What is the cross polarization discrimination of an antenna and how is it measured?
Cross Polarization
Every antenna radiates some energy in the unintended polarization. This cross-polarized radiation is caused by: (1) non-ideal feed radiation (even a good horn has cross-pol peaks of -30 to -40 dB), (2) asymmetric reflector geometry (offset-fed dishes generate cross-pol from the offset angle), (3) surface current distribution that does not conform to the ideal co-pol pattern, and (4) feed support struts that scatter the co-pol field into cross-pol.
| Parameter | Low Gain | Medium Gain | High Gain |
|---|---|---|---|
| Gain Range | 2-6 dBi | 6-15 dBi | 15-45 dBi |
| Beamwidth | 60-360° | 15-60° | 1-15° |
| Typical Types | Dipole, monopole, patch | Yagi, helical, horn | Parabolic, array, Cassegrain |
| Bandwidth | Narrow to wide | Moderate | Narrow to moderate |
| Complexity | Low | Medium | High |
Frequently Asked Questions
How does offset reflector geometry affect XPD?
An offset-fed parabolic reflector generates cross-polarized radiation due to the geometric asymmetry. The cross-pol level depends on the offset angle and f/D ratio. Matched dual-offset (Gregorian) configurations can cancel much of the cross-pol by proper shaping of the subreflector.
Can I improve XPD with the feed design?
Yes. Corrugated horn feeds achieve 35-45 dB XPD by using corrugations to control the aperture field distribution. Dual-mode horns and Potter horns also achieve improved XPD. The feed is often the limiting factor for the entire antenna's XPD performance.
What about circular polarization XPD?
For circular polarization, XPD is the ratio of the desired-sense circular (RHCP or LHCP) to the opposite-sense. Axial ratio and XPD are related: XPD (dB) ≈ 20·log10((AR+1)/(AR-1)), where AR is the axial ratio in linear scale. For AR = 1 dB: XPD ≈ 24.8 dB.