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I had some fun with an RTL-SDR dongle, and I noticed this weird signal with no noticeable pattern at 119.9 MHz. What's even weirder is that the signal is symmetrical. What might this be? Or is this simply a hardware error?

The signal in a waterfall diagram

EDIT: This picture is from an oscillating signal generator/MP3 tansmitter from AliExpress (2SC9018) and it looks pretty similar: That one

The only problem is, that when listening to the signal (In FM, AM, etc.) you cannot hear any music, any speech or any patterns that would sound digital.

  • When you said "some fun", which areas of your computer environment did you sniff? – Ale..chenski Sep 03 '17 at 02:35
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    Where did you put your antenna? – Ale..chenski Sep 03 '17 at 02:40
  • I wandered around with it a bit in my house but outside seemed to be the best reception – technical_difficulty Sep 03 '17 at 02:42
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    Is the signal always there and at similar level? If so, it's probably EMI from the spectrum analyzer itself or from the computer to which it's connected (e.g. [a birdie](http://whatis.techtarget.com/definition/birdie) from the receiver's oscillators.) – reirab Sep 03 '17 at 04:52
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    Make a multi-element directional antenna if you think this is some outside source. It will be more fun. – Ale..chenski Sep 03 '17 at 05:32
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    Are you near to an airport? – RoyC Sep 03 '17 at 07:43
  • When you reduce receive gain by N dB, do the symmetric side spectra reduce by N dB, too, or more, or less? **Where** are you, in which situation, in which country? – Marcus Müller Sep 03 '17 at 10:05
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    Frequency range 108-137 MHz is used for [aircraft communication](https://en.wikipedia.org/wiki/Airband) (in AM). – Curd Sep 03 '17 at 12:27
  • @Curd I know, but it was in the middle of the night and no speech could be heard. – technical_difficulty Sep 03 '17 at 12:31
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    @stendarr: there are planes flying even in the middle of the night. Note that this frequency range is not only used for communication by speech but also for [VOR](https://en.wikipedia.org/wiki/VHF_omnidirectional_range)s (although <118MHz) and [ILS](https://en.wikipedia.org/wiki/Instrument_landing_system) – Curd Sep 03 '17 at 12:35
  • There's all manner of bells and whistles throughout the radio spectrum. Nothing is a surprise. Often it's just a carrier with no modulation ... just to keep the channel clear, or someone fell asleep with the transmitter on. – Chu Sep 03 '17 at 17:32
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    @Chu Maybe it's an alien transmission from outer space And maybe you're one of them, trying to tell us it's nothing special and making us dismiss this as just noise whereas in reality the invasion is imminent. Or maybe it's just an empty carrier wave, who knows – technical_difficulty Sep 03 '17 at 17:36
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    @stendarr aliens don't speak or use the internet; they told me so, in a dream. One of them was called Jack Daniels. – Chu Sep 03 '17 at 17:51

1 Answers1

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This signal is an extremely inefficient use of bandwidth, as you can see because there is an essentially unused area between the center and ±125 kHz. Therefore, I expect that it is almost certainly unintentional radiation (a.k.a. RFI/EMI) rather than a meaningful transmission.

The origin of the signal could be as follows — there are other ways it could arise but this is a simple plausible one:

  • There is an oscillator at 120 MHz, a nice round number probably chosen as a clock frequency. (Your 119.9 is either error in your receiver's oscillator or in the transmitting oscillator — it is likely in an application which does not need less than 0.1% error.) This oscillator is not deliberately connected to a transmitting antenna — it is just part of some circuit that isn't designed well enough to not radiate.

  • That oscillator is being amplitude-modulated by another signal at about 125 kHz (the distance from the carrier to the nearest sideband). This can occur many ways — one of the simplest being if something is varying the load on the common power supply at 125 kHz and the 120 MHz oscillator's output amplitude follows its supply voltage.

  • The 125 kHz oscillator's frequency is being modulated a bit by something else, causing the visible frequency changes. Again, this is fairly easy to have happen by accident.

  • Then if we look outward further to twice the frequency, we see a copy with twice the frequency variation but exactly the same shape other than that. Thus, this is just a higher harmonic of the 125 kHz signal. This is more evidence that this is not a deliberate transmission, as this doesn't efficiently add useful information to the signal.

Kevin Reid
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    It needn't definitely be AM, it could be narrow band FM. Also, looking at the harmonics as a snap shot picture doesn't really give much understanding to the basic nature of the source being square or triangle. I think you might be on the verge of "baffling with science" here. Also, there is no Y axis measurements at all. I'm not going to downvote because you analysis could be right but you are on shaky ground. – Andy aka Sep 03 '17 at 09:54
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    @Andyaka I've revised the answer to claim less. I think the description is still useful to illustrate that just because something is structured doesn't necessarily mean it's an intentional transmission. – Kevin Reid Sep 03 '17 at 14:03