8

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?

uhoh
  • 3,399
  • 2
  • 23
  • 63
  • 6
    Signals in a hurry? I have read of using line-of-sight tower to tower laser links from NY to Chicago to shave a few ms off the fiber transit time to give a competitive edge to stock exchange trading companies. (I've also read of a company adding a 32 km coil of fibre in their switchroom to add a few ns delay to users to limit the rate of transactions.) These thinks matter to some folks. – Transistor Sep 24 '17 at 16:00
  • 1
    The "best" I have seen is around 0.8x and I have never seen a case where it is necessary to have a special factor, just some where you need to know it. – PlasmaHH Sep 24 '17 at 16:01
  • 1
    @Transistor more like a ms than a ns. So this cable was invented for high speed stock trading? – uhoh Sep 24 '17 at 16:02
  • 5
    I haven't a clue. I don't think coax is used for long distances anymore. Fiber took over a long time ago. – Transistor Sep 24 '17 at 16:04
  • @PlasmaHH right. It's often important to know it, and in some situations that it be very stable, but I can't think of any reason why it would need to be as fast as possible. – uhoh Sep 24 '17 at 16:04
  • Links like the ones mentioned by @Transistor also exist between London, Berlin and Paris, if I am not mistaken. – Joren Vaes Sep 24 '17 at 16:21
  • 99% of times when you see cable performance measured in fraction of lightspeed, it's just a marketing gimmick in order to sell veblen goods. Fasten your wallet and look up "audiophile cable". – Agent_L Sep 25 '17 at 09:51
  • 1
    The high velocity specs translates into lower loss, more air 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 – Hoagie Feb 27 '23 at 15:05

5 Answers5

14

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.

The Photon
  • 126,425
  • 3
  • 159
  • 304
  • This is a great explanation of the situation, thank you! – uhoh Sep 24 '17 at 20:45
  • 1
    LMR 1700 http://www.timesmicrowave.com/documents/resources/LMR-1700.pdf has a velocity factor of 0.89. The insulation material is gas-injected polyethylene. – davidmneedham Sep 25 '17 at 15:05
  • @davidmneedham, thanks. One of my references also said ePTFE can reach 0.9 (presumably when made with higher air content). But I find 0.85 is much more common. – The Photon Sep 25 '17 at 16:17
7

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.

Ale..chenski
  • 38,845
  • 3
  • 38
  • 103
  • Makes sense; the only thing better than having the best *is having less of it* :-) – uhoh Sep 24 '17 at 20:46
  • 2
    Just as an extra comment, if you end up working with higher frequencies such as 10 or 20 GHz, the loss per foot for regular coax starts to be very large. – user57037 Sep 24 '17 at 21:08
4

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\$.

analogsystemsrf
  • 33,703
  • 2
  • 18
  • 46
  • That's really interesting! I'd love to read about that or see a picture if possible. As long as the bead-lumping distance was way smaller than the wavelength, I suppose it's no different than the air pockets in foam or the dielectric spacing in ladder lines. – uhoh Sep 25 '17 at 04:12
  • 1
    Show me a coaxial cable with a velocity factor of 1.0. Even "air core" coax has a sub 1.0 vf. – W5VO Sep 25 '17 at 06:33
  • 2
    @W5VO even a beam of light six feet wide traveling in a vacuum will travel slightly slower than the speed of light. See [Wouldn't any structured beam of light be expected to travel slower than a plane wave?](https://physics.stackexchange.com/q/269052/83380). Somehow I missed that last sentence-fragment in the answer, thanks for pointing it out. Indeed Nothing really travels at exactly 1.0 c other than "infinite plane waves". – uhoh Sep 25 '17 at 10:26
  • 1
    @uhoh Some applications use _rigid line coax_, which is essentially a copper tube suspended in a copper tube ([example image](https://www.eriinc.com/wp-content/uploads/2016/09/macx650.jpg)). [EEVblog #569 - Tour of an Analog TV Transmission Facility](https://youtu.be/mR_wJkxKSXU?t=2884) is a fascinating video about high-power analog RF devices, including rigid coax. – marcelm Sep 25 '17 at 19:00
  • @marcelm thanks for the links! I will watch the video tomorrow, I got distracted by [all of the cool coax](https://electronics.stackexchange.com/a/331156/102305) your linked image led me to! – uhoh Sep 25 '17 at 19:56
3

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:

rigid line coax 1 rigid line coax 2

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!

HJ11-50

H5-50

HJ9HP-50

uhoh
  • 3,399
  • 2
  • 23
  • 63
  • 1
    Those spiral spacers are where the "heliax" name comes from. Although nowadays it also encompasses foamed PE dielectric which is more conventional, but still manages 88-91% Vf by being mostly air. – hobbs Feb 27 '23 at 15:29
1

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.

Hoagie
  • 1
  • 4