An Evaluation of the "Back-to-Back" Method of Measuring Adaptor Efficiency

The problem of adaptor evaluation is of considerable interest in the UHF and microwave art where, because of the several different types of transmission line in common use, there is a frequent need to extend an established measurement capability in one type of line to other types of lines, e.g., from rectangular waveguide to coax. In power and noise calibrations, it is the adaptor efficiency which is usually of interest, and one indication of the adaptor losses may be obtained by connecting an identical pair "back to back" and measuring the insertion loss of the combination by the usual techniques. If, then, the losses are assumed to divide equally between the two, the efficiency is thereby determined. As a practical matter, the losses do not, in general, divide equally, which leads to an error whose evaluation is the subject of this paper. In particular it is shown that the method is capable of good accuracy provided that the assumed identity of the two adaptors is satisfied.