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Early radio broadcasting relied on —low-powerhigh-pricedoverweightfast-moving transmitters that could reach only a few kilometres, so stations were built close to the audiences they served. As valve technology improved, engineers were able to —extendextend toexpand intowiden up signal range considerably, allowing a single transmitter to cover an entire region. This shift —enabledobligedresembledwithheld the rapid growth of national broadcasting networks during the 1920s.