Polarization
Understanding Polarization
Polarization is a fundamental property of electromagnetic waves that determines how antennas must be oriented and how signals interact with the environment. Proper polarization management doubles the capacity of satellite links through frequency reuse and is critical for radar target discrimination.
Polarization Types
- Linear vertical (V): E-field oscillates vertically. Used in most terrestrial communications.
- Linear horizontal (H): E-field oscillates horizontally. Used in broadcast TV, some satellite links.
- RHCP: Right-hand circular polarization. E-field rotates clockwise (looking from behind the wave). Used in GPS, satellite communications.
- LHCP: Left-hand circular polarization. E-field rotates counter-clockwise.
Polarization in Practice
- Satellite frequency reuse: Orthogonal polarizations (V/H or RHCP/LHCP) carry independent signals on the same frequency, doubling capacity.
- Rain depolarization: Raindrops are slightly oblate, causing differential attenuation and phase shift between V and H polarizations.
- Faraday rotation: The ionosphere rotates the polarization of linearly polarized waves, making circular polarization preferred for satellite links.
Linear-to-linear (aligned): 0 dB
Linear-to-linear (45 deg): -3 dB
Linear-to-linear (cross): infinite loss
Linear-to-circular: -3 dB
Circular-to-circular (co-pol): 0 dB
Circular-to-circular (cross-pol): infinite loss
Cross-pol discrimination (XPD):
Typical antenna: 25-40 dB
Frequently Asked Questions
What is polarization?
Polarization describes the direction of the electric field of an electromagnetic wave. Linear polarization has a fixed field direction (vertical or horizontal). Circular polarization has a rotating field. The polarization of transmit and receive antennas must match for maximum power transfer.
Why is circular polarization used for satellites?
Circular polarization is preferred for satellite links because: (1) the ionosphere rotates linearly polarized waves (Faraday rotation), but circular polarization is unaffected; (2) antenna orientation does not matter, important for mobile terminals; (3) orthogonal circular polarizations (RHCP/LHCP) enable frequency reuse.
What is cross-polarization?
Cross-polarization is the unwanted radiation in the polarization orthogonal to the desired polarization. An antenna designed for vertical polarization will also radiate a small amount in horizontal polarization. Cross-polarization discrimination (XPD) measures the ratio between desired and undesired polarization components.