F

flyboymd

I'm New Here
Hi guys....without getting into the specifics of equipment, I want to ask about my radio static that only happens whenever I fly near a large antenna array installation. It is so bad I cannot hear transmissions. Things clear up and go back to normal when I get further away from the array. It has plagued me for a while and am now trying to trouble shoot it but don't know where to begin. Any ideas??
 
flyboymd said:
Hi guys....without getting into the specifics of equipment, I want to ask about my radio static that only happens whenever I fly near a large antenna array installation. It is so bad I cannot hear transmissions. Things clear up and go back to normal when I get further away from the array. It has plagued me for a while and am now trying to trouble shoot it but don't know where to begin. Any ideas??
Click to expand...

There is probably nothing you can do about it. Changing the radio to a different type may help or make matters worse depending on the construction of the Radio.

The first possibility is that your array is in fact transmitting at the frequency that you have tuned into. This is normal and due to the fact that any transmitter creates weaker harmonics at higher frequencies (usualy at integer multiples of the actual transmit frequencies but it is possible that other frequencies are also a bit contiminated due to mixing effects in the transmitter).

The second possibility is due to the way modern receivers work. They create an internal frequency that is a fixed offset from the desired frequency (in our radios at 45Mhz offset). This is then "mixed" with the signals present at the antenna resulting in the desired receive signal appearing at the output of the mixer at a fixed frequency (the same frequency as the offset - so 45Mhz in our case).
This is done so we only have to make good filters for a single frequency.
The trouble with mixers is however that they can also receive other frequencies due to imperfections in the mixer or to an effect caused "intermodulation" (I'll ignore the obvious "mirror" frequency here).
Intermodulation happens when a signal (at any frequency) is present at the mixers input at a level that exceeds its "linear" operating region. This creates multiple frequencies of the input signals (all of them) and they can all happily mix with our local oscillator frequency resulting in what you would call a "mess".

This is most likely what you are experiencing when flying close to a powerful transmitter array.

The only way to improve matters would be to find out at what frequencies these transmitters operate and then introduce custom filters in your antenna feed to reduce the power of these frequencies at the mixer input.

Hope that helps (at least a bit).

Rainier
 
Thanks for your reply Rainier, it is really appreciated! Wow, you must be an EE? I was thinking that I needed to replace the antenna coax cables or check for some type of ground issue, certainly not what you proposed!

I used to fly in New York up the Hudson corridor and had the same issue when flying past the Twin Towers due to the massive TV and radio antenna array atop the buildings, while self announcing on 123.05. Now when I fly past Mount Wilson near the eastern most part of the San Gabriel Mountains in Los Angeles County (near Pasadena, CA) [note that it is the home of a huge antenna array used in relay broadcasting of radio and television for the greater Los Angeles area], i talk to SoCal on 135.05, 125.5, or 124.6 and also get the big static on both comm radios. So does your explanation explain the various frequencies I experience this on? I must admit it was a bit more technical for my immediate understanding, and if I sit with it for a while I am sure it'll make sense in the short run.
 
Do you have more than one radio??

...and do you have an intercom installed?? If so, then you can try this...
While flying near the "problem" area, turn off the radios one at a time and note if the "noise" is still present.

Is the noise only on one or two frequencies, or is it across all channels?

Then turn off the intercom...is the noise still present?

Do you get complaints of transmission problems...is this only a receiver issue?

Can you disconnect the coax at the antenna and install a 50 ohm terminating resistor at this end of the coax, then fly again (do not transmit on this radio) and do you still get the noise on this receiver?

Can you disconnect the coax at the com antenna connector, install the 50 ohm terminator at the radio end and is the noise still there??

We are trying to determine if the interfering signals are coming in through the antenna, or if the signals are flooding the entire radio systems and entering through the openings of the case and installation stack.
 
Noelf:


Yes I have 2 comms. I will most certainly try your suggestions to see what happens. In the meantime to answer your questions:

1) Yes I do have an intercom
2) Noise is across all channels
3) Don't remember ever getting complaint about my transmission. It is I who cannot here when receiving during these spells
4) Yes I can disconnect the terminal at the antenna.

WHere do I get this resistor?

Appreciate your input!

