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ATC unable to read certain frequencies

T

Tsabean

Well Known Member
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Recurring problem. Skyview SV-COM-T25, Deltapop antenna installed in RV4.Most frequencies are loud and clear but certain freqs (specifically 124.4 and 123.7), controllers unable to read my transmissions. I can read ATC fine on all frequencies. Local “experts” suggest it may be antenna related. Any suggestions?

Thanks, Tom
 
I had this same problem with the same brand of antenna. I checked the SWRs at the problem freqs and found them to be slightly high. Reworked the antenna ground at the antenna and the problem went away. The antenna itself was not the problem.
 
RVDan said:
I had this same problem with the same brand of antenna. I checked the SWRs at the problem freqs and found them to be slightly high. Reworked the antenna ground at the antenna and the problem went away. The antenna itself was not the problem.
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Thanks Dan. I’ll check on the antenna grounds.
 
Tsabean said:
Recurring problem. Skyview SV-COM-T25, Deltapop antenna installed in RV4.Most frequencies are loud and clear but certain freqs (specifically 124.4 and 123.7), controllers unable to read my transmissions. I can read ATC fine on all frequencies. Local “experts” suggest it may be antenna related. Any suggestions?

Thanks, Tom
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Is the antenna on the top or on the belly?
 
I'm not antenna expert... So take this for what it's worth.

My antennas (bent whip Delta pop) were pretty corroded from years of outdoor living. I know painting antennas can be a no-no, but I figured it's either try and and see or throw them out. So I cleaned them and repainted them, but afterwards along the lines of what you describe I had issues receiving certain bands of frequencies. I got a antenna analyzer off amazon, plugged it in, and sure enough the SWR in that range was quite bad. All it took for me was to straighten out the antennas a bit to get the SWR into acceptable range, and now they work perfectly fine.

If you like tinkering an antenna analyzer is much cheaper than a new antenna, could be with a try!

Again, others here have way more knowledge of this than I do... But can't argue with results.

Alex
 
Double check that it's your problem first. We have a dead zone North East of FNL where the control tower cannot hear. It's bizarre. I always make sure I'm far from the zone before calling.
 
How do you use an antenna analyzer in the air?

Seriously though, never heard of this. Can you post a link to an example?
 
Do a search for "NanoVNA" on Amazon, and there is a lot of videos on YouTube about how to operate it. Also a search here of NanoVNA will come up with some information as well.

I did my testing on the ground.

Alex
 
rv6builder said:
Do a search for "NanoVNA" on Amazon, and there is a lot of videos on YouTube about how to operate it. Also a search here of NanoVNA will come up with some information as well.

I did my testing on the ground.

Alex
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Does it matter what way you are facing, do others complain that they have problems with ATC on those frequencies in the same area. Very strange that you would have problems on a top mounted antenna while flying and only on those two frequencies.
 
Norman CYYJ said:
Does it matter what way you are facing, do others complain that they have problems with ATC on those frequencies in the same area. Very strange that you would have problems on a top mounted antenna while flying and only on those two frequencies.
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My antennas are both bottom mounted. My issues were with the ~125.00 range of frequencies, I could barely pick up some ATIS messages at all no matter how close I was. Since I got the SWR under control I have much better reception (I fly the same route regularly on the way to work). Again I have no idea if what I was doing actually was the fix for the issue. But it at least didn't hurt to get my SWR into the acceptable range.
 
Norman CYYJ said:
Does it matter what way you are facing, do others complain that they have problems with ATC on those frequencies in the same area. Very strange that you would have problems on a top mounted antenna while flying and only on those two frequencies.
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The antenna is bottom mounted and has worked flawlessly for the last 3 years. I cleaned all the connections today and flew it again. Same issues, basically unreadable on 124.4. Tried calling another aircraft on that frequency and they reported strength 1 also.
 
Borrow a handheld, plug it into the coax instead of your radio. Does it work on 124.4? Then you’ll know if it’s the antenna, radio, or black magic.
 
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Tsabean said:
Plugged in a handheld and it came through load and clear. I’m guessing it’s not the antenna.
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Just curious. Where does the problem occur? Can it be repeated? Does anyone else hear it?
Have someone on the ground tune in to ATC Live on the tower and listen.
Just narrowing it down.
 
wirejock said:
Just curious. Where does the problem occur? Can it be repeated? Does anyone else hear it?
Have someone on the ground tune in to ATC Live on the tower and listen.
Just narrowing it down.
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Problem occurs airborne with centre on 124.4. Called another aircraft that was on that frequency and he reported garbled strength 1 also.
 
My bet is the antenna, but check the coax end to end. I rarely saw a radio bad and all of them either worked or failed. Never at one frequency. Although to be fair, a bad radio may be bad across the band but worse on one end or the other. Usually they failed because of coffee or soda. Most issues were antennas, bad or loose connections and damaged coax.
 
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A frequency specific issue is usually in the transceiver (radio) and not the antenna. Especially a "garbled" transmission from your radio and monitored by others.

If you plug a NanoVNA, or MFJ-259D into the radio-end of the coax and run through all the modes, how do things look? (VSWR <2:1, R/C Impedance is <= 50ohms?)

Else - swap the radio out with another one and see if the problem persists -- that will tell you where to look further (coax/antenna - or radio)...
 
bjdecker said:
A frequency specific issue is usually in the transceiver (radio) and not the antenna.
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Yeah, I’m with you on this one. An antenna is a piece of wire. It is either connected sufficiently or not. It is hard to be connected well on one frequency but not on another nearby frequency.

Transceivers on the other hand can have a lot of internal stuff that can change a lot even across a small frequency change. I am thinking of internal frequency synthesizers, switchable filters, and other tuning related stuff here.
 
bbaggerman said:
Yeah, I’m with you on this one. An antenna is a piece of wire. It is either connected sufficiently or not. It is hard to be connected well on one frequency but not on another nearby frequency.

Transceivers on the other hand can have a lot of internal stuff that can change a lot even across a small frequency change. I am thinking of internal frequency synthesizers, switchable filters, and other tuning related stuff here.
Click to expand...
Well, finally got a weather window to get airborne. On the advice of a local shop and some comments from the Dynon forum, I removed the 406 ELT and ATC reported I was loud and clear on all frequencies.
Now I have to figure out where to remount the ELT antenna.
 
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