Archivo:Relay 1 antenna USA.jpg

This type of antenna is called a Hogg or horn-reflector antenna, invented by Albert Beck and Harald Friis in 1941 and further developed by David C. Hogg at Bell Telephone Laboratories in 1961. It consists of a flaring metal horn with a metal reflector mounted in the mouth at a 45° angle to the axis. The radio waves come from the satellite at a 90° angle to the horn and are reflected down the horn to the small building on the left, where they are received and amplified. The reflector is a segment of a parabolic reflector, so the antenna is equivalent to a parabolic dish fed off-axis. The advantage of this design over a standard parabolic dish is that it has very small sidelobes; that is, the horn shields the antenna from radiation coming from angles outside the main beam, such as ground noise, reducing the noise temperature of the antenna. Early satellites had weaker transmitters than modern satellites and required large antennas like this to communicate with them. Telstar was not in a geosynchronous orbit and passed overhead quickly in about 20 minutes, so the huge antenna had to be precisely steerable to track it across the sky as it passed. The antenna is mounted on a huge turntable in an altazmuth mount.