Cassegrain Antenna
Understanding Cassegrain Antenna
Dual-Reflector Optical Geometry
In high-gain microwave and millimeter-wave communication systems, such as satellite ground stations, deep space tracking terminals, and military radar, standard prime-focus parabolic antennas face physical limitations. In a prime-focus system, the feed horn and active receiver/transmitter front-end are suspended at the focal point in front of the dish. This layout requires long, lossy coaxial cables or waveguide runs to route signals to the processing equipment, introducing insertion loss and raising the receiver noise temperature. Dual-reflector designs, specifically the Cassegrain antenna, solve these problems using optical folding techniques.
The Cassegrain configuration is derived from the classic reflecting telescope design. It consists of a large parabolic primary reflector and a smaller hyperbolic sub-reflector. The primary reflector has its focus at the same physical point as the virtual focus of the hyperbolic sub-reflector. The feed horn is positioned at the real focus of the sub-reflector, which is conveniently located at the center, or vertex, of the primary dish, or directly behind it.
Engineering Advantages and blockages
Placing the feed horn and RF front-end behind the primary reflector offers major advantages. First, the feed lines are extremely short, minimizing attenuation and preserving the system G/T figure, which represents the gain-to-noise-temperature ratio. Second, heavy high-power transmitters and cryogenic low-noise amplifiers can be mounted securely behind the dish, eliminating mechanical strain on the feed support arms. However, the sub-reflector blocks a portion of the incoming waves, creating aperture blockage. This blockage increases sidelobe levels and slightly reduces efficiency, which designers mitigate by shaping the sub-reflector and primary dish to optimize the illumination profile.
Key Mathematical Relations
Technical Specifications Comparison
| Antenna Type | Feed Location | Waveguide Loss | Cross-Polarization Performance | Aperture Efficiency | Primary Applications |
|---|---|---|---|---|---|
| Prime-Focus | Focal Point (Front) | High (long run to back) | Good | 55% – 65% | Consumer satellite TV, small terminals |
| Cassegrain (Dual) | Vertex / Behind Dish | Very Low (short run) | Moderate (symmetric block) | 65% – 75% (shaped) | Large ground stations, deep space links |
| Gregorian (Dual) | Vertex / Behind Dish | Very Low | Moderate | 65% – 75% | Large radio telescopes, satellite terminals |
| Offset Reflector | Offset (No Blockage) | Moderate to High | Poor (requires correction) | 70% – 80% | VSAT terminals, high-efficiency links |
Frequently Asked Questions
What is the primary advantage of a Cassegrain antenna over a prime-focus dish?
The primary advantage is that the feed horn and RF electronics are located at or behind the primary reflector. This drastically shortens the waveguide feed lines, reducing transmission losses and lowering the receiver system noise temperature.
What is the difference between a Cassegrain and a Gregorian antenna?
A Cassegrain antenna uses a convex hyperbolic sub-reflector placed inside the primary focus. A Gregorian antenna uses a concave ellipsoidal sub-reflector placed beyond the primary focus. Both allow the feed to sit behind the dish, but Gregorian designs require a slightly longer support structure.
How does sub-reflector blockage affect Cassegrain performance?
Aperture blockage by the sub-reflector scatters some of the electromagnetic energy, which reduces the antenna gain by 0.1 to 0.5 dB and raises the sidelobe levels. This is minimized by ensuring the sub-reflector diameter is as small as possible relative to the primary dish.