As early as 1915 Marconi became concerned that long distance wireless was getting into a rut of longer waves and higher power and he directed research into much higher frequencies.
Greatly aided by Charles Franklin, he established some of the basic characteristics of short wave propagation. I should also say at his point that some credit must also be given to amateur experimenters whose tireless work in pushing the known boundaries of the science are often an inspiration to the professionals. Numerous tests were conducted between Poldhu and Marconi’s yacth “Elettra” leading to the conclusions that reliable day time shortwave communications could be achieved between the UK and South America.
In July 1924 the Marconi Company won a contract from the British Post Office (BPO) for the Short Wave Beam system to communicate with Canada, India, South Africa and Australia.
There were a number of challenging requirements to meet.
The technological architect of the Short Wave Beam System was Charles Franklin. He had responsibility for the development of a 20kW (input power) transmitter, the antenna system and a means of conveying the RF power from the transmitter to the antenna.
It can be seen here how transmitter design has progressed from the panel of valves in the previous picture to a more conventional layout, albeit with minimal safety constraints. The transmitter was called the SWB1 (Short Wave Beam 1) and became well known as a “Swab One”.