High-Speed Frequency-Shift Keying of LF and VLF Radio Circuits

Present VLF transmitters are limited in transmission speed to approximately 20 words per minute because of the bandwidth limitations of the High-Q antenna circuit. To permit an increase in transmission speed, a unique method has been developed which makes use of the variation of reactance of the antenna circuit in synchronism with frequency shift modulation of the carrier. The method makes possible the transmission of teletype or high speed Morse signals at speeds of 100 words per minute or more. This increase by a significant factor in present traffic handling capabilities of VLF stations can be accomplished by increasing the cost of the station less than ten per cent.