on CTAF type frequencies (12x.00) but when I usually switch to a tower frequency (11x.xx) my sidetone goes way down. No reported issues with transmissions. I can even switch from a CTAF frequency to a tower and back and have the issue, no other changes.

I haven't tried it with just Com2 transmit/receive, but I will give that a go today.

Any ideas what could cause something like this?
 
PS Engineering says:

This would represent RFI, in another words, enough RF is getting inside the audio panel and being demodulates and adding to the 750 sidetone audio which in this case, attenuated the sidetone level.
Click to expand...

But I made the harness so they are unable to assist with how to eliminate the problem. Any idea how to fix this?
 
CharlieWaffles said:
PS Engineering says:



But I made the harness so they are unable to assist with how to eliminate the problem. Any idea how to fix this?
Click to expand...

My guess is that it could be one of two things. You are probably already aware of these gotchas.

1. a floating shield, in that neither end is attached to ground
2. a ground loop, which both ends of the shield are grounded

Both of these are going to be a PITA to find.

Also, the shield should be just that and not doing double duty to function as the low side of the connection.
 
Hi Mark,

Bob is likely right on here and it's likely you have a ground issue somewhere/somehow as Bob mentioned, or even something more miniscule like a shield braid pinching into a conductor wire in a shielded wire below a solder splice.

Indeed it will be a royal pain to track down, but if you do it methodically eventually you will find it. Just take the panel out of the rack and unhook the radio so you're not tracing wires or signals through the various boxes themselves (which can sometimes give you false positives). Just start checking wires to see if you can find one showing ground that shouldn't!

This is about all the assistance I can point your way as well; because without knowing how you built the harness or what it looks like in detail it's really hard to know where to start.

Cheers,
Stein
 
But first

Go into the 750 set up and turn the side tone volume to zero or off, and repeat the test. If you hear yourself on one of the frequencies you've confirmed the diagnosis. Then start looking ....
 
rleffler said:
My guess is that it could be one of two things. You are probably already aware of these gotchas.

1. a floating shield, in that neither end is attached to ground
2. a ground loop, which both ends of the shield are grounded

Both of these are going to be a PITA to find.

Also, the shield should be just that and not doing double duty to function as the low side of the connection.
Click to expand...

EXCELLENT advice. As I posted in a similar thread, a Rule of thumb for cable grounds:

If you want to prevent signals from getting OUT of the cable, ground both sides.

If you want to prevent signals from getting IN to the cable, only ground the side closest to the equipment.

:cool:
 
Good afternoon everyone,
The Garmin team has been monitoring this and wanted to provide some feedback for some information on what may be an issue. The change in side tone volume level observed is an indication that the aircrafts antenna system has a slightly different and higher VSWR at the band edge frequencies as opposed to the mid band range where most if not all COMM antennas have their best match.

In measurements that we've made on several different manufacturers antennas the band edges VSWR has always been higher than the middle frequency range and some installations are worse than others but they are typically of no concern with the transceiver that the user has.

Again, there is no concern with either the GTN6XX, 7XX comm. Radios or with the GTR/GNC series transceivers as we designed all of these transmitters to operate into an open and a short circuit without any damage.

A 2.5 :1 antenna VSWR, of which one could experience at the band edges in some installations, is also the point where the side tone would start to change and be noticeable and this VSWR is also within aviation antenna manufacturers standard operating range.

The antenna manufacturers err on the high side of their VSWR ratings to account for issues such as bent whip antennas close to the aircraft bodies, antennas close to an ELT or another style antenna or aircraft structure and also for antennas with smaller than normal ground planes.

Let me know if there is any questions,.

Best Regards,
 
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First I would try to confirm Garmin's hypothesis. Does your antenna have an exposed conductor, with no balls or other gizmos on the end? If so, just clamp (plastic clamps) a stiff wire to the end, so as to add about 1 3/4" to the length. This will move the center frequency down to near 118.0 MHz. Repeat the test. If sidetone is now normal you have confirmed Garmin's diagnosis.

Fix? Make sure you have a good ground plane, good connection between antenna base and fuselage skin (no corrosion). Try swapping antennas by swapping the coax cables behond the panel. Maybe #2 will work better.

Is your antenna mounted near the fiberglass cabin top? If so you may need to extend the ground plane.
 
Thanks Bob, I have 2 comant bent whips on the underside of the fuselage under the rear seats - about 5 feet from each other. I don't have any plastic clamps. but I can I tape the temporary extension on with a simple paper tape like blue painters tape or masking tape?



CI122-600x600.jpg
 
Good morning everyone,
I've been in communication with some of our smarter then me :) RF engineers on this, and basically we would highly recommend not to go and tape or clamp any extensions on the antenna, we don?t know the effect of further coupling from the new antennas wire tip to the body of the aircraft which could throw the resonance further off.

An antenna analyzer is the only safe method to use to get a true visual representation of the antennas bandwidth.

The side tone is designed to change like this so as to alert the user to a change in VSWR, but it is only of a concern if the side tone audio becomes distorted which indicates a shorted or open antenna.

Best Regards,
 
Ok, I have an appointment to take the plane over to a local avionics shop with an antenna analyzer. Anything specific I need to have them check for or just describe the general problem I am having?
 
BobTurner said:
Go into the 750 set up and turn the side tone volume to zero or off, and repeat the test. If you hear yourself on one of the frequencies you've confirmed the diagnosis. Then start looking ....
Click to expand...

So I did this today, turned off the GTN sidetone volume, restarted and tested transmissions on all frequency ranges (118-122) and there was NO sidetone what so ever on any frequency.

If I understand this test properly, this shows that there is no shielding/ground issue at play correct?
 
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