Measurement of loss coatings applied to helix support rods

Most manufacturers of helix TWTs deposit resistive material on the helix support rods to provide the necessary internal attenuators. It is common to evaluate these coatings by passing the rods through a reduced height rectangular waveguide with a short a quarter wavelength beyond the rod position and measuring the return loss as the rod moves through the waveguide. Although the measurement does not give the loss which would result when the rod is inserted into a helix TWT, it measures a quantity which is "proportional" to the loss. The "constant" of proportionality is a function of helix and barrel dimensions, TWT frequency, the measurement frequency, etc.. Nevertheless, this measurement is an effective tool for quality control. It is our intent in the future to relate these measurements to the loss produced on helices with the aid of simulation tools such as HFSS and MAFIA. This paper gives an analysis of this measurement.