During the 1970's and 1980's, anyone who owned a shortwave or ham radio would have been very familiar with a sharp, repetitive "rat tat tat tat" noise.
This interference permeated the airwaves and disrupted over air communications and television signals all over the world. This characteristic tapping sound gave the device(s) its now-famous nickname "The Russian Woodpecker" aka the Duga Radar system.
This "Russian Woodpecker" was a set of massive arrays of antennae constructed in secret in woods close to Chernobyl in Ukraine. Two arrays were built near Chernobyl with a third located on the Russian Pacific Coast near Sakhalinsk. The arrays were built by the Soviet Union to provide an early warning radar system called Duga. The purpose of these arrays was to allow the Soviets to detect any incoming ballistic missiles from America and European NATO members.
These arrays were truly enormous. One of the Chernobyl ones measured 210 meters wide by 85 meters tall. It consisted of over three hundred individual transmitter elements. Each array operated at very high power levels, in some cases as much as 10 million watts. This led to them being able to completely drown out legitimate transmissions over the same frequency, hence the characteristic interference on people's radio sets. Interestingly, it not only interfered with domestic and amateur radio and broadcasting equipment, it also affected Moscow's own radio stations. The tapping noise could also be heard over telephone circuits at times because of the strength of the signal.
Woodpecker sparked a few conspiracy theories
When in use, the Soviets had little regard for which frequency they would use at a particular time but it would range between 3 MHz and 30 MHz. It became such a nuisance that receivers began to include "Woodpecker Blankers" in their circuitry to attempt to filter out the interference.
The Soviet Union also never officially acknowledged the arrays. But it was obvious to any observers that the signals were coming from more than one source over Russia. NATO was very confident that the Russian Woodpecker was some kind of over-the-horizon radar system. They could not, however, be entirely sure what it was being used to scan for. Some conspiracy theorists also let their imagination run riot. Some thought it was being used to jam Western broadcasts or even interfere with submarine communications. Yet others began to think the arrays were actually used to interfere with the weather or even attempt mind control.
Closer analysis of the signals revealed a pseudo-random binary sequence that gave the radar a resolution of around 15km. When a second array was built in Eastern Russia it became obvious it was some form of detection system pointed towards the US. The system fell out of use in 1989 as the Cold War came to a close and the Soviet Union finally collapsed. Not only that but the technology had quickly become obsolete and was replaced by more accurate satellite-based warning systems.
You can still visit the existing The Russian Woodpecker arrays today but as it is located in within the Chernobyl Exclusion Zone, permits need to be obtained in advance.