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Question about interference to aviation frequencies

N

neo11

Hi all, I thought I would turn to the engineering experts on this board regarding a question I have about FM broadcasting and potential interference to aviation frequencies above 108 MHz.

Specifically, I am interested in knowing whether FM stations broadcasting in the upper part of the dial (say above 100 or 101 MHz) up through 107.9 can potentially cause hazardous interference to aviation frequencies.

I'm not talking about pirate stations that might use low-quality equipment, but legitimate FM radio stations, using professional grade equipment and broadcasting with a reasonable wattage.

Are there any parts of the country where FM frequencies, particularly large chunks of the FM dial, have purposely not been allocated to avoid interference with the aviation band?

The reason I'm asking is because I was recently reading about a situation in Greece, a country where the FM broadcast band is fairly anarchic and unregulated. The government there recently announced a new frequency allocations plan for the entire country, and while it remains to be seen whether or not they will actually enforce it, it does cut down on the number of available frequencies versus the amount that is currently operating. Additionally, in the country's two largest cities, huge chunks of the FM dial will be left vacant, citing concerns about "interference" to the respective airports in those regions. In one city, the FM dial will stop at 101.0 MHz and in another, will pretty much stop at 103.2 MHz with only a couple of stations past that point, whereas below 101.0 and 103.2 MHz the dial will be pretty much filled in those respective cities.

Personally it all seems a bit fishy to me, considering that government's previous track record when attempting to regulate the radio landscape (long story), and also as I haven't heard of any city anywhere else in Europe or North America where such a huge chunk of the dial is completely vacant over "interference" concerns. But since I'm not an expert on these matters, I wanted to turn to you guys to get your take on this. Thanks!
 
My town has an FM on 102.3 MHz that was a popular AOR station in the 70s. When commercial aircraft would fly directly overhead they would often interfere with this station. I believe they were transmitting on 113.0, which is 10.7 MHz above 102.3. But I don't know if the problem ever went the other way.

When a new 103.5 booster went on the air here, the airport called me saying planes were getting my 107.7 on their radios operating at 111.9. The problem only occurred in the area near the new 100 watt 103.5 site. Since 107.7 - 103.5 = 4.2 ; and 107.7 + 4.2 = 111.9, I surmised that the problem was intermod in the aircraft receivers, even though they really couldn't hear 103.5 at all. They verified that the problem began the same week that the 103.5 site went on the air.

And finally, one time the airport called me complaining that they were receiving my 103.1 on their Instrument Landing System. That operates just above 108 MHz somewhere. That one really concerned me, but a spectrum analyzer revealed that my transmitter was clean. I don't know why that was ever a problem, and I never heard from them again regarding that particular problem.
 
During the 70's and early 80's I had a 107.9 in Boston that had a couple of problems
with interference into the avaition frequencies above 108 mhz.

The first involved the original FM site which was on the tower of the co owned AM
station at 1430 khz.
In Florida I ran into a situation where a station on 95.1 transmitted from the top
of one of their AM towers on 930 Khz.
The transmitter was a Harris FM 20 H with a TE-1 exciter, the AM got into the
FM composite and appeared 930 plus and minus 95.1 which on the high side put
it right on top of our 96.1 signal across town.
Gates - Harris TE-1's and TE-3's were very prone to this problem.
I have run into this problem at several other stations with the cure being very
short double shielded composite cables.

The 107.9 had the old Gates serrisoid (before direct FM) and stereo generator.
The stations Washington consultants were brought in about the same time I
arrived on the scene and I do not remember the details but the resolution
was filters on the three phase closed delta AC power feeding the FM transmitter.
I remember tunable glass vacuum capacitors on each phase to ground in a shielded box on the wall in the conduit going from the AC panel to the transmitter.
The Gates 5 KW AM transmitter was also fed with the same 230 volt closed delta feed but required no filtering.
Once this was installed there were no more complaints from the FAA until the FM moved to the top of Prudential Tower in downtown Boston about 2.5 miles from Logan International.

The first 3 years at the Pru were trouble free, then we got another call from the FAA that involved a different type of interference involving WMEX 1510 whose
transmitter was in Quincy just across Boston harbor.
After much aggrivation the problem was traced to a rusted ground clamp on the roof of one of the airline terminals. The rusted clamp was rectifying WMEX and mixing with our 21 KW (unity gain 2 bay antenna) at the control tower 2.5 miles away.
The mess was finally located with a field strength meter, but the problem being intermittent in nature caused lots of grief.

