coaxial cable thermal conductivity

Be aware that thin coax hardlines are usually Teflon-insulated, and so suffer from the "Teflon knee" effect, where the electrical length changes abruptly around 20 C (crystal structure phase change). Don't know if this matters in your application. If the cables are short, the effect is small.

The good news is that Teflon can be run a good bit hotter than polyethylene et al.

Joe Gwinn

Calculate thermal resistand of inner and outer conductor and you have a very good first estimate

I mentioned the Teflon effect to my customer (who is providing my trigger) and they'd never heard of it. And they are harassing me over picoseconds.

My block will only run at 35 or 40C, so any cable should survive. I'll probably measure prop delay of candidate cables vs temperature.

I measured the electrical resistance of the shield and multiplied by

Maybe I can find some super tiny cable for the bias signals, which are slow and drive the hi-z part of the EOM.

Teflon's mechanical properties go all over the shop at the glass transition, but the dielectric properties don't. So if you've got a bunch of mechanical stress due to bending, say, I expect it'll relax all at once at T_g, causing mechanical motion that may be quite noticeable. However, it ought to stay still after that if it's undisturbed.

Cheers

Phil Hobbs

The semi-flex ones are tinned braided copper.

The 150K factor is pretty good for most metals. Some alloys seem to be higher, so the tinned braid might be even higher theta. I'd want to use flexy stuff for the bias runs anyhow.

Something like this:

I suspected the part about picoseconds.

If your stuff is always above 25 C while in operation, you won't see the knee effect. The customer cables coming to your stuff may be another issue. I assume that your requirements are worded such that the delivery cables are not your problem.

Hmm. Any idea how long these delivery cables are?

Coaxial cables insulated with foamed polyolefin have no such knee.

I assume that the relevant cables in your product are less than a foot in length.

The small-diameter rigid coax (solid copper shield, solid Teflon, solid or stranded copper center) is actually pretty flexible, and is easily formed by hand, or in a wooden form if multiple assemblies will be made. One can hand wind baluns using such coax insulated with a heat-shrink outer jacket (needed to prevent shield-to-shield shorts).

Joe Gwinn

This is cool, especially page 2.

Should be fun. Coax and fiber delay tempcos have the opposite sign!