Friday, February 12, 2021

Tracking Cold War Signals

Adcock four element antenna array
WWII Naval direction finding station
(source: Frontline Ulster)
The Cold War was also a war of signals. This battle comprised chatter over radio, Morse, data and technical signals. Eavesdropping on enemy communications and analyzing their technical signals was a vital part of that battle. However, to know where those signals came from was just as important.

Directional antennas find the bearing of a signal. Early simple loop antennas had to be turned mechanically to find the signal bearing. With two or more such antennas on different locations, the target is located at the crossing of those bearings, but it was a cumbersome task. Later, double loop antennas and Adcock antenna arrays with four elements improved performance, but many more special  direction finding (DF) antennas were built to locate signals, and some were quite extraordinary.

German Wartime Research

The Wullenweber antenna array
(source: FGCRT)
Significant progress was made during the Second World War by Dr. Hans Rindfleisch, who invented the Circularly Disposed Antenna Array (CDAA). Rindfleisch also headed the Communications Research Command of the German Navy, and together with Telefunken they developed his antenna array under the codename Wullenweber.

The first Wullenweber, build in Skibsby in northern Denmark, was designed as high-frequency direction finding (HF/DF) antenna array and operated in the 6-20 MHz range. The above drawing shows, at the top, the view of the antenna array and the reflector screen wires behind them (click image to enlarge)
First Wullenweber at Skibsby site
(source unknown)
The antenna consisted of 40 vertical radiator elements, each supported by a wooden structure and placed in a large circle, 120 m (392 ft) in diameter. Inside that circle was a reflector screen of wires, supported by 40 poles and arranged in a smaller circle, 105 m (344 ft) in diameter.

The Foundation for German communication and related technologies (main page) has a description of the Wullenwever (original spelling), including German Naval research on Wullenwever (pdf p11-20) and the Landsberg-Lech conference (pdf) with technical details and several photos of  the Skibsby Wullenweber site.

German Technology in Soviet Hands

Many German scientists were rounded up by US and Soviet forces in the final days of the war. Both were interested in this new CDAA technology, but the Soviets were the first to start building them in 1951 with assistance of German scientists.

The Soviets eventually build 31 CDAA's of various types and called them KRUG. They were placed in Russia, Warsaw Pact countries, Mongolia, Cuba, Vietnam and Burma. These KRUG stations tracked radio communications of US and NATO reconnaissance aircraft and nuclear bombers. GlobalSecurity has info and photos on Soviet KRUG antenna arrays.

The Global U.S. Antenna Network

One of the German antenna researchers was moved to the United States to assists in the development of a CDAA. The US version of the Wullenweber was the AN/FLR-9 antenna, nicknamed "Elephant Cage". The first was built in 1962 at the RAF Chicksands base in the UK, leased by the US Air Force.

FLR-9 at USASA Field Station Augsburg, Germany (source: US Air Force ISR)

The huge FLR-9 antenna had an outer diameter of 440 m (1,443 ft) and height 37 m (121 ft). A network of eight FLR-9 was constructed in Alaska, England, Germany, Italy, Japan, Philippines, Turkey and Thailand. This network could accurately locate HF signals anywhere on Earth, to track enemy airplanes, ships or ground based transmitters, but also to follow own or friendly targets.

The US Naval Security Group operated the AN/FRD-10, also a Wullenweber antenna but smaller than the FLR-9. Its outer diameter was 263 m (863 ft). A network of sixteen FRD-10's was located at coastal lines of the Pacific and Atlantic on US mainland and Alaska, Hawai, Puerto Rico, Canada,  Panama, Japan, Spain and Scotland.

More about the Wullenweber

FLR stands for Fixed Countermeasures Receiving. FRD stands for Fixed Radio Direction-finder (see JETDS designations). More about the FLR-9 on FAS and Freedom Through Vigilance Association (USAFSS). Navy Radio has details of the AN/FRD-10. There's a report on the dismantling of the AN/FLR-9 at Misawa air base in Japan and the decommissioning of Joint Base Elmendorf-Richardson in Alaska (video).

More on WWII direction finding at the RSS Secret Listeners website and on Frontline Ulster's WWII Aircraft Direction Finding in the UK.

The NSA video below explains the history and purpose of the FLR-9.

The American Forces Network Pacific gave a look inside Misawa’s FLR-9, build in 1962. The antenna was demolished in 2014.

More on Signals

Many different signals were sent, received and analyzed during the Cold War. Below some posts on this blog about Signals Intelligence (SIGINT), but there's much more to discover...

Visit also the Cold War Signals page about the battle over radio waves on our website


LCDR M(Ret) said...

I was stationed at NSGA Misawa twice. Spent about 10 years altogether in "Missy" and, as a Russian linguist, was quite a consumer of the antenna. Sad to see the ole "Elephant Cage" go, but all good things must come to an end.

Thanks for this article.

Dirk Rijmenants said...


Thanks for your comment. Always hard to see such things go, when you've put so much work in it. Linguist is one of the things people don't think of on a "tracking station", but always in demand. Did you checked out the FSB Teufelsberg post? There's also link to Lew's story as liguist in Berlin. Field Station Berlin - Teufelsberg

As I always say, sad to see our barracks vanish or neglected, but no WWIII, and mission accomplished! Thanks for your service.