Measurable resistance between center and shield on LMR 240 coax

Discussion in 'General Technical Questions and Answers' started by K0SIK, May 18, 2021.

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  1. K0SIK

    K0SIK Ham Member QRZ Page

    I received a 50’ roll of LMR-240 flex coax
    I prepped both ends to install PL-259 connectors
    After terminating I checked to ensure continuities
    Center to center , shield to shield, and center to shield
    It was at C to S where the cable showed resistance
    I dis-assembled both connectors and I still had resistance between the center conductor and the shield
    My question is there something wrong with this new coax? The RG-8X section I have reads correctly
     
  2. KG7WGX

    KG7WGX XML Subscriber QRZ Page

    The dielectric should not break down at the voltages used by a DMM to measure resistance.

    Unless the dielectric was damaged during soldering, it should show open, like your RG-8X.

    If all else fails, cut it in half and see if you can salvage a 25 ft section.
     
    KU3X likes this.
  3. KE5MC

    KE5MC Premium Subscriber QRZ Page

    If nothing found on cable ends causing resistance then visual check along the length of the cable for physical differences. One inch of cable should look the same as the next and repeat. Anything looking different might be the source. On the other hand easy for a single stand of shield to gum up the works and not be seen.
    Good Luck!
     
  4. K7JEM

    K7JEM Ham Member QRZ Page

    What is the actual resistance that you are measuring? Is it a few ohms, or a few hundred thousand ohms?
     
    KB0MNM likes this.
  5. G8FXC

    G8FXC XML Subscriber QRZ Page

    Anything less than several millions of ohms should be cause for suspicion...

    Martin (G8FXC)
     
    WA9SVD and SPEEDSKATER like this.
  6. KP4SX

    KP4SX Premium Subscriber QRZ Page

    Is this brand new cable?
     
    NN4RH likes this.
  7. WQ4G

    WQ4G Ham Member QRZ Page

    Short...

    WQ4G
     
  8. KU3X

    KU3X Ham Member QRZ Page

    This is where time domain reflectometer comes into play. It would tell you where the short or open is so one does not waste coax.

    Barry
     
    WZ7U, K6EFA, K1XH and 1 other person like this.
  9. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    How did you disassemble the connectors? They should have been soldered or irreversible crimps.

    If you have a NanoVNA or a friend who does, it has a mode for calculating time domain reflection and can be used to find cable faults.

    Cut an inch off of each end beyond where the connectors were and use a good magnifier to look very carefully at the ends for any wayward shield strands or foil causing shorts, or melted insulation, then measure resistance again before attempting to install connectors.
     
    KB0MNM and KP4SX like this.
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    What is the measured resistance?

    How did you measure it?

    The phrase "it showed resistance" makes perfect sense -- it should show resistance (center conductor to shield), a whole lot of resistance. C to C and S to S should show "no" resistance (short circuit, or milliohms).

    I've never seen UHF connectors that could be disassembled; whether solder type or crimp type, once they're installed, they'd need to be "cut off," as trying to take either design apart would be destructive. However, non-crimp (clamp type) BNCs and Ns can be disassembled, sacrificing only the rubber gasket that will be in two pieces instead of one.

    A photo or two of your setup would be helpful.
     
    AB6RF and KB0MNM like this.

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