Ringway Manchester does a great job of explaining how, since the dawn of broadcasting, the U.S. Federal Communications Commission has been tracking radio transmissions by using direction-finding installations in remote, rural areas.
Many employ a unique antenna known as a Wullenweber array that looks sort of like a wireframe Stonehenge. Viewed from above, some of these circular setups could pass for center-pivot irrigation systems.
L.T. Hanlon 
At the 5:07 mark in the video, a postcard is featured of the Grand Island, Nebraska station. If you do a search on eBay for: “Federal Communications Commission” postcard, there are currently four postcards being sold. I was hoping to see other postcards, but so far only the Nebraska one is available.
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At 6:23 he mentions an array at Kenai, Alaska. But he couldn’t find it!
Mount Diablo’s Amateur Radio Club lists it as
60°43′26.0″ N. Latitude, 151°20′15.0″ W. Longitude
And at 11:30 he couldn’t find Waipahu, Hawaii
21°22′33.6″ N. Latitude, 157°59′44.1″ W. Longitude
There’s something so fascinating about these stations being able to monitor the local radio waves for illegal usage.
Their remote location where someone’s just is just to monitor the airwaves? What is that job like? Adding to the strangeness of all this is how all these arrays look like a modern Stonehenge. Some falling apart like Stonehenge, but still visible. Others not even to be seen at all.
A little bit of insight into the jobs of the people is in this 1992 Popular Communications magazine.
Maybe I missed it in the video. But what distance can these monitoring stations observe?
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To my knowledge the FCC use mobile radio vans with directional antennae to pinpoint locate an illicit transmitter. They visit every major city at least once per quarter.
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What’s odd is how many pirate radio stations slip through. There’s a low-power FM pirate here on Chicago’s North Side who’s been broadcasting for years, and written about in local media multiple times.
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A link in that dnainfo article points to an FCC map of violators from 2003-2013. Most states have about 1 to 3 violators. Some of the bigger states, like Texas have 10, and California with 20. But boy howdy, what’s up, Florida? 176 violations!
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