Is velocity factor independent of frequency in coaxial cable?

 

That's a question I asked on my local club FB page, here are the answers. Anyone know better?

http://www.radio-electronics.com/.../coax_velocity_factor... "Hope this helps".
 Coax cable velocity factor is the speed an electromagnetic wave travels along a coax cable relative to the speed in a vacuum.
"I don’t think so but if the frequency is outside the capabilities of the dielectric, then it might have an effect. If the frequency is within the capabilities of the coax, I would say no".
"Not sure Dave, it is dependant on the dielectric used, however I have never seen cable where they quote velocity at a specific frequency".
Good Question, http://owenduffy.net/transmissionline/concept/mvf/index.htm Measuring the velocity factor of coaxial transmission line Implementation of some transmission line systems… owenduffy.net
"When we used to set up phasing harnesses we used to use a vector analyser to get the right lengths but we where being super critical. Thus did not rely on manufactures specs".
"There’s nothing for frequency in the velocity equation but it might be worth finding out if it’s used in the dielectric constant equation. Thinking about it, a good dielectric should look like the correct impedance within the working frequencies".
"I've cut two lengths of coax for use in a 144Mhz Wilkinson divider. One slightly longer than the other. The longer one is a 1/4w*vf and dips at 138MHz and the other shorter one which is vf less 10% dips at 144MHz. Just need to confirm this is right, don't have a vna, access to a antenna analyzer might help. Only reference to this I can find http://www.dg7ybn.de/Phasing/Phasing.htm DG7YBN - Phasing Stacking and matching with coax lines by DG7YBN dg7ybn.de".
"Are the legs made of the same coax?"
"Same coax RG179. VF is measured at 1MHz apparently and it seems to be assumed that it is fixed figure".
"I would think that for all practical purposes, you should be able to assume that. And besides, 144MHz is way higher than 1MHz and not much different to 138MHz so it probably doesn’t make enough difference to worry about".
"Trouble is I'm combining two 300watt amps and would like to be sure it's right but perhaps its not that critical, I'll find out⚡"
"Found an article for adjusting vhf phase in lines using a t adaptor 50 ohm load and swr meter will bring it up".
"Using the club analyzer I've reached the conclusion that the VF of the coax I'm using is 65% and not 69.5% in the data sheet. This ties in with the dip meter and swr tests although the results are not as clear as the MFJ. If the VF is indeed 69.5% at 1MHz which is where it's measured apparently, and 65% at 144MHz it cannot be independent of frequency me thinks". 👍
"Or is the test equipment varying with frequency? I would be highly surprised to find an swr meter flat, mine are not I know that much. As for MFJ, I bought the expensive one it is (from a professional perspective) junk".
"You have to make the best you can from what you have. I'm not a professional but an amateur. This is the swr test".
"I know. I long for a suite of professional grade test kit. I am merely cautioning about not believing everything your test kit tells you as accurate".
"Understood, that's why I've used various test methods. All indicate that the VF is less than the data sheet states so I had to decide on a final figure".

"Here's a photo of the mfj connected to a final cut of coax. Impedance is low but swr high but that's how it should be so I'm happy with that ".☺
 THANKS TO THE ORIGINAL AUTHORS FOR THEIR COMMENTS