Microwave horn antenna in New Jersey, US
The Holmdel Horn Antenna is a large microwave horn antenna that was used as a satellite communication antenna and radio telescope during the 1960s at the Bell Telephone Laboratories facility located on Crawford Hill in Holmdel Township, New Jersey, United States. It was designated a National Historic Landmark in 1989 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 cosmic microwave background radiation (CMBR) that permeates the universe. This was one of the most important discoveries in physical cosmology since Edwin Hubble demonstrated in the 1920s that the universe was expanding. It provided the evidence that confirmed George Gamow's and Georges Lemaître's "Big Bang" theory of the creation of the universe. This helped change the science of cosmology, the study of the universe's history, from a field for unlimited theoretical speculation into a discipline of direct observation. In 1978 Penzias and Wilson received the Nobel Prize for Physics for their discovery.
Description.
The horn antenna at Bell Telephone Laboratories in Holmdel, New Jersey, was constructed on Crawford Hill in 1959 to support Project Echo, the National Aeronautics and Space Administration's passive communications satellites, which used large aluminized plastic balloons (satellite balloon) as reflectors to bounce radio signals from one point on the Earth to another.
The antenna is in length with a radiating aperture of and is constructed of aluminum. The antenna's elevation wheel, which surrounds the midsection of the horn, is in diameter and supports the structure's weight using rollers mounted on a base frame. All axial or thrust loads are taken by a large ball bearing at the narrow apex end of the horn. The horn continues through this bearing into the equipment building or cab. The ability to locate receiver equipment at the horn apex, 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 located in the 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 turntable track in diameter. The track consists of stress-relieved, planed steel plates individually adjusted to produce a track that is flat to about . The faces of the wheels are cone-shaped to minimize contact friction. A tangential force of 100 pounds (400 N) is sufficient to start the antenna rotating on the turntable. The antenna beam can be directed to any part of the sky using the turntable for azimuth adjustments and the elevation wheel to change the elevation angle or altitude above the horizon.
Except for the steel base frame, which a local steel company made, the Holmdel Laboratory shops fabricated and assembled the antenna 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 Arthur Crawford.
When not in use, the turntable azimuth sprocket drive is disengaged, allowing the structure to "weathervane" and seek a position of minimum wind resistance. The antenna was designed to withstand winds of , and the entire structure weighs 18 short tons (16 tonnes).
A plastic clapboarded utility shed with two windows, a double door, and a sheet-metal roof, is located on the ground next to the antenna. This structure houses equipment and controls for the antenna and is included as a part of the designation as a National Historic Landmark. 
The antenna has not been used for several decades. 
Technical.
This type of antenna is called a Hogg or horn-reflector antenna, invented by Alfred C. Beck and Harald T. Friis in 1941. It was built by David C. Hogg. It consists of a flaring metal horn with a curved reflecting surface mounted in its mouth at a 45° angle to the long axis of the horn. The reflector is a segment of a parabolic reflector, so the antenna is a parabolic antenna that is fed off-axis. A Hogg horn combines several characteristics useful for radio astronomy. It is extremely broad-band, has calculable aperture efficiency, and the walls of the horn shield it from radiation coming from angles outside the main beam axis. Therefore, the back and side lobes are so minimal that scarcely any thermal energy is received from the ground. Consequently, it is an ideal radio telescope for accurately measuring low levels of weak background radiation. The antenna has a gain of about 43.3 dBi and a beamwidth of about 1.5° at 2.39 GHz and an aperture efficiency of 76%.
Preservation.
In 2021, the Crawford Hill site was sold to a developer who was interested in building a residential development. In reaction, this triggered a "Save Holmdel's Horn Antenna" petition to preserve the property as a park. Advocates felt that a better fate than the horn antenna or its site encountering destruction to make way for a planned real estate development.
As of October 2023, the site is now planned to be preserved. After public support for the preservation of the horn antenna emerged—demonstrated in part by more than 8,000 signatures on a petition disseminated by community groups—the Holmdel Township Committee agreed to pay $5.5 million for of land, including that which the antenna sits on. The town plans to turn the land into a public park.
References.
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External links.
 Media related to at Wikimedia Commons