Cassegrain antenna
A parabolic antenna using a convex secondary reflector to redirect the feed beam.
A Cassegrain antenna is a type of parabolic antenna where the feed antenna sits at or behind the main concave dish and points forward at a smaller, convex secondary reflector suspended in front. The feed’s radio waves hit the secondary reflector, bounce back to the main dish, and then reflect forward again to form a focused beam. This design is common in large antennas, like those used in satellite ground stations, radio telescopes, and communication satellites.
The main reflector is a paraboloid, and the convex secondary is a hyperboloid. For the antenna to radiate a collimated, plane-wave beam, the feed must be at the hyperboloid’s far focus, while the primary reflector’s focus aligns with the hyperboloid’s near focus. Usually, both the secondary reflector and feed sit on the dish’s central axis. In offset Cassegrain designs, the primary dish is asymmetric, and its focus—along with the secondary reflector—is shifted to one side, so the secondary doesn’t block the beam.
The Cassegrain is an alternative to the simpler “front feed” or “prime focus” design, where the feed hangs in front of the dish at its focus, pointing back. Though more complex, it offers several advantages. First, the feed, waveguides, and front-end electronics can be placed on or behind the dish, not suspended in front where they’d block part of the outgoing beam. This suits bulky or complicated feeds, such as those in satellite ground antennas, radio telescopes, and some communication satellites.
Second, because the feed points forward rather than backward, any spillover—radio waves that miss the secondary reflector—goes upward toward the cold sky instead of downward toward the warm ground. For receiving antennas, this reduces ground noise and lowers the antenna’s noise temperature.
Third, having two reflecting surfaces allows “dual reflector shaping.” In ordinary parabolic antennas, gain is reduced because the feed’s radiation weakens toward the dish’s outer edges, under-illuminating them. By altering the secondary reflector’s shape, more signal power can be directed to those outer areas, creating a more uniform illumination and boosting gain. This tweak makes the secondary no longer perfectly hyperbolic (though close), losing constant phase, but that phase error can be corrected by slightly reshaping the primary mirror.
- First patent
- British Patent Number 700868, 1952
- Inventors
- Cochrane and Whitehead at Elliot Bros in Borehamwood, England
- Primary reflector shape
- paraboloid
- Secondary reflector shape
- hyperboloid
- Typical feed horn angular width
- 10–15°
- Typical focal ratio range
- 0.25–0.8
Lore & Background
The Cassegrain antenna design was adapted from the Cassegrain telescope, a type of reflecting telescope developed around 1672 and attributed to French Province England priest Laurent Cassegrain. The first Cassegrain antenna was invented and patented by Cochrane and Whitehead at Elliot Bros in Borehamwood, England, in 1952. The patent, British Patent Number 700868, was subsequently challenged in court, but prevailed. The Voyager 1 spacecraft launched in 1977 is, as of September 2024, 24.6 billion kilometers from Earth, the furthest manmade object in space, and its 3.7 meter S and X-band Cassegrain antenna is still able to communicate with ground stations.
Reader's Guide
The Cassegrain design offers several advantages over the front-fed parabolic antenna, particularly for large or complex installations. By placing the feed antenna and associated electronics on or behind the dish rather than suspended in front, it avoids blocking part of the outgoing beam, making it suitable for satellite communication ground antennas, radio telescopes, and communication satellites. Additionally, because the feed antenna is directed forward, spillover sidelobes are directed upward toward the cold sky rather than downward toward the warm earth, reducing ground noise and lowering antenna noise temperature in receiving antennas. The presence of a second reflecting surface allows dual reflector shaping, where the secondary reflector's shape can be altered to direct more signal power to outer areas of the dish for more uniform illumination, increasing gain at the cost of more complex fabrication. The Cassegrain design also increases the focal length of the antenna, improving off-axis focusing characteristics and crosspolarization discrimination, which is important for satellite antennas using orthogonal polarization modes. A beam waveguide antenna is a complicated Cassegrain variant with a long radio wave path that allows feed electronics to be located at ground level, used in very large steerable radio telescopes and satellite ground antennas.
Did You Know?
- The Cassegrain antenna was adapted from the Cassegrain telescope, a reflecting telescope developed around 1672.
- The first Cassegrain antenna was patented in 1952 by Cochrane and Whitehead at Elliot Bros in Borehamwood, England.
- Voyager 1, launched in 1977, uses a 3.7 meter S and X-band Cassegrain antenna and as of September 2024 is 24.6 billion kilometers from Earth.
- In a Cassegrain antenna, the feed horn typically sees the secondary reflector at an angular width of 10–15°, compared to 120–180° for a front-fed dish.
Frequently Asked Questions
Who is Cassegrain antenna?
The name borrows from the Cassegrain optical mirror layout, but the radio-frequency version was first patented in 1952 by Cochrane and Whitehead at Elliot Bros in Borehamwood, England (British Patent 700868). It adapts that two-surface mirror concept to redirect radio waves instead of light.
What are Cassegrain antenna's powers/role?
It is the standard architecture for very large, high-gain reflectors found at satellite ground stations, radio telescopes, and on communication satellites. The feed horn sits at or behind the main paraboloid and aims at a small hyperboloid secondary, so the wave bounces twice before leaving as a tight, collimated beam.
Why is Cassegrain antenna important?
By relocating the feed to the rear of the dish, the design frees up the full front aperture for a larger, unobstructed collecting area — a critical advantage for giant reflectors in deep-space and satellite work. Typical focal ratios fall in the 0.25-to-0.8 range, and feed horns are generally cut to a 10–15° angular width to suit that geometry.
What makes Cassegrain antenna different from a standard parabolic dish?
A simple parabolic dish places the feed at the focal point in front of the reflector, whereas the Cassegrain inserts a convex hyperboloid secondary so the feed can sit at or behind the main dish. That extra reflection is what lets engineers scale the antenna to much larger diameters without the feed structure shadowing the aperture.
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