Complex sentence: It boasted the most powerful privately owned transmitter in the world (1,300 kW broadcasting on medium wave) in the late 1930s, and again in the 1950s and 1960s, it captured very large audiences in Britain and Ireland with its programmes of popular entertainment.
Split into simpler sentences: It boasted the most powerful privately owned transmitter in the world (1,300 kW broadcasting on medium wave). In the late 1930s, and again in the 1950s and 1960s, it captured very large audiences in Britain and Ireland with its programmes of popular entertainment.