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B Identifying an RF signal...

  1. Dec 12, 2016 #1
    Looked at a RF spectrum that came from the IF output on a radio so it was not tuned thru the radio filters, just whatever the HF antenna was capable of.

    A pulse went across the entire spectrum slow enough to see it sweep across the screen.

    The pulses were not regularly spaced but came in bursts on average prolly 1 every 30 seconds.

    Lots if people in my circle know it.

    The hypothesis is Chinese OTHR, purpose unknown.

    How can one identify such a thing.

    I can video the computer screen this Friday if that helps.
     
    Last edited: Dec 12, 2016
  2. jcsd
  3. Dec 12, 2016 #2

    sophiecentaur

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    What is the frequency range of the frequency spike?
    Does its rate of change of frequency change if you scan at different rates? That may or may not apply to different forms of frequency analysis.
    If you could operate your receiver at two or more different sites, you could see if the level changes much. If not much then probably a distant source but if a lot then probably a local source.
    I assume you have tried turning on and off any local possible sources of interference - before looking for the 'exotic' cause. Interesting, though.
     
  4. Dec 12, 2016 #3
    Good questions I will follow up on, thanks.

    Not sure what I am looking at given we have a fixed antenna that is working so far off resonance. Does that change how the pulse looks. Does an off resonant antenna work like a resonant antenna just "weakerly".

    I will reference yr screen name live so you know my video is legit.
     
    Last edited: Dec 12, 2016
  5. Dec 12, 2016 #4

    tech99

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    The spectrum is so polluted now that it is very hard to identify pulse-like transmissions without really good detective work with known reliable equipment. On the face of it I can't see why an OTH would have such a slow PRF and transmit simple pulses.
     
  6. Dec 12, 2016 #5

    davenn

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    you really have given far too little info to be helpful

    describe you exact setup ( preferably with diagrams) starting at the antenna and going through
    to what ever you have connected to the IF output

    1) what is the receiver ?
    2) what frequency was it tuned to
    3) does it have more than 1 IF frequency ? if so what are they

    what were you using to look at the spectrum ??

    if you say an oscilloscope, then my response will be you should be using a spectrum analyser
    an oscilloscope scans in time, a spectrum analyser scans in frequency

    the pulse on the oscilloscope could be just indicating a burst of signal on the freq set by the receivers IF
    it tells you nothing about the frequency of the signal


    Dave
     
    Last edited: Dec 12, 2016
  7. Dec 12, 2016 #6
    We used an RF spectrum analyzer and a HF antenna. The frequency of the pulse went across the entire spectrum.

    Sort of like a navigation beacon but we know all them so something artificial but not known yet.
     
  8. Dec 12, 2016 #7

    davenn

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    and what else ?
    you commented on an IF
    your description of a pulse didn't match using a spectrum analyser
     
  9. Dec 12, 2016 #8

    davenn

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    the Chinese radar sits on one freq for a period of time before freq hopping to a different freq with better propagation
    the old Russian OTHR did the dame thing .... unaffectionally known as the "woodpecker"
     
  10. Dec 12, 2016 #9

    If you have a spare few hours I will tell all about the station. What else would you like to know specifically?
     
  11. Dec 12, 2016 #10

    Will post a video if I get permission. You won't see many better stations in civvy or non commercial world.
     
  12. Dec 12, 2016 #11

    davenn

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    have already asked twice ... what was the IF thing you spoke of
    an IF assumes a receiver, and I asked about that too

    IF you are really using a spec an, then it alone should be connected to the antenna ... NOT via a receiver
     
  13. Dec 12, 2016 #12
    Wrong, the IF internal port bypasses the receiver filters. The spec is connected to the antenna and so is the radio to the same antenna.
     
  14. Dec 12, 2016 #13

    davenn

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    not normally wrong

    so AGAIN, WHAT is the receiver ??

    PLEASE stop making it difficult for people to help you and answer the Q's when asked
    dragging every bit of info out of you tiny bit at a time is a pain in the butt

    WHAT is the IF freq?
     
  15. Dec 12, 2016 #14
    You mean model/ brand??
     
  16. Dec 12, 2016 #15

    davenn

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    YES!!
     
  17. Dec 12, 2016 #16

    sophiecentaur

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    The word "pulse" implies a short burst in time. A spectrum analyser shows the frequency spectrum. The frequency spectrum of a pulse is not a single peak of frequency. What your trace is showing is a signal of narrow bandwidth with a centre frequency that's sweeping about slowly.
     
  18. Dec 12, 2016 #17
    Yes that is what I am seeing and pulse seemed a more efficient way of saying it.

    Dave it's a Kenwood, will find out the model.
     
  19. Dec 12, 2016 #18

    sophiecentaur

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    "Spike" might be less confusing, perhaps. That has no 'temporal' connotations - as in time domain.
     
  20. Dec 13, 2016 #19

    Tom.G

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    Try turning off the Kenwood. It may be scanning for signals and leaking interference back to the antenna.
     
  21. Dec 13, 2016 #20

    davenn

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    why have the radio there ? .... it's only confusing the issue and isn't doing anything to help the situation

    yes, you could easily be seeing signals from the radio .... take it our of the equation and connect the spec an directly to the antenna coax
    With NOTHING else connected to the same antenna
     
  22. Dec 13, 2016 #21

    sophiecentaur

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    Yes.
    In the absence of filtering, that signal could be getting mixed in from anywhere at a different frequency. You have to hope that the spekky annie can cope with what you're looking at or it will involve an extra home-built RF filter to make sure you know what's being looked at.
    There are checks you can do, like seeing if the displayed signal actually drops by 10dB for a gain change of 10dB. My (fairly significant) experience of RF spectrum analysers is limited to high end analogue versions from the past but the 'rules' still apply to digital versions (plus extra stuff because of the sampling).
     
  23. Dec 13, 2016 #22
    There is a signal that I pick up on my SDR (a lot like a Analyzer) on HF. Looks rather like what you describe. Called CODAR. Do you live "near" the ocean?



    Google CODAR.

    DC
     
  24. Dec 13, 2016 #23
    Well, that was supposed to be a youtube link. Just check out CODAR on line plenty of info.
    DC
     
  25. Dec 13, 2016 #24
    Nice, my signal is not periodic like yours and scans a bit slower.

    Link worked fine.
     
  26. Dec 13, 2016 #25

    berkeman

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    Sorry if I missed it, but did you post your video, along with a block diagram of your setup and pictures of your antenna? That would help a lot...
     
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