Hope this sheds a bit of light
 
Here in New Zealand, the area between 106.7 and 107.7 is reserved for low-power FM stations only. Maximum EIRP in this group of frequencies is limited to 500 mW.
No licence is required and choice of frequency is on a "If it is not being used, it's yours" basis.

The department that administers radio frequencies here (Radio Spectrum Management) has been talking about raising the EIRP for low power frequencies. [The other section of the band that is currently available for low power use is 88.1 - 88.7]

They suggested that they *MAY* consider raising the EIRP to 5 watts - but only for the lower end of the band. When I queried them as to why they would not raise it to 5 watts for both ends of the band, they cited "interference issues to the sensitive aviation band".

They are really worried (read = paranoid) about some guy sitting in his bedroom poking 5 watts out on 107.5 and causing a 767 to fall out of the sky.
They did say they were looking at perhaps raising the EIRP in the top end of the band to 1 watt, but that more testing, evaluation and consultation was needed before they could make a decision.

We have high power FM stations working in the 100 - 106 MHz region (one is 5kW EIRP on 105.4) and the point of transmission (Sky Tower) is only 30 km line of sight from a major international airport.

This has obviously been engineered to avoid interference to aircraft - although I don't know exactly what in the way of filtering etc they have on the TX.
 
2 years ago I received a call from the FAA. They wanted to put a spectrum analyzer on our FM transmitter to check for interference they could not identify.

They had pilots (as in multiple aircraft - large passenger planes) reporting hearing our type of music on their cockpit radios instead of the Indianapolis tower.

A senior tech drove in from Texas. Lots of knowledge on interference and mixing of fm signals.

This was reported as an air emegency or some such designation because the tower could not be ehard over the music. Air traffic is AM and we broadcast FM. We are not near an AM site.

They visited many of the transmitter sites in the area with a large truck and lots of equipment. We never received a report on what they found but apparently it wasn't us. They said they never really found anything but were still looking.

For many years the FAA has been running with the FCC interference charts for aircraft interference before granting specific cp's.
 
Most of these problems with FM broadcast interference to aircraft system is due to poor receiver design on the part of the aviation radio manufacturers. This was a huge issue in the eighties when a company came out with a new line of inexpensive transceivers for general aviation aircraft. These receivers experienced significant front end overload problems whenever the plane flew anywhere near a high end of the dial FM station. The FAA wanted stations to reduce power but thankfully that never happened.

From personal experience poor receiver designed followed by mixing products (and external Intermod) are the two main causes of VHF aircraft interference from FM broadcasts.
 
Bad receivers, poor design? Who'd a thunk it?
If aviation receivers are designed this poorly, can anyone wonder why consumer broadcast car radios are so miserable?

What is it about an RF preselector stage that's so dang hard to include in a design?

Oh, that's right, we refuse to employ physically adjusted L or C in new radios.
A varactor diode is NOT a capacitor, though it may be mis-used as one.
The results are about what should be expected from such a kluge.

Where does go about trimming such a capacitor for each unique installation?

Oh, that's right, we don't need to peak anything anymore...it works as well as it works.
What, am I worried there's some mistracking?
Yes, I KNOW there's mistracking, those step-output voltages are just a best-guess for input reasonance in a varactor "tuner-emulator".
I would think the FAA should have some RF engineers on staff, but like all engineering, no wants to pay for it,
so things slowly go to hell and the remaining engineers must to try to correct it and explain it to increasingly
ignorant users.
 
Studio1 said:
We have high power FM stations working in the 100 - 106 MHz region (one is 5kW EIRP on 105.4) and the point of transmission (Sky Tower) is only 30 km line of sight from a major international airport.
Click to expand...

At Auckland International the frequencies are...

tower: 118.1
ground: 121.9
approach: 124.3
ATIS: 127.0

I don't think any aviation communication frequency is put so close to 108.0

I could be wrong, but I think that frequency band is used for navigational aids like Instrument Landing Systems etc. In Auckland their ILS frequencies are 109.9 and 110.3 which are much closer.
 
Thank you for your replies so far.

Based on what I'm reading and what I'm getting out of everything that's being posted, while it does seem that interference with the aviation band which is adjacent to the FM band is quite possible, in many cases it seems to be the result of some sort of minor fault either on the radio station's end, or at the airport end. Additionally, it also seems that a possible culprit is cheap aircraft receivers that get overloaded in high RF fields. It also seems that most cases of interference can be easily solved/prevented. Even in New Zealand, where the very top part of the dial is reserved for only very low-power stations, it does not seem like there is an 8 MHz "gap" in the FM dial anywhere, nor does it seem to be necessary to have such a gap to prevent interference, especially since any cases of interference seem to be preventable. Nor is the top part of the dial completely vacant, even if the stations are operating without a license and with just a few watts.

