What is inside VNA test cables - Page 1
Forum section: electronics / rf-microwave

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: October 28, 2021, 02:20:00 pm
content: 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?

author: TimFox
author_rank: Super Contributor
timestamp: October 28, 2021, 04:01:49 pm
content: 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.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 28, 2021, 08:05:40 pm
content: https://patents.google.com/patent/US5477011

author: TimFox
author_rank: Super Contributor
timestamp: October 28, 2021, 08:10:11 pm
content: 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.

author: coppercone2
author_rank: Super Contributor
timestamp: October 28, 2021, 10:31:58 pm
content: 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

author: rfclown
author_rank: Frequent Contributor
timestamp: October 29, 2021, 12:44:24 am
content: Years ago I got a piece of teflon at a Gore exibit booth at a trade show (probably MTT-S). It was like the dielectric that would be inside one of their cables exept that it was a solid cylinder (no center hole) about 3/4 in diameter and about 3 inches long. It was amazing how it could bend without getting deformed on the sides. Although you could bend it (like you bend a cable) very easily, you couldn't easily squash the cylinder. Made you wonder "How do they do that?". It was facinating and fun to play with. Good phase stability I assume comes from a good design off all the materials and construction parameters.

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: November 13, 2021, 02:03:00 am
content: Or you could go with articulated lines. HP used to make them for the HP8409 (the first VNA I worked on, as a summer intern). Here's a modern version from my friends at Spinner.https://products.spinner-group.com/2x-articulated-line-18-ghz-n-male-to-n-male-with-2x-portsaver-and-case-bn533626C1111