Velocity factor of co-ax

I did.

Solid polyethylene isn't too bad but foamed has a nasty habit of moving under the influence of its own 'memory'. You solder the end of a slightly bent centre conductor and, as the heat travels down it, the foam springs back to the straight position, leaving you with a slot in the foam and a bare centre conductor shorted to the screen.

That was why I asked about it here, I suspected the measurement.

However... my method of finding the first reactance swing in the reflection from an open circuit should give me a measurement of the electrical length of the cable that is independent of the terminating impedances, calibration etc

The cable was physically 6.39 metres long and the first 'resistive' impedance point was at exactly 15.000 Mc/s. (That's another reason I was suspicious, it really was spot-on 15.000 Mc/s, give or take nothing.)

Can you not do it by TDR? Use some fast switching logic, tee piece to a scope input and a dead short at the far end of the cable.

Looking for the max resistance may not be ideal.

If you jack up the frequency and get multiple wavelengths in the cable, resolution will improve.

I don't have a VNA. I use TDR to measure time delays.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

That's why we have crimped connectors.

Maybe. The VNA needs to be calibrated to move the reference plane to the start of the cable, which is probably not at the same place as the VNA output connector. At lowish frequencies, it probably doesn't matter,

Jeroen Belleman

True, but the OP also wants to measure the length of the cable AIUI, and for that, you want as high a frequency as possible for greatest accuracy. It's a trade-off (as ever).

I always do. Wikipedia pages get edited from time to time, and it's wise to check for changes.

There was more than one screen, but 29 sounds a bit high. I read faster than most people do, but not that fast.

It did list a number of ways of making coaxial cable, some of which included schemes where the dielectric was mostly air.

That's always a potential explanation for odd results, but it's well known that the propagation delay through regular cables is a bit slower than the speed of light in a vacuum.

I have some thin 50 Ohm coax with Teflon inside, used outside on my satellite dish. So that I can feed it through the window and close it, bends a bit, reception is great!

So you didn't read all of it, but you expected me to.

Where did you find the information on that webpage about air dielectric affecting the Velocity Ratio, which was the reason you referred me to the page? If you can give me an unique quoted phrase, I can soon find it.

There was no mention of this affecting the Velocity Ratio. I wasn't asking for advice on making co-ax cables, I was asking how the construction affected the Velocity Ratio.

Yes, that is called the Velocity Ratio. I would have though it was obvious from my question that I realised what the VR was - my question was about the physical properties of the cable that affected it.

I could; and some time ago I did try this with some cable from the same drum. The results were similiar, around 78%.

Perhaps I didn't explain that very clearly. It wasn't the point of maximum resistance, it was the point where the capacitive reactance swung through zero to become an inductive reactance; it was quite sharply defined. At that point the impedance was purely resistive but it was the reactance that I was measuring, not the resistance.

True, but the electrical errors in measurement may increase too. An accuarcy of around 1% would be good enough for the present purposes - after all, where exactly is the 'end' of a piece of co-ax that is splayed out for connection to something else? I also wouldn't expect a length of cheap co-ax to be particularly homogenous.

Yes, that worried me.

My VNA will also function as a TDR and I have a telephone-testing TDR for longer lines. If all else fails, a square-wave signal generator and an oscilloscope will work too.

Yes, I did that., the maximum error after that would have been the length of an adaptor, around 1 cm in 6 metres of cable.

Having used both VNAs and TDRs for cable length measurements, I've always found the VNA measurements much superior to those of a TDR. My HP8753 would resolve 1 degree @ 1GHz with ease. That corresponds to a little under 3ps. TDRs are much too noisy to do that.

I've got a picture of a TDR and a VNA-derived measurement of the same setup here. Note how much cleaner is the trace from the VNA. You can't beat a VNA for S/N.

I made these measurements in the context of constructing a wide band beam transformer for the Proton Synchrotron at CERN.

Jeroen Belleman

My VNA is a relatively cheap hand-held device but it is good enought for the measurements I want to make and a lot better than the previous rule-of-thumb and guesswork method.

My 11802/SD24 is about two generations behind the times; your S-series rig is three. The 11801 series has both smoothing and signal averaging, and can easily resolve 1 ps of delay accurately. And has an RS-232 interface to export waveforms.

But yes, data from a good VNA can be crunched to make a good TDR. But TDR is DC-coupled and VNAs aren't.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

The problem becomes "where does the cable actually start and end" ? With my TDR I can resolve within, say, a transitiion from an edge-launch connector to a PCB trace, so one has to decide where inside the connector the handover happens.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

I read through all of it. Most of it was familiar and predictable. You may well have needed to read it more carefully, and you clearly didn't

You have to read down to the section labelled "important parameters", then work through that to the section "derived electrical parameters" where you will find "velocity of propagation"

"The velocity of propagation depends on the dielectric constant and permeability (which is usually 1)" and they give the formula

v= one over the square root of the product of the permeability and the dielectric constant.

The permeability of free space,and the electric constant of the spacer aren't "variables" but parameters. and they should be familiar concepts to any body who has had any kind of education in physics or elementary electronics. It is remarkably basic stuff.

You were actually asking how the construction affected the velocity of propagation. Your "velocity ratio" is actually the ratio of the velocity of propagation to the speed of light. A "ratio" compares two numbers and you didn't mention the speed of light.

The formula tells you exactly how the parameters affect the velocity of propagation. Telling you that in a way you can follow hasn't been easy

And you got told, repeatedly. Sadly, we didn't realise that you hadn't had much formal education in the area.

My VNA collection also includes a cheap hand-held device. Because ... why not? One of the hams in my club uses his VNA exclusively for SWR measurements. So he simply skips Smith charting altogether. On the other hand, it's all about the Smith chart for me. The VNA's sort of a shortwave Swiss Army knife you can use as you see fit.

Your combo square-wave signal generator and oscilloscope as a TDR is intriguing. Thank you for sharing.

Are you *sure* it's standard RG58? No suffixes/prefixes? Does it physically (I mean from visual inspection) conform to the published specs if you peel back the layers and have a good look? And preferably a measure-up.