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Detector is unstable after installing coaxial cable

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  • #1

    Detector is unstable after installing coaxial cable

    I tested the metal detector on my table. I used 15 cm normal wire (no coax) to connect the coil to the pcb.
    By adjusting the potentiometer I got a nice constant sound, very very low freq - a single frequency, close to zero Hz.


    Then I put the coil at the end of the stick (plastic) and used coax cable to connect the coil to the pcb. About 1.6m, 75ohm (3C-2V coax for TV).
    Now the sound is not stable anymore. It fluctuates between low and high frequency.

    How to get rid of this problem?
    I WILL make the coax cable shorter but there is a minimum length I need (90cm).

    Maybe I should also readjust the R6 trimmer with the coax cable installed.
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  • #2
    Update: I have tried it with 1m speaker wire and it works without problems.
    Why is coaxial cable recommended when it adds such a high capacity to the circuit?

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    • #3
      Can't answer your question directly but I have had trouble with TV coaxial cable when used for TV. Some of the cheaper one have a lot of loss of signal over quite short runs.

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      • #4
        Since the speaker wire works so well I will stick with this solution for the moment. I guess you need coax cable to protect the weak signal in the TX coil (well, I have a monocoil setup). But out in the field there are no electrical devices to produce electric noise. So, I seems ok to use unshielded wire.

        I will try also UPT (cat5) shielded cable.

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        • #5
          with coaxial cable connected you got a change of self resonance of the system coil+cable. try to retune the damper resistor to another upper critical value.

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          • #6
            Originally posted by kt315 View Post
            with coaxial cable connected you got a change of self resonance of the system coil+cable. try to retune the damper resistor to another upper critical value.
            Ok. I will try that. What should I look for (on the oscilloscope screen) when I adjust R1?

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            • #7
              you should see the signal at preamps output and set FIRST STROBE DELAY in minimal value for it. with mine english I can not able give to you flustrated
              story what you have to see. you can buy ITMD and learn needed pages on that.

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