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[Kevin Maxwell]

Bad coax?

I was working on an install of some stereo mics in a church for recording purposes. The owner of company I was working for hands me a solder type of BNC connector and said they asked him if while I was up dealing with one location (up on a 12 foot ladder) if I could solder this connector on the coax up there that the electricians had run along with the other wiring. This is for a camera so 75ohm coax. I have crimped a lot of BNC connectors on coax cable over the years and not too much solder on connectors.

So I proceed to strip the coax. It looks like your standard coax with a braided shield and a foil shield under that. I unbraided the shield and then I go to remove the little bit of exposed foil shield. The foil shield seems to be one with the white dielectric. I can’t even scrap it off cleanly it looks like at the tiny stripped off part of the center conductor that it is getting too close for comfort. I clean it up what I hope is enough to not short out, but I am not happy with the way it looks. I then go to solder the braided shield wire to the connector. This wire will not take solder, it just rolls off of it. I am using very good quality flux core solder, I use a thin gauge solder. I have never seen wire react like this. I looked very closely at the printing on the wire and it indicated it is Chinese made coax. This stuff might work ok when crimped but I had to give up on soldering it.

Has anyone else experienced anything like this? 

[Henry Cohen]

The coax was not necessarily bad.

It's not uncommon to have the foil bonded to the dielectric in cables designed for install. The bonding helps the foil remain uniform distance from the center conductor, helping to maintain characteristic impedance, and from moving [much] relative to the braid when the cable is disturbed, minimizing noise. Proper stripping procedure of the coax leaves the foil on the dielectric right up to the dielectric cut; the neck of the connector body goes over the foil. This way the center conductor has proper impedance and shielding right up to the pin.

As far as the braid not taking solder, also not unusual, though it is indicative of a lesser expensive construction using an aluminum alloy material for the braid. For these types of coaxial cable the only proper type of connector is crimp style. The coax cable spec sheet will indicate such.

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