The dial is packed with signals and half of them do not exist: they are images and subharmonics made by the receiver itself.
To find out whether a signal is real, switch the oscillator: a real station does not move on the dial; a ghost jumps.
Somewhere on the short-wave band a voice reads out groups of figures and then falls silent. The station receiver is old and manufactures signals that do not exist: images of other stations and subharmonics that sound just as clear. The real voice has to be found before the transmission ends.
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