Radio stations detected on oscilloscope

[Gary350z]

In the past I have noticed a very small (2mV) 100 MHz background signal on my Rigol DS1052E scope. It is not a problem, just a curiosity. I could never identify the source, and thought maybe it's just a clock signal radiating out of the scope itself.

I recently bought a Rohde & Schwarz HMO1212 scope and noticed the same background signal on it. So it's not coming from the Rigol scope. So where is it coming from? Could it be the local FM radio station (99.9 MHz)?

So using the HMO1212, I attached a 2 foot wire to the scope probe, set the probe to X10, and using the FFT, I found the signal was 99.9 MHz.

On the FFT, I noticed a bunch of signals around the 99.9 MHz signal. I wondered if these could be radio stations on the FM radio band.

Using the FFT, cursor measurement, and the "Next Peak" button, I could identify 18 signals in the area around the 99.9 MHz signal, so I wrote down the frequencies, and then compared them to the radio station frequencies on my digital FM radio tuner. All 18 signals on the scope were a 100% match for radio stations on my tuner!!!

So the scope, using the FFT was showing the FM radio band.

I thought this was amazing.

The scope screen caps below show the stations in the FM radio band, using the uV scale and the dBV scale.
The frequency span is compressed to show the whole FM radio band.

[tautech]

Nice, with a bit more playing around with a directional antenna you'll have compass bearing for them too. 

[chris_leyson]

I once heard a galvanized steel garage roof and some twisted steel wire tied onto one corner demodulate radio 2 or radio 4 (UK) and a French radio station at the same time. It only lasted for 20 minutes or so. It was a really hot day and I walked around the outside of the garage a few times listening for someone who might have a radio, not a thing, just the sound of a few insects buzzing and very distant traffic noise, but walk back into garage and in the corner at the back where to steel wire was tied to the roof I could hear at least two radio stations. To this day I still can't get my head around how the sound was being generated, I can see that dissimilar metals would at as a rectifier and maybe the "rectifier" was acting as a speaker. Who knows.

[danadak]

There have been reports of people with fillings in their teeth acting as detectors (diodes)
and being able to hear radio stations.

Of course this is in keeping with reports of alien invaders periodically appearing, and
Armageddon event forecasting.....


Regards, Dana.

[chris_leyson]

It definitely wasn't fillings as I could hear it at only one location, the other end of the discarded washing line was tied to a steel post some 4 to 5 meters away so the "antenna" may have been quarter wave resonant on either the 22m or 25m SW broadcast band. I could make out a male voice with a British accent reading the news and another voice in French.

[Richard Head]

It's due to intermodulation created at a poor electrical connection. The poor electrical connection has to behave non linearly (like a diode). If two powerful stations are close in frequency one of the intermodulation products can fall in the audio range. In this case it was probably the difference in frequency between the two stations. I would expect you to hear both stations modulation on top of each other. This is technically referred to as passive intermodulation distortion. It was more common in the days of very high power AM radio stations, but they are getting far less common these days.

[chris_leyson]

Hi Richard, thanks for that, I had not thought of intermodulation, that would fit the circumstances, I'm still puzzled by what was generating the actual sound, given that both the roof and "antenna" were galvanized steel I suspect it would have been a magnetic mechanism rather than an electric. Never experienced anything like it since and it would have been some 45 years ago.

[Richard Head]

Chris

It was probably one or both of the steel conductors vibrating in sympathy with the varying magnetic field, just like a loudspeaker. I'm just a little surprised that a station in France was powerful enough to cause this.

[vk3yedotcom]

It's due to intermodulation created at a poor electrical connection. The poor electrical connection has to behave non linearly (like a diode). If two powerful stations are close in frequency one of the intermodulation products can fall in the audio range. In this case it was probably the difference in frequency between the two stations. I would expect you to hear both stations modulation on top of each other. This is technically referred to as passive intermodulation distortion. It was more common in the days of very high power AM radio stations, but they are getting far less common these days.

