Huff Duff Elephant Cages
Go to Google Earth and type in Elmendorf Air Force Base. To the northwest of the runways, near the water, you’ll see a round thing that looks like a big eye. What in the world is that?
If a satellite can see it, it must be big. In fact, it covers about 40 acres, which is the area of a circle with a diameter of about 1489 feet, or nearly 0.3 miles. This was AN/FLR-9, a High Frequency Direction Finding (HFDF, pronounced “huff duff”) station. Built at the height of the Cold War in 1966, it was used to detect the direction from which radio signals were emanating. Overcome by newer technology, it was decommissioned in 2016. You can see a picture of it at:
https://www.16af.af.mil/News/Legacy/Article/779267/silencing-the-arctic-mammoth/
So what was it exactly? To answer that question, I went looking for a reliable source and found the technical manual for the AN/FLR-9 at http://www.premium-rx.org/ref/wullenweber.pdf, which spelled things out in detail.
First, the AN/FLR-9 is a passive, circular, high frequency (HF) three-band antenna array. Remember that we talked about passive radars, those radars that sense ambient signals without transmitting any signal themselves. Passive antenna systems don’t emanate EM waves; they only collect incoming signals.
An antenna array is a set of connected antenna elements that are designed to work together as a single antenna to collect (or transmit) radio waves. An antenna array laid out in a circle like the AN/FLR-9 is also called a circularly disposed antenna array (CCDA).
Second, the AN/FLR-9 consists of three concentric rings of antenna elements with reflectors. The outer ring (band A) covers the 2 to 6-MHz range and has a diameter of 1198 feet. The center ring (band B) covers the 6 to 18-MHz range and has a diameter of 1146 feet. The inner ring (band C) covers the 18 to 30-MHz range.
The original design for these HFDF antennas came out of World War II. The Germans designed this HFDF antenna and gave it the code name Wüllenweber. As Supreme Commander of the German Navy, Admiral Dönitz used this antenna to communicate with his submarines. After the war, the Wüllenweber in Denmark was destroyed while the one in Germany was disassembled and brought back to the United States for analysis. Of course, the United States already had its own HFDF capabilities which you can read about here:
https://stationhypo.com/2019/01/05/early-direction-finding-part-1-of-2/
Third, the arrangement of antenna elements is complex. Band A contains 48 105-foot vertical antenna elements spaced 78.4 feet apart. Band B contains 96 35-foot vertical antenna elements spaced 37.5 feet apart. Band C contains 48 horizontal antenna elements.
That certainly is a lot of big antennas! But remember the performance goal. This design was intended to pinpoint the direction of HF signals around all 360 degrees of its circular array to a distance of 4000 nautical miles, which is about 4603 miles.
Fourth, the AN/FLR-9 includes an arrangement of reflecting screens which improve the performance of the HFDF. A gigantic (120-foot high) screen, located inside and concentric to the band B antenna array, increases the power gain of individual antennas in both bands A and B. There is also a band C screen, supported by the band C antenna support structure. Finally, a mesh ground screen for bands A and B extends out from the base circumference of the bands A and B reflecting screen.
Now go back and look at the picture in the reference at the beginning of this article. That giant 120-foot high reflecting screen does look like a round cage, doesn’t it? Now you know why these antennas came to be known as elephant cages!
Today we’ve talked about the use of direction finding for opposing forces. We’ve seen that one side can use HFDF to detect enemy signals, signals which may provide warnings about potential threats. Are there other applications of direction finding?
Direction finding is also very useful for navigation. Such a system was developed during World War II. It was known as LORAN, which stands for Long Range Navigation. Next week we will investigate the science behind Loran. We’ll also trace its path from invention to proliferation, retirement, and resurrection, all in the last 80 years.