Michael "flyboymd"
 
Isn't the FCC supposed to exist to prevent just this kind of interference? Since this could be a safety of flight issue, I would call the FCC and the FSDO and ask them to investigate whether this facility is in compliance with their permit. I wouldn't assume it to be your equipment if this is the only place you have a problem.
 
Yah - Some places are just bad. For instance in a number of different planes/radios I can determine if the Stanford Linear Accelerator is operating. If flying over it, the San Carlos twr freq always breaks squelch when they are running...
 
The 50 ohm terminator...

...is like the p/n A24564-ND from Digi-Key. You can also construct your own with a 50 ohm resistor soldered into a BNC coax connector. When using one of these terminators, just be sure you do not use the transmitter function of the radio that this is attached to. Your transmitter has enough power to make the resister smoke!

If you happen to know someone who is an amature radio operator, get in touch with them. They will either have, or know someone with, a "proper" termination or even a shielded "dummy load". They can also be quite helpful in tracking this noise issue down.

Now, for a few more questions:

1. Is the "problem area" noted on a sectional chart as a hazardous radiation area?

2. When you key the mic to transmit, do you still hear the noise in your headset when you are talking...and / or in the "dead space" between your words? (Looking for a sidetone problem here).

3. Does your intercom have a "Fail Safe" feature, and can you try that... (intercom powered off, and tx /rx on one of the radios.

4. What kind of radio stack installation do you have, and are you using ANR type headsets?

5. While flying in the problem area and hearing the noise, if you unplug the mic cable (all of 'em), does the noise go away? (high power RF could be impinged upon the mic headset wires and cause the intercom to "think" you were talking (sidetone) noise.)
 
Noah said:
Isn't the FCC supposed to exist to prevent just this kind of interference? Since this could be a safety of flight issue, I would call the FCC and the FSDO and ask them to investigate whether this facility is in compliance with their permit. I wouldn't assume it to be your equipment if this is the only place you have a problem.
Click to expand...

FCC dictates the limits of spurious emmisions for various classes of transmitters.
Unfortunately it is not technically possible to create a signal at a few hundred or thousand watts that is spectraly clean (i.e. only transmitts at the intended frequency).

Typical limits range from -40 to -60db allowable harmonics - to put that in easier numbers - between 100th and 1000th of the main transmitted power. So if you have a shortwave transmitter (as example, these tend to be VERY powerful and have big antenna arrays) at say, 50.000 Watts power and it transmits at 30Mhz then it is likely you will get 500 Watts of signal at 60Mhz and somewhere around 100W perhaps at 120Mhz.
Now, if you are flying close to that transmitter, even if your radio is not tuned to 120Mhz, it is quite likely that the 120Mhz signal is simply going to flood the mixer in your receiver and render it useless. Depending on the construction it will now receive just about anything it sees at its input. This is often recognized as static but you can often hear partly demodulated sound like music or voice.

Now, having said this - your transmitter at perhaps 6-10W is also a problem. It also generates harmonics and it can interfere with nearby TV receivers. Of course, you would have to transmit and fly very low over the TV arial. Perhaps you can try this next time you have an argument with your neighbour and he wants to watch the game...
Does not work so well with cable or sat TV though...

The FCC regulates what is reasonably possible using current technology but does not make impossible demands.

I recall an accident related to this - this was in Germany, Bavaria of all places. The then new fly-by-wire Tornado spun out of control whilst flying close to a powerful radio antenna. It was quickly learnt then what kind of havock a powerful radio signal can do with any kind of sensitive electronics (not just radios).

On a second, similar note, I performed an emergency landing on a field as both CHT and EGT readings all of a sudden went way out of bounds. At the time I had two sets of electronic gauges (no, not made by us - MGL was still in the distant future then).
Once on the ground, everything was just normal and eventually I decided to risk a take-off (it was a good field). Just after take-off the same thing happens but this time I looked at my fuel flow - normal and also the engine sounded just fine so I pressed on. a few minutes later the problem went away. How strange. On another flight in the same area the same thing happened (except this time I ignored it). Behold, close by was a very large short-wave transmitter array. So that was it...

Once MGL was founded and we made our first instruments back in 2000 - guess where I would take them for a good, practial RF interference test...

