Circularly Polarized
Understanding Circular Polarization
Circular polarization solves the practical problem of maintaining consistent signal reception regardless of antenna orientation. For satellite links, the satellite and ground antenna orientations are generally different, and for mobile terminals, the orientation changes continuously. CP eliminates the need for precise polarization alignment.
CP Generation Methods
- Helical antenna: Naturally produces CP. Simple, wideband.
- Crossed dipoles: Two dipoles at 90 degrees with quadrature feeding.
- Patch antenna: Square patch with truncated corners or dual-feed with 90-degree hybrid.
- Septum polarizer: Converts LP waveguide to CP at the aperture.
Axial Ratio
AR measures CP quality: 0 dB = perfect circle, 3 dB = acceptable, 6 dB = nearly linear. AR is usually best on-axis and degrades off-axis.
AR = E_max / E_min (linear)
AR (dB) = 20 log10(E_max/E_min)
Perfect CP: AR = 0 dB
Good CP: AR < 3 dB
Acceptable: AR < 6 dB
Polarization efficiency:
e_pol = 0.5 (1 + 1/AR^2) for CP-to-LP
CP to LP loss: 3 dB (theoretical)
CP to cross-CP loss: infinite (theoretical)
Frequently Asked Questions
What is circular polarization?
CP is a polarization state where the electric field rotates at the carrier frequency with constant amplitude. It eliminates polarization mismatch loss from antenna orientation, making it ideal for satellite, GPS, and mobile communications.
Why use circular instead of linear polarization?
CP eliminates the 3+ dB loss that occurs when a linearly polarized antenna is rotated relative to the signal polarization. For satellite links, this is critical because the satellite and ground antenna may have any relative orientation.
What is the 3 dB CP-to-LP loss?
When a CP signal is received by a linearly polarized antenna, exactly half the energy is captured (the LP antenna sees only one component of the rotating field). This 3 dB loss is inherent and unavoidable.