Ask SAIL: Thumb Trick Test Coax

I am doing a retrofit on my old CRT radar to a new Garmin LCD radar system. All my wiring is replaced, except for a spaghetti-sized coax cable that was original
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THUMB TRICK TESTS COAX

Q: I am doing a retrofit on my old CRT radar to a new Garmin LCD radar system. All my wiring is replaced, except for a spaghetti-sized coax cable that was original, with the correct connectors. How can I tell if that cable is good or waterlogged?

D. Munroe, Sunset Harbor, CA

GORDON WEST REPLIES

West-head-250x300_2

Check coax continuity and dryness with an analog needle-reading multimeter. Measure resistance on the R x 10,000 scale. Wet your fingers, touch the leads, and you will get about half-scale reading. Get a shipmate to apply a wet thumb at one end of the cable, up at the radar arch, with you measuring it from down below. You should, again, see half-scale reading, until your shipmate “un-thumbs” the far end. You should then see the meter drop all the way down to infinite resistance—an open circuit. If the meter still detects a high resistance continuity, you likely have water intrusion within the coax, and it must be totally replaced.

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