FEASIBILITY OF END-TO-END PHASE-SHIFT CORRECTION FROM THE OUTSIDE OF SEALED CESIUM BEAM TUBES

In the case of primary Cesium beam frequency standards realized in the laboratory the so-called "end-to-end" phase shift is usually taken into account on paper, and sometimes corrected by arm symmetrization, after it is carefully measured in beam reversal experiments, as part of the accuracy assessment procedure. It was suggested recently that hardware compensation might also be obtainable by introducing a suitable asymmetry in the symmetric port of the waveguide junction which feeds the resonator. In this way it may become feasible to correct end-to-end phase shift from the outside of commercially available tubes, which are sealed and therefore do not allow beam reversal measurements. The purpose of this paper is to show that such external compensation of end-to-end phase shift is possible for existing standards. Experimental results are reported for the effect of an asymmetry in the feeding port, which are obtained from the measured frequency difference of the standard upon reversal of the whole feeding structure. These results are compared to theoretical predictions based on electromagnetic calculations of the asymmetry introduced in the microwave structure by such an asymmetry in the feed. A discussion on how well the absolute size of the effect can be judged by observing the structure in reflection is also given. The relevance of this work toward accuracy statements on unidirectional beams is underlined.