[BC] Assistance With A PI Network Definition

[Thomas G. Osenkowsky]

> Ok, I'm REALLY going to go out on a limb here and show some ignorance
> of basic ATU theory. What EXACTLY is meant by, and what is the
significance
> of the phase rotation of a PI, T or any other network as it relates to an
> ATU?

For a non-D station the phase shift of the ATU, transmission line
and transmitter output network should provide an optimum load
to the final amplifier(s). The choice of Pi, Tee or L network in the
ATU is governed by several factors. First, the load impedance. In
a directional array the self-Z is NOT the drive point Z. A DA is a
coupled system. A change in any component is reflected in all towers,
some more than others.

In one three tower dogleg, unequal height array I had a daytime
drive point Z of -1200+j2000. I used a leading Pi network design.
I used a 40 uh coil to shunt the base, thus transforming the
-1200+j2000 to a more reasonable -21+j306. A Tee would never
match that high Z well at all. I could have used an L network however,
the bandwidth would not have been as good as the Pi.

Each case is different. Use what works for you. In the case of a DA,
you must consider impedance AND pattern bandwidth. Careful choice of
networks is crucial to producing a broadband, stable pattern.

Tom Osenkowsky, CPBE