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Need help from RF experts

lr172

Well Known Member
Recently on my 6A, I have started to pick up audible noisy receptions on my Comm radio while taxiing. It is not classic noise, but actual reception that is not voice. It only happens on the ground and never after the wheels leave the ground. Receptions are intermittent and typically around 5 seconds in length. All voice receptions are clear as can be and never hear complaints about my transmissions.

It is a 430W with RG-58 cables and a commant CI-138 antenna mounted on the belly. Has worked flawlessly for the previous 800 hours.

Any ideas on where to start troubleshooting? Antenna failure, ground plane, coax, etc.

Appreciate all input.

Larry
 
Not much to go on, but I’ll guess this is intermod (cross talk from a nearby transmitter). For example you are on the ramp listening to Unicom, but near the ADIS transmit antenna.

Carl
 
Carrier

If it's like a radio keyed up, maybe a handie talkie in a bag keying up? Just a thought.
Any electronics should have a TX light. Maybe see if anything is keying somehow.
 
Larry,

Can you inspect the interface between the CI-138 and the airframe?

I can, but will have to pull the floor panel. It is my next step. My first thought was to check for resistance from the BNC shell to the airframe to see if I have an issue with the ground plane.
 
Not much to go on, but I’ll guess this is intermod (cross talk from a nearby transmitter). For example you are on the ramp listening to Unicom, but near the ADIS transmit antenna.

Carl

That is an interesting idea. This seems to mostly or only happen at my home field and they recently had issues with the AWOS and did a bunch of work on it. The AWOS is 128.27 and the unicom is 123.0. Is that close enough for bleed over?

Larry
 
avionics

When comm or analog avionics are good, then start going bad when nothing had been touched, in my experience, it is usually grounds or mechanical electrical connections that are corroding.
If this started after a configuration change, I would suspect something other than grounds.
 
Getting .05 ohms from bnc outter shell on antenna to frame ground. Getting . 3 ohms from outter shell of bnc on cable to frame ground. Haven’t gotten under the panel yet to measure other side of cable. Is this enough resistance to cause my problem?

EDIT: Ok, If I wiggle the connector end of the cable around, I see it moving between .1 and .4 ohms. I will clip off the end and put a new BNC. Seems like my issue.
 
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Typical COAX end connectors are crimped. Easy to get corrosion in there. Also easy to do a really poor job of it that didn't securely attach the braid or center pin. Also possible for corrosion between the coax connector and the sockets on the radio and antenna.

In ham radio - for outside use every connector has to be wrapped with stuff similar to silicone "rescue tape" to ensure water (and hence corrosion) doesn't get in. A nasty problem that's hard to isolate.
 
Have you recently introduced a USB socket into the aeroplane via a cigarette socket or hard wired ?

They spew out dirty RF unless you get a decent one.
 
USB Charger noise

I recently had a similar problem in my RV-8 with a Dynon SV radio. I found that my 12 VDC / USB adapter was causing significant RF, especially when the I-pad was a low charge. The radio was showing a near constant RX, squelch had almost no affect. Once I removed the USB charger and replaced it with an "aviation quality" 12 VDC / USB charger, the noise went away. I was amazed at how well the Dynon radio works now....
 
That is an interesting idea. This seems to mostly or only happen at my home field and they recently had issues with the AWOS and did a bunch of work on it. The AWOS is 128.27 and the unicom is 123.0. Is that close enough for bleed over?

Larry

If it only happens at one airpprt then the problem is with the airport equipment not your airplane. Does it happen only at a specific area on your airport? A misadjusted transmitter can easily cause this problem depending on how bad it is misaligned and how far away you are from the transmitting antenna. :cool:
 
If it only happens at one airpprt then the problem is with the airport equipment not your airplane. Does it happen only at a specific area on your airport? A misadjusted transmitter can easily cause this problem depending on how bad it is misaligned and how far away you are from the transmitting antenna. :cool:

airport isn't very big, but have gotten it at various places on the airport.
 
So, I crimped on a new bnc. Now a very stable .2 ohms on the bnc outter shell. Did a test flight. Got just a very small amount of noise on my field, but way better. Went to another airport and did 3 full stop taxi backs with 0 noise. No noise on landing back at my home airport.

Thinking that I had a wire issue, but also maybe an issue with the AWOS at my airport.

Thanks for the help.

Larry
 
So, I crimped on a new bnc. Now a very stable .2 ohms on the bnc outter shell. Did a test flight. Got just a very small amount of noise on my field, but way better. Went to another airport and did 3 full stop taxi backs with 0 noise. No noise on landing back at my home airport. Asked the tower for a radio check adn they said 4 to 5 out of 5. Would have expected better being only 8 miles from the field. Maybe I need to go under the panel and check the coax connection to the 430. Would have expected less than .2 ohms (measured with bnc disconnected from antenna) on the coax ground out at that antenna.

Thinking that I had a wire issue, but also maybe an issue with the AWOS at my airport.

Thanks for the help.

Larry
 
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