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Thread: SDR FFTs for frequency detection

  1. #1

    Default SDR FFTs for frequency detection

    Hi All,

    I am using a Noolec SDR to collect data and plot FFTs (Fast Fourier Transform). This is my way of detecting or seeing what is out there but it does not work. I do not wish to decode anything, I just want to detect it. So I live near a council office which uses TETRA I believe, when I collect data around the TETRA frequencies I should be able to see a spike on the FFT but it doesn't work. The things I have tried to verify if the software I am using works is by using a TV remote nearby or car key fob or just your regular radio (e.g. FM). There was an ambulance standing nearby the other day and I tried to collect the data and perform an FFT of the data but it showed nothing.

    How can I find out what I am doing wrong? Is there a way to tune the antenna or something?

    Thanks,

  2. #2
    Super Moderator 5B4AJB's Avatar
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    In a word, yes.

    The solution, is of course, more complicated,
    Antennas naturally resonate at one particular frequency, which is their "sensitivity". A harmonic of that resonant frequency usually has some sensitivity too.

    Receiving a couple of ranges of frequencies is possible, it normally requires two (or more) antennas and a diplexer.

    There are limitations and bonuses to this approach.

    You are probably trying to receive V.H.F. & U.H.F. - tuning this kind of antenna is far more sensitive than an H.F. one, it can be done, but is not really practical.

    Probably your best bet is to go for a preamplifier with a simple diplexer and a couple of antennas.
    This will introduce a lot of noise and will swamp your receiver with strong local signals, but will give you a lot of frequency coverage.

    If you don't mind building/fiddling, look up "antenna tuning unit".

  3. #3

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    Quote Originally Posted by 5B4AJB View Post
    In a word, yes.

    The solution, is of course, more complicated,
    Antennas naturally resonate at one particular frequency, which is their "sensitivity". A harmonic of that resonant frequency usually has some sensitivity too.

    Receiving a couple of ranges of frequencies is possible, it normally requires two (or more) antennas and a diplexer.

    There are limitations and bonuses to this approach.

    You are probably trying to receive V.H.F. & U.H.F. - tuning this kind of antenna is far more sensitive than an H.F. one, it can be done, but is not really practical.

    Probably your best bet is to go for a preamplifier with a simple diplexer and a couple of antennas.
    This will introduce a lot of noise and will swamp your receiver with strong local signals, but will give you a lot of frequency coverage.

    If you don't mind building/fiddling, look up "antenna tuning unit".
    Thank you for your reply. I am looking at VHF I think as they're in the region of 390-470MHzI think I might start off with your suggestion about a preamp. I looked at an LNA4ALL preamp for SDR. Can you suggest me a rookie guide to filters etc to cancel out the noise, can I do this?

  4. #4

    Default

    Quote Originally Posted by 5B4AJB View Post
    In a word, yes.

    The solution, is of course, more complicated,
    Antennas naturally resonate at one particular frequency, which is their "sensitivity". A harmonic of that resonant frequency usually has some sensitivity too.

    Receiving a couple of ranges of frequencies is possible, it normally requires two (or more) antennas and a diplexer.

    There are limitations and bonuses to this approach.

    You are probably trying to receive V.H.F. & U.H.F. - tuning this kind of antenna is far more sensitive than an H.F. one, it can be done, but is not really practical.

    Probably your best bet is to go for a preamplifier with a simple diplexer and a couple of antennas.
    This will introduce a lot of noise and will swamp your receiver with strong local signals, but will give you a lot of frequency coverage.

    If you don't mind building/fiddling, look up "antenna tuning unit".
    I have read more about LNAs now. I have a few more questions if you can help please.

    Q1. LNAs seem to require bias tees. As I understand it, it's to supply power to the lna through the coaxial cable. So what if I don't mind connecting the LNA to my systems power supply through the gnd and vcc, do I still need a bias tee?

    I might be able to activate bias tee by software as the nooelec sdr has his feature I think.

    Q2. Can I just use any cheap LNA? I'm not interested in L band and really only need to look at UHF. Do I need to care about which LNA I get ?

    Q3. For the circuit would I connect the LNA between the nooelec sdr dongle and nooelec antenna? And then connect the LNA to a power supply of 9Volts?

    Thanks

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