Cec,

Alternatively, place orders for one or two reels of coax with all USA,
Japanese, Chinese, Korean, German, French, Russian, British cable
manufacturers.

When your back yard is stacked high with full cable reels, cut off a
measured length of cable from each real, lay them in straight lines on
the ground and measure the velocity factors using a hand-held antenna
analyser at around 10 MHz. You will also need a good steel measuring
tape and a sharp cable cutter. Don't use an axe!

Discard outliers of the distribution - they are probably faulty. Use
your pocket calculator to find the average, max and min, and the
standard deviation and you will have the answer to your question.

Then set up a shop.

But I'll save you the trouble and make an uninformed guess. The
Standard Deviation will be around 4 percent of the VF with the 3-Sigma
limits at plus and minus 12 percent.
----
Reg.

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

All the info is very interesting. Thanks folks.

Just want to make two 50-ohm 1/4-wave section lengths to transform the
impedance of two linear-loaded 40 meter verticals (spaced 1/4 wave) from
around 32 ohms to about 75 ohms, and then extend the cable from the
"director" vertical element with another 1/4-wave with 75 ohm coax for the
90/90 phasing. Then I just "tee" the two feedpoints together to get one
common feed point that's close to 50 ohms. Have done this previously with
good results, and had a much better than expected match to the 50-ohm
transmitter feedline. The last time I did this, I used fence wire that was
staked into some flat farmland turf for the ground system. This time I will
have to create an artificial ground plane or counterpoise, as the present
mountain terrain does not lend itself well for creating a good low-loss
real-ground system. Still have my old RG-11 matching section for the one
75-ohm coax 1/4-wave extension cable, but the two RG-8 sections are no
longer around. So, thought I would use the smaller and cheaper RG8X this
time. Maybe I won't. HI
AK

Reg Edwards wrote:
But I'll save you the trouble and make an uninformed guess. The
Standard Deviation will be around 4 percent of the VF with the 3-Sigma
limits at plus and minus 12 percent.

Thanks Reg, I can live with 56 ohm coax.
--
73, Cecil http://www.qsl.net/w5dxp

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
--