Horn Antenna

A large horn antenna used as a radio telescope at Bell Telephone Laboratories in Holmdel Township, New Jersey, now known as the Holmdel horn antenna, was designated a National Historic Landmark in 1988 because of its association with the research work of two radio astronomers, Arno Penzias and Robert Wilson. In 1965 while using this antenna, Penzias and Wilson discovered the microwave background radiation that permeates the universe. This was one of the most important discoveries in cosmology since Edwin Hubble demonstrated in the 1920s that the universe was expanding. It provided the evidence that confirmed George Gamow's and Abbe Georges Lemaitre's "Big Bang" theory of the creation of the universe. This helped change the science of cosmology, the study of the history of the universe, from a field for unlimited theoretical speculation into a subject disciplined by direct observation. In 1978 Penzias and Wilson received the Nobel Prize for Physics for their discovery.


The Horn Antenna at Bell Telephone Laboratories in Holmdel, New Jersey, was constructed in 1959 to support Project Echo—the National Aeronautics and Space Administration's passive communications satellite project.

The antenna is 50 feet (15 m) in length with a radiating aperture of 20 by 20 feet (6 by 6 m) and is made of aluminum. The antenna's elevation wheel is 30 feet (10 m) in diameter and supports the weight of the structure by means of rollers mounted on a base frame. All axial or thrust loads are taken by a large ball bearing at the apex end of the horn. The horn continues through this bearing into the equipment cab. The ability to locate receiver equipment at the apex of the horn, thus eliminating the noise contribution of a connecting line, is an important feature of the antenna. A radiometer for measuring the intensity of radiant energy is found in the equipment cab.

The triangular base frame of the antenna is made from structural steel. It rotates on wheels about a center pintle ball bearing on a track 30 feet (10 m) in diameter. The track consists of stress-relieved, planed steel plates which are individually adjusted to produce a track flat to about 1/64 inch (0.4 mm). The faces of the wheels are cone-shaped to minimize sliding friction. A tangential force of 100 pounds force (400 N) is sufficient to start the antenna in motion.

To permit the antenna beam to be directed to any part of the sky, the antenna is mounted with the axis of the horn horizontal. Rotation about this axis affords tracking in elevation while the entire assembly is rotated about a vertical axis for tracking in the azimuth.

With the exception of the steel base frame, which was made by a local steel company, the antenna was fabricated and assembled by the Holmdel Laboratory shops under the direction of Mr. H. W. Anderson, who also collaborated on the design. Assistance in the design was also given by Messrs. R. O'Regan and S. A. Darby. Construction of the antenna was completed under the direction of Mr. A. B. Crawford from Freehold Borough, New Jersey.

When not in use, the antenna azimuth sprocket drive is disengaged, thus permitting the structure to "weathervane" and seek a position of minimum wind resistance. The antenna was designed to withstand winds of 100 miles per hour (160 km/h) and the entire structure weighs 18 tons.

This type of antenna is called a Hogg horn antenna, invented by D. L. Hogg at Bell Labs in 1961. It consists of a flaring metal horn with a reflector mounted in its mouth, at a 45° angle. The reflector is a segment of a parabolic reflector, so the antenna is really a parabolic antenna fed off-axis. A Hogg horn combines several ideal characteristics: it is extremely broad-band, has calculable aperture efficiency, and the back and sidelobes are so minimal that scarcely any thermal energy is picked up from the ground. Consequently it is an ideal radio telescope for accurate measurements of low levels of weak background radiation.

A plastic clapboarded utility shed 10 by 20 feet (3 by 6 m), with two windows, a double door and a sheet metal roof, is located next to the Horn Antenna. This structure houses equipment and controls for the Horn Antenna and is included as a part of the designation as a National Historic Landmark. It is not in use anymore.


The original material in this article was taken from a National Park Service publication which in turn used the following sources:

  • Aaronson, Steve. "The Light of Creation: An Interview with Arno A. Penzias and Robert W. Wilson." Bell Laboratories Record. January 1979, pp. 12–18.
  • Abell, George O. Exploration of the Universe. 4th ed., Philadelphia: Saunders College Publishing, 1982.
  • Asimov, Isaac. Asimov's Biographical Encyclopedia of Science and Technology. 2nd ed., New York: Doubleday & Company, Inc., 1982.
  • Bernstein, Jeremy. Three Degrees Above Zero: Bell Labs in the Information Age. New York: Charles Scribner's Sons, 1984.
  • Chown, Marcus. "A Cosmic Relic in Three Degrees," New Scientist, September 29, 1988, pp. 51–55.
  • Crawford, A.B., D.C. Hogg and L.E. Hunt. "Project Echo: A Horn-Reflector Antenna for Space Communication," The Bell System Technical Journal, July 961, pp. 1095–1099.
  • Disney, Michael. The Hidden Universe. New York: Macmillan Publishing Company, 1984.
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  • Hey, J.S. The Evolution of Radio Astronomy. New York: Neale Watson Academic Publications, Inc., 1973.
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  • Learner, Richard. Astronomy Through the Telescope. New York: Van Nostrand Reinhold Company, 1981.
  • Penzias, A.A., and R. W. Wilson. "A Measurement of the Flux Density of CAS A At 4080 Mc/s," Astrophysical Journal Letters, May 1965, pp. 1149–1154.