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Cross Polar Discrimination
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## a. OBJECTIVES

1.  To study the phenomenon of Linear and Circular polarization of antennas.

2.  To determine the Cross Polar Discrimination (XPD) for the antenna systems in the lab.

3.   To study polarization diversity.

## b. THEORY

Antenna Polarization for linear antennas is in direction of its elements so if the dipole is mounted in horizontal plane it is horizontally polarized. If it is made vertical using polarization adapter then it is vertically polarized.    Linear polarization of an antenna is measured with reference to a dipole antenna. So if maximum signal is received from a given antenna with test dipole horizontal, then the given antenna is horizontally polarized. A diagram illustrating polarization is shown below

FIGURE 1:- A Diagram Illustrating  Polarization

Ã¯Â»Â¿As the plane of either of the antennas is changed using a polarization adapter the received signal strength reduces. A vertical antenna radiates vertically polarized wave as a vertical whip/ vertical dipole/monopole discone/endfire/broadside. A horizontal antenna radiates horizontally polarized waves as a horizontal dipole/biconical/square loop/quad/Vee/Yagi/ Log-periodic. Cross polarization discrimination is the change in received signal strength with change in polarization direction for a linearly polarized antenna. In the case of measurements between dipoles and yagis upto 20dB of change can be observed on changing plane of polarization. Good polarization discrimination reflects the purity of an antenna pattern. A circularly polarized wave front has equal power in its plane. Hence when a dipole antenna is rotated from horizontal to vertical using polarization adapter in front of a crossed dipole antenna or an axial mode helix antenna. No appreciable change in signal strength is observed – concluding that crossed dipole and helix antennas are circularly polarized. The axial ratio of a crossed dipole and helix antennas would be close to 1 and signal variation would be a few dBs around all directions in vertical plane. Received signal strength is maximum between circularly polarized antennas at Tx and Rx when both have same handedness. Thus it is maximum between RHCP and RHCP or between LHCP and LHCP antennas. It will be lesser in case of communication between RHCP and LHCP antennas indicating the polarization discrimination antennas.  A practical antenna using polarization diversity is shown below

FIGURE 2:  A Practical Antenna Using Polarization Diversity

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