training course on weather radar systems - RTC, Regional Training ...

MODULE A- INTRODUCTION TO RADAR
differ. First, the two components may not have the same amplitude. Second, the two components
may not have the same phase that is they may not reach their maxima and minima at the same
time in the fixed plane we are talking about. By considering the shape traced out in a fixed plane
by the electric vector as such a plane wave passes over it, we obtain a description of the
polarization state. By considering that issue, three types of polarization of electromagnetic
waves can be defined. These are Linear, Circular and Elliptical Polarizations.
a) Linear polarization
If the electrical vector remains in one plane, then the wave is linearly polarised. By convention,
if the electric vector (or field) is parallel to the earth's surface, the wave is said to be horizontally
polarized, if the electric vector (or field) is perpendicular to the earth's surface, the wave is said to
be vertically polarized.
Linear polarization is shown in Figure-3.5. Here, two oorthogonal components (Ex-red and Eygreen)
of the electric field vector (E-blue) are in phase and they form a path (purple) in the
plane while propagating. In linear polarization case, the strength of the two components are
always equal or related by a constant ratio, so the direction of the electric vector (the vector sum
of these two components) will always fall on a single line in the plane. We call this special case
linear polarization. The direction of this line will depend on the relative amplitude of the two
components. This direction can be in any angle in the plane, but the direction never varies.
Linear polarisation is most often used in conventional radar antennas since it is the easiest to
achieve. The choice between horizontal and vertical polarisation is often left to the discretion of
the antenna designer, although the radar system engineer might sometimes want to specify one or
the other, depending upon the importance of ground reflections.
Figure-3.5
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TURKEY RADAR TRAINING 1.0 / ALANYA 2005