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into one national service from London. The reasons given included the need to prevent enemy aircraft from using differentiated output from the Regional Programme's transmitters as navigational beacons. To this end, the former regional transmitters were synchronised in chains on (initially) two frequencies, 668 (South) and 767 kHz (North), with an additional chain of low-powered transmitters (known as "Group H") on 1474 kHz appearing later. Under this arrangement,

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