Two backward wave local oscillators

The present paper describes the development of two low-power backward wave oscillator tubes covering the frequency range 7.6-12.4 kmc/sec, and 11.6-18.0 kmc/sec. The initial design of the tube geometry is based on a Pierce type analysis for the starting current and frequency, which includes the effects of space charge and circuit loss, as well as the effect of the dielectric loading of the support structure. After successful completion of the X-band tube, scaling was employed to arrive at the proper dimensions of the high frequency tube. These tubes employ unifilar helix circuits, hollow beams, modified Pierce type electron guns using impregnated cathodes. A sectioned air-cooled electromagnet package consuming approximately 200 watts provides for the necessary confining magnetic field, while permitting the use of waveguide coupling throughout. The total weight of the magnet and tube assembly is 7 pounds. A helix-to-waveguide match with a maximum VSWR of 2:1 was developed resulting in very smooth tuning and power output curves. Minimum power outputs in the X- and Ku-band tubes are approximately 50 and 25 milliwatts, respectively, and power variations in the frequency range are about 4:1. Ten such tubes in each range were built and the results are discussed in detail.