Comm Noise Question

Half Fast

Half Fast

Touchdown! Greaser!
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Half Fast
When my shop installed a new radio and did some panel cleanup last year, I had them route the coax for an unused VHF antenna to a BNC connector on the panel. I've used that with a short jumper to connect my Yaesu FTA-550 handheld to the external antenna. This lets me have a second comm radio (handy at times) which is battery operated and therefore makes a decent emergency backup.

Initially it worked well, but over the last few months I've been picking up bad noise on the Yaesu, so loud that I can't squelch it out, but only in flight. It works fine on the ground, making troubleshooting very difficult. The noise is broadband white noise; I don't think it's coming from the ignition.

At SNF, I spoke with the Yaesu rep and he suggested doing a flight with the regular short whip antenna attached to the radio, rather than wiring to the external one. I tried that today, and sure enough the noise cleaned up. The radio worked just fine, but of course I'd get better range if I could use the external antenna.

Any idea what might be going on? Any troubleshooting suggestions?
 
I'm a mechanical guy and have no idea really about this sort of thing....but just thinking based on my troubleshooting intuition, the adapter cable between your radio and the panel is the first place I'd think to look...my thought is that it's the most likely to become compromised in some way.....could be the connectors being dirty or damages, could be the cable itself has a bad shield form perhaps being bent and flexed, etc........
 
Agree the shielding is incorrectly installed.
 
Shielding is a possibility, but why would the problem only appear in flight? It's quiet on the ground.
 
Half Fast said:
Shielding is a possibility, but why would the problem only appear in flight? It's quiet on the ground.
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The aircraft electrical system goes through cycles from start to flight to shutdown. Sometimes with shielding issues it only becomes apparent at a specific point in those cycles. Then again it could simply be a gremlin or two with your number.....;)
 
Bell206 said:
The aircraft electrical system goes through cycles from start to flight to shutdown. Sometimes with shielding issues it only becomes apparent at a specific point in those cycles.
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Could you be a bit more specific? It's quiet on the ground even during runup, so I don't think it's anything ignition related. The shielding doesn't change in flight, therefore it must somehow be the noise environment that's changing. It's not radiated emissions from the transponder or the other radio, as I've tried switching them off for a moment without any change.

I wouldn't be at all surprised to learn it's a problem with a coax shield or with a ground connection, but I'm stumped for why I can't duplicate it on the ground.
 
I don't know how, but maybe something is bending a tiny bit it flight vs ground, and that's pulling the shield loose. Intermittent connections on coax cable can be a PITA to troubleshoot. Used to do computer wiring with RG-58 and RG-62, and just things sitting on desks in offices could sometimes be weird. On the antenna, if those are soldered inside, they can be failure points, too. If they flex, they open, and relaxed they can touch together under pressure and work fine. We had that with a batch of network cards once...the solder joints looked fine visually, but were cracked and were intermittent. Re-flowing they were fine.
 
I'm surprised nobody has mentioned static buildup yet. Does this aircraft have any static wicks on it?
 
Half Fast said:
Could you be a bit more specific?
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That would require more info from you. When does the noise start? How long does it last? Is it intermittent in flight? When does the noise stop? Etc, etc. For example, I've seen audio noise generated once the charging system complete a charge cycle which was 20 minutes after takeoff (regulator issue). Other issues could be heat related that would only manifest after X-number minutes in flight. The inability to duplicate on the ground can simply be a feature of the fault and not the cause if that makes sense to you. Best play to TS in my opinion is to make a single change to narrow things down like install a good coax.

EDIT; and being it is a handheld radio adds another level to the mix.
 
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Bell206 said:
Temp install a 2nd coax in place of the original from the antenna direct to the radio and see what happens. Sounds like the coax shielding has a fault.
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Yeah, I think that was ultimately the problem, sorta. Connecting a coax all the way from the radio to the antenna would be a PITA and require pulling the headliner, so I started with the easy part. I just replaced the short coax cable between the handheld radio and the panel (BNC to BNC) and that cured it. The bad coax checks okay for continuity on both the signal line and the shield, but the shield braid must be separated internally or not making a 360 degree connection at one of the connectors.

Flew today and it worked perfectly. I'm happy with this arrangement, as it gives me a decent Com 2 radio that's a poor man's nav/com and has its own internal battery.

I'm still not sure why the noise was only manifesting in flight, but as long as it's working I'm not going to worry about it.
 
When you say you "can't squelch it out", does that mean the "RX" icon is illuminated when you're getting this noise?

If so, what's probably happening is your handheld is positioned close enough to an RFI source that it is actually breaking the squelch on your FTA550. Digital handhelds are especially prone to this. Try moving the handheld around while this is happening. You may need to make up a coax extension in order to move the handheld far enough away from the source to make a difference.

I've had this problem in my little Sonerai. I pretty quickly realized it was the USB battery pack I was using to power my iPad. I finally just quit using USB battery packs in the airplane at all.
 
wilkersk said:
When you say you "can't squelch it out", does that mean the "RX" icon is illuminated when you're getting this noise?

If so, what's probably happening is your handheld is positioned close enough to an RFI source that it is actually breaking the squelch on your FTA550. Digital handhelds are especially prone to this. Try moving the handheld around while this is happening. You may need to make up a coax extension in order to move the handheld far enough away from the source to make a difference.

I've had this problem in my little Sonerai. I pretty quickly realized it was the USB battery pack I was using to power my iPad. I finally just quit using USB battery packs in the airplane at all.
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Yes, that's what I meant by not being able to squelch it. But read my post #11. Problem solved, bad coax jumper.
 
Water (moisture) can get in coax as well. Your antenna presents different impedance close to the ground than it does inflight, it’s possible that’s the reason you cant duplicate.

You’d need an antenna analyzer or SWR meter at the least to formally troubleshoot all that.
 
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