Wired Audio on Airband Freq

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TampaTyron

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Guys,
I have a customer who is receiving a weird noise on their unicom frequency whenever someone transmits on the same frequency within several miles of their facility. I have swept the area with a spectrum analyzer and the noise is not present when no-one is transmitting. Ever heard anything like it before? Thank you, TampaTyron

Noises as recorded from their console:
F120.wav
F1203.wav
 

Mike_G_D

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A couple of things come immediately to mind after hearing those files.

Firstly, I take it that this is a relatively new issue. If so, did they (your customers) install any new equipment within their facility (tower?) recently that may coincide with the start of this interference?

Secondly, when you did the sweep with the spec an did you try using it right in the same room where the customers' receiver is? As a subpart to this, locate where the antenna is an do a sweep right there as well - perhaps you can even connect the spec an to the actual user's antenna (during some appropriate time, of course) and try a sweep then.

To me, since you can't see anything on the spectrum analyzer when in the general area it sounds like it might be very local to either where the antenna is or where the radio receiver is. Possibly a new piece of equipment such as a computer or another radio. In fact, it sounds exactly like a heterodyne caused by a local oscillator leakage from a nearby radio receiver that is scanning between two or more frequencies (could they have a scanner or commercial radio that is in a scanning mode nearby?). But it could be a heterodyne from nearly any source very close to either the radio or the antenna. Try using a very short antenna or an RF probe with the spec an and sniff around the radio equipment and the area where the antenna is (if possible).

If the antenna is located far away and not easily accessable and you can't see anything on your analyzer within the radio room then try and find out if any new equipment has been installed nearby where the antenna is.

Also, be sure and check for harmonic sources - try looking with the spectrum analyzer at frequencies of Fc/n especially at odd values of n.

Also, be sure and check the power, data, and audio cable connections nearby and/or connected to the radio for conducted sources using an RF probe as the interference could be coming through those and not be easily received, if at all, by an antenna even close by.

And finally, a long shot but if all else fails - read the specs for the radio receiver they are using. Find out what the internal intermediate frequencies are for the receiver. Sniff around those frequencies with the spectrum analyzer as well, both conducted and radiated, as I outlined above. Make sure you check all IF's (if the receiver is triple conversion then look at all three, etc.). Leakage into the receiver's IF stages from external or connected sources is possible though a well constructed and shielded commercial aircraft radio shouldn't likely have this problem unless the signals were extremely strong nearby but it's worth a shot.

Just some suggestions - good luck!

-Mike
 
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wa8vzq

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In my opinion, it sounds a lot like it is coming from a nearby strobe light. The ones in ground vehicles make a similar sound.
 

TampaTyron

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wierd airband audio noise

I did sweep the room where the radio is located, swept the actual antenna that the customer uses, also had a different outside antenna that I brought for testing. All came up clean for noise sources 10mhz either side of their assigned frequency. I did not sweep IF frequencies (awesome suggestion). Strobes will be fun to track down as facility is very near a downtown area. Customer has been dealing with this issue for several months since another facility (5 air minutes away) upgraded their dispatch position. Since said upgrade, that other facility has been unable to use same unicom frequency due to interference. Said other facility has just muted the unicom frequency at the console and never uses it. Just not used to signals that only appear when there is another active transmission on the same frequency. I can easily resolve intermod, desense, etc but this one has me earning my keep. Keep up the suggestions. Coming. I will schedule a sweep at the "other" facility" this week and report back. FYI, I am the third vendor on this, other shops have long since given up. TampaTyron.
 

TampaTyron

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weird air band audio.

Customer is a large medical center. I cannot hear it on my handheld on the roof near their console radio antenna, but they can hear it when I talk on my portable on the roof. The helos that are close can hear it from my portable, from the console, and from each other. I think my antenna on the portable needs to be better (read that as directional with good front/back). Trying to get the go-ahead for more funding on this thing. ......................Customer just told me that they had some strobe work done. And we are off................TampaTyron
 

TampaTyron

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weird airband noise-update

UPDATE:
Ok, determined that the strobes had zero effect. Also was able to idependently confirm that the other hospital facility is completely not at fault (although their tx/rx audio sounds bad!). Furthermore, was able to verify the noise IS being re-radiated outside of the facility (copied noise on my personal portable on the flight line 100m from customer's tower. Doubt it is IF related due to the number of different types of airband makes/models affected. Starting to look like one of the 20 800mhz freqs that is co-located is generating a IMD product. So, basically any rf on this airband freq that is radiated locally (within a few miles) is received and re-radiated by this site. I am making a small loop probe and am going to be poking around the site equipment with the Anritsu/IFR WHILE my assistant is on the flightline transmitting. The 800mhz site is set up as a fill-in site with panel antennas that only tx/rx about 120 degrees away from the facility. Developing some test procedures now. Unfortunately, I am not able to just unplug the 800mhz rf cables 1 at a time to see which one is the offender. Will advise, TampaTyron.
 
