Hi Sandra
I don't have a classical textbook with
me, but without here goes:
a) Polarization: every
real antenna establishes E and H vectors around
it when the energy in the feeder reaches the antenna
structure. Conventionally, we think of the E field,
because the H field is directly related to it, but
90 degrrees apart in space. Due to the way the current
flows on the surface of the conductive elements
making up the antenna, most of the E field will
be oriented in one vector direction relative to
a fixed set of orthogonal co-ordinates, on earth
taken to be the horizon. If the direction is parallel
to the earth's horizon, conventionally we define
this as horizontal polarization, and of course if
vertical to the horizon, vertical polarization.
If current can be pursuaded by design of the structure
to follow a particular curved path, then circular
polarization results, and this is right hand or
left hand depending on which sense the curved path
follows.
b) and c) In the desciption above, I
deliberately said "most of the E field", because
the flow of current will establish E field in both
the hozizontal plane and the vertical plane simultaneously.
The ratio of these two vector magnitudes in dB is
the cross-polarization level. In a circular polarized
antenna with perfect circularity, this will be zero
dB, because the E field level that can be measured
is the same in the horizontal as well as the vertical
plane. If you imagine a fat dipole, however, with
the elements horizontal, most of the E field vector
will be horizontal. However, some current will flow
across the cylinder, around the curved direction,
and thus it will set up a vertical E field. Let
us assume this level is -15dB compared to the horizontal
level. If an ideal horizontally polarized antenna
were to be used as a far field source, with no vertical
polarization, the level of energy received in the
fat dipole is taken as 0dB when they are both in
the same plane. If now the perfect antenna is rotated
to the vertical, the level received should fall
to zero, but will actually fall to -15dB. This is
the cross polar discrimination. In my book, it is
also the cross-polar isolation, but purists might
disagree and argue the point.
Essentially, cross
polar discrimination shows the ability of an antenna
to separate, in the above example by rotating it,
or discriminate, the pure vertical from the pure
horizontal polarization of a received signal.
