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Measuring Velocity Factor w/ MFJ-259

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November 3rd 03, 01:21 AM

J M Noeding

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On 31 Oct 2003 13:43:32 -0800, (Tom Bruhns) wrote:

"Reg Edwards" wrote in message ...
The velocity factor of ALL solid polyethylene coax cable, regardless of
impedance, is 0.665

And this comes from someone who I could swear posted not long ago a
table that had velocity factors for solid polyethylene cable that were
significantly different from this magic number?

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.

It's quite accurate figure, but the cables are not so accurate.
Remember trying to make two RG213/U halfwave stubs for 100MHz, and was
surprised to learn that the 'halfwave' length varied about an inch

Suppose it wasn't only for the bubbles...

73
Jan-Martin, LA8AK
http://home.online.no/~la8ak/
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