Circular Polarization Feed
Understanding Circular Polarization Feed
Circular polarization requires two orthogonal linearly polarized components with equal amplitude and 90-degree phase difference. In a waveguide-based feed system, the challenge is generating this precise amplitude and phase relationship across the operating bandwidth. The most common approach uses a circular waveguide supporting two degenerate TE11 modes (horizontal and vertical), with a polarizing element that introduces a 90-degree differential phase shift between them. The polarized signal then radiates through a conical or corrugated horn antenna, which preserves the circular polarization purity across its radiation pattern.
The septum polarizer is the gold standard for compact, high-performance CP feeds. A stepped metallic vane (septum) divides the circular waveguide into two D-shaped ports at the input end. When excitation enters one port, the septum's graduated steps progressively convert the input mode into two orthogonal TE11 modes with the required 90-degree phase offset. The stepped profile acts as a multi-section impedance transformer, broadening bandwidth. Modern 5-step septum designs achieve axial ratio below 0.5 dB over 12 to 18% bandwidth, return loss better than 25 dB, and RHCP/LHCP port isolation exceeding 30 dB. For applications requiring dual-CP simultaneous operation (transmit on RHCP while receiving on LHCP), both D-ports are connected through a 3 dB hybrid coupler, providing independent access to each polarization sense.
CP Feed Design Parameters
AR = 20 · log10((1 + ε) / (1 - ε)) [dB]
where ε = √((ΔA)2 + sin2(Δφ/2))
Cross-Polarization Isolation:
XPI ≈ 20 · log10((AR + 1) / (AR - 1)) [dB, AR in linear]
Septum Phase Shift (single section):
Δφ = (2π/λg1 - 2π/λg2) · L [rad]
Where ΔA = amplitude imbalance, Δφ = phase error from 90°, λg1 and λg2 = guide wavelengths of the two orthogonal modes, L = septum section length. AR = 0 dB is perfect CP; AR < 1 dB requires Δφ < 6° and ΔA < 0.5 dB.
CP Feed Polarizer Comparison
| Polarizer Type | AR Bandwidth | Axial Ratio | Size | Application |
|---|---|---|---|---|
| Septum (3-step) | 10 to 15% | < 0.5 dB | Compact | Satcom, radar |
| Septum (5-step) | 15 to 20% | < 0.3 dB | Moderate | Wideband satcom |
| Dielectric plate | 5 to 10% | < 1.0 dB | Simple | GNSS, low-cost |
| Corrugated iris | 20 to 30% | < 1.0 dB | Long | Earth station feeds |
| Meander-line (printed) | 15 to 25% | < 2.0 dB | Planar | Phased arrays |
Frequently Asked Questions
How does a septum polarizer generate circular polarization?
A stepped metallic fin divides circular waveguide into two D-channels. Excitation entering one channel is progressively converted by the stepped profile into two orthogonal TE11 modes with 90° phase difference, producing CP. Left D-channel gives RHCP; right gives LHCP. Five-step designs achieve AR < 0.5 dB over 15 to 20% bandwidth with isolation > 30 dB.
What determines axial ratio bandwidth?
AR depends on maintaining the 90° phase difference across frequency. Single-section dielectric plates give 5 to 10% bandwidth. Stepped septums (3 to 5 steps) achieve 10 to 20%. Multi-section corrugated designs extend to 20 to 30%. The feed horn must also maintain symmetric patterns; corrugated or smooth-walled conical horns are preferred.
Why is CP preferred for satellite communications?
CP is immune to Faraday rotation in the ionosphere (up to 10° at C-band). It enables frequency reuse: RHCP and LHCP carry independent data on the same frequency with 25+ dB isolation, doubling capacity. Rain attenuation affects both polarizations equally, simplifying link budgets versus linear polarization.