Right. There's some discussion of this in the ON4UN book, where I found
the Christman matching. As published there, it's 84 degrees of 50 ohm line
in each element, plus 71 degrees of 50 ohm line in the element facing the
desired forward direction,change directions by switching the 71 degree
section.
It is critical to understand that a 71 degree line does not necessarily
delay phase 71 degrees.
Also, if the lines to the elements are not multiples of 90 degrees, those
lines will alter phase relationship between the two elements based on the
impedance mismatch on the lines. Since the elements in a two element
unidirectional array have grossly different impedances, and since the lines
to the elements are not multiples of 90 degrees, phase delay and current
distribution changes in the element feedlines themselves are different.
This means we cannot look at the system and conclude anything.
Along the same note, almost every Ham transmitting antenna setup I have seen
does not have the phase shift and current ratios the builders or designers
think. It's common to think a hybrid gives 90degrees phase shift, that a
3/8th line gives 135 degrees delay, and other similar things....but that
rarely is true.
<I guess 90 degree phase shift might result in a cardioid pattern?
It might, but a cardioid with a null at zero degrees elevation straight off
the back is a pretty poor pattern for skywave use, as well as for gain. Gain
increases and the effective or useful F/R ratio increases significantly when
phase shift between the elements is some value greater than 90 degrees. I
can't think of a reason to pick 90 degrees as a target value, unless it is a
system designed to protect something on groundwave that happens to be
straight in line with the elements.
73 Tom
All good topband ops know how to put up a beverage at night.
_________________
Topband Reflector
