Feedhorn
Understanding Feedhorns
The feedhorn is the interface between the waveguide/receiver and the reflector antenna. Its pattern shape determines how efficiently the reflector converts the feed's radiation into a focused beam. Feed design is an art that balances illumination efficiency, spillover, and cross-polarization.
Feed Illumination Design
- Edge taper: The feed pattern level at the reflector edge. Typically -10 to -12 dB for optimal efficiency.
- f/D ratio: Focal length / diameter. Deep dishes (f/D < 0.35): wide-angle feeds. Shallow dishes (f/D > 0.5): narrow-angle feeds.
- Illumination efficiency: Typically 50-70%. Combination of spillover, taper, and phase efficiency.
Feed Types
- Conical corrugated: Symmetric pattern, low cross-pol. The gold standard for satellite and radio astronomy.
- Smooth-wall: Simpler fabrication, adequate cross-pol. Cost-effective for commercial systems.
- Multi-mode: Potter horn. Combines TE11 and TM11 modes for symmetric pattern.
Frequently Asked Questions
What is a feedhorn?
A feedhorn is a horn antenna at the focal point of a reflector that illuminates the reflector surface. Its pattern must match the reflector's f/D ratio. Edge taper of -10 to -12 dB gives optimal efficiency.
Why are corrugated feeds preferred?
Corrugated horns produce a symmetric radiation pattern with very low cross-polarization (< -30 dB), essential for dual-polarization operation and high aperture efficiency. The corrugations support the hybrid HE11 mode.
What happens with poor feed design?
Under-illumination: wasted reflector area, wider beamwidth, lower gain. Over-illumination: energy spills past the reflector, increasing system noise temperature and reducing efficiency. Both reduce overall antenna G/T.