wrote:
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?

I went through this exercise about 15 years ago using my MFJ-949E.
I assumed linearity for the capacitor setting numbers on the front
panel, i.e. setting 10 = 208 uf. I don't remember what I assumed
for the minimum capacitance zero setting, maybe 10%?. Using a dummy
load and setting the caps near half scale, I then calculated the
different values of inductance for the various switch settings.

For instance, with C1 set to 3.1, C2 set to 3.2, a match was
achieved on 3.5 MHz with the coil set at J. A match was
achieved on 4.0 MHz with the coil set at I. I calculated
52uH for position J and 40 uH for position I.

I wrote a BASIC program that took the switch settings as inputs
and output the estimated impedance looking into the feedline.
I don't know how accurate it was, but it gave me a useful
ballpark value. Ballpark values are often good enough to
indicate what needs to be done next. Shortly after the above
exercise, I bought an MFJ antenna analyzer.

What I found to be a more useful technique is to adjust the length
of the feedline until a purely resistive value is achieved. That
happens at the current minimum points on the SWR circle. An MFJ
antenna analyzer will read that resistive value. Knowing that
resistive value, the Z0 and loss characteristics of the feedline,
and the length of the feedline allows a fairly accurate calculation
of the antenna feedpoint impedance which, at my QTH, agrees closely
with the feedpoint impedance predicted by EZNEC.
--
73, Cecil http://www.qsl.net/w5dxp

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