A frequency-domain directivity enhancement of beam position stripline detectors

Stripline detectors are commonly used in beam position monitoring (BPM) systems and can be used as dual directional couplers for beams passing in opposite directions. Directivity between 21-26 dB is typical near the Tevatron operating frequency of 53.1 MHz, and these values are limited by mismatches on the upstream ports. Improvements to the directivity can be made with external corrections to these mismatches, but in the Tevatron the four ports (two striplines, proton upstream and downstream) are available only after a series of discontinuities (tapers, feedthroughs, cables) forming unknown frequency-dependent networks. A method has been developed to characterize the networks and compensate the mismatches, at a single frequency. No beam is needed, so this simplifies implementation. The technique can be applied to a narrowband BPM system operating in an approximately steady-state condition. Conditions for directivity, development of the method, and a large-scale implementation procedure are described with applicable results, including an explanation of shortcomings in a transient beam situation