How do I select a feed horn for a parabolic reflector antenna?
Feed Horn Design
The feed horn illumination pattern determines several key antenna parameters: aperture efficiency (how much of the dish area is used effectively), spillover efficiency (how much feed energy misses the dish), and cross-polarization (which limits dual-pol performance). The ideal feed provides uniform illumination across the dish with zero spillover, but this is physically impossible, hence the optimum edge taper compromise.
| 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 |
Design Considerations
For front-fed parabolic reflectors: the feed subtends a half-angle of θe = arctan(D/(4f)) at the dish edge. The feed pattern must be designed so that its -10 dB point corresponds to this angle. For a typical f/D = 0.4 dish: θe = 32°, so the feed needs a -10 dB beamwidth of 64°. A simple conical horn with aperture diameter of approximately 1.5λ achieves this beamwidth.
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
- Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture
- Margin allocation: include sufficient design margin to account for manufacturing tolerances and aging effects
Performance Trade-offs
For Cassegrain and Gregorian dual-reflector systems: the feed illuminates the subreflector (not the main dish). The subreflector subtend angle is much smaller, requiring a higher-gain (narrower beamwidth) feed. This allows the use of a larger, more efficient corrugated horn while keeping the overall antenna compact.
Frequently Asked Questions
What if the feed pattern is too narrow?
The dish will be under-illuminated: the edges of the dish contribute less, reducing gain (lower illumination efficiency) but also reducing sidelobes and spillover. The net effect depends on the degree of under-illumination. Slight under-illumination (> -15 dB taper) is sometimes desirable for low-sidelobe applications.
What if the feed is too wide?
Excessive spillover: feed energy misses the dish edge and is wasted (lost spillover efficiency), the noise temperature increases (the feed 'sees' the warm ground past the dish edge), and the sidelobe level rises. This is worse than under-illumination for most applications.
How do I match the feed to the polarizer?
For circular polarization: a septum polarizer or orthomode transducer (OMT) is inserted between the feed horn and the waveguide. The polarizer converts linear polarization to circular. The feed horn and polarizer must be matched in waveguide size and operating bandwidth. Corrugated horns with OMTs are the standard for satellite communication feeds.