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FAA Air Interference

C

ChiefEngineer

I have at least several weeks invested in FAA interference.

Several years ago Dr. Nasser of the FAA brought his truck to Indiana. He was looking for interference. Didn't find any.

Aircraft uses open unsquelched Am communications. No PL. Interference is not then anything that covers air traffic, it is anything they hear.

A translator in Mount Carmel Illinois was shut down last week as it was re radiating the signal of 93.9 which according to the FAA actually covered the tower at Livingston TN. Now that it's down they are tracking anything they hear.

They are using an airplane and flying low to hear signals on aircraft frequencies. This is less an attenuator, cavity filter, etc. They may hear intermod which would be 80db down.

FAA visited our site and was listening on an AOR (no cavity, unfiltered) and we shut the transmitter down for tests. They continued to note the intermod with our facility off. This is a 150 watt non com at 60 feet. At 35.000 feet this is los to the aircraft 300 miles distant.

Spectrum Analyzer showed no spur on the Anritsu from our transmitter, it continued after our equipment was shut down. It seems the signal is adopting our audio as it is somewhere close. We have tried an additional transmitter to eliminate the transmitter.

They haven't heard the audio enough to isolate the station. We broadcast 30 minutes of alternating 700 hz and 1 khz tone to help the pilots identify it and they did not report hearing the tone.

Part of the problem is determination of interference being simply heard. Even a transmitter with a pure fundamental and emissions 80db down off channel could join a nearby transmitter. Fm becomes Am on the aircraft radio.

Hearing a relatively low power station at 300 miles? This is a whole other question. We can't hear it 15 miles. I will let the board know if an inadvertent discovery on propogation is developed here.

The biggest problem is the inability to hear the interference at either the tower or at our facility as the only receivers hearing this are at 35,000 feet 300 miles distant. No recordings, no clear link to our station. They report hearing oldies which is what our station plays.

Last year both our stations were heard (two different broadcast frequencies miles apart on separate days on an unrelated aircraft frequency) which was determined to be an internet feed to someone with an aircraft transmitter. (A disgruntled person with an all mode ham rig?)

I have noted Channel master TV amplifiers reradiating and have located same. There are no tv homes within a quarter mile.

We have SA displays and the FAA noted a spur that remained after our transmitter was silenced. What is the best way to locate a spur and has anyone had a similar issue in the past?

I want to be able to eliminate our facility as the culprit. Ideas?
 
I would assume you not only shut down your transmitter but the exciter as well. That being done, I can't see how they could hear your station except if your stream were still up and someone was rebroadcasting it on an aircraft frequency to get you in trouble. Or they were using an old recording of your station.

If you can get a recording of the audio heard in the plane at 35,000 ft it might help determine what was up . If someone were rebroadcasting you using an aircraft radio the audio quality would obviously be poor and you would know it wasn't caused by your station. Likewise, the recording would give you a hint if it was intermod due to somewhat better bandwith. There is always the possibility that the problem is being generated in equipment aboard an aircraft and that's why it is heard so far away, at that height I've picked up incredable FM broadcast signals hundreds of miles away. If the intermod was being produced in an aircraft even a very small amount of power would radiate far.

Tha same malfuncting aircraft flying over the midwest would create the signal heard on other aircraft passing nearby.

The fact that the offending signal can't be heard at the tower or at your station kind of exonerats you but doesn't explain it. Good luck.
 
I agree. If you're transmitter is turned off and they still get the "oldies", they might want to look elsewhere. The good news is that it's no longer your problem really. That's the wonderful thing. The monkey has returned to the overpaid FAA guy's back. :)
 
Unfortunately seeing a spur and having the transmitter off at the same time is only real and not the imagined "aha" here is the solution to our problem. Presumed guilt as in our legal system.

Even though we are sure last year's problems were internet feeds introduced into an aircraft frequency radio, we were on the radar. Hearing two geographically isolated stations with independent programming and frequencies on consecutive days over 150 miles away helped them determine the problem and the interference stopped.

The problem is that most people don't know how to properly set up a Spectrum Analyzer. Tom Silliman and staff demonstrated this years ago. Antenna, attenuator, bandpass cavity, attenuator, Spectrum analyzer. Some people run their SA above the rail which introduces intermod in the SA.

The FAA uses an airplane with a standard radio to fly suspected trouble areas. This means when they "hear" noise in the near field of an Fm because they are flying at 100 feet and possibly within 50 feet of an antenna they have the epiphany this is the culprit. Leave out the problems from airspace perspectives on the flight that resulted in hearing the noise.

Using this method it would be probable that any noise on an airplane radio will require justification from any station playing oldies if a pilot hears anything from the Beach Boys.

I am asking that the FAA record the noise. This process has left out anything reliable or repeatable. I can't listen at 35,000 feet 300 miles distant. Because we worked with them last year our name came up. They can't identify any of the songs by title, just that it was oldies and country music. I mentioned the half hour of 1 khz tones we played that they couldn't hear.

Having dealt with intermod most of my adult life I thank God for HD tv. People never hear FM on their televisions now. I bet someone would make a ton of money switching aircraft radios from AM to digital encrypted FM. This would make the skies safer as terrorists couldn't hear traffic. Scanners too. Whole FAA thread on laser pointers. Biggest culprit caught was a fired air traffic controller.
 
Are they hearing I.F. Beats, such as 10.7 and 21.4 MHz above your channel. We had a big SBE meeting once, with nearly everybody including the FCC there to discuss RFI. It was noted that some aircraft radios are very susceptible to I.F. Images with strong signals present, and many are even single-conversion receivers....very susceptible!

Also, if they are flying in the beam of a strong signal, it may overload their receiver, and cause a beat between your signal and another station, WITHIN their radios. The Spectrum Analyzer might be better if placed in the aircraft.
 
I had a complaint from a control tower that our Class A FM on 98.3 was intermittently interfering with their CTAF of 119.4. All that was heard was garble. It remained evn after I turned off our tx (filaments, exciter, etc.). Culprit was an FM booster amp a guy driving an SUV had installed. The FAA tech found it using DF gear.
 
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