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Old 06-23-2018, 05:05 AM
rgmwa rgmwa is offline
Join Date: Feb 2010
Location: Perth, Western Australia
Posts: 1,597
Default Radio transmit problem

Looking for some Comms 101 troubleshooting assistance.

I have an RV-12 with a Garmin SL-40 and standard com antenna installed as per the plans except for using nutplates to secure the antenna rather than the supplied 8-32 hex nuts and split washers.

I can receive OK but get a loud tone/feedback noise in the headset when I push the PTT button to transmit, and am variously reported as `garbled/readability 2/unintelligible' etc. It's been a problem since day one, and I need to get to the bottom of it.

The problem occurs the instant I press the PTT button, whether I speak or not. The intercom works fine, and I've swapped headsets, tried different headsets, etc. I've also tried transmitting from the right seat, but the problem occurs on that side too. I'm thinking maybe a faulty antenna cable or connectors, grounding problem or antenna issue, but not having any expertise in this area, don't really know what the problem might be or how to troubleshoot it, other than finding someone who knows about these things which is easier said than done. Any ideas?
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Old 06-23-2018, 08:30 AM
BobRv4 BobRv4 is offline
Join Date: Nov 2012
Location: Gilbert, AZ
Posts: 94
Default radio

Can start with basic check of wiring. Check the antenna coax make sure the center conductor is not shorted to ground. Or can you run another coax to antenna just to try that? It might come down to replacing one item at a time.
Did you wire the head set jacks correctly? Remove all the jacks, don't remove the wire, just remove from the air frame, maybe you got one grounded wrong?
the shielded wire should only be ground on one end (floating ground)
Can you put in a different radio or antenna to eliminate these items?
Have you tried with the engine not running, any difference?
Assuming the radio is ok, I'd check the coax, headset jacks/wiring first most problems I've seen these are the most common. Good luck report back!
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Old 06-23-2018, 09:07 AM
Mel's Avatar
Mel Mel is online now
Join Date: Mar 2005
Location: Dallas area
Posts: 10,084

I might suspect the use of platenuts on the antenna. Are you sure you are getting a good ground? Split washers tend to cut through primer and help the ground.
Find someone with a meter and check the VSWR.
Mel Asberry..DAR since last century
A&P/EAA Tech Counselor/Flight Advisor/Nat'l Test Pilot School
Specializing in Amateur-Built and Light-Sport Aircraft
North Texas (8TA5)
RV-6 Flying since 1993, 172hp O-320, 3-Blade Catto (since 2003)
Legend Cub purchased 12/2017
FRIEND of the RV-1
Eagle's Nest Mentor
Recipient of Wright Brothers "Master Pilot" Award
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Old 06-23-2018, 09:33 AM
Carl Froehlich's Avatar
Carl Froehlich Carl Froehlich is online now
Join Date: Dec 2007
Location: Dogwood Airpark (VA42)
Posts: 1,660

Your platenuts are not the problem.

Your symptoms most likely indicate a failed antenna cable/connector. If you donít have access to an antenna analyzer you could disconnect the antenna cable at the antenna and try after connecting a hand held rubber antenna to see if the problem still exists. Similarly you can make a test antenna cable and install directly from the radio to the antenna to eliminate your existing cable run as the problem.

After that, the other simple trick is to swap radios with someone with the same radio and try that.

Every EAA chapter should have a simple MFJ antenna analyzer in the shared tool bin. Iím surprised few do.

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Old 06-23-2018, 01:27 PM
Rainier Lamers Rainier Lamers is offline
Join Date: May 2007
Location: Somerset West
Posts: 963

The effect you describe is quite common with AM transceivers and you have in fact identified what it is: Feedback - more accurately RF feedback.

What happens is that some of the transmitted RF signal is finding its way back to the audio (microphone) input of your transmitter.
If level of this feedback is large enough you get howling, if somewhat lower you can get "bathroom sound" or distortion.
The signal is of course high frequency but AM modulation is nasty in that it can be converted back to audio by your transmitters audio input, your headsets or intercom system.

This means the fix is to reduce the RF getting back into your transmitter audio input.

This means you need to find where it is getting in and why it is there in the first place.

As mentioned, a bad antenna match can indeed be a contributing factor as the antenna reflects any energy it cannot radiate back to the transmitter where it enters and contaminates grounds and power supplies.

Lack of shielded wires for the sensitive microphone signals or incorrect shielding techniques can also be a cause. Modern transceivers generally do not like microphone/headset jacks to be grounded to the airframe so make sure the sockets are insulated (also from each other).

Finally, you can get some nice clip-on ferrite beads that you can place over your audio signals at strategic locations - these can help attenuate any RF on these wires - but make sure the types you use are usable at VHF frequencies they need to be formulated for this.
I view this type of fix a applying a "plaster" - but it may help in locating the source of the trouble.

Finally - don't despair - there usually is a simple fix - you just have to find it. You are in good company - this type of problems can sometimes even flummox experts in the field...

CEO MGL Avionics
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Old 06-23-2018, 01:59 PM
rgmwa rgmwa is offline
Join Date: Feb 2010
Location: Perth, Western Australia
Posts: 1,597

Thanks for all the replies. I should have added that it makes no difference whether the engine is on or off.
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Old 06-24-2018, 12:36 AM
rgmwa rgmwa is offline
Join Date: Feb 2010
Location: Perth, Western Australia
Posts: 1,597

Problem fixed! I pulled the DB connector off the back of the intercom this morning (Flightcom 403), thought “well that won’t do much”, and plugged it back in again. No noise and clear reception! I couldn’t believe that after months of on and off fiddling with with headset jacks, the antenna, staring at the wiring looking for smoke and muttering various magical spells I composed myself, the problem vanished so quickly. I sprayed some Deoxit on the pins for good measure. If I’ve learned anything from this lengthy and frustrating exercise, it’s that a bad connector may actually mean a good connector but a bad connection. As Rainier said, it’s often a simple fix. I’ve been very lucky this time but would not have found the fault without the prompts provided by you guys. I’m leaving on a long trans-Australia trip next week and wasn’t happy about going with a radio that wasn’t functioning 100%. Thanks again!

Last edited by rgmwa : 06-24-2018 at 12:39 AM.
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