Electronics > RF, Microwave, Ham Radio

What is inside VNA test cables

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TheUnnamedNewbie:
The title says it all. I don't want to argue about the performance being worth it or whatever - just: What do the manufacturers do to give it amazing phase stability wrt bending and temperature, and why is the cable so thick?

TimFox:
Probably the main thing is strict control on the dimensions of the inner conductor, dielectric, and shield conductor, including variations of dimension along the length of the cable.  This is easier when the dimensions are larger.  The good cable I used with an -hp- 8753 at my former employer was also armored with a heavy metal layer outside the cable proper to protect against denting and other damage due to bending.

joeqsmith:
https://patents.google.com/patent/US5477011

TimFox:
That patent covers an improved cable against triboelectric effects, including mechanically-induced signals when there is a DC voltage present.  Traditional construction used a layer of graphite or similar conductor bonded to the outside surface of the dielectric, but it’s not clear what effect it has on small changes in the high-frequency transmission-line performance due to flexing, etc.  The bonded layer ensures that no temporary air gaps between the dielectric and the shield occur during flexing.

coppercone2:
I don't think triboelectirc effect is the main problem, I think its the stuffing. Those cables are made of a type of foam insulation quite often, which gives it the flexibility required. However, getting this foam to be stable is hard, to ignore bending , and so forth. You won't find a VNA cable better then a standard piece of coaxial hardline, but the hardline gives you 1 bend, whereas the VNA cable gives you many more.

They are typically made with solder, the braid is soldered and the center pin is soldered. They might make extra effort to get wire that is drawn to a better specification, tighter temperature control on the insulation process, tighter purity control on the reagents used for creating the materials as well.

I imagine its somewhat like a good cappuccino or swiss cheese, good control of the bubble foam  :-DD

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