FLR-9

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AN/FLR-9 at Gablingen, near Augsburg, Germany

The AN/FLR-9 is a type of very large circular "Wullenweber" antenna array, built at many locations during the cold war for HF direction finding of high priority targets. The world-wide network, known collectively as "Iron Horse", could locate on HF communications from almost anywhere on the planet.

[edit] Description

The AN/FLR-9 Operation and Service manual describes the array as follows: "The antenna array is composed of three concentric rings of antenna elements. Each ring of elements receives rf signals for an assigned portion of the 1.5 to 30-MHz radio spectrum. The outer ring normally covers the 2 to 6-MHz range (band A), but also provides reduced coverage down to 1.5 MHz. The center ring covers the 6 to 18-MHz range (band B) and the inner ring covers the 18 to 30-MHz range (band C). Band A contains 48 sleeve monopole elements spaced 78.4 feet apart (7.5 degrees). Band B contains 96 sleeve monopole elements spaced 37.5 feet (11.43 m) apart (3.75 degrees). Band C contains 48 antenna elements mounted on wooden structures placed in a circle around the central building. Bands A and B elements are vertically polarized. Band C elements consist of two horizontally polarized dipole antenna subelements electrically tied together, and positioned one above the other."

The array is centered on a ground screen 1,443 feet (439.8 m) in diameter. The arrangement permits accurate direction finding of signals from up to 4000 nautical miles (7 408 km) away.

FLR-9s were constructed at the following places:

Augsburg, Germany
Chicksands, England
Clark AFB, Philippines
Elmendorf AFB, Alaska
Karamursel, Turkey
7th Radio Research Field Station/Ramasun Station, Udon Thani Province, Thailand
Misawa Air Base, Japan
San Vito dei Normanni Air Station, Italy

Advances in technology have made the FLR-9 obsolete. Two FLR-9 arrays remain in use in 2007, although they are expected to be decommissioned soon. In 1997, the FLR-9 at the former Clark AFB in the Philippines was converted into a 35,000-seat fabric-covered amphitheatre. In late 2001, the U.S. Air Force sought bids for the demolition of the site in San Vito, Italy.