FM radio spy?

[Dragon Tamer]

I was working on my radio's power supply the other day and listening to a station on the radio, and all of a sudden I heard someone on the radio (obviously not part of the song that was playing). It sounded like an aircraft radio, which wouldn't surprise me considering how many planes fly over my house on a daily basis.

I was just wondering how common it is to get interference like this with this kind of radio. Don't ask what kind of receiver it is, I don't know. I would also like to know if I could tell the pilot of the plane, or the airport about the problem to see if the problem could be fixed (all in a legal fashon of course)

I've had this problem with several radios, one really old one, and one that is newer. But my good suround sound radio gets no interference at all.

 

[Gary B]

The bottom of the VHF aircraft navigation band is just above the top of the FM commercial band. Some FM radios are also sensitive to AM. If you were tuned to the top of the band, you could have picked up a message on the local VOR.

 

[transistor495]

The problem may be with your low quality receiver(it just receives the ghost signals). Better you won't interfere with them

 

[Nigel Goodwin]

It's nothing strange, it's 'image interference', the radios you're hearing it on are poor quality ones.

 

[Dragon Tamer]

That's what I thought, but the frequency was at 102.9MHz. Is that still high enough to pick up these radio waves?

Almost every radio in the house was turned on at the same time and tuned to the same station. Out of the 8 radios that were on, 3 of them picked up the signal. One radio (the one I was working on) was made some time in the 80's, The second radio was bought sometime in the late 90's, and the third radio was brand new. The only conclusion I had was that they all had the same kind of reciever in them.

It surprised me though that the radio on my night stand (made some time in the 70's,) did not pick up the transmition.

I think that I'm just going to stay out of the whole thing since only a few radios were affected.

 

[Nigel Goodwin]

That's what I thought, but the frequency was at 102.9MHz. Is that still high enough to pick up these radio waves?

Almost every radio in the house was turned on at the same time and tuned to the same station. Out of the 8 radios that were on, 3 of them picked up the signal. One radio (the one I was working on) was made some time in the 80's, The second radio was bought sometime in the late 90's, and the third radio was brand new. The only conclusion I had was that they all had the same kind of reciever in them.

It surprised me though that the radio on my night stand (made some time in the 70's,) did not pick up the transmition.

I think that I'm just going to stay out of the whole thing since only a few radios were affected.

Cheap radios don't track the tuning of the front end, so image rejection is poor - better quality radios track the front end, maybe even with two tuned circuits, this gives greatly improved image rejection.

 

[audioguru]

My cheap Sony Walkman radio has an untuned RF input amplifier that easily gets overloaded by many strong local stations because its input does not have automatic gain control. So it has a manual "local-distant" switch instead to attenuate the signal.
In the distant position it does not pickup image interference from the nearby international airport but has the strong local stations all mixed together smeared all across its dial instead.

My cheap clock radio does not have enough sensitivity to pickup anything but strong local stations.
My home stereo FM tuner picks up many distant and all strong local stations without any interference.

 

[JimB]

Not surprising at all.

The radio will have poor image rejection.

Assuming an IF of 10.7Mhz and the radio using "high side injection" (ie the local oscillator is above the received frequency), when your radio is tuned to 102.9Mhz, the image will be at 102.9 + (2 x 10.7) = 123.4Mhz, which is nicely in the middle of the aircraft band.

What is the name of your local airport?
It is quite easy to find the frequencies used.

JimB

On edit:

Looking on Google Maps, your nearest airport appears to be Wilmington New Castle
The tower frequency is 126.0Mhz, UNICOM 122.95Mhz and Ground 121.7Mhz.
All the info is here:
http://worldaerodata.com/wad.cgi?id=US95651

 

[MikeMl]

Do home FM receivers use high-side injection? I thought that they specifically used low-side injection just so they don't screw up the aircraft Nav band between 108 and 118MHz.

Another possibility is audio rectification in the audio stages of the entertainment system. Aircraft radios are AM; AM can be detected by any non-linear rectifying junction, including a cat-whisker on galena crystal, blued razor blade, galvanized gutters, fillings in teeth, and IC audio amplifiers...

