Re: OT: High Frequency coax?? Copper tubing??

Reginald Beardsley
 

On Sun, Nov 3, 2019 at 12:46 PM, Dennis Tillman W7PF wrote:


These two sizes of Semi-Rigid Copper Coax cable are very common:
RG-402 which is 3.58mm OD (0.141" OD)
RG-405 which is 2.20mm OD (0.0865" OD)

Working with it is very different compared to typical coax we are all used to.
Tools to cut it properly and to bend it properly are expensive. Using common
tools to cut it like a pipe cutter, and to bend it like an inexpensive bending
jig, will almost certainly affect the high frequency performance of whatever
you make.

Dennis Tillman W7PF
I've not made up any cables yet, but I have bent it by hand. I've been using TDR, so I don't have the dynamic range of a VNA, but I've never seen a reflection from a bend which was even within an order of magnitude of the connectors.

The UCB EE department uses soldered braid RG402 for cables for student use. They do point out that the cables are not phase stable and to tape the cable down before doing a cal and inserting the DUT. There is a very good document about making up RG402 SMA cables from UCB, but google is unable to find it at the moment.

Properly ending the solid tube cable only requires a suitable equivalent of a copper tubing bender. It *does* require bending dies of the proper diameter and radius. They need not be expensive, but they often are because the buyer is not spending their own money. Also, bear in mind that it *only* gets difficult if you are working with very wide BW.

Cutting it is a bit more fiddly as tubing cutters deform the outer shield, but there is nothing about it that requires expensive tools. An Exacto or jeweler's saw works fine. However,you *do* need to understand what you are doing and the constraints imposed by Maxwell's laws.

Reg

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