Thursday, April 03, 2008

Polarization Primer

Radio waves are polarized. Many laypeople know this, even if they don't know what polarization is. The polarization of radio waves relates to wave propagation, so lets begin there. A moving wave is also called a transverse wave when it propagates in a direction perpendicular to the direction in which the oscillations that produce the wave are also moving. That's a lot to digest. Let's look at a dynamic diagram.

Now radio waves don't normally travel in just one direction, they ravel outward like ripples in a puddle. But in terms of wave propagation "outward" is one direction. Radiowaves are electromagnetic waves, both AM and FM waves are polarized. This starts at the antenna. The antenna is basically a transducer it converts electrical currents into electromagnetic waves. So here at the physical antenna, polarization refers to the orientation of the electric field or E-Plane. (E-Plane just refers to the wave in reference to the surface of the Earth. ) The term H-Plane will also come up. That's the direction of the maximum magnetic field. More here.
For example in a linearly polarized antenna the "H" plane lies at a right angle to the "E" plane. For a vertically polarized antenna, the H-plane usually coincides with the horizontal/azimuth plane. For a horizontally-polarized antenna, the H-plane usually coincides with the vertical/elevation plane. OK More diagrams here.

Linear Polarization: The polarization of the magnetic field vector to a given plane along the direction of propagation.

Vertical Polarization: This is used for AM & FM. Vertical polarization is most often used when it is desired to radiate a radio signal in all directions.

Horizontal Polarization: This is used for Television. The polarization of the magnetic field vector to a given plane primarily in the vertical plane sending much of it's energy into the ground and outer space, but it is more noise-resistant.

Circular Polarization: (There is both Left and Right circular polarization) The electric field vector of a constant magnitude rotates in the x-y plane.

Elliptical Polarization: The polarization of magnetic field vector such that the "tip" of the variable magnitude of the electric field vector describes an ellipse in a fixed plane intersecting, the direction of propagation. Linear propagation is a subset of this, and true elliptical polarization does not occur in radioland.