AN/FLR-9
Cold War HF/DF array nicknamed "Elephant Cage."
The AN/FLR-9 is a very large circularly disposed antenna array built for high-frequency direction finding (HF/DF) of high priority targets. Developed during the Cold War, eight installations were constructed worldwide, forming a network known collectively as "Iron Horse" that could locate HF communications almost anywhere on Earth. The array is notable for its exceptionally large outer reflecting screen, composed of 1056 vertical steel wires supported by 96 towers each 120 feet (37 m) tall, which earned it the nickname "Elephant Cage."
- Designation
- AN/FLR-9 (9th design of Army-Navy fixed ground countermeasures passive detection equipment)
- Frequency range
- 1.5 to 30 MHz (three bands: A 2–6 MHz, B 6–18 MHz, C 18–30 MHz)
- Number of installations
- 8
- Network name
- Iron Horse
- Outer screen
- 1056 vertical steel wires on 96 towers, each 120 ft (37 m) tall
- Ground screen diameter
- 1,443 ft (440 m)
- Maximum range
- 4,000 nmi (7,400 km)
Lore & Background
The AN/FLR-9 was constructed in the early to mid 1960s at eight locations: USASA Field Station Augsburg (Germany), RAF Chicksands (England), Clark AB (Philippines), Joint Base Elmendorf-Richardson (Alaska), Karamursel (Turkey), Misawa AB (Japan), Ramasun Station (Thailand), and San Vito dei Normanni Air Station (Italy). The array consists of three concentric rings of antenna elements, each covering a portion of the 1.5 to 30 MHz spectrum. The outer ring (band A, 2–6 MHz) uses 48 sleeve monopole elements spaced 78.4 feet apart; the center ring (band B, 6–18 MHz) uses 96 sleeve monopole elements spaced 37.5 feet apart; and the inner ring (band C, 18–30 MHz) uses 48 antenna elements mounted on wooden structures. Bands A and B elements are vertically polarized, while band C elements consist of two horizontally polarized dipole subelements positioned one above the other. The entire array is centered on a ground screen 1,443 feet in diameter, enabling accurate direction finding of signals from up to 4,000 nautical miles away.
Reader's Guide
The AN/FLR-9's significance lies in its role as a key Cold War signals intelligence asset, forming a global network that could intercept and locate high-frequency communications worldwide. Its distinctive design—a massive circular array with a towering outer screen of steel wires—made it one of the largest antenna systems ever built. The nickname "Elephant Cage" reflected its imposing physical presence. Over time, advances in technology rendered the FLR-9 obsolete. Several installations were dismantled: the Clark AB site was converted into a 35,000-seat amphitheater in 1997; San Vito dei Normanni was systematically dismantled in early May 2002; Misawa AB demolition began in October 2014; and the last operational FLR-9, at Joint Base Elmendorf-Richardson, was decommissioned on May 25, 2016. The FLR-9 can be confused with the US Navy's AN/FRD-10, which also used a circularly disposed antenna array.
Did You Know?
- The AN/FLR-9 was nicknamed "Elephant Cage" due to its exceptionally large outer reflecting screen of 1056 vertical steel wires.
- The worldwide network of FLR-9 arrays was known collectively as "Iron Horse."
- A decommissioning ceremony for the last active FLR-9, at Joint Base Elmendorf-Richardson, was held on May 25, 2016.
- The FLR-9 at Clark AB was converted into a 35,000-seat fabric-covered amphitheater in 1997.
Frequently Asked Questions
What is the AN/FLR-9?
The AN/FLR-9 is a massive circular ground-based antenna array built during the Cold War to perform high-frequency direction finding on priority targets. It was the ninth fixed ground passive-detection design in the joint Army-Navy series.
What is the Iron Horse network?
Iron Horse is the collective name given to the eight AN/FLR-9 sites erected around the globe in the Cold War era. Working together, those arrays formed a worldwide HF/DF grid capable of triangulating high-frequency communications from virtually any point on Earth.
Why is the AN/FLR-9 nicknamed "Elephant Cage"?
The nickname comes from the array's enormous outer reflecting screen, made up of 1,056 vertical steel wires held aloft by 96 towers that each stand 120 feet tall. The sheer scale and cage-like silhouette of those towering wires gave the system its memorable moniker.
How large is the AN/FLR-9's ground screen?
The circular ground screen spans 1,443 feet (roughly 440 m) in diameter, making it one of the largest passive-detection structures ever erected. Its outer reflecting screen is supported by 96 towers, each reaching 120 feet (37 m) in height.
What frequency bands does the AN/FLR-9 cover?
The array operates across the 1.5-to-30 MHz HF range, divided into three operational bands: Band A (2–6 MHz), Band B (6–18 MHz), and Band C (18–30 MHz).
More in Antenna Types 1-24
Spotted an error? Know more?
Reader corrections go straight into our review queue. Suggest an edit · How this site is sourced