Abstract: A transmitter station has a data source, a crypto unit, a buffer store for encrypted signals and a transmitter with an aerial, all with a common power supply. A switching control unit has a position in which it connects data source and crypto unit to the power supply but isolates transmitter therefrom, and another switch position in which transmitter is connected to the power supply while data source and crypto unit are isolated therefrom. In this way, if any signals output from data source pass directly (i.e. not via crypto unit) to the input of transmitter, they will not be sent on into the network because transmitter is "off" (i.e., de-energized) at that time.