General hint on mixing / Interference on 40m

Hello world :)

Is there a general way to calculate where mixed up interference may possible come from?

I receive sea weather report on 40m 7193 RTTY - sometimes even 3 or 4 times on 40m with 40khz in between. I know it isn't really there. I first thought it was from DDH47 on 147kHz since I receive them with -30dbm. So I build a notch for that freq. and have them down 40db now. Surprisingly I still have some mixed up signal from seaweather in the 40m band. That weather report is transmitted on different bands at the same time so now I don't have any idea how to possibly calculate where the source frequency may be.

How is the math behind that? :-)

Feel free to tune in and hear how it sounds.




  • do you have an LPF that cuts off at 2MHz or so? That would elimnate the L and MW as poential sources as a test.

  • Yes, I inserted this bandstop filter (1st picture) for testing but I still get those RTTY signals on 40m (probably somewhere else, too)

    The second picture is my quickly built notch for DDH47 148kHz. Doesn't help either.

    Even adding another 10db att to the kiwi doesn't help. The "ghost" signal drops 10db but still appears.

  • If you add 10dB of attenuation and the 'ghost' signal only reduces by 10dB, then it's likely to be something ahead of the attenuator (at the antenna) that is causing the spurious signal.

    Most intermodulation products will decrease at 2, 3 or more times than the value of attenuation that you apply, so this has got to be something external occurring before it reaches the receiver.

    In cases like this it is most usually an active antenna that is being overloaded, but it can also be some (usually rusty) metal objects near your antenna that are re-radiating mixing products.

    I have a barbed wire fence around one of my remote sites that occasionally does this, but the level of interference varies with changes in weather conditions, and how dry or wet (near the sea so salty) the fence is.



  • Andy,

    I sent you mail..

  • @G8JNJ Hm, I'm afraid I didn't think of the whole chain here. The wire is indeed hanging in parallel to a kind of fence and could possibly fade since the fence wire is tight and fixed while my antenna is not. Interesting stuff. I have never heard of mixing products coming from passive metals. I really have to dig into that!!

    @WA2ZKD Jim, thank you, looks cool (what did you use to do the plot?) but I didn't see how that helps here. At least it helps to find the strongest signals, which could be helpful. The plot looks way better than mine :D

    I will continue investigating and let you know what happens :)

    73, Andy

  • I use gnuplot to create that... let's chat on e-mail

  • edited November 2022

    Hi Andy,

    It's often called the rusty bolt effect, where the layer of rust forms a crude diode junction, a bit like the old cats whisker, coherer or carborundum detectors used in the early days of wireless. The diode being non-linear then works as a crude mixer when presented with strong enough radio signals.

    Second and Third order products are usually the most common, but the more frequencies that are present, the more products will be produced. This is a particular problem on radio sites that use multiple closely spaced transmit channels, as the combined intermodulation products can easily fall within the receive channels, causing lots of unwanted interference.

    Try these links to keep yourself amused :-)



  • FWIW over the last few days there has been an 'intruder' on 7193 kHz - FSK with 250 Hz shift.

  • Ha,

    Good catch Richard, I should have thought of the obvious explanation first :-)



  • Hello again :)

    Sorry for not responding for a while.

    We got things sorted out with all your help and a few interesting chats with Jim, WA2ZKD.

    I still should notch out strong areas to not overload the frontend. But for now I am really satisfied with the overall reception. It turned out that it's a bad idea to power the preamp (that is designed for 12V) with 5V, limiting it's headroom.

    We did some this and that but hooking up the amp to 12V made the ghost signals disappear.

    So I have to thank you all for your help. Now this is so obvious to me but it wasn't before :)

    73s, Andy / DG7LAN

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