Sensor cable design

You're playing with words. You didn't measure the impedance, you calculated it.

that was my point with the question. I believe complex and more in the range of 72 ohms stuff, not that kiloohms touted earlier.

Your method of quoting implies I said that statement. I did not. I refuted it with a 'verbal' description of the complex relationshi between loss, Z, and frequency.

Actually you can, that's around 100 ppm effect shows up gangbusters in my work. Actually a bit more since the velocity down a cable is less than speed in air.

Describe a practical method of determining the characteristic impedance of a lossy transmission line, at a specific frequency, without calculation from indirect measurements.

The use of specialized instruments, such as vector network analyzers, (which invariably do the calculations for you), isn't allowed.

The insets and ">" quote characters say the rest.

I was replying to John, and his reply to me shows that he knew that.

Looks clear to me.

Maybe your newsreader doesn't honor the usual quote characters.

Or sampling voltage and current, multiplying individual samples, and integrating to derive power?

Quick and dirty simulation with a 50 ohm load on both network and line shows a difference in amplitude of 0.053dB and phase difference of 0.113 degrees at 10kHz. Using Belden 8262 figures for cable constants, the same values for the lumped network.

Difficult to measure. Yes, there *is* a difference, and it depends on load resistance. Above about 1k, there's little difference.

John's assertion is close enough, the way he described it, for readily measurable quantities.

Evidently not! Glad you were responding to someone else.

I just didn't want to leave the impression that I did not understand the effect of 'skin effect' on characteristic Z, etc.

The phenomenon I was describing is not due to skin effect. The departure from a resistive, zero-phase, characteristic impedance increases

For example.Belden 8262 (RG58C/U) looks like 199 -j193 at 1KHz, 773 -j53 at 10kHz, 51 -j8 at 100kHz, and doesn't look approximately resistive until above 2MHz.

Increasing attenuation with frequency is a separate thing. Skin effect plays a part in that, together with dielectric absorption.

Yes, I remember everything goes kind of wonky at low frequency diverging away from the concept of characteristic impedance, just didn't remember how wonky.

I was actually referring to skin effect changing characteristic impedance, not just changing the losses. From about 100kHz upwards 1GHz, you get not just the frequency related attenuation, but also a shift in Zo because the carriers on the center conductor are no longer uniformly distributed inside the center conductor, but move out to the surface of the center conductor, thus changing the inductance per foot, from memory, can change more than 4% total.

Have you measured it?

On a sunny day (Tue, 22 Oct 2013 13:43:01 -0700) it happened RobertMacy wrote in :

It is in fact easy to see. At frequencies low enough where the L part has no influence, the cable becomes simply a capacitor. Z = 1/j.w.c

You can make a capacitor of a few pF with some length of coax...