Design Analysis of a TM-Mode Piston Attenuator

The design analysis of a transverse magnetic mode piston attenuator which may be utilized as a microwave absolute standard in the comparison method of measuring attenuation is described. The system employs a coaxial section input and a coaxial section output separated by a cylindrical waveguide section operating below cutoff. The electromagnetic coupling between the two coaxial sections is comprised of exponentially decaying field components of the TMommodes associated with the waveguide section. A method of satisfying input and output boundary conditions based upon circular symmetry is detailed and applied in approximate form to obtain numerically the attenuation, input impedance, and VSWR characteristics of a model designed for a wavelength of 10 centimeters. The predicted data apply to coupling separations of 0.5 cm and higher.