Axial Ratio Measurement
Understanding Axial Ratio Measurement
Circular polarization requires two orthogonal electric field components with equal amplitude and exactly 90 degrees of phase difference. Any deviation in amplitude equality or phase quadrature produces elliptical polarization, quantified by the axial ratio. The measurement is performed in an anechoic chamber or compact range by illuminating the antenna under test (AUT) with a known-polarization source and analyzing the received signal amplitude as a function of the source polarization angle.
The most common technique is the spinning linear method, where a linearly polarized source antenna rotates continuously about its boresight axis at 5 to 30 RPM. As it rotates, the received amplitude traces out a sinusoidal envelope whose peak-to-valley ratio equals the axial ratio. This method is simple and requires only a single-port source, but it cannot distinguish between right-hand and left-hand circular polarization. For sense determination, a dual-port method using separately measured RHCP and LHCP components is required.
Axial Ratio Formulas
AR (dB) = Pmax - Pmin (from received power envelope)
AR from orthogonal components:
AR = (Emajor / Eminor) = (|ERHCP| + |ELHCP|) / (|ERHCP| - |ELHCP|)
Cross-Polarization Discrimination:
XPD = 20 × log10((AR + 1) / (AR - 1)) dB
At AR = 1.26 (2 dB): XPD = 19.3 dB
At AR = 1.41 (3 dB): XPD = 15.0 dB
Polarization Efficiency Loss:
ηpol = (1 + 1/AR2) / 2
At 3 dB AR: loss = 0.5 dB. At 1 dB AR: loss = 0.04 dB
AR Specifications by Application
| Application | AR at Boresight | AR Beamwidth | Why It Matters |
|---|---|---|---|
| GPS/GNSS | ≤ 2 dB | 3 dB over upper hemisphere | Multipath rejection, satellite visibility |
| SATCOM (Ku-band) | ≤ 1.5 dB | 1.5 dB over 3 dB beamwidth | Cross-pol isolation, adjacent satellite interference |
| Weather Radar | ≤ 0.2 dB | 0.2 dB over aperture | Differential reflectivity (ZDR) accuracy |
| Radio Astronomy | ≤ 0.5 dB | 0.5 dB over primary beam | Stokes parameter calibration |
Frequently Asked Questions
How does the spinning linear method measure axial ratio?
A linearly polarized source antenna rotates continuously about its boresight at 5 to 30 RPM while the AUT remains stationary. The received amplitude varies sinusoidally as the source sequentially aligns with the major and minor axes of the AUT's polarization ellipse. The peak-to-valley ratio in dB equals the axial ratio. Perfect CP shows zero variation (0 dB AR), while linear polarization shows infinite variation. This method is simple and needs only a single-port source, but cannot distinguish RHCP from LHCP.
What axial ratio values are acceptable for CP antennas?
GPS/GNSS antennas require 3 dB or better over the upper hemisphere, with 1 to 2 dB at boresight. SATCOM antennas typically need 1 to 2 dB over the 3 dB beamwidth. Weather radar requires better than 0.2 dB across the aperture for accurate differential reflectivity. At 3 dB AR, the cross-pol discrimination is only 15 dB and polarization loss is 0.5 dB. At 1 dB AR, XPD improves to 24.8 dB with negligible polarization loss.
How does axial ratio vary with angle off boresight?
AR almost always degrades as the observation angle moves off boresight. A CP patch antenna might achieve 1 dB at boresight but degrade to 6 dB at the edge of the 3 dB gain beamwidth. This occurs because maintaining equal amplitude and 90-degree phase difference between orthogonal components becomes increasingly difficult off-axis. The 3 dB AR beamwidth is typically 50 to 70 percent of the 3 dB gain beamwidth for single-feed CP antennas.