Electronics > RF, Microwave, Ham Radio

15m long RG-213 with a notch at 450 MHz

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A ham friend gifted me with a 15m length of RG-213 coax with a strange problem. The insertion loss is within a few tenth of a dB from the specification (blue dashes) at HF and 2m (see attached graph). Above 200 MHz, the loss begins to deviate from spec. and the deepest null occurs at 450 MHz. Above 700 MHz, the loss reverts back to spec. The cable ends are connected to Telegartner BNC connectors (attached photo). I have checked the exposed shield / braid for signs of water ingress, e.g. green residue, but didn't find any. Any idea what is wrong? TIA

[edit] Return loss is better than -25 dB with the opposite end terminated with a 50 ohm load

Lay it out flat in one big loop instead of rolled up and see if the 'notch' moves.

Thanks for the suggestion.
Unfurling the cable into one large loop did not eliminate the notch, but there is an upward shift in the notch frequency (red trace in attached graph). The notch depth is reduced by ~1 dB. What does it mean?

this is the same fault I see with my LNA for a spectrum analyzer, with a notch in the same range. I still have not investigated it (its dealing with the arc of the covenant) but people were somewhat optimistic that it was some kind of issue with the internal hardline/connectors/solder. I got screwed out of the VHF-UHF transition for some reason after owning it for a while.

Since its a long cable have you tried redo the connectors?

That's the kind of thing you can get if you have a bad spot in the cable, like someone drove across it, and it smashed down the insulating dielectric or broke the shield.  Run your hand along the cable while sweeping it and see if the notch changes at any particular point.  A break in the ground shield can act like a tuning stub put an effect short on the line (not a real short so you can't ohm it out, but a broken stretch of ground shield can act like an open which rotates to a short at some frequency).  Tough to find these things.  To quote myself  (from the book) "Cables are like dogs, either they are bad, they've been bad, or they're going to be bad.  And if they're good, they only stay good with great care"


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