Warsaw radio mast

The Warsaw radio mast was the world's tallest structure until its collapse on 8 August 1991. It is the second tallest land-based structure ever built, being surpassed as tallest by the Burj Khalifa, completed in 2010. The mast, which was designed by Jan Polak, was 646.38 metres (2,120.67 ft) tall. Its construction, started in July 1970, was completed on 18 May 1974, and its transmitter entered regular service on 22 July of that year. It was located in Konstantynów, Gąbin, Poland, and was used by Warsaw Radio-Television (Centrum Radiowo-Telewizyjne) for longwave radio broadcasting on a frequency of 227 kHz before 1 February 1988 and 225 kHz afterwards. Its base, according to Google Earth, was 115.2 metres above sea level. Because a voltage potential of 120 kV was desired between the mast and ground, it stood on a 2-metre-high insulator. It operated as a mast radiator (half wave radiator), so its height was chosen in order to function as a half-wavelength antenna at its broadcasting frequency. The signals from its 2 megawatt transmitters could be received across all of Europe, North Africa and as far away as North America. Its weight is debated: Polish sources claim 420 tonnes.