• Get involved.
    We want your input!
    Apply for Membership and join the conversations about everything related to broadcasting.

    After we receive your registration, a moderator will review it. After your registration is approved, you will be permitted to post.
    If you use a disposable or false email address, your registration will be rejected.

    After your membership is approved, please take a minute to tell us a little bit about yourself.
    https://www.radiodiscussions.com/forums/introduce-yourself.1088/

    Thanks in advance and have fun!
    RadioDiscussions Administrators

EMI in aircraft band AGAIN

C

ChiefEngineer

Hello all. Within the past few years FAA personnel began using scanners with front end overload to "identify" EMI.

A pilot hears "noise" and the FAA uses a large number of sources not to identify the specific noise but to track ANY noise. There is no attenuator or band pass filter on the receivers.

In Jan this year one of our stations was shut down (150 W at 50 feet) as it was causing interference in Livingston TN. (170 miles distant) Later another station was found to be the real culprit. Our station was included (and shut down) as near field measurements identified a drifting noise in the 120-137 Mhz spectrum that went away when the transmitter was shut down. Measurements were taken with no attenuator to place the offending signal below the rail or use of band pass filter.

The same issue has resulted in FAA removing power to another FM transmitter. Licensee was not contacted prior to. In this case the FAA had interference AGAIN, then began looking for any source of rf and any signal using non tuned radios (scanners) which may be the product of intermod within the receiver.

Even though the FAA admitted the previous case of interference was not the result of the FM station they shut down before, they shut another of our stations off with no prior notification. They also royally irritated our leased site owner. When shut down they found an rf drift (which could be intermod) with a scanner that was not interfering with aircraft.

My questions include:

1) What might cause rf drifting in the 120-137 Mhz spectrum? The disabled Transmitter is rock solid stable, has a band pass filter, and is FM. The interference is reported to be AM in the aircraft band but the FAA notes the signal they hear on scanners is FM.

2) What can be mixing to cause such a signal? The FAA notes no instability in the FM signal but notes on intermod prone scanners the drifting signal (not neccessarily the same signal heard in aircraft) goes away when they have disabled our transmitters.

3) Is there a way to define any signal which might drift as a product of intermod through research papers or scientific evidence?

4) Here I am with another improbable problem and I have gone through this before. The FAA will n ot use the protocol of tuned receiver (using scanners), will not use band pass filtering, will not use attenuators to lower the signal below the rail. The transmitter has no spurs noted on my SA using protocol, has a band pass filter, and is not drifting.

I have gone through this previously. I have a transmitter that the FAA has shut down without notification. I have had a slew of FCC Agents through that have no background in receiver overload. Any ideas would be appreciated.
 
Spectrum analyzer, consultant, and a good lawyer.

Also, if you have a local piss-ant grass strip airport, campaign to shut the thing down as a nuisance.
 
CE, anything else at the site? Any antennas still connected to non or intermittently running transmitters (on any frequency)?
 
Other than cooperating with them, do they even have the RIGHT to shut you down? Shouldn't that actually come from the EB at the FCC? Tampering with your equipment without first contacting you to me sounds techincally like a federal crime. Am I wrong on this? To be nice, most of us broadcasters will kill the carrier for a few seconds to help them determine if our signal is truely the problem child or not. Unless they can prove with real test equipment like a decent spectrum analyzer employing attenuation to prove even the spectrum analyzer isn't being overloaded, I wouldn't be inclined to let them keep me turned off. I'd tell them to come back with someone that actually has real test equipment, and preferrably the FCC themselves. It sounds like you have someone in your area that thinks they have athoriti when they really do not.
 
That's where the good communications lawyer comes in. Along with the consultant to show tests that indicate no spurious emissions out of band.

Otherwise the bureaucrats will just bulldoze past you. Try to fight them on your own and they will scream about "danger to lives" even if it just involves some weekend pilot in his 20 year old Cessna momentarily loosing contact on approach.
 
120.0-21.4 (2 IF's) = 98.6
129.4-21.4 (2 IF's) = 108
Simple images in the Aircraft radios often do this. The "wandering" may be the instability of the Local Oscillator in the Aircraft receiver. One of our local (now retired) FAA tower chief's who was also a ham indicated that a large number of the interference complaints received in the Las Vegas Area were a result of just this problem. Front end overload accounted for several others. I have 3 AM's within 2 miles of the North Las Vegas Airport (VGT) and 1 20KW on 1100 can even cause problems on some poorly constructed 120 MHz plus radios. (note I didn't say cheap). A quality spectrum analyzer is your first defense. Sounds like you have a less than competent FAA tech trying to pass the buck. Be friendly, show them and DOCUMENT the SA results and see what happens. They suddenly get a lot more cooperative when you show you have docmented and are prepared to defend your position. Good luck
Bill Croghan, Lt. Col, Civil Air Patrol,
Chief engineer, Lotus Broadcasting, Las Vegas.
 
