It's a common misconception that the use of disks or foamed dielectric
reduces loss because of reduced dielectric loss.

But, up to a frequency of somewhere around 1 - 10 GHz or so, that isn't
true.

Below 1 - 10 GHz, the conductor skin effect loss is much greater than
the dielectric loss of solid PE or PTFE (the common coax insulating
materials), so further reduction of dielectric loss makes no appreciable
difference.

BUT, going to disks or foamed dielectric does reduce the effective
dielectric constant of the insulator. This results in the requirement of
a larger diameter center conductor for the same Z0 and cable diameter.
The cable does have lower loss, but because of the larger center
conductor, not because of decreased dielectric loss.

Roy Lewallen, W7EL

Ed wrote:
I was just given a coil of 7/8 hardline coax that I have not seen
before.
It has plastic discs inside to separate the core from the outer
aluminum tube which I suppose uses air as the dielectric. Anybody got
any info on this stuff
and any advantages it may have over the normal hardline cable?
Regards
Art


That's nothing real new. Yes, is air dielectric and slightly less lossy
than regular foam dielectric heliax. Since you described the outer
conductor as aluminum, I suspect it is 75 ohm cable.... but not
necessarily. There should be info printed onto the outer plastic jacket
material mthat would help in specifically identifying that cable.


Ed K7AAT