What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Garmin GTN650 side-tone

Weasel

Weasel

Well Known Member
I have always had an annoying issue with the side tone on the GNT 650 in the RV-10.

The audio is routed through a PS 5000 audio panel and works great.

The problem is that the side-tone volume is set perfect when using some radio fequencies but when on other frequencies it is too loud. I assume this has to do with some RF harmonics at specific frequencys. It is not squint your eyes loud but more like slightly louder than perfect....makes you talk abnormal.

I used sheilded wire for the intercom audio with only 1 end of the sheild grounded at the intercom.

I tried to keep the RF signal wires seperate from the audio but there are a few places that they run together for short distances.

Also I used RG-58 instead of RG-400 on the comm.

Have I self diagnosed the problems (lower grade coax and wire run proximity)? or is there other causes for this problem.

I may consider fixing it sometime but I want to know exactly what it is before I just change something to give it a try.

Just to re-cap. The problem is is something that 90% of people would never notice.....:eek:
 
I have the same problem and have used RG400 so the wiring isn't the issue. I have just learned to live with it as an artifact of the GTN or antenna issues. I have also swapped antennas and that didn't resolve the issue. I have the 750 but I believe there is enough similarities that the problems are probably the same.
 
I would think that the audio circuitry within the GTN would be the culprit. I can't see how external items would impact sidetone.

What does Garmin say? Have you called them?

I haven't noticed this issue on my 650, but then with the squelch issues I'm troubleshooting, sometimes I'm lucky to hear any sidetone.
 
I have had a similar issue as Weasel on one occasion and believe it may have been due to both my radios tuned to a high range frequency's only a couple spaces apart, transmitting on one and monitoring the other. The side tone was extremely distorted and caught me off guard. It only happened the one time though.
 
Hello everyone,
We do have a great deal of interest in this and have been watching the thread as you an imagine.
One of our design engineers contacted me about this and provided me these details on the internal operation of the system.

"The sidetone on the GTN is sampled from the RF output forward power which under higher than normal VSWR conditions may also includes some reflected power.

When the sidetone volume increases or becomes slightly increased along with some audio distortion this is a audible indication that the VSWR of the Comm antenna is high and the operation of the GTN was intended to do just this.

The GTN transmitter can operate just fine into such a high VSWR so this is just an indication to the user of a possible impending antenna problem but it is not a concern from the radios operation. "

Let me know if there are any questions and I'll be happy to investigate them.
 
CharlieWaffles said:
I have also swapped antennas and that didn't resolve the issue.
Click to expand...

TrekLawler said:
this is a audible indication that the VSWR of the Comm antenna is high
Click to expand...

Charlie, did you try swapping the antenna for a different type of antenna or the same?

If it is a result of VSWR then going to a strait blade instead of a bent whip may help.

For what its worth I have a lower mount (bent whip) like this mounted on the bottom.
 
Weasel said:
Charlie, did you try swapping the antenna for a different type of antenna or the same?

If it is a result of VSWR then going to a strait blade instead of a bent whip may help.

For what its worth I have a lower mount (bent whip) like this mounted on the bottom.
Click to expand...

Thanks Weasel,
very good question, the problem with what we've seen here in the field service team is when some swap out an antenna they do so with the same style. after speaking with our design engineer he thought this problem may stem around a possible antenna issue. during his testing he found that the typical thin aircraft whip and even more so with bent whip bottom/belly mount antennas or for that matter any antenna with a insufficient ground plane/ counterpoise such as will be found in most aviation installs typically measure a band edge VSWR around 2.9 :1 or higher and a even a midpoint VSWR over 2.3:1.
The best antennas found were the wide blade antennas which typically have an overall VSWR from 118 to 136 MHz under 2:1 VSWR , but the design of the transmitter of the GTN/GTR/GNC series is to operate and survive into a 10:1 VSWR.

Let me know if there are any other questions,
 
Try an antenna by Don Pansier who runs Delta Pop Aviation. Love mine and I use it with a GTN 650 and it performs great with no sidetone issues. Don has done a ton of work with his antenna analyzer to get the best out of his designs.
 
Last edited:
RF OUT and RF back....

