Axial Ratio Pattern
Understanding Axial Ratio Patterns
When an antenna radiates circularly polarized (CP) waves, the electric field vector traces an ellipse (ideally a circle) at each point in space. The shape of this ellipse varies with observation angle. On boresight, a well-designed CP antenna produces nearly perfect circular polarization (AR close to 0 dB). As the observation angle moves off boresight, the two orthogonal field components that combine to create CP change in relative amplitude and phase, causing the polarization to become elliptical and eventually linear at extreme angles.
The axial ratio pattern captures this angular variation by plotting AR (in dB) versus observation angle, typically in the principal cuts (E-plane and H-plane) and diagonal cuts. A good CP antenna maintains AR < 3 dB over its coverage cone. The 3 dB axial ratio beamwidth defines the angular range over which the polarization is acceptably circular. Outside this range, the increasing cross-polarization causes signal loss when communicating with a CP receiver: a receiver matched to RHCP loses 3 dB when receiving a signal with AR = 3 dB, and increasingly more as AR degrades further.
Axial Ratio Calculations
AR = (|Eco| + |Ecross|) / (|Eco| − |Ecross|)
AR (dB) = 20 log(ARlinear)
Polarization Loss Factor:
PLF = ½(1 + 1/AR²) for CP antenna receiving CP signal
PLF (dB) at AR = 3 dB: −0.46 dB loss
PLF (dB) at AR = 6 dB: −1.26 dB loss
Cross-Pol Discrimination (XPD):
XPD = 20 log((AR + 1)/(AR − 1)) dB
AR = 1 dB → XPD = 24.8 dB
AR = 3 dB → XPD = 14.9 dB
Axial Ratio Requirements by Application
| Application | AR on Boresight | AR Coverage Cone | Measurement Method |
|---|---|---|---|
| GNSS (GPS/Galileo) | < 1.5 dB | < 3 dB over ±85° | Spinning linear / dual-port |
| Satellite Comm (GEO) | < 1 dB | < 2 dB over coverage | Compact range / near-field |
| Weather Radar (Dual-Pol) | < 0.2 dB | < 1 dB within main beam | Precision far-field |
| Satellite TV (DBS) | < 2 dB | < 3 dB over ±1° | Outdoor far-field |
| RFID (UHF CP) | < 3 dB | < 6 dB over ±30° | Anechoic chamber |
Frequently Asked Questions
What does axial ratio measure?
AR is the ratio of the major to minor axis of the polarization ellipse. Perfect CP has AR = 1 (0 dB). Perfect linear has AR = infinity. AR < 3 dB is considered good circular polarization. GNSS antennas require AR < 1.5 dB on boresight. Satellite antennas require AR < 1 dB boresight and < 3 dB within the coverage cone. Higher AR degrades link margin because a CP receiver loses signal proportional to the polarization mismatch.
How is the axial ratio pattern measured?
The traditional method uses a spinning linearly polarized probe that rotates rapidly while the AUT is stepped in angle. The max/min response ratio gives AR at each angle. Modern dual-port methods measure with both RHCP and LHCP reference antennas, computing AR from co-pol and cross-pol amplitudes and phase. The dual-port approach is faster, avoids mechanical rotation, and provides full Stokes parameter characterization.
Why does axial ratio degrade off boresight?
CP is generated by combining two orthogonal linear components with equal amplitude and 90° phase difference. Most designs achieve this precisely only on boresight. Off-axis, the two mode patterns have slightly different shapes and the phase relationship shifts, making the polarization elliptical. Array antennas maintain better AR over wider angles by using sequential rotation of elements, where each element is physically rotated and phase-compensated to cancel element-level AR errors.