Roy is right. You must meaure your cable to be sure.
The stuff I have now calculates to .73 using a regular MFJ analyzer.




Roy Lewallen wrote:
My experience is that the velocity factor of foam dielectric cable like
RG8X varies considerably from batch to batch and manufacturer to
manufacturer. Apparently they have poor control over the foam density in
the manufacturing process. For any application where it really matters,
you really have to measure it for the particular piece of cable you'll
be using.

Roy Lewallen, W7EL

AK wrote:

Is 78% the correct velocity factor for RG8X coax?

AK wrote:
Is 78% the correct velocity factor for RG8X coax?

NO.

RG-8X is really not an RG cable, and has no manufacturing standard.

Foamed cables can vary from near .7 vF all the way to near .9 vF

The manufacturer normally adjusts the ratio of air to dielectric to get
the correct impedance. Since RG8X has no standard dimensions or
conductor diameters, and it has no standard impedance range, the vF
will vary all over the place.

RG8X is normally a much denser foam (less air) than other foam cables.
The RG8X cable I have is in the low .7 range as measured on a network
analyzer. I've never found any with higher than the upper .7 range.

Since there is no manufacturing standard (meaning it is NOT really
suitable for an RG number) you should measure the cable.

73 Tom

Depending on the manufacturer the specs list VF from 0.72 to 0.81 in the
Wirebook IV. Not to mention various manufacturing variations.

73's

Guenther VE3CVS

"AK" wrote in message news:KRUZf.88176$oL.627@attbi_s71...
Is 78% the correct velocity factor for RG8X coax?

I have one piece, and it measures 0.745.

Roy Lewallen, W7EL

On Sun, 09 Apr 2006 16:00:48 -0700, Roy Lewallen
wrote:

I have one piece, and it measures 0.745.


This is a bit like the previous discussions on Buryflex!

I read an interesting article on the 'net by NEC on a production
system for foam dielectric coax for low microwave frequencies... it is
clearly a greater challenge than extruding PE for solid dielectric.

Reg's response is interesting, I draw the conclusion that a 'quick and
dirty' confirmation of quality of a foam cable is whether its measured
VF is close to spec, it is it off, so is the Zo likely to be off.

Owen
--

Owen Duffy wrote:
. . .
Reg's response is interesting, I draw the conclusion that a 'quick and
dirty' confirmation of quality of a foam cable is whether its measured
VF is close to spec, it is it off, so is the Zo likely to be off.

I don't agree that this is a good test. The Z0 of cables with solid PE
insulation varies considerably, and it's doubtful that the variation is
due to variable dielectric constant of the solid insulation. So why
should we assume that Z0 variation in foamed dielectric cable is due
solely or mostly to the dielectric density?

Roy Lewallen, W7EL

Owen Duffy wrote:
. . .
Reg's response is interesting, I draw the conclusion that a 'quick and
dirty' confirmation of quality of a foam cable is whether its measured
VF is close to spec, it is it off, so is the Zo likely to be off.

Roy Lewallen wrote:
I don't agree that this is a good test. The Z0 of cables with solid PE
insulation varies considerably, and it's doubtful that the variation is
due to variable dielectric constant of the solid insulation. So why
should we assume that Z0 variation in foamed dielectric cable is due
solely or mostly to the dielectric density?

That's right.

More important, didn't he say he is matching 35 ohm elements in a
vertical array. I'd think mutual coupling would cause much bigger
errors than the cable unless he is feeding the elements either at 0 or
180 degree phase. Why worry about Vf and cable impedance when the
lements are all over the place in impedance?
:-)

73 Tom

On Sun, 09 Apr 2006 18:02:28 -0700, Roy Lewallen
wrote:

Owen Duffy wrote:
. . .
Reg's response is interesting, I draw the conclusion that a 'quick and
dirty' confirmation of quality of a foam cable is whether its measured
VF is close to spec, it is it off, so is the Zo likely to be off.

I don't agree that this is a good test. The Z0 of cables with solid PE
insulation varies considerably, and it's doubtful that the variation is
due to variable dielectric constant of the solid insulation. So why
should we assume that Z0 variation in foamed dielectric cable is due
solely or mostly to the dielectric density?

The thinking is more the case that the foam density / dielectric
constant is apparently very hard to control, and in cheap cables with
poor QA, a probably source of failure to meet spec.

Owen
--