Traveling-wave amplifiers and backward wave oscillators for VHF

The principal aspect of the design of TWT´s and BWO´s for frequencies below 500 Mc is the problem of obtaining a tube of reasonably small physical dimensions. Hollow beams of greater perveance than is obtainable with solid beams offer one method of increasing the gain per wavelength in such tubes and hence reducing the size. Some aspects of the design of minimum size forward wave amplifiers using single helix circuits and backward wave oscillators using bifilar helix circuits, both in combination with hollow beams, will be discussed. Several tubes of these types for operation below 500 Mc have been built and performance data will be presented. Bandwidths and tuning ranges respectively, for TWT´s and BWO´s, in excess of 4 to 1 in frequency have been obtained experimentally. Efficiencies in excess of 20 per cent have been obtained.