Rainier
CEO MGL Avionics
 
Hey guys: I will get back to you with some answers (Noelf) this evening....in the meantime I really appreciate all your comments and inputs!

Michael
 
noise

I have a similar problem with my SL40 and GPS.
Every time fly within 2-3 miles of the Cedar Hill TV towers in south Dallas my radio starts to crackle and my GPS 396 says Satalite Reception Lost.The other RV's I fly with dont have this problem. I suspect I have a ground loop problem in my wiring that is picking up the towers.
 
Rainier Lamers said:
Typical limits range from -40 to -60db allowable harmonics - to put that in easier numbers - between 100th and 1000th of the main transmitted power.
Click to expand...

Ranier,

Unless I'm mistaken, the Decibel formula for power is:
dB=10LOG(Pi/Po)

So -40 to -60 dB is 1/10000 to 1/1000000.

Some transmitters have more spectral purity issues than others. Some newer AM transmitters use a modulator based on switching technology which really creates some wide band artifacts. But if you use enough power it really doesn't matter if its a pure signal, the raw field strength is likely to infiltrate the receiver through any connection with the outside world, and once it gets into a P-N junction in the receive path, you have your own harmonic and wide band noise generator.

Paige
 
PaigeHoffart said:
But if you use enough power it really doesn't matter if its a pure signal, the raw field strength is likely to infiltrate the receiver through any connection with the outside world, and once it gets into a P-N junction in the receive path, you have your own harmonic and wide band noise generator.
Click to expand...

And the P-N junction doesn't have to be inside the receiver. Dissimilar metal interfaces on your antenna mount can act like a rectifier too.

--Paul
 
?ELT antenna?

My home field (AXH) is quite close to a large group of commercial transmission towers southwest of Houston and produced similar problems for me when in the pattern in my Cardinal.

An avionics tech suggested I disconnect the ELT antenna and give it a try. Sure enough, the spurious signal was apparently resonating with the ELT antenna and effecting the audio in some way. Disconnecting the antenna eliminated the problem and shortening the antenna by 1" significantly reduced the interference so that I can maintain communications while in the pattern. My ancient ELT continues to provide a strong signal with its slightly detuned antenna.
 
First confirm it is not an audio problem..

If it is is RF interference identifying the actual mode of the interference will be difficult. Many interference mechanisms are difficult to identify without some sophisticated equipment. Some simple tests will eliminate IF (intermediate frequency) interference.

I believe it is primarily an issue with the design of the radios. If it becomes intollerable then let the manufacturer know.

In my experience the most likely cause of the interference is due to saturation of the first stage of the radio by the strong out of band signals. If the transmitter farm is in the VHF band - these signals are not too far from the airband our radios are optimised for.

The first active stage of the receiver (the stage that is likely to be saturating) has to perform across the baseband of the receiver 118 to 136 MHz in our case but must reject very strong signals only tens of MHz away to an extent that the first active stage does not saturate.

To achieve this a passive filter is inserted in the front end, the quality of this passive filter will determine how well these signals are rejected. If the receiver was required to operate at only a single frequency then cascadeing several same frequency hi-q filters will achieve very good rejection.

Unfortunately, the airband radios have an 18MHz bandwidth at a center frequency of 127MHz - it is difficult to build and tune passive filters to pass this band and brickwall the large out of band signals causing the interference. To cover such a band several filters are cascaded but each tuned to slightly different frequencies within the band. Out of band rejection performance suffers.

The sort of filter required tends to be large is physical size taking valuable PCB real estate. Hand-helds tend to peform poorly compared with aircraft radios because they generally don't have the space for these filters.

Radio designs have had to improve as the frequency spectrum becomes more congested by the competing signal sources ever increasing in power to keep their own heads above water.

One practical sugestion is to put your antenna on the top of the aircraft, the airframe will provide some shielding. This would give you 3-4dB that may just keep it from saturating. Also, I have long suspected that the gearlegs may be resonant at some RF frequencies not far from the airband - keeping the antenna away from these may also help.

Doug Gray
 
Rainier,

We also lost some UH-60A Blackhawk helicopters back in the 80's from rf interference, causing the stabilator to slew down in flight while flying nap of the earth(noe). The Army installed manual slew switches to override on the control stick.
 
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