On-line Book

Book Cover
Cover Page







Other Sites

Existing National Historic Landmarks


Astronomy and Astrophysics
Horn Antenna
National Park Service Arrowhead

Horn Antenna
Horn Antenna — Holmdel, New Jersey.
Horn Antenna, circa 1960.
(Photo Credit: Bell Labs)

Name:Horn Antenna (Horn Reflector Antenna)
Location:Holmdel, New Jersey
Classification:Private, building, structure

Areas of Significance:National Register: science, NHL: science, Subtheme: physical science, Facet: astronomy

Builder:Mr. A. B. Crawford


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. [1]

The antenna is 50 feet in length with a radiating aperture of 20 x 20 feet and is made of aluminum. The antenna's elevation wheel is 30 feet 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 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. The faces of the wheels are cone-shaped to minimize sliding friction. A tangential force of 100 pounds 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, 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 and the entire structure weighs 18 tons.

The Horn Antenna 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 x 20 feet, with two windows, a double door and a sheet metal roof, is found next to the Horn Antenna. This structure houses equipment and controls for the Horn Antenna and is included in this nomination.


The Horn Antenna, at the Bell Telephone Laboratories in Holmdel, New Jersey, is significant because of its association with the research work of two radio astronomers, Dr. Arno A. Penzias and Dr. Robert A. Wilson. In 1965 while using the Horn Antenna, Penzias and Wilson stumbled on the microwave background radiation that permeates the universe. Cosmologists quickly realized that Penzias and Wilson had made the most important discovery in modern astronomy since Edwin Hubble demonstrated in the 1920s that the universe was expanding. This discovery provided the evidence that confirmed George Gamow's and Abbe Georges Lemaitre's "Big Bang" theory of the creation of the universe and forever changed 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 momentous discovery. [2]


"We live in an ocean of whispers left over from our eruptive creation, physicist George Gamow and his colleagues had said. Nobody was listening." [3]

By the middle of the 20th century cosmologists concerned with the creation of the universe had evolved two leading theories to explain their views. Some astronomers supported the steady-state theory of creation, which stated that the universe has always existed and will continue to survive without noticeable change. Others believed in the "Big Bang" theory of creation which taught that the universe is the glowing debris of a huge fireball that was created in a massive explosion about 16 billion years ago. No one knew for sure which theory was correct.

At Holmdel, New Jersey, in 1964 Dr. Arno Penzias and Dr. Robert Wilson were experimenting with a supersensitive, 20-foot horn-shaped antenna originally built to detect radio waves bounced off Echo balloon satellites. To measure faint radio waves from the Telstar communications satellite, they had to eliminate all recognizable interference from their receiver. They removed the effects of radar and radio broadcasting, and suppressed interference from the heart in the receiver itself by cooling it with liquid helium to -269°C, only 4° above absolute zero--the temperature at which all motion in atoms and molecules stops. [4]

When Penzias and Wilson reduced their data they found a low, steady, mysterious noise that persisted in their receiver. This residual noise was 100 times more intense than they had expected, was evenly spread over the sky, and was present day and night. They were certain that the radiation they detected on a wavelength of 7.35 centimeters did not come from the Earth, the Sun, or our Galaxy. After thoroughly checking their equipment, the noise remained. Both men concluded that this noise was coming from outside our own galaxy--although they were not aware of any radio source that would account for it.

At that same time, Robert H. Dicke, Jim Peebles, and David Wilkenson, astrophysicists at Princeton University, just 40 miles away, were preparing to search for microwave radiation in this region of the spectrum. Dicke and his colleagues reasoned that the "Big Bang" must have scattered not only the matter that condensed into galaxies but also must have released a tremendous blast of radiation. With the proper instrumentation, this radiation should be detectable.

When a friend told Penzias about a preprint paper he had seen by Jim Peebles on the possibility of finding radiation left over from a fireball that filled the universe at the beginning of its existence, Penzias and Wilson began to realize the significance of their discovery. The characteristics of the radiation detected by Penzias and Wilson fit exactly the radiation predicted by Robert H. Dicke and his colleagues at Princeton University. Penzias called Dicke at Princeton, who immediately sent him a copy of the still-unpublished Peebles paper. Penzias read the paper and called Dicke again and invited him to Bell Labs to look at the Horn Antenna and listen to the background noise. Dicke, Penzias, and Wilson visited the antenna and immediately recognized the significance of their discovery--they had stumbled on to the "embers" of creation predicted by their Princeton colleagues.

To avoid potential conflict, they decided to publish their results jointly. Two notes were rushed to the Astrophysical Journal Letters. In the first, Dicke and his associates outlined the importance of cosmic background radiation as substantiation of the Big Bang Theory. In a second note, jointly signed by Penzias and Wilson titled, "A Measurement of Excess Antenna Temperature at 4080 Megacycles per Second," they noted the existence of the residual background noise and attributed a possible explanation to that given by Dicke in his companion letter.

Harvard physicist Edward Purcell read this announcement and concluded that "It just may be the most important thing anybody has ever seen." [5]

Astronomer Robert Jastrow echoed this conclusion by stating that Penzias and Wilson ". . . made one of the greatest discoveries in 500 years of modern astronomy." [6]

In 1978, Dr. Arno Penzias and Dr. Robert Wilson were awarded the Nobel Prize for Physics for their joint discovery.


1. The descriptive material for this section was taken from the following sources:

J.S. Hey, The Evolution of Radio Astronomy (New York: Neale Watson Academic Publications, Inc., 1973), pp. 98-99.

A.B. Crawford, D. C. Hogg, and L. E. Hunt, "Project Echo: A Horn Antenna for Space Communication," Bell System Technical Journal (July 1961), pp. 1095-1099.

2. Marcus Chown, "A cosmic relic in three degrees," New Scientist, September 29, 1988, pp. 51-52.

Richard Learner, Astronomy Through the Telescope (New York: Van Nostrand Reinhold Company, 1981), p. 154.

3. Timothy Ferris, The Red Limit: The Search for the Edge of the Universe, (New York: Quill Press, 1983), p. 141.

4. Herbert Friedman, The Amazing Universe (Washington, DC: National Geographic Society, 1975), p 166-167.

5. Ferris, op. cit., 151.

6. Robert Jastrow, God and the Astronomers (New York: W. W. Norton & Company, Inc., 1978), p. 20.


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 Degree 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.

Ferris, Timothy. The Red Limit: The Search for the Edge of the Universe. 2nd ed., New York: Quill Press, 1978.

Friedman, Herbert. The Amazing Universe. Washington, DC: National Geographic Society, 1975.

Hey, J.S. The Evolution of Radio Astronomy. New York: Neale Watson Academic Publications, Inc., 1973.

Jastrow, Robert. God and the Astronomers. New York : W. W. Norton & Company, Inc., 1978.

Kirby-Smith, H.T. U.S. Observatories: A Directory and Travel. Guide. New York: Van Nostrand Reinhold Company, 1976.

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.


Horn Antenna Horn Antenna

(click on the above photographs for a more detailed view)

Cincinnati Observatory


Last Modified: Mon, Nov 5 2001 8:00:00 pm PDT

National Park Service's ParkNet Home