What is Coax Velocity Factor?

Do we need to know or even understand the velocity factor of a coax line? In the main, no - although very often, velocity factor often follows loss - but that is not the point today. You shouldn't (normally) have to worry about coax velocity factor. Let me explain. Callum.
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[@vk2gpg746]

Velocity factor is the velocity by which RF waves travel through the coax. Because the dielectric which is the insulation between the conductors has a particular density and not a vacuum RF travels slower through it. The number is is referenced so "1" is the speed of light in a vacuum, so .68 would be 68% the speed of light. As a side effect because frequency is referenced to time the wavelength compresses by the same factor so a VF of .68 will mean a wavelength in the coax is .68 x (300/frequency MHz) to give wavelength in metres. In practice cutting phasing sections and coaxial transformers by the above will not be perfect. Using a VNA to measure and match a resistive load at the end of your matching section at the target frequency will yield superior results, as will using the VNA to measure phase shift for phasing harnesses.

[@DXCommanderHQ]

Perfect!

[@dacodemonk]

Something I never understood just explained in 4 minutes. This is why I love this channel!

[@thomasperina2990]

Top of the Evening to you Callum: Thank you for the introduction to the velocity factor of coax cable. With my new CB Antenna install I was able obtain some LMR 600 FOR more energetic transmitting & if I'm not mistaken the VF IS IN THE High 80's. I know it's a real pain in the butt to move around. God Bless & I look forward to more of your videos. TMP, UNIT 22, from N.J.

[@jasonodette8607]

Thanks Calum 👍

[@DXCommanderHQ]

You bet!

[@michaelmohr9700]

Thank you Calum for this Video it is nice and so simple description !!!! Thank you !

[@DXCommanderHQ]

Glad you liked it!

[@DonzLockz]

Awesome Callum. ;) My velocity factor decreases considerably every year😅... does that happen to cable age too?🤔

[@DXCommanderHQ]

Yes! :)

[@billybiker1383]

Thanks Callum 😊

[@DXCommanderHQ]

You’re welcome 😊

[@kd5smf]

I use a Rig Expert AA-35 Zoom to analyze my antenna's. There is a VF under the Set Up menu to allow you to enter the correct VF for the cable you choose to use. If you do not have the correct VF entered then your measurements will be erroneous and you can mess up your antenna project. I found that out the hard way :( So VF is important when measuring for the coax length and to see how your feed line affects your SWR! Best to invest in a high quality feed line with low loss for the frequencies you want to transmit on. 73's de kd5smf

[@mewrongwayKOCXF]

Awesome Callum!! 👍👍👍

[@DXCommanderHQ]

Legend!

[@Hamradio101]

Thanks for another upload cal

[@DXCommanderHQ]

You bet!

[@pixeluser175]

My rig has a knob named CAL. I don't get a response however.

[@eliotmansfield]

you need to lube it

[@DXCommanderHQ]

Shout louder!

[@fingers5100]

Cheers cal

[@dnel83]

I tried cutting a 6m dipole in the summer feed was 10m of rg213 into 7m rg58 air choke and my measurements for the leg length were not even close, it was then I found rg213s VF is very different but mixing two mediums in the feed probably isnt helping either. Never did figure out what length the dipole legs should be 😣

[@DXCommanderHQ]

So physical elements also have a velocity factor. I don't think your coax worried about it. The velocity factor of most PVC coated dipoles is around 93% so if you cut the dipole ends off, it would have probably come in..

[@dnel83]

@@DXCommanderHQ I did tune it, I think I started around 0.97 and was getting close to 0.9 without any noticeable improvement so figured something else must be going on. This was using speaker cable which is a first for me but so is using rg213 as a feed I just never got around to working it out. Probably would be easy with a VNA but alas it's not something I have (yet).

[@markramsay6399]

Liking these videos and looking forward to the next ones. Mark, 2E0MSR

[@DXCommanderHQ]

Awesome, thank you!

[@bassangler73]

My velocity factor was about 15% getting out of bed this morning...lol

[@DXCommanderHQ]

HAHA!

[@haraldlonn898]

But if it slows down in the coax how does it speed up in the antenna and does it mean we have a trafic jam in the coax?

[@DXCommanderHQ]

Haha.. No traffic jams, no.. When it leaves the coax, it get's a speeding ticket :)

[@jolebole-yt]

Why would a velocity factor shorten the wave length? I thought because of the coax (or any conductor) resistance signal travels slower, so it takes longer time to reach the same distance.

[@TrevorClarke]

The frequency will remain constant but the propagation speed slows which changes the wavelength. λ=c/f * VF The frequency doesn't change as it is the drinking characteristic of the wave and the relationship to wavelength is not fixed.

[@jolebole-yt]

@@TrevorClarke Thanks for the formula. I should do more reading on this.

[@jolebole-yt]

@@TrevorClarke So this practically happens in the antenna elements too? For the element to be resonant on a specific freq the elements are usually shorter then the actual values on paper.

[@TrevorClarke]

@@jolebole-yt yes. Every medium has a velocity factor and every medium that isn't pure vacuum has a VF

[@jolebole-yt]

@@TrevorClarke Great info. Thanks fro sharing it.

[@mark2w0jmk7]

Hi Callum I have a 2M power beam antenna with 37m of ultraflex 7 coax. I cut 7.5 meters off it to help with db loss and my VSWR shot up to 1.9. WHY? Could you or do you know why the length of a run of coax would do this on VHF 2M. Mark 2w0jmk

[@g0fvt]

Loss in your feeder will lower the SWR seen at the transmitter end, my guess is that you have a problem at the antenna. Taking an extreme example, if you have 3dB loss in your feeder the return loss would be 6dB (25%) which corresponds to an SWR of 3. So with NO antenna the SWR is a long way short of infinite in the shack. Reducing the length of you feeder will have increased the power at the antenna but you are seeing more of the reflected power. Genuine Ultraflex 7 is a moderately low loss cable but according to my quick mental arithmetic you should be seeing about 2dB more reflected power than previously.

[@DXCommanderHQ]

So G0FVT has one solution, I have another.. Firstly, that's quite a run of 7mm for 30m or so run.. But I digress, it is my view that assuming your antenna was matched, ANY length of coax would have worked (shorter better) so it it more likely that you created a small issue with the connector. Double check continuity between earth and centre and make sure it's zero..

[@g0fvt]

@@DXCommanderHQ I don't disagree (for a change) I would guess he has a problem close to the antenna, I did miss that the cable was 37m long. The one way loss will be in the region of 3dB on 2m, so my rather extreme example fits rather well. I think as you say he needs to inspect the connector at the antenna carefully..73