Cross-Polarization
Understanding Cross-Polarization
Cross-polarization limits the performance of dual-polarization systems that use both polarizations to double capacity. High cross-pol from the antenna causes interference between the two polarization channels, degrading signal quality.
Cross-Pol Sources
- Antenna asymmetry: Physical asymmetries in the antenna structure generate cross-pol radiation.
- Feed offset: Offset-fed reflector antennas generate more cross-pol than symmetric designs.
- Array errors: Element rotation errors in an array generate cross-pol from the array factor.
Cross-Pol Specifications
- Standard gain horn: -25 to -35 dB (excellent).
- Microstrip patch: -15 to -25 dB (moderate).
- Reflector antenna: -25 to -30 dB on-axis. Degrades off-axis.
- Dual-pol satellite: Requires -25 dB or better for frequency reuse.
Frequently Asked Questions
What is cross-polarization?
Cross-pol is the unwanted polarization component orthogonal to the intended polarization. XPD (cross-pol discrimination) measures the ratio of wanted to unwanted polarization, typically 20-40 dB. Critical for dual-polarization frequency reuse.
Why does cross-polarization matter?
In dual-polarization systems (carrying independent data on H and V pol), cross-pol causes interference between channels. XPD of 25 dB limits the achievable SNR to ~25 dB, regardless of how much power is transmitted. Better XPD enables higher-order modulation.
How do you reduce cross-polarization?
Symmetric antenna design (centered feeds), corrugated horn feeds (which have inherently low cross-pol), precision manufacturing, and cross-pol cancellation algorithms in the receiver. Potter horns and dual-mode feeds provide excellent XPD.