I need to make some RF probe and instrument coax cables. I have some old RG-8X coax that I can cut pieces out of that is pretty flexible but what is the coax used to build scope probes? It's a lot smaller diameter than RG-8X and very flexible. Any ideas? I do have some RG-142 double shielded 50Ω coax but that stuff is pretty stiff (solid #14? center conductor) although about the same diameter as RG-8X. I do like the idea of well shielded instrument cables. RG-316 seems pretty small in diameter but is a possibility that I'm looking at.

 

If you want low loss, oscilloscope probe line is not what you want - it is usually very lossy:

https://cleverscope.com/downloads/articles/THE_SECRET_WORLD_OF_PROBES_OCt09.pdf

 

what is the coax used to build scope probes?

Tektronix has a patent for a resistance wire so that the distributed R and C gives the cable a flat frequency response. Patent #2,883,619.

 

Hmmmm... Quite a bit more to the problem than I expected.

 

Hmmmm... Quite a bit more to the problem than I expected.

Have you considered stranded coax?

 

Some RG-174:has a copper-plated steel core. Avoid this version (RG-174 with a steel core) where a cable with be flexed a lot. The steel core can slice through the dielectric when heavily flexed, resulting in shield shorts. You've not had a headache until you try to troubleshoot this.

 

Kept doing a bit of reading and found RG-400U. Very flexible multi strand silver plated copper center conductor w/ dual silver plated copper braid shields, PTFE dielectric, and 5mm in diameter. Pricey but I don't need a lot of it. Looks interesting but probably overkill for what I need.
Flexible RG400 Coax Cable Double Shielded with Tan FEP Jacket (pasternack.com)

 

@SamR these prepared patch cables from Times Microwave are only ~$1800.00 each and they are good out to 70GHz, so probably perfect for your hobby applications.

 

5mm in diameter

The RG-174/U I mentioned has a 1.52mm diameter and, IIRC, can be bent to a 1cm radius. Probably under $1/ft in small quantity.

 

The RG-174/U I mentioned has a 1.52mm diameter and, IIRC, can be bent to a 1cm radius. Probably under $1/ft in small quantity.

Beware of the steel core. Shield shorts are a real problem. Not all variations have a steel core.....

 

I use copper core RG-174/U for ion beam setup Faraday' current pickups that move in/out from the beamline on every implant cycle of wafers. With a nice loop the stuff lasts for years in continuous 24/7 operation of thousands of cycles.


The setup faraday is between L1 and L2 connected to the pneumatic rod assy on the bottom.
https://www.mouser.com/datasheet/2/46/7805_techdata-2323555.pdf

 

I went ahead with what I had which is some RG8X for the lead and a piece of RG132 for the tip since it has a solid center conductor. The Heath T-3 signal tracer I restored was missing it's leads so I used the manuals schematic and instead of a switch shorting the 1N34 diode I built both an RF and an AF probe. I did use 630V poly caps since this will be used around tube gear. Used 3/4" copper pipe and end caps for the probe bodies. Almost finished but discovered the male BNC connectors I have are for RG59 and the RG8X is too big for their ferrule. Always something. Used 2.5mm solder wick braid for the front ground connection and also ran a piece out of the back cap to solder to the body to ground it to the coax braid and circuit ground. Will also put some heat shrink around the body when I get the right size in. Used 6" piece of coax for the probes and may shorten them if needed. I'll see how these work before modifying with better coax.