Ground testing to help isolate cause of weak radio transmissions

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bkspero

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bkspero
Starting a few weeks ago I began getting responses to some ATC calls telling me that my transmissions from Comm 1 (GNS-530w) were weak, scratchy/garbled, and nearly unreadable. Comm2 (KX-155) was usually better, but sometimes I was told only slightly better. These calls are all generally medium range (ca. 20 nm). I can hear their responses loud and clear. Local radio checks always get a Loud & Clear response on both Comm's

My problem is that troubleshooting this in the air is impractical. Changing equipment is either impossible or impractically difficult, and the responses are qualitative and not consistent. So, I'd like to troubleshoot the parts of the transmission train on the ground, and I could use suggestions on how to do that. I've spoken to a few local avionics shops and they haven't been interested in this kind of troubleshooting.

So, what can I do or specifically request a shop to do, and what devices should I look to test? My guess is that I should be looking at the headset mic, the mic socket in the panel, the audio panel, intercom (mine are separate units), the radio, and the antenna. Anything to add or eliminate?

In particular I'd love to have someone bench test the VHF output power and quality of the 530w transmission. Is there a way to do that short of sending it to Garmin along with a check for $2200?

So far I've treated the panel sockets with DeOxit on a cotton swab and tugged on the wire connections to confirm that they were solid. I've also sprayed the push to talk button with DeOxit. I've tested the SWR of the Com1 antenna by removing the 530w from its tray and connecting a handheld through an SWR meter to the antenna connection of the tray and it was good (between 1.1 and 1.2). I've tested 4 different headsets in the air, and have gotten comments of weak transmissions from all but one (the Lightspeed Zulu 3 that is my usual headset...on a recent flight using it I got a 5x5 response from ATC at about 25 nm). One of the 3 headsets that have been in use during weak-transmission episodes (a QT Halo) went back to the manufacturer and no problems were found with it.

I am at a loss as to what to do next. How can I get other parts of the system checked, and the parts I have addressed checked better?
 
Could it be the female receptacle the plug goes into. Could be a slightly loose connection that just happened to connect good with the Zulu. Maybe the spacing on the male plug is slightly different from Zulu to others but they should be the same. The female tangs could be slightly off too. Does it happen from the other pilots position?
 
Turn up mic gain?

Given that you stated it began a few weeks ago, it sounds like somethings changed. Anything changed you know of? This really sounds like a failing antenna, but your low SWR isn't consistent with a failing antenna.
 
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FORANE said:
Turn up mic gain?

Given that you stated it began a few weeks ago, it sounds like somethings changed. Anything changed you know of? This really sounds like a failing antenna, but your low SWR isn't consistent with a failing antenna.
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There was a change, but it may have been a red herring. I use the Zulu 3 headset from the autumn through spring, and in the summer I change to the in-ear Halo headset. The first ATC comment happened on my first flight with the Halo headset. I presumed that the problem was the Halo's, and that's why I sent them back to Quiet Technologies to be "fixed". Instead QT found no problems with the Halo's, and at the same time I tested two more different Lightspeed QFR headsets which also produced the same problem as the Halo's. Otherwise nothing has changed.

I did start a test of turning up the mic gain on one of the QFR headsets (the Halo's don't have field adjustable mic gain). Lightspeed advised very small changes (like less than 5 degrees of rotation), and the first attempt didn't help. It was also complicated because Lightspeed couldn't tell me with certainty which way to turn the potentiometer to increase gain. Their newer models call for clockwise rotation. But the QFR's have a counterclockwise arrow around the gain access hole. No one at Lightspeed tech support could remember for certain what way to turn the gain, and they advised to follow the arrow. So for now mic gain is still a possible answer. I just need to make bolder changes.
 
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You know the 530 has a setting for mic gain too, correct?
 
How can your transmission on the ground be fine and airborne be bad if it’s the headset? You’ve tried 2 or more with similar results.

Seems to be a range issue, hence transmitter or antenna. You’ve ruled out the 530 transmitter because your KX155 is not much different. And you have 2 dedicated separate antennae, right? (If you want to rule em out further, swap radios with someone or put yours in someone else’s, but that’s a bit like trying 4 headsets and getting the same result).

Something changed to degrade your transmit, and while coincidences do occur, it not likely both radios or 4 headsets. Check the plane (ignition noise?) and wiring (ungrounding of any avionics wiring that has shielded wire, but not just avionics grounding). Vibration and age and very tiny wires.

This article may be of help? https://www.kitplanes.com/get-grounded/

I hope you find it. It’s tough without hands on help.
 
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FORANE said:
You know the 530 has a setting for mic gain too, correct?
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I did not, thanks. I have a set of configuration screen images and see it on the Com Setup page. Mine is set to 42.
 
