On 31 Jul 2005 21:01:19 -0700, "
wrote:

Hi Group,

I have a pi-network, or at least I believe it is a pi-network in my
antenna tuner. It has two caps to ground and one in series inductor.

If I tune my SWR for 1:1 and go back and measure each value of the
capacitors and inductor, is there a equation or computer program I can
use these measured values to obtain my complex impedance at the input
of the coax cable?

You ought to be able to do it from first principles, the computation
is not that difficult.

Some of the issues include your estimate of the value of the
capacitors, the value of inductance, and the Q of the inductor. You
could try to form a value for those by substitution of some known
capacitors and "matching up" a known load, working back to estimate C,
L, then use those values to solve the unknown case. The accumulated
error might render the answer of little value.

One program that springs to mind that solves a pi network is in the
Hamcalc suite, but IIRC, it treats all components as ideal (ie
lossless). Reg Edwards has another that you could play with, and IIRC,
it does allow you to specify unloaded Q of the inductor.

Have a play, you have nothing to lose and everything to learn!

Owen
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