Back when I operated AM on the 2meter Ham band, I got into lots of my neighbor's consumer grade electronics and even the phone line. It took lots of 0.001 disc ceramic capacitors and ferrite beads to cure the r.f. rectification in the neighbor's equipment. With narrow-band FM on VHF, the most that my transmitter causes is a "thump" when I key the transmitter; no voice audio is detected.

As a pilot, I don't spend enough time over anybody's house (at 170mph) to worry about it

 

[JimB]

Do home FM receivers use high-side injection?

I cant help but think we have had this discussion before?

Anyway, I just checked a portable radio, and it does use highside injection on the FM broadcast band.

JimB

 

[MikeMl]

...

Anyway, I just checked a portable radio, and it does use highside injection on the FM broadcast band.

That's why all FM broadcast receivers are banned on all commercial aircraft flights. You dont want the Local Oscillator radiation blocking the Aircraft's ability to track a VOR, LOC or ILS at 108 to 117.95 MHz.

 

[JimB]

Listening to the safety brief on comercial flights (yes I do pay attention, not read the paper or whatever!), it is interesting the number of variations of what is and isn't allowed.

Last week, the variations of what you could do with cellphones with "flight safe mode" just had me wondering why cant people just switch their toys off for an hour or so and relax?
Generally KLM state that if it has an antenna it has to remain switched off. But then how many would even know if some device had an antenna or not?
On an internal flight in India a couple of years ago, the list of things which must be switched off included "amateur radio transceivers". Only once did I hear specific mention of this during the course of a dozen or so flights over a period of 3 months or so.

JimB

 

[Damo666]

It's nothing strange, it's 'image interference', the radios you're hearing it on are poor quality ones.

+1 ... I run 300w on various HF/UHF frequencies without harmonics and have had numerous reports of " intermod/if image " problems. All of whom have had poor old tv's, et al. Regards. Damian.

 

[Dragon Tamer]

As a pilot, I don't spend enough time over anybody's house (at 170mph) to worry about it

I only hear the interference for a second or two and then it will go away for anywhere between a few hours or a few months.

Listening to the safety brief on comercial flights (yes I do pay attention, not read the paper or whatever!), it is interesting the number of variations of what is and isn't allowed.

Last week, the variations of what you could do with cellphones with "flight safe mode" just had me wondering why cant people just switch their toys off for an hour or so and relax?
Generally KLM state that if it has an antenna it has to remain switched off. But then how many would even know if some device had an antenna or not?
On an internal flight in India a couple of years ago, the list of things which must be switched off included "amateur radio transceivers". Only once did I hear specific mention of this during the course of a dozen or so flights over a period of 3 months or so.

JimB

On most flights I prefeer to listen to my iPod or something to that effect just to keep myself from dying of boredom. However on a 7 hour flight to Alaska, my brother thought it would be a good idea to take out his Nintendo DS and start playing it mid flight. Normaly not that big of a deal, except he was using the wireless connection on it to play against another kid in first class. I told him that doing that was a very bad idea. Even though nothing happened, it still makes me nervous. (I am a little curiouse as to the frequency of the RF signal that they use)

 

[MikeMl]

...On most flights I prefer to listen to my iPod or something to that effect just to keep myself from dying of boredom. However on a 7 hour flight to Alaska, my brother thought it would be a good idea to take out his Nintendo DS and start playing it mid flight. Normaly not that big of a deal, except he was using the wireless connection on it to play against another kid in first class. I told him that doing that was a very bad idea. Even though nothing happened, it still makes me nervous. (I am a little curiouse as to the frequency of the RF signal that they use)

As a practical matter, enroute, there are only a couple of frequencies that matter. Nowadays, enroute navigation is dependent on GPS, on L1. If an onboard device radiates (harmonics of an oscillator) happen to land on 1575.42 MHz, the GPS signal could be blocked.
Close to the destination, the landing guidance happens on 108 to 135 MHz. All of the electronic device restrictions in the cabin are to protect these frequencies.