Home Forum Support Long GM tube Coaxial cable noise

Viewing 9 posts - 1 through 9 (of 9 total)
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  • #6911
    R_A_Ghosh
    Participant

    Hi all,
    I am using an LND712 with an 8.5 meters long RG6 coaxial cable. The anode resistor is exact near to GM tube at the other end of cable. I have used compensation capacitor as well.
    The system works and gives no issues but as soon as I touch the cable with my hands, spurious counts appear. Why such noise gets introduced just by touching. Using a long cable has been a problem with my project. Any ideas in why the problem on how to solve it ??? Is it due to cable capacitance ? Or any other reasons ?

    #6912
    edodd55
    Participant

    The standard answer is to put some ferrite chokes on the cable.
    These things just snap around the cable
    See https://www.ebay.com/b/Ferrite-Choke/7288/bn_7023291411 if you are unsure of the nature of this part.

    #6913
    R_A_Ghosh
    Participant

    How many for 8.5 meters cable ?

    #6916
    Radu
    Keymaster

    Are you installing it in a shelter? Hope you’ll never need it but we live troubled times.

    You’ll need to increase your compensation capacitor. Calculate your cable’s capacitance then add that in the equation posted on the blog. Let me know if you need help.

    #6917
    edodd55
    Participant

    I’d put one on each end. You want to suppress the errant signal and stop it entering the device(s). In theory one at the receiver would work.

    #6919
    R_A_Ghosh
    Participant

    RG6F capacitance is 53pF/meter so for 8.5 meters it is approx 450pF.
    Find attached circuit. Please guide how to make improvements.
    R anode is exact near GM tube as recommended.

    #6923
    R_A_Ghosh
    Participant
    #6927

    Hi,

    Good you included that drawing to make it clear what you meant.
    It is no wonder you get spurious counts when you touch the caox shield, since you connected the very high impedance cathode to it.
    If possible, you should connect the shield to the HV supply, and the cable core to the cathode!

    However, I am a bit concerned about carrying a voltage that high over a coax cable designed for SAT receivers…

    Cheers
    Wolferl

    #6928
    R_A_Ghosh
    Participant

    I have also done the same, but no improvement. Core has cathode, braid has the 500V DC.

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