Coaxial cable leakage

A few weeks ago, there was some talk about how interference leaks into coaxial cables, and I promised to dig up some measurements. (Thread "Are ferrite cable shields on coax a good idea?")

The results are at: . I measured only a few cable types, chosen from those that are commonly used around here, and which would fit into my test jig.

I've been trying to come up with a spice model for this, with checkered results. The low-frequency end is easy enough: It just depends on the DC screen resistance. But the drop of Zt at medium frequencies is apparently caused by the skin effect, and not, as I formerly believed, due to the common-mode inductance of the cable under test.

With just the cable geometry and screen resistance measurement, I can't predict at which frequency the breakpoint will be, and I don't readily know how to model the skin effect in spice, either. Anyone care to comment?

Jeroen Belleman

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Jeroen Belleman
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Robert LaCoste made considerable experiments into different grounding patterns with different types of co-ax, which is a closely related area. It was written up in a very recent edition of Circuit Cellar - December 2008 IIRC. You should look it up, because he presented models that explain much of what he measured.

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Clifford Heath

Did I miss it or did you state what the termination of the outer transmission line is?

Skin effect looks quite a bit like a parallel RL circuit. A many stage lumped constant line will match a cable fairly well if you do parallel RL and series RC circuits to model the increased losses at higher frequencies.

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this may or may not help

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