Topband: Coax velocity factor

Tom Rauch w8ji at
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 
> typical?

Hi Charles,

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.

73 Tom

73 Tom

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