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TampaTyron

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weird airband audio

EDACS site (who is co-located) 800mhz freqs went through rebanding AND the site went from 15 to 20 ch about the same time the problem started. TampaTyron
 

hillscoflx49

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Is this on 122.825?

Or one of the other UNICOM's?

122.700,122.725, 122.800,122.950,122.975,123.000,123.050,123.075

H A/A 123.025, 123.050, 123.075?

Does any of these sites have internal or local cable plant? I would look a local internal CATV ops if its localized to an immediate area as well.

The coax side of things can still leak and uses the airband area for their RF for video and other things... Could be a leak in the local facility or the plant...

Not that it can't be from a lot of things... but its possible bad connectors, cable and etc. could cause the problem.
 

TampaTyron

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122.825..............can only hear/cause it within a mile or so of the facility. Noise seems to favor 800mhz panel antennas facing the west.

cable plant, that is a new one. Will investigate that one on the next trip. Thank you, TampaTyron.
 

hillscoflx49

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122.825..............can only hear/cause it within a mile or so of the facility. Noise seems to favor 800mhz panel antennas facing the west.

cable plant, that is a new one. Will investigate that one on the next trip.
Not that is... interference like this can be a PITA to find at times, but CATV channels A2 to C are in the 108-136 area.

Leakage here can cause all kind of grief and the proximity of 2 major airports, 1 very active AFB, and 2 smaller airports, the CATV operators tend to go after this pretty quick and it gets noticed quick.

Keep us advised with any more details or outcome...
 

mdulrich

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I would think the FCC and FAA would be interested in tracking this down since there is a potential hazard to life issue.

Mike
 

kb2vxa

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Doesn't sound like radar or cable leakage, not like anything I've heard before. Well, maybe a cartoon character stuck in mud. (;->) Being aviation frequencies are given special attention by the FCC and when there's an interference complaint they JUMP, quite frankly I'd get them involved since so far all efforts have failed.

Speaking of cable, since the FCC is so picky about signal egress in the aviation band cable providers conduct an annual aerial survey. I don't recall the particulars at the moment but basically they fly a grid pattern at a particular specified altitude to detect leakage, then if it is found in goes a ground unit to pinpoint the source. Put it this way, the survey may be expensive but the fine is far worse!
 

TampaTyron

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Swept the 20ch 800mhz EDACS system that is in the same room with the equipment. I did not find an issue, also had the EDACS tech turn off all of the channels (one by one) to make sure the mixer/oscillators went quiet (holy crap is the EDACS system noisy on RF!!!!!!!!)...........and no joy. Swept any/all CATV amps and fiber/media converters that I could locate. Have contacted the FCC as this has been determined to be a safety issue. Will advise once they contact me. TampaTyron
 

TampaTyron

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weird audio on airband freqs

Solved the "Zetron hanging up on the EDACS radio" issue. The Orion was not programmed for dual-head operation as it was supposed to be. Fixed the programming and it is working fine. Still waiting to hear from the Friendly Candy Company. TampaTyron
 

TampaTyron

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Weird Airband Audio

UPDATE!!!! Fixed interference problem. Two very nice guys showed up from the FCC in a very well equipped Crown Vic. It took them a bit to understand the nature of the problem, but once they had a grasp on the issues at hand they solved it very quickly! Turns out, it was the other hospital that had just recently undergone a console change out. It looks like whenever the aviation radio at that facility rx'd a signal, it then went into tx mode for a period of time. I am not sure if it was radio programming, mis-wiring of the console equipment, or bad console/radio...........as soon as they shut it off, the problem ceased. Wished I was there to see the radio managers face when he got the message that the FCC was waiting for him downstairs! The major issue is we requested he shut the unit off early on in the troubleshooting (weeks ago), but he would just say "It is not us!" and hang up the phone. All of the equipment connected to his console sounds very poor on tx and his dispatchers are always telling people that they cannot be understood on the radio. Oh well. Thank you for your interest. Same poop, different toliet..........TampaTyron.
 

Mike_G_D

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Thanks for the update!!!! I have been VERY curious and interested in your issue and have been following it in this thread as closely as possible. Talk about a strange one! Well, it certainly explains it all anyway; odd that the radio at the hospital was set up to transmit when it received on single frequency - a real mess in the making!

Again, thanks for the update and good work - have a beer or ten now!;-)

-Mike
 
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