AN/FPS-26 Radar

The AN/FPS-26 Radar was a height finder radar used by the United States Air Force Air Defense Command. It was an early application of a concept called "Frequency Diversity", which took advantage of a tunable output power device (a 3-cavity power klystron) to allow the radar operator to easily and quickly change the frequency of the radar to avoid enemy jamming. (Earlier generation radar transmitters typically used a magnetron as a high power output generating device; the output frequency of a magnetron cannot be controlled at will.) The output power of the transmitter pulse was approximately 5 megawatts.

The FPS-26A antenna was hydraulically driven in azimuth. The hydraulic drive system operated at high pressure levels typical of aircraft hydraulic systems. Azimuth selection made by the radar tactical operator aiming the antenna toward a target of interest. Antenna "nodding" could also be operator controlled, although in practice it was usually left in a default mode. The waveguide that conveyed the high power transmitter pulse to the antenna feedhorn was pressurized with sulfur hexafluoride (SF6) to maintain a high dielectric. Technicians were warned that a byproduct of waveguide arcing was fluorine, a deadly gas.

The radome was inflatable, and needed to maintain a minimum internal pressure at all times in order to avoid contacting the antenna. Radome contact with the antenna, even if the antenna were not in motion, would result in damage to both the radome and the antenna. Technicians entered the pressurized radome via an air lock to access the antenna deck.

Avco Corporation built this height-finder radar that operated at a frequency of 5400 to 5900 MHz. This radar deployed in the 1960s.

In Operation


This image shows several types of radar systems, operated by several agencies, co-located at a single radar site. The two radars without radomes are US Army FPS-6 heightfinders; the radome with an open steel grid support structure is a search radar operated by the Federal Aviation Administration; the two radars with radomes and white clad support structure are US Air Force FPS-6A heightfinders; and the radome in the foreground with dark-clad support structure is a US Air Force FPS-26A, just completing construction here circa late 1962. The radar site was located on Fort Lawton, in Seattle, Washington.