A new modulator design for the BMEWS transmitter

The Ballistic Missile Early Warning System (BMEWS) is a family of radars that was designed and built in the late 1950s and early 1960s. Originally, there were three BMEWS sites: Thule, Greenland; Clear, Alaska; and Fylingsdale, England. Currently, there is only one remaining site, at Clear, Alaska, that still uses eighteen of the original design transmitters. Each transmitter uses a pair of modulating-anode klystrons which provide a combined peak output of 2.5 MW at 425 MHz with 2 ms pulses at 27 pps. Very few modifications have been implemented on the transmitters in the thirty years since the site became operational. Many of the parts required to service the equipment are now difficult to obtain and cost prohibitive. Southwest Research Institute (SwRI) has redesigned the pulse modulator for the transmitter final-stage power amplifier. The redesigned pulse modulator operates at -120 kV, uses a floating deck configuration, and is a form-fit-and-function replacement for the original unit. The design approach included: (1) minimizing the size of the floating chassis to reduce self-capacitance for reduced switch-tube power dissipation; (2) keeping support circuitry simple for fewer failures and ease of maintenance; and (3) using conventional parts to make the electronics supportable for many years to come.