Information on the site. There is a 50 kw fm 1 mile away at 97.3, with a co-located Am at 1460. Our NCE is at 90.5 at 200 watts approximately at 100 feet.

There are many empty antennas on the tower plus several non licensed pizza box dishes.

Had this happen several months ago at another site. (3 transmitters had the same noise. All recent models. All have a low pass filter installed.) When I mention overload I get a blank stare. I ask what was causing the problem and tech says,"your transmitter." "It went away when i turned it off."

"I listened on a scanner and heard the audio of the station in wideband mode." He did say the audio was not identical to the 90.5 audio. Months ago it was the same. Overload on a scanner doesn't mean anything to the tech. When I mention a channel specific filter again it seems foreign.

I was told that overload can't occur except in the enar field. "It can't take place 5 miles away." The problem was reported to be causing "interference" with the St Louis tower. A 10 w tower transmitter could not overcome it over a hundred miles distant. Interference sometimes mean a pilot hears noise, not always that it covers a signal so I still don't know what interference is yet for sure in this case.

I am a ham also, since 1981. I have been in Broadcast Engineering since around the same time. After seeing mixture and front end overload I am shocked that people finding rf for the FAA aren't aware of this.

Because this drifting noise sometimes doesn't contain fm stations in other areas I am hopeful I can find someone who has had the same problem and willing to comment on it.

I sent an email to ask about relocation to another frequency which might fix the problem. Commission Staff had no knowledge of the station being shut down. In our local site we shut down, even broadcast tones so pilots could identify us, and all while I listened on the tower radio. Never did hear our tones coming back and neither did pilots. In the rush to "find something" it seems when there is interference they drive to towers and listen on a scanner.

When I asked about the math team in the FAA noting that this was a mix I was told they had no way of compiling the many licenses and unlicensed frequencies to determine where the problem originated.
 
My "scanner" (an old Yaesu FRG-9600 "Communications Receiver") overloads on FM signals at 17 miles from the transmitters.
Clicking in the RF Attenuator makes most of the phantom frequencies go away.

Do they have a way to switch an attenuator in and out on their "test" scanners?
Also, a good way to test might be to somehow find a scanner, or some aircraft radio, that has a non-standard I.F. frequency (not the usual 10.7 MHz I.F.), and see if there are image frequencies (the FM station, +/- 10.7 and 21.4 MHz) that show up at a different spot on the "odd-I.F." radio.
 
If you are still shut down, better ask for an STA from the Commission to avoid future problems.

The FCC field inspectors we've come across in recent years have not been engineers--just glorified clerks. Suspect its the same with the FAA folks. Trying to give an engineering explanation to these folks is not going to get you anywhere. They are right and you are wrong.

As Bill suggests, get documentation showing that your site is clean (which might also identify some local mixing re-radiation that might eliminate an actual spur), then give it to this "tech" & tell him you are turning the station back on. If they still object, time to get a communication lawyer involved.
 
I appreciate the information. I have searched and have found FAA circulars on mixing and IF but this seems to be totally ignored in our discussions with the FAA.

I found several useful FAA bulletins on using an attenuator. Seems also foreign to the tech.

I would like to find a presentation on the proper method for looking at interference. Tom Silliman demonstrated this many years ago using an antenna, bandpass filter, attenuator, then spectrum analyzer. The best method is a port from the transmitter that bypasses external sources. This seems to be an ERI standard but I haven't found anything in writing. An outline of this process and the reason for doing this (rail of the signal, overload, other signals mixing) would be helpful. Some of the FAA material was helpful but didn't explain why you isolate the frequency to look at only the "offending" transmitter.

It hasn't been 30 days so I haven't asked for an STA. This is a full power station.

The tech seems to be willing to have another engineer look at it as he is stuck on : "I shut it down it must be you as it went away." I am quite stuck here as I went through this witch hunt already this year at another station. It was blamed on a faulty transmitter. (3 different transmitters had the same problem.) When the SA was attenuated the noise went away but the understanding of the process is not there.

The station is not a rich company and does not have the resources. I am not paid but volunteer my time. Any diagrams, educational amterial would be appreciated. I have googled extensively.

