If you look carefully at this picture, from the May 1960 issue of Electronics Illustrated, you’ll see at the top of the mast a Yagi antenna for 89.55 MHz. The ship is the Cheeta, and flies the Panamanian flag.

The ship is in international waters off the Danish coast, and is serving as the transmitter site for Radio Mercur, a pirate radio station. From studios in Copenhagen, programs were recorded in Danish, Swedish, and English, and brought to the ship anchored in international waters.

According to the magazine, the sale of FM receivers boomed in the ship’s listening area of about 50 miles, with an ERP of about 1500 watts. The Danish government had been unable to silence the station, but this changed a few years later with legislation forbidding anyone in Denmark from cooperating with the broadcasts.