What is circular polarization and axial ratio?
Circular polarization (CP) is produced when two orthogonal linear field components have equal amplitude and a 90° phase difference. Its quality is measured by the axial ratio (AR), the ratio of the major to minor axis of the polarization ellipse: AR = 1 (0 dB) is perfect CP and AR → ∞ is linear.[1][2] Antennas for GNSS and satellite links typically specify AR below 3 dB over the coverage region.[3]
How is circular polarization generated?
- Axial-mode helix: inherently CP; sense set by winding direction.[4]
- Patch with two feeds 90° apart via a hybrid, or a single feed with a perturbation (truncated corners, near-square patch, diagonal slot).[5]
- Quadrifilar helix: four arms fed in phase quadrature.
- Crossed dipoles fed in quadrature, spirals, and septum or polarizer-equipped horns.
What axial ratio is acceptable?
AR in dB is 20·log10(major/minor). AR ≤ 3 dB is the common definition of CP bandwidth; at AR = 3 dB the loss to an ideal CP receiver of the same sense is only about 0.13 dB. Single-feed CP patches typically have AR bandwidths of only one or two percent, while dual-feed designs are much wider.[5]
Why does a reflection reverse the sense?
Reflection from a conductor reverses the direction of propagation while preserving the rotation in space, so RHCP becomes LHCP. A GNSS RHCP antenna therefore partially rejects single-bounce multipath, one reason CP is used for satellite navigation.[3]
Frequently asked questions
Is GPS RHCP or LHCP?
GPS signals are right-hand circularly polarized.
What is the loss between RHCP and linear?
3 dB in theory for a perfect CP wave and ideal linear antenna.
How do you measure axial ratio?
Rotate a linear probe antenna and record the ratio of maximum to minimum received signal, or measure two orthogonal components with phase.
See also
References
Online sources accessed October 21, 2026; printed works cited by edition. See all sources.
- “IEEE Standard for Definitions of Terms for Antennas (IEEE Std 145-2013)”. IEEE Antennas and Propagation Society. 2014.
- “Antenna Theory: Analysis and Design, 4th ed.”. C. A. Balanis, Wiley. 2016.
- “Understanding GPS/GNSS: Principles and Applications, 3rd ed.”. E. D. Kaplan and C. J. Hegarty (eds.), Artech House. 2017.
- “Antennas for All Applications, 3rd ed.”. J. D. Kraus and R. J. Marhefka, McGraw-Hill. 2002.
- “Microstrip Antenna Design Handbook”. R. Garg, P. Bhartia, I. Bahl and A. Ittipiboon, Artech House. 2001.