An S-band variable attenuator for high-power working

The description is given of a waveguide insertion attenuator in which the loss in r.f. power is produced by the action of a column of water flowing in a glass tube mounted in the waveguide, continuous variation of attenuation being obtained by adjustment of the water column height. Resonance effects in the water column are exploited by choosing the cross-sectional dimensions of the glass such that an optimum attenuation per unit length of tube is obtained. Experimental results are given for a model covering the range 0 to 30 db, the input v.s.w.r. at ¿a = 10·00 cm being ¿0·87 at any setting and ¿0.97 over the greater part of the range. Satisfactory operation has been achieved in an evacuated test-system fed by a magnetron delivering 2 MW peak power (200 W mean) at a wavelength of 10 cm. Various modifications in design are suggested for use with higher mean powers.