Thanks
 
Well, as a former broadcast engineer now working with non-FAA towers on comm. And navigation systems, I can tell you that they do have the right to shut you down until your gear is ruled out as a cause. I also suggest you comply. Yes, a lot of them need more hard engineering knowledge and are tough to deal with BUT…IF they are right and your refusal to shut down results in an aircraft accident, woe be unto you and anyone else who can be held liable. There are still very strict logging requirements in the navaids and aeronautical comm world. I keep meticulous logs, and yes, I’ve been sued over an air crash. Those logs saved me.
 
I did a bit of Google Searching for some of those FAA documents, and suddenly had a thought...
Have you looked for "Talking Winnebago's" around the area?

Many RV's have amplified antennas that are/were used for TV reception. These will often go in to oscillation, especially when there is no TV set connected (the amplifier is "un-terminated"). I've seen them radiate for many city blocks. Add a parked RV and a battery charger, and you have a nice, but ugly, modulated signal...most anywhere in the bands. Turning off a nearby, strong signal source (like am FM transmitter) can change the frequency of the spur, or eliminate some of them.
 
Oddly enough here is what has happened to date:

Checked transmitter at the monitor port and there are no emissions outside of the proper (fundamental).
The FAA technician was a guy named Mike out of Kansas City. After he shut the transmitter off he promised to provide detailed information. To date he has not provided it, and later emailed if I wanted information to contact the FCC in Chicago. (No contact person reported) There has not even been a phone call with the reason for the shut down or any engineering.

We had already contacted Washington after the shut down. They indicate the station cannot return to air and suggested a new frequency. We still don't know why.

Apparently this is a mix somewhere outside of our transmitter. I have found other cases of this but it is usually found before a frequency is allowed used. Because of the secrecy and lack of anything in writing (or due process), the addition of the FAA to homeland security seems rather National Socialist.

For those with money to fight it I suppose this is less a hassle.
 
Seems to me, if they can let LightSquared run on frequencies near GPS for a test (in Las Vegas), they could let you run while doing tests, too. They'd have to put out a NOTAM, but ought to be able to let you run at several power levels and do some work to locate the problem. Could be a "Talking Winnebago", could be mixing in somebody's PA, could be mixing in some dissimilar metals someplace.
You can't find it if they just shut you down. And, the next transmitter (of any kind) to go on the air someplace could start it up again.
 
In fairness the new frequency is far departed from 3 channels. It is also much higher power. If we had to mvoe 3 channels this would be a mess. It also involves a new tower site but this is okay also.

The problem I see is a lack of due process. Not sure why this is the case, or, the unwillingness to let us know why and what is involved in the mix.
 
There has not been anything in writing yet.

Currently 2kw A going to 10 kw B1 if approved. My math shows the other part of the mix a local 50K B and Class 4 D 1 kw. I believe the AM is mixing with the exciter (RG-59) of the B and spewing fundamental plus fundamental plus fundamental and 2nd harmonic AM. The FM shows up several places on the FM band where the AM and it's 2nd harmonic add to the FM.

Transmitter site move 15 miles. Only a sliver of the FM's current NCE will still receive 60dbu service. The 54 should complete the task.
 
ChiefEngineer said:
There has not been anything in writing yet.

Currently 2kw A going to 10 kw B1 if approved. My math shows the other part of the mix a local 50K B and Class 4 D 1 kw. I believe the AM is mixing with the exciter (RG-59) of the B and spewing fundamental plus fundamental plus fundamental and 2nd harmonic AM. The FM shows up several places on the FM band where the AM and it's 2nd harmonic add to the FM.

Transmitter site move 15 miles. Only a sliver of the FM's current NCE will still receive 60dbu service. The 54 should complete the task.
Click to expand...

I've heard that before -- an AM transmitter at the same site as an FM, hearing the AM audio on FM on frequencies offset by the AM frequency. (i.e., with the AM on 1480 and the FM on 98.1, I was hearing AM audio on 96.6 and 99.6) Heard through the grapevine the FM exciter drawer had been left open. No harmonics of the AM were involved (best I could tell) but I can certainly see that happening.

The FCC has been known to allow things not ordinarily permissible in cases of FAA impasse.
 
I had this problem with the AM signal getting on the FM and being heard + - the AM frequency. Changing the cable that went from our Optimod to the exciter to a good double shielded cable fixed that problem for me.
 
Try this one...An FM broadcast station showing up on Ku Satellite?

Had an FM station getting in to the 70-MHz I.F. cable on a Ku uplink at the same site. It was being broadcast all across America, offset from the satellite uplink frequency by the difference of the FM channel from the 70 MHz I.F.
A new cable, and some ferrites, fixed it.
 
Status
This thread has been closed due to inactivity. You can create a new thread to discuss this topic.


Back
Top Bottom