Re-entrant transmission-line filter using printed conductors

The re-entrant line filter section first described by Alford in 1939 is employed. The theory of these filters is expanded, and a novel procedure is described for the design of microwave low-pass filters possessing a relatively high stop-band insertion loss over bandwidths of 3 : 1 or more. The filter may conveniently be realized in strip-transmission-line form utilizing printed-circuit techniques. In this form it appears to possess certain advantages over strip-line filters of more conventional design, notably the large operating bandwidth, and the accuracy with which the cut-off frequency can be computed. A variety of these filters have been constructed both in microstrip and triplate line, the latter form being more suitable for packaged units. A very high degree of electrical reproducibility comparable with the best waveguide practice has been observed, with the advantage of reduced manufacturing costs.