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Hum in Radio?

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Goodone

Joined: 13 Sep 2008
Posts: 49

Posted: Sat Sep 03, 2011 8:27 am Post subject: Hum in Radio?

I have an  Icom IC-A24 Aviation Transceiver mounted in my aircraft with an external antenna.  The radio sends and receives while in the air very well. However, when I am on the ground at airports with a control tower or Unicom I get a hum in the radio. This also occurs when I am on the ground near another aircraft that has its radio on the same frequency. Where is this hum coming from? I assume that it is some type of feedback but I do not know what to do. Suggestions?

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douglist(at)macnauchtan.c
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Posted: Sat Sep 03, 2011 12:23 pm Post subject: Hum in Radio?

At 09:27 -0700 9/3/11, Goodone wrote:

 	  Quote: 	
		  

I have an  Icom IC-A24 Aviation Transceiver mounted in my aircraft with an external antenna.  The radio sends and receives while in the air very well. However, when I am on the ground at airports with a control tower or Unicom I get a hum in the radio. This also occurs when I am on the ground near another aircraft that has its radio on the same frequency. Where is this hum coming from? I assume that it is some type of feedback but I do not know what to do. Suggestions?


	


Some things to check:



Most non-engineers describe "hum" as a low frequency audio noise that is an octave of so lower than middle C.  It is usually 60 Hz or 120 Hz noise that comes from a commercial power  source. That would be very unusual in a light airplane. But at an airport with big mercury lights, perhaps.



If what you hear is a higher frequency it could be originating in your generator or alternator.  The test is to move the throttle and report that  the tone of the noise changes. Alternators run about 400 Hz which would sound like an A.



It's possible for a nearby aircraft to be transmitting because of leakage in the local oscillator of it's receiver.  That kind of thing is pretty rare unless the radio has been subjected to unprofessional servicing. You would notice it and remember the aircraft involved. Or do you mean that that other aircraft is deliberately transmitting?



I have seen cases in which the aircraft antenna is broken and works only when forward airspeed makes it bend and effectively connect the fine copper tape inside of the fiberglass tube that is the antenna.  I don't see how that would generate "hum" though.  Perhaps noise because of some automatic gain control though,



In short, you need to be more explicit about your problem.



-- 



--> From the U S of A, the only socialist country that refuses to admit it. <--

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Goodone

Joined: 13 Sep 2008
Posts: 49

Posted: Sat Sep 03, 2011 4:13 pm Post subject: Re: Hum in Radio?

I would say that the "hum" that I am hearing is about one octave below middle C. It sounds a lot like the noise that you hear when folks are having trouble with feedback with a microphone in a nightclub. I doubt very much that it is from some noise source on the ground. Maybe it is being generated from something in my aircraft that I do not hear while in the air. I also don't think that it is from a bad connection in my antenna since I do not hear it at all on the ground when I am on the grass strip that I operate from.

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douglist(at)macnauchtan.c
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Posted: Sun Sep 04, 2011 3:34 pm Post subject: Hum in Radio?

At 17:13 -0700 9/3/11, Goodone wrote:

 	  Quote: 	
		  I would say that the "hum" that I am hearing is about one octave below middle C. It sounds a lot like the noise that you hear when folks are having trouble with feedback with a microphone in a nightclub. I doubt very much that it is from some noise source on the ground. Maybe it is being generated from something in my aircraft that I do not hear while in the air. I also don't think that it is from a bad connection in my antenna since I do not hear it at all on the ground when I am on the grass strip that I operate from.


	


 I have never played with an Icom IC-A24 but I suspect that it has an automatic squelch scheme that is supposed to turn off received audio until there is a sufficiently large signal coming in on the selected frequency.



If the squelch is set too low it's conceivable that it works properly at your home base where there may be only a tiny bit of RF activity in the neighborhood.  On a large airport there may be a lot of transmitters such as ILS equipment  or radar that are "breaking squelch".  That kind of noise would not  usually get described as hum.  But ILS and VOR use low audio frequency modulation tones that might come thru.



If your receiver has a squelch control you might need to adjust it differently. If it doesn't there is probably an internal adjustment that could be mucked with but somewhere it will say that needs to be done by a man with a radio license. It isn't true though so long as you don't tune the transmitter.



Someone on this list probably knows about that particular radio.



OK on grass strip plus NG on a big taxiway seems to rule out something in your aircraft power system.



-- 

-->A flashlight: is a metal tube used to store dead dry cells. <--

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Goodone

Joined: 13 Sep 2008
Posts: 49

Posted: Tue Sep 06, 2011 9:16 am Post subject: Re: Hum in Radio?

Thanks for your suggestion. I have not tried to adjust the squelch and that certainly is worth a try.

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