This paper presents two new structures of four-port microwave directional filters incorporating microstrip-line resonators, uniplanar 180/spl deg/ phase-reversal units, and two or four identical p-i-n diodes. These filters provide substantially improved frequency characteristics compared to the conventional constructions. The unique feature of them is that they ensure high-level isolation independent of frequency between opposite ports. Due to this electrical property the proposed filters may be suitable for some pulse-radar applications.