Menu

Measuring Velocity Factor w/ MFJ-259

Search this Thread

#1

Report Post

Old

November 3rd 03, 06:49 PM

Tom Bruhns

Posts: n/a

Default

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

Even if you just ignore the fact that at low frequencies, R dominates
over omega*L and affects the velocity factor, even if you just say
VF=c/sqrt(LC), I believe there are changes with frequency. That's
because with a good dielectric like polyethylene, C doesn't change
much with frequency (AFAIK), but L decreases as the skin depth
decreases. On large line, it's unlikely to be noticable, but on a
line with an inner conductor radius only a few skin depths at the low
end of HF, the change can be perceptable. Of course, the R term does
dominate over omega*L at low frequencies. If the velocity factor were
independent of frequency, I'd expect no dispersion.

BTW, I'd be a little surprised to find an inexpensive line which did
not have polyethylene dielectric...polyethylene isn't expensive, and
it's a good dielectric except for its low softening/melting
temperature. You should be able to get a pretty good idea what it is
from carefully measured VF, if it's a solid dielectric. There are
very few inexpensive and low-loss plastics with as low a relative
dielectric constant as polyethylene.

Cheers,
Tom

Reply With Quote

Reply With Quote

Thread Tools	Search this Thread
Show Printable Version	Search this Thread: Advanced Search
Display Modes
Switch to Linear Mode Hybrid Mode Switch to Threaded Mode

Posting Rules
Smilies are On [IMG] code is On HTML code is Off Trackbacks are On Pingbacks are On Refbacks are On

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
End effect, velocity propagation question	Tac	Antenna	3	May 25th 04 10:00 PM
Measuring radiation resistance	Reg Edwards	Antenna	11	December 13th 03 12:51 PM
Shielded Loop - Velocity Factor?	Loopfan	Antenna	4	July 16th 03 07:33 AM

All times are GMT +1. The time now is 07:58 AM.

About Us

"It's about Radio"

Contact Us RadioBanter forum home Privacy Statement

Copyright © 2017