FLR-9

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The AN/FLR-9 is a type of very large circular "Wullenweber" antenna array, built at many locations during the cold war to gather signals for western signals intelligence (SIGINT). The world-wide network, known collectively as "Iron Horse", could eavesdrop on HF communications from almost anywhere on the planet.

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 AS, Italy

Advances in technology have made Iron Horse obsolete. It is not known how many antennae remain in existence—some have already been demolished such as the one at Chicksands. 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 Italy.