"AI4QJ" wrote in
:


"Owen Duffy" wrote in message
...

You have 4.124m of RG58C/U, the far end is s/c, what is the
apparent inductance at 4MHz looking into the line.

Did you get 1.1uH?

I got 1.025uH using 250nH/m.

Well, it is about the right answer, but could be for the wrong
reasons.

Well, I used a manufacturers' spec.. The same manufacturer I think
used 53 Ohms for Zo. I like your method better although mfr's spec. is
probably accurate over a very wide frequency band.

RG58C/U would usually be 50 ohms Zo.

You cannot use the inductance/metre figure in the general case. If you
approached it that way, though your answer is close, the method is
inaccurate for all but very short lines. Compare the results of both
methods on 16.496m s/c stub of the same line at 4MHz, your method gives
4uH, bit the correct answer is that it is not inductive, it looks like a
capacitive reactance around 86 ohms.

The next dimension is the loss or equivalent series R of the impedance
looking into the stub. The maths is a bit uglier, and is implemented in
the calculator at http://www.vk1od.net/tl/tllc.php . Playing with the
calculator will reveal why transmission line elements don't necessarily
make good reactors.

This concepts above will be explained in any decent transmission line
text. Some of these concepts are the reason you are having difficulty in
understanding Corum's transmission line representation of the coil.

Owen