Velocity factor of co-ax

Liz Tuddenham · 2026-01-22T09:29:10+00:00

What physical properties determine the velocity factor of co-ax? Mostof the amateur radio books give around 60% as the velocity factor for'common' types of 50-ohm co-ax. I recently bought a drum of fairly cheap 50-ohm co-ax with the screenmade from a metallised plastic tape and a loosely-woven copper braid.Using a VNA I measured the reflected impedance of a known length (about6 metres), open circuit at the far end, and found the frequency at whichits reactance first swung through purely resistive. From this Icalculated its effective electrical length and the velocity factor,which turned out to be 78%.This seems so different from the 'conventional' value that I amsuspicious of my measurements - but this type of screen construction wasnot in common use when the original 'words of wisdom' were written. Are there any physical properties of the co-ax could I check, whichmight explain my measured velocity factor?

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Liz Tuddenham 5 months ago

That's the one I used, there are several models with different frequency ranges but they are all priced much the same. The only negative points are the flimsy connectors and the lack of information.

Mine came with two short extension leads and I find these should always be used so as to take the strain off the sockets. For high frequency work, the device needs to be calibrated with the terminations on the far end of the leads, so the leads don't become part of the measurement.

The only information was an A4 sheet giving a map of the menus - nothing else at all! There is plenty of information on the Web, including the supplier's own website, but the device does so many things that the purchaser could easily be swamped with unnecessary information.

What does it measure? Impedance Resistance Inductance Conductance Susceptance SWR Smith Chart Time domain reflectometry Pulse response Series resonance Parallel resonance Q factor Crystal resonance Signal generator R.F. Frequency response & bandwidth etc.

All with a choice of Log or Lin scales and frequency range. Frequently-used settings can be saved in 6 onboard memories and the results downloaded to an SD card.

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Phil Hobbs 5 months ago

I’d probably be spending a few bucks on a calibration kit, then. Cheap, uncalibrated VNAs can produce lots of reasonable-looking wrong answers.

Cheers

Phil Hobbs

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Jan Panteltje 5 months ago

Fun site, I have the same small mp3 player, but am using a Creative Muvo with a 1.5 V eneloop rechargable AAA cell lasts 10 hours on a charge...

As to QO100, I dunno where exactly you are, but it is not accesable from the US, see 'coverage':

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I think there are many small Amsat satellites?

I got into digital transmission systems because I worked many years here in broadcasting when all was analog, needed deeper understanding to keep up, so hands on, designed my own and wrote the DVB-S encoder code myself:

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No PCB needed, GHz out..

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Bill Sloman 5 months ago

The satellite isn't directly overhead for very long - if it every is - an equatorial orbit is the default case and that only gets directly overhead if you live on the equator.

Pointing a parabolic antenna slightly above the horizon gives you signal as soon as the satellite pops over the horizon, and it keeps on giving you signal as the satellite gets closer (giving you a bigger signal) and moves off the axis of the parabola (making the antenna less sensitive).

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Jan Panteltje 5 months ago

Get error message?

Very nice!

I have a dish with a motor that can swing east to west and pick out satellites with TV channels or QO100 (Eshail2) I did modify the LNB (downconverter) for crystal lock so I could receive SSB. Normal LNBs often use ceramic oscillators, a bit of drift from those does not matter much for DTV, but for SSB we are talking about a few Hz drift in a GHz carrier. Lots of stuff on my site about that, or was posted here, locked one to my 10 MHz Rubidium reference:

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the spectrum analyzer is xpsa, uses an RTL_SDR stick:

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that is an old version... Added FM stero to it, more functions, moving target:

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Really guys you do not need a fortune for equipment. What amazes me is that a LNB and dish and dish motor I bought 26 years ago still works perfectly Freezing cold, storms rain, hot sun,... I moved house 2 times in that period and just re-installed the thing - works perfectly.

The QO100 carrying satellite is a "geostatic satellite' it moves around the earth above the equator at the same speed as the earth rotates, so always is in the same (for us in the Northern hemisphere) south position. It's altitude above the equator is always the same The further you are north the lower angle to the sat above the equator.

Here is a list of many commercial satellites (Radio + TV)

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The little icon at the top gives the east-west postion of each, and all te htransonder channels.

QO100 transponders are on Es'hail 2 26.0°E Es'hail 2 43700 130 66 25.78°E -0.02° 0.02° N/A N/A 2026-01-26 02:55

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You need a calculator to enter your location so you can see what vertical and horizontal angle your yagi or dish needs to point to:

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to QO100 is at 2.4 GHz.

The house across the road from my dish had the roof insulated, it became a bit higher likely some metal in it ,,, since then my receptioon of Eshail 2 is bad. So I will have to move the dish / yagi a bit higher some time.

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Cursitor Doom 5 months ago

Better signal capture.

If you're in the N. hemisphere, your antenna probably points towards somewhere above the equator. Same if you're in the South.

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Jasen Betts 5 months ago

you probably want to do wavelengths shorter than the cable for best results but crazy high frequencies aren't needed.

You run a frequency sweep and record the reflected amplitude and phase, then do a Fourier transform, (which is what the NanoVNA does in TDR mode) This will give you good answers if the cable has a linear response.

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Velocity factor of co-ax

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