Am I correct/incorrect in any of these conclusions? As I mentioned in my original post, I am not an engineering expert by any means, but it just seems to be a bit of an extreme step to vacate such a huge portion of the FM dial, especially when I've never seen it done anywhere else in North America or in Europe. Forgive my relative ignorance on these issues and thank you again for your replies!
 
Airports tend to be located near cities, and cities have a lot of high powered FM stations usually located close together. If the airplane flies near the transmitters, intermod products in the aviation band may be heard. The plane will also be in line of sight for a long distance, so there could be interference 20 miles away from a cluster of 100kw stations. An FM trap would solve the intermod problem.
 
Pardon my ignorance but what is an FM trap? From your description I'm guessing that it's added to the plane's receiver to filter out intermod and interference from FM stations?
 
Proposed by the FAA: http://forms.faa.gov/forms/faa7460-1.pdf

ITEM #3.
New Construction would be a structure that has not yet been built.
Alteration is a change to an existing structure such as the addition of a side mounted antenna, a change to the marking and lighting, a change to power and/or frequency, or a change to the height. The nature of the alteration shall be included in ITEM #21 “Complete
Description of Proposal”.
Existing would be a correction to the latitude and/or longitude, a correction to the height, or if filing on an existing structure which has never been studied by the FAA. The reason for the notice shall be included in ITEM #21 “Complete Description of Proposal”.

ITEM #21.
•For transmitting stations, include maximum effective radiated power (ERP) and all frequencies.
For antennas, include the type of antenna and center of radiation (Attach the antenna pattern, if available).
For microwave, include azimuth relative to true north.
For overhead wires or transmission lines, include size and configuration of wires and their supporting structures (Attach depiction).
For each pole/support, include coordinates, site elevation, and structure height above ground level or water.
For buildings, include site orientation, coordinates of each corner, dimensions, and construction materials.
For alterations, explain the alteration thoroughly.
For existing structures, thoroughly explain the reason for notifying the FAA (e.g. corrections, no record or previous study, etc.).
 
Some VOR navaids operate on 108.2 MHz, for example "ARD" in Yardley, PA and "GEE" in Geneseo, NY.

Keep in mind that an FM station licensed for 107.9 MHz would transmit its upper HD carriers across an approximate range from 108.0 to 108.1, which falls in the aeronautical band. The proposed 10 dB digital power increase is likely to increase interference to these VORs, which typically operate with much less power than FM stations.
 
It seems to me that allocation of frequencies "way back when" does not line up with modern transmission issues such as high power.

Perhaps they couldn't have predicted just how much power FM broadcast stations would be pumping out - perhaps they didn't count on FM being used at all?

This reminded me of something. 100 - 108 Mhz has not always been used for FM broadcast here.
It was used for land mobile VHF communications using AM modulation. Only in the last 5 years have they cleared the land mobile users out and regained the spectrum to be used for FM.

We had taxis, couriers, the fire service, security companies, glaziers and plumbers plus many others, transmitting up to 25 watts in this band!

Obviously it wasn't an issue. But now it is. Maybe this ties in with the earlier comments about how receivers have become cheaper and less selective.

Why have sensitive communications services near high power bands anyway? Why not move the aviation services elsewhere? Anywhere but near the FM band!
 
It is true that 100-108 MHz were not always used in many parts of the world, though I believe that they were in use in the United States since the 1950's. I even recall reading that in the UK, it used to be illegal to listen to transmissions above 100 MHz as police, fire, etc. were located in those bands.

It does seem pretty risky that they would stick the FM band next to the aviation band (or the aviation band next to the FM band).

At the same time, however, with so many stations broadcasting around the world from 100-108 MHz, what has the effect on aviation frequencies and safety really been? It seems to me that even high powered stations in that part of the dial can avoid causing harmful interference as long as their engineering is in order and quality equipment is being used.
 
Time for the Greek broadcasters to get organized!

With a large number of FM stations broadcasting in the upper end of the band near airports in the US, there is plenty of real world experience. As mentioned, the problems are created by:

1. Cheap AM receivers used by general aviation (e.g.= small private or corporate aircraft) in the aircraft band that are overloaded by strong signals;

2. IF beat interference (FM station operating frequency plus 10.7 = mirror image signal appearing in the aircraft band;

3. (Less a problem in the US because of FCC regs.) Poorly designed FM transmitters with second harmonic or spurious signal issues.

