UHF phantom signals

HB9TMCHB9TMC
edited August 22 in Signals Received

I'm using an Airspy to receive radiosondes in the frequency range 400-406 MHz, and I was very surprised when I heard a number station transmitting CW in that spectrum.

I thought maybe it is intermodulation inside the airspy receiver, but I could receive it independently with a second portable receiver.

After two days puzzling, I heard the number station transmit "VVV VVV VVV DE FAV22 FAV22 FAV22"

Turns out that is a number station of the french military on 6825 kHz, making S9+20. And for some unknown reason, it is radiated by the KiwiSDR on 406.822 MHz. Not very strong, but I can receive it up to a distance of 20 meters.

It seems to be mixing with the 5th harmonic of the 66.66 MHz oscillator. I also found the signal mixed with the fundamental, e.g. 6825 + 66666 kHz.

Comments

  • G8JNJG8JNJ

    Which Airspy model is it, as the architectures vary sightly.

    From memory, the R2 uses a local oscillator and mixer with a 5MHz IF, but I'm not sure about the Mini.

    I also think there used to be block diagrams and technical descriptions of the Airspy products on their website, but I can't find them, so maybe I saw them somewhere else instead.

    Although generally good for their price, they have their Fanboy's, who attribute them with them almost mystical properties.

    Maybe better to ask on the dedicated Airspy forum ?

    https://groups.io/g/airspy

    But don't criticise their products, or you will face the consequences...

    Regards,

    Martin

  • HB9TMCHB9TMC

    The signals are actually radiated from the Kiwi, so it had nothing to do with the Airspy.

  • G8JNJG8JNJ

    Ah OK, sorry my misunderstanding.

    I have KiWi's and other receivers sharing the same antennas, so I'll see if I can find anything similar.

    Regards,

    Martin

  • HB9TMCHB9TMC

    It's not radiated through the antenna connector, as far as I can see, but from the Kiwi board itself. I was only mentioning it, in case someone else is stumbling across unexpected signals on VHF/UHF.

    It already helps a lot, if the kiwi is in a metal case.

  • G8JNJG8JNJ

    My V1 KiWi in its original clear plastic case did produce quite a lot of radiated noise and other signals, which could be problematic if you had other radio kit in proximity.

    Putting it in the metal case, or better still a die-cast box, with additional filtering, made a huge difference.

    The V2, as supplied, is much better.

    Regards,

    Martin

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