Both Rebecca II and Eureka II were developed by Murphy Radio, with early pre-production of Rebecca II by Dynatron Radio. A system using a selection of tuned capacitors was used to select the operational frequency from a set of four. Looking for a controller, Murphy selected a General Post Office 5-position electromechanical system used in their telephone exchange systems. A similar selection of four channels was available in the Eureka units, but these were selected manually. Rebecca was powered off the aircraft mains, whilst Eureka was battery powered with a lifetime of about six hours.