Superconducting transmission lines of controllable properties

It is well known that the measured properties of superconductors are sensitive to magnetic field, whether generated by self-currents or externally applied. The authors suggest that it may be possible to take advantage of this sensitivity, by applying it to develop microwave components whose characteristics can be varied by an externally applied control field. This field could be produced by a current-carrying conductor lying near the component whose properties are to be controlled. They envisage this technique as being applicable to structures containing superconductors in bulk or thick-film form, as well as to thin-film systems. It appears straightforward to switch high-T_c material between superconducting and normal states by applying a magnetic field from a coil or other conductor. Digital control of this form might be used for phase shifting, for switching delay elements or for signal routing, preferably with balanced circuitry to maintain impedance values. The development of continuously variable (analogue) control will make it possible to control the electrical length of delay lines and sections of transmission line, and the properties of other passive devices including filters and resonators