At least in terms of RV installs, I can't agree with the ground plane/ counterpoise statement. I do see the issue in many airframes though.
On belly of the RV's, if the antenna is not placed over on one side... there is plenty of conductive aluminum to form a counterpoise for the wavelengths we are using. A very GOOD bond to ground, at the base of the antenna is mandatory. ANY resistance to ground will not benefit the situation.
Something I rarely see, even in the law enforcement avionics installations that I inspect is a good reflected power measurement. Borrowing a VSWR meter is usually doable. A sample of readings at the edges and center of the band will tell how much power is getting out. As mentioned, the final amps in the radios now days are built to tolerate reflected power. Trying the Delta Pop antenna is a good idea. The bent whips are the least sophisticated... and cheapest. Expecting wideband performance is a stretch from these.
Sidetone is a blessing. I learned to fly shouting into a hand held mic. And a speaker on the ceiling. So all in all... we are much better off now.
 
If you have a Delta Pop antenna, take a good look at the install and contact Don if you don't find something. His antennas are well engineered and he uses a high dollar analyzer to validate them. He is also a super nice guy and will ensure your getting the best out of his product.
 
For what its worth, I have a GTN-750 and GNC-255A with a GMA-35 audio panel and RG-142 coax going to 2 RA Miller bent whips on the belly. I originally had the same squelch issue everyone else is experiencing but that was resolved after following the service bulletin from Garmin and adjusting to 75%. I have no side tone volume issues as others have mentioned. I did notice one issue where the received audio would suddenly change volume levels in flight and discovered a feature in the GMA-35 configuration called "Ambient Noise Sensor". Once I turned that off, everything worked great.
 
TrekLawler said:
"The sidetone on the GTN is sampled from the RF output forward power which under higher than normal VSWR conditions may also includes some reflected power.

When the sidetone volume increases or becomes slightly increased along with some audio distortion this is a audible indication that the VSWR of the Comm antenna is high and the operation of the GTN was intended to do just this.
Click to expand...

In my case, I appear to have the opposite problem. The vast majority of my frequencies used result in a normal level of side tone. However, some frequencies result in a lowered/hushed volume of side tone.
 
Interesting email from Garmin yesterday. They seem to think it is in the GNT 650...

Dear Joel Gaber,


Thank you for contacting Garmin International.

From your description I would not consider this normal operation. I recommend returning to your Garmin Aviation dealer for further evaluation.


Best Regards,

Ken


>> Subject:
>> Sent: 05/13/2015 11:04 AM
>>
>>

HI Garmin,



I have a GNT 650 that has a noticeable difference in side tone volume when transmitting on different frequencies.



When on some frequencies the side tone audio seems normal (I have adjusted it in the menus) but on other transmit frequencies it seems too loud.



Is this normal?



Joel
 
Joel,

Did Garmin work with you to fix the sidetone issue on your GTN-650? I'm having exactly the same problem with my GTN-650.
 
PixelWorker said:
Joel,

Did Garmin work with you to fix the sidetone issue on your GTN-650? I'm having exactly the same problem with my GTN-650.
Click to expand...
This was there response.
Dear Joel Gaber,


Thank you for contacting Garmin International.

From your description I would not consider this normal operation. I recommend returning to your Garmin Aviation dealer for further evaluation.


Best Regards,

Ken

Garmin Aviation Support

I did not return it due to cost and also because I have no SWR data on my actual instalation.

The antenna itself had a SWR survey but not the installation.

I am assuming it is my instalation until proven different.


Also, on that particular antenna I am running RG56 coax because that is what I had at the time. I have considered changing it to RG400 but have just been living with it.
 
Weasel said:
Also, on that particular antenna I am running RG56 coax because that is what I had at the time. I have considered changing it to RG400 but have just been living with it.
Click to expand...

I haven't noticed this attribute on mine yet. I do have RG400 installed and have tested SWR to be well within specification.

SWR meters aren't that expensive. The only issue is that most don't come with TNCs, so you have to buy an adapter. You also need to validate that the unit supports the correct frequency. Some of the cheap units have limited bandwidth capabilities.
 
Version 6.21 fixed problem

Have been flying for a while now with version 6.21 which allows the installer to select a side tone source of internal or external. By switching to internal source the variances in side tone volume across the band are now constant.

Thank you Garmin for this!!!
 
Weasel said:
Have been flying for a while now with version 6.21 which allows the installer to select a side tone source of internal or external. By switching to internal source the variances in side tone volume across the band are now constant.

Thank you Garmin for this!!!
Click to expand...

Amen! Amen!
 
You must log in or register to reply here.
Back
Top