Why does GPS use right-hand circular polarization?

GPS satellites orbit at 20,200 km, and the receiver antenna can be at any orientation on the ground, in a vehicle, or on an aircraft. Circular polarization eliminates the orientation dependency that would cause a linearly polarized signal to fade to zero when the receive antenna rotates 90 degrees. RHCP was chosen as the convention; a single reflection from a building reverses the sense to LHCP, so a RHCP receive antenna naturally rejects single-bounce multipath signals by 20 to 30 dB (the cross-pol rejection). This is a major reason GPS works reliably in urban environments.

What axial ratio is considered good CP performance?

Perfect circular polarization has an axial ratio of 0 dB (1:1). In practice, 3 dB axial ratio is the boundary between circular and elliptical polarization. For GPS antennas, the L1 spec requires less than 3 dB axial ratio across the upper hemisphere. High-performance satellite antennas achieve 0.5 to 1.5 dB. Phased arrays typically achieve 2 to 4 dB at broadside, degrading to 5 to 8 dB at scan angles beyond 45 degrees as the element pattern becomes more elliptical.

What happens when you receive CP with a linear antenna?

A linearly polarized antenna receives exactly half the power of a circularly polarized signal, regardless of the linear antenna's orientation. This is a 3 dB polarization mismatch loss. The advantage is that the received power is constant as the antenna rotates, unlike the case of two misaligned linear antennas where the loss varies from 0 dB (aligned) to infinite (crossed). Many commercial systems accept this 3 dB loss because it simplifies the receive antenna.

Antenna Technology

Circular Polarization

Point a linearly polarized antenna at a satellite and rotate it: the signal fades to nothing every 90 degrees. Now switch to a circularly polarized antenna and rotate it: the signal stays constant. That immunity to orientation is why GPS, satellite communications, weather radar, and RFID all use circular polarization. The electric field vector rotates at the carrier frequency, tracing a helix through space. A single reflection reverses the rotation sense, giving CP antennas a natural 20 to 30 dB rejection of single-bounce multipath, a property linear polarization cannot provide.

Category: Antenna Technology

Senses: RHCP, LHCP

Key Spec: Axial Ratio (dB)

Generating a Rotating Field Vector

Circular polarization requires two orthogonal linearly polarized components of equal amplitude with a 90-degree phase difference. If the horizontal component leads the vertical by 90 degrees and you look toward the source, the field rotates clockwise: that is right-hand circular (RHCP) by IEEE convention. Swapping the lead gives left-hand circular (LHCP).

CP Generation Methods Compared

Method	Mechanism	Axial Ratio (typical)	Bandwidth	Application
Truncated-corner patch	Opposite corners trimmed to split mode	2 to 4 dB	1 to 3%	GPS receivers, RFID tags
Dual-feed patch + hybrid	Two orthogonal feeds via 90° hybrid	0.5 to 2 dB	5 to 10%	Satellite terminals
Crossed dipoles + phasing	Two dipoles at 90° with λ/4 feed offset	1 to 3 dB	15 to 25%	FM broadcasting, telemetry
Helical antenna	Traveling-wave helix radiates axially	0.5 to 1.5 dB	40 to 70%	S-band telemetry, CubeSat
Septum polarizer + horn	Waveguide septum converts LP to CP	0.3 to 1.0 dB	10 to 20%	Satellite feeds, radar

Polarization Mismatch Loss

Loss between two CP antennas of the same sense: 0 dB (perfect match)
Loss between RHCP and LHCP: ∞ dB (complete rejection in theory)
Loss between CP and linear: 3 dB (constant, orientation-independent)

Axial ratio effect on cross-pol rejection:
For an antenna with axial ratio AR (dB), the cross-pol rejection is approximately:
XPD ≈ AR + 20·log₁₀((AR_linear+1)/(AR_linear−1))
At AR = 1 dB: XPD ≈ 25 dB | At AR = 3 dB: XPD ≈ 15 dB | At AR = 6 dB: XPD ≈ 10 dB

Common Questions

Frequently Asked Questions

Why does GPS use RHCP?

CP eliminates orientation fading (a linearly polarized antenna loses all signal at 90-degree rotation). RHCP rejects single-bounce multipath by 20 to 30 dB because a single reflection reverses the sense to LHCP. This is why GPS works in urban canyons where multipath is severe.

What axial ratio counts as good CP?

3 dB is the boundary between circular and elliptical. GPS L1 specs require <3 dB over the upper hemisphere. Satellite feeds achieve 0.5 to 1.5 dB. Phased arrays reach 2 to 4 dB at broadside, degrading beyond 45-degree scan.

What happens receiving CP with a linear antenna?

Exactly 3 dB mismatch loss, constant regardless of antenna rotation. Many commercial systems accept this because it simplifies the receive antenna and the received power is stable (unlike two misaligned linears where loss varies from 0 dB to infinite).

Related Terms

← Circular Patch CP DRA →

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