ABSTRACT
The earliest example of a small and short-range radar device is the proximity fuse (Fig. 8.1). This fuse is mounted on top of an artillery shell and contains a small CW Doppler radar made up of four tubes operating at approximately 100 MHz. First tube is a free-running 100 MHz oscillator, second and third are amplification stages, and the fourth is a thyratron (basically a high current switch with a threshold). When the shell is fired from a gun a glass capsule breaks open, spilling acid onto the contacts of a lead acid battery. Soon thereafter the tube filaments warm up and the oscillator turns on in free space while the shell is traveling through the air, well outside of the gun. The scattered Doppler signal is coupled off of the free-running oscillator. This Doppler is low-pass-filtered, amplified, and fed to a thyratron tube that actuates when a sufficiently high level of scattered Doppler return is detected. The thyratron blows the smaller explosive within the shell that causes the larger one to explode, thus damaging or destroying whatever was within close proximity to the shell [1]. These devices were developed for air defense so that the shell would explode at just the right time near an aircraft.