455 Bravo Uniform said:
How can your transmission on the ground be fine and airborne be bad if it’s the headset? You’ve tried 2 or more with similar results.

Seems to be a range issue, hence transmitter or antenna. You’ve ruled out the 530 transmitter because your KX155 is not much different. And you have 2 dedicated separate antennae, right? (If you want to rule em out further, swap radios with someone or put yours in someone else’s, but that’s a bit like trying 4 headsets and getting the same result).

Something changed to degrade your transmit, and while coincidences do occur, it not likely both radios or 4 headsets. Check the plane (ignition noise?) and wiring (ungrounding of any avionics wiring that has shielded wire, but not just avionics grounding). Vibration and age and very tiny wires.

This article may be of help? https://www.kitplanes.com/get-grounded/

I hope you find it. It’s tough without hands on help.
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I thought one possibility for the headset causing poor results at 20 nm and good results at 1/2 nm was mic gain differences causing different output power levels.

Yes, there are separate antennas. Comm1 on top of the plane and Comm2 on the bottom.

I've tried the long vs. short distance comparison on about 18 flights with ground checks before departure and either unsolicited ATC comments or my radio check request from ATC on the flights. I used the Zulu 3 on about 8 of the flights, the Halo on about 6, and the 2 QFR's on about 2 each. So I have fair confirmation of an issue, but the individual results are not always clear cut. I might speak with 3 or 4 controllers on a flight and only 1 will complain about weak reception. Was performance better while I was talking with the non-commenters, or was I just closer to their antennas, or were they just more tolerant of a weaker signal? That's why I am asking for bench tests, particularly quantitative bench tests, that can be used to determine if the different pieces of equipment or parts of the signal chain are operating properly.

I agree about your recommendation to check the grounds. I've done that where I can, but there are connections behind the panel that I don't have the knowledge or tools to access. So I'll continue to search for a shop that is willing to work on it.
 
On your ground checks, have you tried bringing your engine up to 2000rpm or so? Ignition or alternator interference will increase at higher rpms.

Also, check your engine ground strap.
 
Half Fast said:
On your ground checks, have you tried bringing your engine up to 2000rpm or so? Ignition or alternator interference will increase at higher rpms.

Also, check your engine ground strap.
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I've gone up to about 2300 (I don't recall the exact number) in a static runup recently. No whine or squal in the headset. Just the sound of the running engine muffled by my headset. Ground strap is something I can check and didn't think of. Thanks.
 
There are tools like Bird RF Watt meters. I've never used one but, I believe you could use it to measure the power output to the antenna. Maybe you could rent one from a test equipment shop.

There are some cheap hand held RF signal meters on Amazon too. I've never tried using one of them either. If you have two radios with basically the same antenna setup, you might be able to use one to see if one radio is transmitting at a significantly lower power than the other for some reason. Bad connectors and damaged coax will affect the transmitted power. Antenna needs to be properly grounded too.

If you have two radios maybe you can cross the antenna connections at the back of the radio stack to rule out the antenna and coax connections.
 
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kontiki said:
There are tools like Bird RF Watt meters. I've never used one but, I believe you could use it to measure the power output to the antenna. Maybe you could rent one from a test equipment shop.

There are some cheap hand held RF signal meters on Amazon too. I've never tried using one of them either. If you have two radios with basically the same antenna setup, you might be able to use one to see if one radio is transmitting at a significantly lower power than the other for some reason. Bad connectors and damaged coax will affect the transmitted power. Antenna needs to be properly grounded too.

If you have two radios maybe you can cross the antenna connections at the back of the radio stack to rule out the antenna and coax connections.
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Thanks for the input. My SWR meter is actually capable of measuring RF power output. But I need the help of a shop to do the work (will explain later). So I contacted three prominent local avionics shops and each one told me that transmission output power couldn't be measured reliably and dismissed my request.

My problem is that I need a shop to break into the coax antenna cables while they are connected to the 530w and KX-155. My problem is that the cables for both radios do not appear to have breakable connections to the radio trays. They appear to be hard wired. I don't know about the connections at the antennas, but I can't access them without removing major parts of the interior (which I am not comfortable doing). So I will keep looking for an avionics shop that is willing and capable of troubleshooting.

This is also the reason why I can't swap antenna connections to see if the ATC complaints change with that. But I think I already know that the coax and antennas are good. I have been able to measure SWR for both coax/antenna chains by removing the radios and connecting my handheld radio via a short piece of coax to the antenna connection on the insides of the trays. Both were very good. So I think the cables, antennas, and antenna grounds are good.
 
bkspero said:
each one told me that transmission output power couldn't be measured reliably and dismissed my request.
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That's BS - measuring output power is simple on an AM transmitter. They're just blowing you off (or else are only minimally competent and don't really understand basic radio). +/- few Watts 5 (typical VHF com power) will not make a noticeable difference in your signal.
 
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