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What is polarization in VSAT System

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Apr 28th, 2013 at 8:10am  
Can anyone explain what is polarization in VSAT systems.

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USN - Retired
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Reply #1 - Apr 28th, 2013 at 12:19pm  
Polarization is the orientation of radio waves as they leave a transmitting antenna. This is true across the entire RF spectrum, of which VSAT frequencies only occupy a small segment. Common polarization types within the VSAT segment include linear and circular. Receive antennas must be oriented on the same plane as the transmitted signal.

Linear polarization is either  horizontal and vertical. It might be easier to think "sideways" or "up and down". In horizontal, the radiation pattern comes sideways off the transmit antenna. In vertical, it comes off up and down. These waves maintain that relationship to the transmit antenna as far as they can travel, and the receiving antenna must be oriented on the same plane; horizontal transmitter to horizontal receiver, vertical transmitter to vertical receiver.

Circular polarization is either left hand or right hand. It might be easier to think of a corkscrew spiral. In left hand, the radiation pattern comes spiraling off the transmit antenna in a left hand spiral. In right hand, it's the opposite. Again, the receiving antenna must be oriented on the same plane; left hand transmitter to left hand receiver, right hand to right hand receiver.

To understand this in VSAT, you have to consider that the "dish" is not the antenna. It's simply a reflector; focusing the RF plane on the transmit, collecting the RF plane on the receive. The actual antennas are tiny little dipole antennas in the waveguide. In most cases these miniature antennas are situated at the point where the waveguide attaches; BUC output, and LNB input. It is the orientation of these little antennae that determines horizontal or vertical. In circular, an additional device called a polarizer is added to change from linear to right hand or left hand.

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