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(Bob-Stanton) wrote in message . com...
(Svante) wrote in message

Now we're talking, these were the nuts and bolts I wanted to see. I
don't know if I think that this is easier than the RLC implementation,
but I guess that would depend on the computer implementation that is
available. One thing in the above explanation that seems akward is
that it seems like you have to have a TABLE of the s-parameters for
each and every frequency? In the RLC model it is easy to calculate the
impedances for ANY frequency. Is there any method to CALCULATE the
S-parameters, rather than having a large table?

I'd still like an answer here... :-)

In a sense, you don't have to calculate the S-parameters. What you
have to calculate is the *loss of the cable*. For example, suppose
you had 2000 ft of 12 gage cable and wanted to know the S-parameters
at the frequency of 100 Hz.

What we would have is this:

Cable

-------4 Ohms--------
|
100 Ohms (The line must be terminated
| in it's characteristic
--------------------- impedance.)

Calculated Loss = 0.9615 (-0.34 dB)

Knowing that the loss is -0.34 dB we could write the S-parameters:

! Hz. S11 deg S21 deg S12 deg S22 deg

100 -120 0 -0.34 -0.0732 -0.34 -0.0732 -120 0

We don't need rules for writing S-parameters, what we need is rules
for finding cable loss.

Cable loss is caused by: the resistance of the center conductor, the
conductance of the dialectric, and any deviation from the
characteristic impedance. A general rule is: cable loss(in dB)
increases with the sqrt(F). If the frequency doubles, the cable
loss(in dB) increases by 1.41 times. This rule works well above 1 MHz.

Cable loss at audio frequenices? I wouldn't comment on that. Not
unless I first had a opportunity to verify my comments, by actual
measurements.

A circuit analysis program will have a set of rules that define the cable's performance, at all frequencies.

What would these rules look like? They wouldn't be a table?

Using a table of actual measurments, would be the most accurate basis
for calculating cable loss.

I once used an HP Network Analyzer to automatically do this. I
programed the analyzer to measure the S-parameters from 5 Mhz to 1
GHz, at 500 frequecnies. The analyzer would then write out a file to a
floppy disk, in the ".S2P" format. All I had to do was connect the HP
Analyzer to a cable, and in a couple of minutes I had a perfect, 500
point, two-port model of that cable.

Using this method, I made two-port models 10 different RF directional
couplers and a length of 0.5 in coax. (From 5 Mhz to 1 GHz). I
cascaded the 10 two-port couplers and 10 two-port cables in the
computer.

The computer results agreed *closely* with the actual measurements of
a real asacade of cables and couplers. I saw that two-port modeling
was pretty damn accurate!

OK, I think I got it now. And the "deg" of -0.0732 would correspond to
the delay of the cable (in this case you assumed propagation speed of
299 Mm/s which would be the about 100% of the speed of light in
vacuum, right?)
I'll just have to work out the S-to-Y conversion, but you already
wrote about that so I think I can figure it out.

Thanks a lot for your time.