Superconductive Microwave Meander Lines

Abstract

This paper describes the fabrication, testing, and application of superconductive meander lines. The meander line is a slow-wave circuit which consists of a number of parallel tapes uniformly spaced and connected in series. The tape length is equal to one-quarter wavelength for the center frequency of the line. Microwave signals propagate in the form of TEM waves along the tapes. Interaction between adjacent tapes leads to a dispersion curve which is in general nonlinear, but which possesses some nearly linear regions. Thus, either linear or dispersive performance may be obtained. Meander lines find application as slow-wave elements in low-noise amplifiers such as traveling wave masers, as linear delay lines, or as dispersive delay lines in pulse-compression filters. The present investigation deals with lead meander lines fabricated by the photoetch process. Reflection from and transmission through the meander line were measured by conventional techniques at room temperature, 77°K, and 4.2°K. A striking decrease in insertion loss was noted when the line became superconductive. Insertion loss and VSWR of the superconductive line were rapidly fluctuating functions of frequency, generally increasing and decreasing together. At numerous frequencies between 4.0 and 4.5 GHz, a 30 nsec delay line had insertion loss of 0.5 dB and VSWR less than 2. (Most of this insertion loss was probably due to nonsuperconductive input lines inside the cryostat.) For a comparison, a copper meander line, identical in geometry to the superconductive line was operated at 4.2°K, and was found to have insertion loss that was as much as 20 dB greater than that of the superconductive line. Meander delay lines should readily lend themselves to operation a closed cycle refrigerator.