Theoretical and experimental TWT helix loss determination

As efficiency and power requirements for broad-band traveling-wave tubes (TWT) increase, the necessity to reduce losses becomes more and more important. It is particularly important to reduce circuit losses because, when efficient multistage depressed collectors are used, each 2-percent increase in circuit efficiency yields a 1-percent increase in overall efficiency! Attempts to predict losses in helix structures have produced answers that were a factor of at least three below the measured loss. Historically, loss measurements have been very difficult to make, primarily, because of the small values of the quantities being measured. This paper presents an accurate theoretical analysis of the loss of a helix structure. The effect on loss of the dielectric support rods for the helix is taken into account. In addition, helix losses are separated from barrel losses. Functional variations are given of loss with frequency, helix pitch, helix radius, barrel-to-helix diameter ratio, helix and barrel resistivity, and the effective relative dielectric constant of the helix support rods. Measurements of loss, made with a Hewlett-Packard automatic network analyzer and a new automated measurement procedure, are presented. It is shown that excellent agreement between the predicted loss and the measured loss is obtained when helix surface roughness effects on resistivity are taken into account. Also, a correction is made for the increase in measured loss resulting from impedance mismatches between the helix and the measurement circuit and from loss in the sliding short.