Axial Ratio: Antenna Polarization Ellipticity Metric
Axial ratio (AR) quantifies the ellipticity of a polarized electromagnetic wave. It is the ratio of the major to minor axes of the polarization ellipse traced by the electric-field vector. An AR of 1 (or 0 dB) indicates perfectly circular polarization, whereas larger ratios represent elliptical or nearly linear polarization.
Engineers often express axial ratio in decibels: AR(dB) = 20 log₁₀(major/minor). Lower values correspond to purer circular polarization and reduced mismatch loss when communicating with circularly polarized antennas.
Definition and Measurement
Axial ratio is derived from the orthogonal components of the electric field. If Ex and Ey are the amplitudes and δ is their phase difference, then AR = (|Ex| + |Ey|)/(|Ex| − |Ey|) for circularly polarized states. Vector network analyzers with polarization-capable antennas sweep amplitude and phase to compute AR across frequency and angle.
Antenna datasheets typically specify axial ratio either over the main beam (e.g., ≤ 3 dB) or at boresight. Some standards, such as those used for GNSS antennas, define “coverage” as the angular region where AR stays below a threshold.
Historical Notes
Early radio astronomers in the 1950s coined “axial ratio” to characterise horn and helical antennas used for circular polarization. Space programs adopted the metric to ensure spacecraft could maintain links regardless of orientation, since linear polarization would suffer deep fades if the spacecraft rolled.
Modern polarimetric radars and GNSS constellations still rely on the same definition, though measurement equipment has advanced from rotating dipoles to fully digital polarimeters.
Concepts and Considerations
Polarization Mismatch Loss
When a transmitting antenna with axial ratio ARt illuminates a receiving antenna with ARr, the mismatch loss depends on the angular difference between their major axes. Link-budget tools incorporate this loss term, which can approach 3 dB if one antenna is linear and the other circular.
Wideband Behaviour
Maintaining low AR across wide bandwidths requires dual feeds, hybrid couplers, or sequential rotation arrays. Designers study AR versus frequency alongside gain and impedance plots to avoid resonances that distort polarization.
Measurement Uncertainty
Reflections in anechoic chambers can skew AR readings. Time-gating or near-field scanning reduces multipath, while referencing measurements to SI-traceable field probes maintains accuracy.
Applications
Satellite Communications
GEO, MEO, and LEO systems use circular polarization to avoid orientation penalties. Mission planners feed AR requirements into the downlink budget calculator to assess fade margins.
Radar and Remote Sensing
Polarimetric radars exploit axial ratio to distinguish targets, since metallic spheres reflect circular polarization while elongated debris alters the ellipse. Calibrated AR also improves GNSS multipath rejection.
Optical Communications
Free-space optical links rotate polarization to mitigate scintillation. The laser interlink availability tool factors polarization into pointing budgets.
Importance
Axial ratio condenses polarization quality into a simple number engineers can specify, test, and monitor. Keeping AR within tight limits preserves link margin, reduces interference, and unlocks dual-polarization techniques that double capacity.
Coupling axial ratio metrics with visibility planning via the LEO visibility calculator ensures end-to-end communications resilience.