Are there types of standard coaxial cable with a propagation velocity of 0.9c? What would be the application?

Question

Reading this answer I was surprised to hear that there are "excellent" kinds of coaxial cable with a propagation speed of 0.9c; 90% the speed of light.

The bargain basement number is about 2/3c, and coaxial cable with a faster propagation velocity would have to have a lower dielectric constant. Assuming \$v/c\$ scales as \$1/\sqrt{\mu_r \epsilon_r}\$ as it would in free space, that would mean the cable would have a relative dielectric constant of 1.2 for example.

Does this exist as a standard product? If so, are there applications where having "excellent cable" with such a high propagation velocity would be important? Or would it be a side-effect of other desirable properties of the dielectric?

Answer 1

Air lines still exist, with velocity factor very close to 1.0. These are AFAIK mainly used in old-fashioned VSWR measurements. The advantages are that the dielectric constant of air is fairly stable and well-known, and that you can insert a probe (a tiny antenna) into the middle of the transmission line without damaging the dielectric.

ePTFE (aka "Teflon foam") dielectric typically gives a velocity factor of about 0.85. These cables are, in my experience, used because they maintain low loss to fairly high frequencies and their phase delay is quite stable under variations of temperature and flexure, not specifically because of the high phase velocity. I've used them in test and measurement applications, and I imagine they're also used in things like radar and avionics.

I found a reference saying that "foam polystyrene" dielectric gives a velocity factor of 0.91, but I have no experience with such cables, and I don't know what applications they're favored in. In fact I couldn't (with 2 minutes googling) find any vendor actually selling them.

Answer 2

The high propagation velocity would be a "side effect" of more desirable property - signal loss/dissipation along a cable. Losses are dependent on properties of isolation material. If you already have the best low-loss material, the next thing is to make it porous, so it will mostly contain dry air. The higher propagation velocity is a side effect of this.

Answer 3

Some (older?) coaxes suspend the center conductor with glass beads every so often. Thus most of the dielectric is air. Propagation velocity is almost 1.0 \$c\$.

Answer 4

Thanks to the link to the image from the eriinc.com site in @marcelm's comment (from which another page links to commscope.com) I've been enjoying some amazing pics of coax instead of doing work.

Here's the rigid line coax and dielectric space from the comment and a similar one:

but they also have a link for coaxial cable at Commscope where there are some very nice low volume dielectric spacers; HJ11-50, H5-50 and HJ9HP-50 with propagation speeds of 92%, 92% and 96% the speed of light!

Answer 5

The high velocity specs translates into lower loss, more air-core than plastic and more copper $$ more rigid and more bandwidth were all standard rigid coaxial trunks for 300+MHz CATV in the 1980’s before fibre was more cost-effective.

Reducing C from lower mu was the goal while maintaining impedance error tolerances from O/I diameter ratio damage.

This also raises impedance from higher L/C suitable for some apps unless the centre diameter is increased to compensate with lower L thus more $$ to get back to 75 ohms for CATV or 50 ohms for almost everything else.