Contribution from various types of losses to total damping in an ultradirectional emitter

A previously described superconducting quadrupole emitter which possesses a moderate degree of ultradirectivity is examined in order to analyze the different types of loss which effectively limit the use of superconductivity in the design of compact ultradirectional emitters. The emitter consists of a resonant section of a four-write transmission line shorted at one end and loaded by a 'shortening' capacitor at the other. The resonance length of the emitter was varied from about 0.25 to 0.08 wavelengths by changing the capacitance of the shortening capacitor. The antenna gain was found to vary continuously from 1.5 to 2.5 during transition form quarter-wave resonance to the 'strong shortening' regime. Equations for the total emitter Q-factor, Q-factor due to resistive losses, and quality factor determined by losses in the shortening capacitor are given. Calculated values for various Q's are shown and discussed. It is concluded that the major contribution to losses in an ultradirectional superconducting emitter may be caused by the rf-transparent cryostat.