RadioBanter - View Single Post - Measuring Velocity Factor w/ MFJ-259

November 1st 03, 01:24 AM

In article , (Tom
Bruhns) writes:

But even if we just limit ourselves to HF and above, there's a
problem: most "solid poly" cable I've encountered has small gas
bubbles in the dielectric, and the v.f. does not measure exactly
0.665. Most of the time, the difference doesn't matter, but sometimes
it does, and then it's not safe to assume it's 0.665. And of course a
lot of cable these days uses foam dielectric, which can be noticably
different from batch to batch.

I tend to agree, based on past experiences in finding VP of lots of
different RF-video cables. That includes some "economy" type
cables where the dielectric looked like polyethylene but a burn-and-
sniff test proved it was some other polymer.

There's some good formulas for finding out the VP of coax in
the May, 1978, issue of HAM RADIO magazine. The title is
"Antenna Bridge Calculations" with my byline and was for folks
using a calibrated Noise Bridge and had access to an HP pocket
calculator. A Noise Bridge nulls for admittance but this will convert
to impedance with a complex inversion...much handier on the newer
HP 32 SII calculator.

The formulas in the May '78 issue of HR will work with the MFJ
analyzer's impedance read-out values.

Len Anderson
retired (from regular hours) electronic engineer person