Topband: Coax velocity factor
w8ji at contesting.com
Wed Mar 28 06:50:28 EST 2007
> a dozen or more Z measurements and then double checked the
> length of
> the cable. Every time I get the same results: VF=84%.
> This just
> seems a tad high to me, especially when 75% is the
> published value
> for RG-6, though I know the VF of foam cable is higher.
> Could the
> frequency display on the RF-1 be off, or is this value
First, most people using the designator "RG-6" should not be
doing it. RG numbers are joint army navy (JAN) numbers for
standard cables that are built to a certain detailed spec.
The only real RG-6 is actually a silver plated copper
/copper double shield and copperweld center video cable.
As for foamed cables, the VF can range anywhere from very
near .66 to up near unity. The actual VF depends on the
ratio of air and dielectric and what type of material is in
the dielectric. This is true for any standard construction
foam dielectric coaxial line. I have some foam cables that
measure near .9VF, and others that measure near .72.
Many times the ratio of air to plastic is adjusted to a
certain value to produce the correct impedance with the
conductor size used. My RG8X foam has very little air, and
measures in the low .7 range. My CATV F-6 type cables (often
incorrectly called "RG-6") measure in the .8 range. A few of
my large hardline cables measure around .9 VF.
The .84 you measured sounds very reasonable.
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