Efficiency of tapered backward-wave oscillators

The effect of a tapered circuit phase velocity on the efficiency of an O-type backward-wave-oscillator (BWO) is investigated. Circuit tapers based on a hard-kernel-bunch approximation are derived. It is shown that when such tapers are applied to realistic BWO's higher efficiencies result. An experimental
S
-band BWO with a variable pitch helix designed on the basis of the hard-kernel-bunch theory was constructed. An efficiency improvement of 20 per cent to 80 per cent over a frequency range of 2 to 3.75 Gc has been obtained.