coax cable testing


Thread Starter

Hugo Ahrens

On a current project I have run in to various problems with the workmanship in completing coax crimp-on connectors (for ControlNet RG6 coax). The end result is that the system works, or does not work with the tiniest disturbing of the cable. The physical pull-off test only confirms that the outer crimp is strong. The resistance test is done at too low a voltage to be meaningful. TDR testing is also done at low pulse voltage and has not helped to point out bad connectors. I am wondering if there is a test instrument that can stress the termination (especially in the area of the foil cutback or displaced inner core strands), so that we can be more positive about the long term performance of the cable system.

Thanks for sharing your experience.


Jake Brodsky

Resistance is resistance. Voltage has little to do with it until you break down the dielectric. Likewise, TDR voltages really don't tell you much, but pulse width can.

If you're seeing this performance it's probably because the center pin is pistoning in and out either from mechanical and/or thermal extremes or because the center pin wasn't installed properly. I suspect the latter. The body of the connector can be physically tight, but the center pin may not be well secured.

Crimp-on connectors have been certified for spacecraft. They can be very reliable. The only reason you're having problems is probably
because someone took a shortcut.
If workmanship was truly an issue, I would have all of the connections redone according to established procedure with proper tools.
Consider this cheap insurance.
While your answer is no doubt the least costly and the most expedient to get the desired end result, there are alas other complicating circumstances. Others, (contractors) are responsible for the installation. I'm pushing the
envelope of on site relations by jiggling the cables. The owner is not likely to want to spend money to repeat an installation that looks fine to
them. The electrical consultants to whom I report will be more inclined to listen if I can point at some test results. I would be happier if I could
prove that not only did my system work when I left, but that there is a better than likely chance that the coax cable system will continue to
perform long after. That's why I am exploring to see if there is a method, or a test that I could use to qualify the coax terminations.

I am not a coax expert. That is why I went looking for a tool kit when I wanted to investigate and possibly use ControlNet. What I found and purchased is Allen-Bradley's ControlNet Coax Tool Kit, catalog number 1786-CTK/B. It came
with a video tutorial on CDRom that made it so easy, even I could do it. But being new to this, I wanted to be absolutely sure I had gotten it right so I also got their media checker (1788-MCHKR). I even did a poor job crimping on a connector to test the checker and it found the problem and told me which end of the cable was wrong. I have been pretty happy with both.

Jake Brodsky

I wish I had better news for you, but no TDR is going to demonstrate a mechanical problem with the center pin in a manner that your customers will comprehend.

The best way to approach this problem is to demonstrate the cable's intermittent behavior by wiggling it and then if it causes the network to fail, declare the connector installation deficient. There is no excuse for a crimped connector to be intermittent in this fashion. You need to make it clear to all parties involved if this problem isn't addressed, that reliability and utility will suffer.

I must reiterate, this problem can't be fixed by leaving it alone or securing the cables. You can either address it, hide it, or run away screaming. Something has to give.
Giving a tug on a connector and watching communications wack out seems to have produced a meaningful test result, regardless of how low-tech and inelegant it seems.

Sporadic communications failures will cause the owner (and operators) to quickly lose faith in the system and start a blame game.

Discuss with the installation contractor why the job wasn't completed properly and offer assistance to rectifying (redoing connections) the problem. You each save face, reputations go on unblemished, lessons are learned, and the customer is happy.

Our client did purchase the 1786-CTK/B, and I agree it's tutorial is one of the better training aids I have seen come out of A-B. From my inspection of the connectors I was able to pull off, I think the problem is a less than desirable cleaning off of the foil around the center conductor (and with the fine multiconductor core style the exclusion of one of the conductors in the pin-crimp). These stray conductors can then of course cause a short. That's why I was looking for a test to stress the termination. The 1788-MCHKR you mentioned is able to test coax? When I first looked for solutions I came across it but had the impression it was for RJ45 Ethernet only.

Thanks for you input.