Generally AM stations in a given service area are much further than audio frequencies apart (if they were closer radios wouldn't be able to separate them without hearing a high whistle).  Eg they may be 40 or 50 kHz at least.  But what I could see happening is that non-linear elements can cause harmonics which can mix and create all sorts of products.  So if there was a station on 702 kHz and a station on 1395 kHz in the same service area the difference between the harmonic of 702 and the fundamental of 1395 is only 9 kHz - ie audible.  I'd imagine that spectrum planning would seek to minimise this within a given service area.

[bitslice]

works with grass too





Don't spectrum analysers have a demodulator function? I used to use the work one to listen to the radio (and the old style mobile phones)

[chris_leyson]

It was probably one or both of the steel conductors vibrating in sympathy with the varying magnetic field, just like a loudspeaker. I'm just a little surprised that a station in France was powerful enough to cause this.

I was surprised by that as well, I sort of concluded at the time that it was a LW broadcast, you could pick these up during the day and the signal strength was quite high and I don't recall hearing many French broadcasts on MW. SW broadcasts might fit in line with the "antenna" dimensions, could it have been some atmospheric ducting phenomenon, I'm not really convinced as this sort of thing happens at higher frequencies. I also ruled out any local transmitters, these would have been Wenvoe and St Hillary, they didn't broadcast AM only TV and FM. I suppose the other thing that has hampered my understanding of this phenomenon is my simplistic "electric field" view of radio propagation and reception, I've never really been able to visualize magnetic fields from a RF propagation point of view.
I suppose its a bit like the "rusty bolt" effect causing inter-modulation from co-sited transmitters, but for some reason several orders of magnitude higher.

[altaic]

works with grass too

What the hell?! Crazy Russians

[Powermax]

I once heard a galvanized steel garage roof and some twisted steel wire tied onto one corner demodulate radio 2 or radio 4 (UK) and a French radio station at the same time. It only lasted for 20 minutes or so. It was a really hot day and I walked around the outside of the garage a few times listening for someone who might have a radio, not a thing, just the sound of a few insects buzzing and very distant traffic noise, but walk back into garage and in the corner at the back where to steel wire was tied to the roof I could hear at least two radio stations. To this day I still can't get my head around how the sound was being generated, I can see that dissimilar metals would act as a rectifier and maybe the "rectifier" was acting as a speaker. Who knows.

I heard a story about how one old woman kept hearing music and it was driving her crazy, literally (she thought she was crazy) but I guess her sons or dentist figured out it was old cracked tooth fillings that were generating sound from the radio station. I also heard of a simalar story of some metal drawers and a old oven with loose sheet metal doing the same thing. With that one the house owner thought the house was haunted until closer listening and investigation he was able to determine it was the talk radio station or something.

That can happen if you live where a radio station is nearby, and in WWII they used fox hole radios where a carbon lead from pencil made a point contact with a blued or rusted razor blade, Although I could never get that to work.

[JPortici]

vatican's radio. The only station you could pick up anywhere, even inside caves.
Powered by the holy spirit



but seriously, I am also picking up a 100MHz signal (and i have a 70's mixer that picks up radio transmissions when the input is in "mic" mode -> high gain, but i don't think it demodulates fm, i think it picks up shit from my neighbours radio)
-why at 100 MHz? is it because the probe's cable lenght?
-can i make the environmend quiter without covering the room with nice metal wallpaper?

[HighVoltage]

works with grass too

That is pretty amazing
But why would there be such high RF energy in the base of this tower?

[bitslice]

But why would there be such high RF energy in the base of this tower?

It's sitting on insulators and it's an AM broadcast tower where the whole mast is used as an antenna

https://en.wikipedia.org/wiki/Mast_radiator

[System Error Message]

its good to know that scopes are wifi/radio ready.

[bson]

I see the FM band also without taking precautions.  That's what living within line of sight of a massive terrestrial transmitter will do for you...

[raupi]

Wow... One meter of wire attached to the probe and it's a mess   

Living in an appartement building with lots of SMPS, computers, wireless gadgets and tons of other electronic gear around sure takes its toll... Wonder what the pileup at ~938 MHz is about? 

[chris_leyson]

Best guess GSM-900 maybe.