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.

  • 2
    \$\begingroup\$ Where did you put your antenna? \$\endgroup\$ Commented Sep 3, 2017 at 2:40
  • 2
    \$\begingroup\$ 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 from the receiver's oscillators.) \$\endgroup\$
    – reirab
    Commented Sep 3, 2017 at 4:52
  • 3
    \$\begingroup\$ Make a multi-element directional antenna if you think this is some outside source. It will be more fun. \$\endgroup\$ Commented Sep 3, 2017 at 5:32
  • 4
    \$\begingroup\$ Are you near to an airport? \$\endgroup\$
    – RoyC
    Commented Sep 3, 2017 at 7:43
  • 2
    \$\begingroup\$ Frequency range 108-137 MHz is used for aircraft communication (in AM). \$\endgroup\$
    – Curd
    Commented Sep 3, 2017 at 12:27

1 Answer 1


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.

  • 5
    \$\begingroup\$ 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. \$\endgroup\$
    – Andy aka
    Commented Sep 3, 2017 at 9:54
  • 3
    \$\begingroup\$ @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. \$\endgroup\$
    – Kevin Reid
    Commented Sep 3, 2017 at 14:03

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.