What is the difference between a Cassegrain and a Gregorian reflector antenna?
Dual-Reflector Antennas
The Cassegrain antenna was originally developed for optical telescopes. The convex hyperboloidal subreflector reflects the feed energy back toward the main paraboloid. The second focus of the hyperboloid coincides with the focus of the paraboloid, so the subreflector acts as a virtual equivalent parabola with a longer focal length. This longer equivalent focal length provides more uniform illumination across the main aperture.
| 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
The Gregorian antenna uses a concave ellipsoidal subreflector. The feed is placed at one focus of the ellipse, and the other focus coincides with the paraboloid's focus. Because the ellipse has a real second focus between the two reflectors, the Gregorian system has a real intermediate focal point, which is useful for placing polarizers, filters, and frequency-selective surfaces.
- 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
Both dual-reflector systems allow the feed and receiver to be located behind the main reflector, reducing the feed support structure and minimizing aperture blockage. They also provide a magnification effect: the subreflector magnifies the equivalent focal length, allowing the use of larger, more efficient feed horns.
Frequently Asked Questions
Which is better?
For most applications, performance is similar. Cassegrain is more common for large Earth station antennas because the feed location behind the dish simplifies maintenance. Gregorian is often preferred for offset-fed designs where the geometry naturally accommodates the ellipsoidal subreflector without blockage.
What about shaped reflectors?
Both Cassegrain and Gregorian can use shaped (non-standard) subreflectors to optimize the illumination distribution across the main aperture. Shaped dual-reflector antennas achieve 75-80% aperture efficiency by tailoring the illumination to be nearly uniform, significantly better than standard designs.
What is the subreflector blockage?
The subreflector and its support struts block a portion of the main aperture, reducing gain by 0.5-2 dB. The blockage effect is minimized by: making the subreflector as small as possible (limited by diffraction effects), using thin struts, or using an offset geometry that places the subreflector outside the main beam path.