Antenna Fundamentals and Integration Advanced Antenna Topics Informational

What is the axial ratio of a circularly polarized antenna and how do I measure it?

Q: What axial ratio is acceptable for communication systems?

GPS receivers: < 3 dB AR (specification), most commercial GPS antennas achieve < 2 dB at broadside. Satellite communications: < 1-2 dB AR for professional systems, < 3 dB for consumer. RFID: < 3 dB AR for reliable tag reading. Weather radar: < 0.1-0.5 dB AR for accurate precipitation measurement. Radio astronomy: < 0.5 dB AR for accurate polarization measurement. The 3 dB AR point (where the polarization efficiency drops to 50%) is the standard 'edge' of the CP bandwidth.

Q: How does axial ratio affect link budget?

When a CP antenna with finite AR communicates with another CP antenna (or a linearly polarized antenna), there is a polarization mismatch loss. For two CP antennas facing each other, both with AR in dB: the worst-case polarization loss (in dB) is approximately (AR_1 + AR_2)/2. For a CP antenna with 3 dB AR receiving from a perfect CP source: polarization loss approximately 1.5 dB. This loss is in addition to the free-space path loss and must be included in the link budget.

Q: Does axial ratio change with frequency?

Yes. The AR of an antenna is frequency-dependent because the amplitude and phase balance between the orthogonal electric field components changes with frequency. Most CP antennas have a narrow frequency range where AR < 3 dB (the 'AR bandwidth'). The AR bandwidth is typically 1-5% for single-feed CP patches, 15-30% for sequential rotation arrays, and 50%+ for spiral antennas.

The axial ratio (AR) of a circularly polarized antenna is the ratio of the major axis to the minor axis of the polarization ellipse traced by the tip of the electric field vector as the wave propagates. For perfect circular polarization, the axial ratio is 1.0 (0 dB), meaning the electric field traces a circle. For elliptical polarization (imperfect CP), the axial ratio is greater than 1 (> 0 dB). For linear polarization, the axial ratio is infinite. The axial ratio in dB is: AR_dB = 20 x log(E_major / E_minor), where E_major and E_minor are the magnitudes of the major and minor axes of the polarization ellipse. To measure the axial ratio: the standard method uses a linearly polarized reference antenna (a standard gain horn or dipole) that is rotated about the line of sight while recording the received signal level. As the linearly polarized antenna rotates through 360 degrees, a circularly polarized signal produces: a perfectly CP signal produces no variation (constant received power), while a non-ideal CP signal produces a sinusoidal variation where: AR_dB = P_max - P_min (the difference between the maximum and minimum received power levels). This spinning-linear method can be performed on a far-field antenna range or in a near-field scanning system. Modern vector network analyzer-based antenna ranges measure both orthogonal linear components (vertical and horizontal) simultaneously, from which the axial ratio is calculated: AR = (|E_R|^2 + |E_L|^2 + 2|E_R||E_L|) / (|E_R|^2 + |E_L|^2 - 2|E_R||E_L|) where E_R and E_L are the right-hand and left-hand circular components.

Category: Antenna Fundamentals and Integration

Updated: April 2026

Product Tie-In: Antennas, Arrays, Feeds

Axial Ratio Measurement for CP Antennas

Axial ratio is the primary specification for any circularly polarized antenna. GPS, satellite communications, RFID, weather radar, and radio astronomy all require specific axial ratio performance to ensure proper signal reception or transmission.

Measurement Methods

Spinning linear method: The reference linearly polarized antenna is physically rotated (or electronically rotated using a dual-polarized antenna with phase shifter). The power variation as the reference rotates gives the AR directly. Simple and accurate. Best for far-field ranges
Dual-linear method: Measure the antenna's radiation pattern with two orthogonal linearly polarized reference antennas (H and V). Convert to circular components: E_R = (E_H + j E_V)/sqrt(2), E_L = (E_H - j E_V)/sqrt(2). Calculate AR from the ratio of |E_R| and |E_L|. More information (RHCP vs. LHCP identification) but requires phase-coherent measurement of both linear components
Near-field scanning: Measure the near field using two orthogonal probes. Transform to far field, then calculate AR at each angle. Provides full 3D AR pattern

Axial Ratio Calculations

Axial ratio: AR = E_major/E_minor (linear), or AR_dB = 20log(AR)
Perfect CP: AR = 1 (0 dB)
3 dB AR: E_major/E_minor = 1.414 (boundary of "acceptable" CP)
From linear components: AR = sqrt((|EH|^2+|EV|^2+C)/(|EH|^2+|EV|^2-C))
Where C = sqrt((|EH|^2-|EV|^2)^2 + 4|EH|^2|EV|^2 cos^2(delta_phase))

Common Questions

Frequently Asked Questions

What axial ratio is acceptable for communication systems?

GPS receivers: < 3 dB AR (specification), most commercial GPS antennas achieve < 2 dB at broadside. Satellite communications: < 1-2 dB AR for professional systems, < 3 dB for consumer. RFID: < 3 dB AR for reliable tag reading. Weather radar: < 0.1-0.5 dB AR for accurate precipitation measurement. Radio astronomy: < 0.5 dB AR for accurate polarization measurement. The 3 dB AR point (where the polarization efficiency drops to 50%) is the standard 'edge' of the CP bandwidth.

How does axial ratio affect link budget?

When a CP antenna with finite AR communicates with another CP antenna (or a linearly polarized antenna), there is a polarization mismatch loss. For two CP antennas facing each other, both with AR in dB: the worst-case polarization loss (in dB) is approximately (AR_1 + AR_2)/2. For a CP antenna with 3 dB AR receiving from a perfect CP source: polarization loss approximately 1.5 dB. This loss is in addition to the free-space path loss and must be included in the link budget.

Does axial ratio change with frequency?

Yes. The AR of an antenna is frequency-dependent because the amplitude and phase balance between the orthogonal electric field components changes with frequency. Most CP antennas have a narrow frequency range where AR < 3 dB (the 'AR bandwidth'). The AR bandwidth is typically 1-5% for single-feed CP patches, 15-30% for sequential rotation arrays, and 50%+ for spiral antennas.

Keep Reading