Hi Mr Whatever. It is me again.
The antenna is
usually configured as an array of a minimum of 4
elements. The signals are fed into a matrix of hybrids
in such a way that there is a sum of the element
patterns for the horizontal plane, and for the vertical
plane, and there is a difference port for each.
The difference port produces a deep null along the
array boresight for each plane. To one side of boresight
the phase is positive, but when the source is to
the other side of antenna boresight the phase reverses,
the sum ports providing the phase reference. Detectors
on the difference ports can be connected to a servo
mechanism that drives the antenna so that it oscillates
very slightly either side of boresight in each plane
ie azimuth and elevation.
I have the hybrid coupler
configuration I could upload if anyone is interested.
Polarization purity is important only to the extent
that if severe cross-polarization occurs, the null
in the difference channel is not so deep. At least
I seem to recall that was the argument when I was
involved in tracking radars.