Suggested responses to the bureaucrats:

A. Allow new stations near airports in the upper end of the band as long as the tower is not within a reasonable buffer zone that includes the IFR flight path in/out of the airport;
B. Allow new stations so long as they use equipment certified as FCC type accepted or comparable EU regs regarding harmonic and spurious emissions. (Most European manufacturers offer FCC type accepted equipment, indeed, a number of US companies import & re-brand European made FM eqipment).

With the considerable US experience operating stations in crowded metro areas (think Boston to Washington D.C. corridor with hundreds of FM stations and an equal number of airports from JFK and Dulles down to various small general aviation fields; the argument can be made that FM stations and airports can co-exist with some reasonable precautions.

Unless, of course, your regulatory agency is imitating the FCC in its "pay to play tactics."
 
Hi Tom,

From what I know, there is a "problem" with one airport in particular, in the northern city of Thessaloniki, Greece's second largest. Apparently, the airport's main runway points in the direction of a mountain northeast of the city, where the FM transmitters are currently located and slated to remain under the reassignment plan if it is ever enforced. The argument that is being made is that planes taking off will fly over the mountain and the RF will overload the front end of the plane's receivers.

The thing that is a bit bizarre about the whole thing is that from 1988, when radio was privatized in Greece, until today, the FM band there has been jam packed with stations, broadcasting at high powers (2-30 kw) from that very mountain, up to and including 108.0 MHz. It seems strange to me that they've allowed such a situation to exist for 20 years if it is as dangerous as they are now claiming to be. Yet, the claim is that not more than 28 stations can operate from the site, and no one over 101.0 MHz. It also seems strange to me because I can think of several airports in the US which might have similar issues in terms of proximity to the transmitter sites, e.g. in Boston, where Logan airport seems to be literally next to the transmitters on top of the Prudential Building, etc. And I know from personal flying experience here in New York that many times planes approaching LGA fly north directly over Midtown Manhattan and over the RF monster that is the Empire State Building, before making a turn to the east and another turn to land at LaGuardia.

To answer your other point, from what I understand, the Greek version of the FCC mandates that all broadcasters use equipment that is CE certified. Whether or not this is always true in practice is beyond my knowledge, but that is what is required by law. However, most big-city stations in Greece are known to use transmitters and equipment of international standards.

Not to become a conspiracy theorist, but past experience following the radio scene in Greece and the government's attempts to regulate broadcasting seem to indicate that the "aviation safety" excuse is being used for other reasons (political, patronage, etc.). As I alluded to in my original post, when the new Athens International Airport began operations in 2001, the government shut down a large percentage of the radio stations which were operating in the city at the time, concurrent with the completion of its licensing process for 20 stations in the city. Before that, stations were operating under a complicated quasi-legal status I won't get into.

Anyhow, the argument then was that more than 20 stations would interfere with the new airport's aviation frequencies. That number almost immediately grew to 28. The following year it grew to 35, a bit later to 39. The new study calls for 40 and there's still a gap at the end of the dial. And, many of the stations that received those new licenses just happened to belong to the country's largest media groups. Some of them had been broadcasting before 2001 and were shut down, others were completely new. There were some who accused the government there of corruption and patronage, of bowing to the wishes of the largest media groups who wanted the "little guys" out of the way to maximize their own profits and ratings. And to be honest, it certainly seems that was the case, especially as I find it very hard to believe that engineering studies can produce such a wildly diverging amount of available frequencies for the same region (20 to 40), and hard to believe that the "best qualified" stations for new licenses always just happened to belong to Greece's versions of Clear Channel, Emmis and CBS.

But that's all politics, though it seems that politics and engineering *may* be getting uncomfortably close in this situation. But purely from an engineering standpoint, how do you evaluate the situation I described, with the runway on the same general path as the transmitters and with the dial slated to end at 101.0 whereas for years there have been stations up to 108 MHz?
 
Concur with your initial analysis that the main issue is $$$$$$$

(I know, should be Euro symbol, but this is a US keyboard)
 
neo11 said:
It is true that 100-108 MHz were not always used in many parts of the world, though I believe that they were in use in the United States since the 1950's. I even recall reading that in the UK, it used to be illegal to listen to transmissions above 100 MHz as police, fire, etc. were located in those bands.
Click to expand...

88-108MHz has been assigned for broadcasting in the U.S. since the closing months of World War II. The 106-108MHz segment was assigned for *facsimilie* broadcasting for a few years - not sure when they gave up on that but no later than 1955 and probably quite a bit earlier.

In the early 1950s some U.S. FM stations were a LOT more powerful than what's permitted today. There were a number of 400kw+ stations.
 
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