I. INTRODUCTION

Rectangular waveguide E-plane diplexers are indispensable in many microwave and millimeterwave systems and subsystems used in radar and communication. With increasing activity in mil- limeter-wave personal communication systemsPCS., there has been a strong need for low cost E-plane diplexers. Considering the tradeoff between ease of mass production and performance, E-plane metal insert filters are the best. A good overview of the subject can be obtained in w1x. Unless the bandwidth of an E-plane filter is extremely narrow, it rarely needs postproduction fine tuning. During the 1970s E-plane filters were designed experimentally w2x. With the advent of faster and cheaper computers, today any waveguide component can be designed with a very

Correspondence to: Dr. P. Pramanick

high accuracy. During the 1980s, E-plane waveguide diplexers were designed using the brute force computer aided optimization method w3x based on repeated analysis. A large number of papers were subsequently published on computer aided waveguide diplexer and multiplexer designs between 1985 and 1994 w4]12x. This stream of papers shows a gradual reduction of computational effort in the design of such diplexers and multiplexers. A breakthrough paper by Morini and Rozzi w13x in 1995 showed how to exactly design a waveguide common junction diplexer without having recourse to a brute force optimization method. The method is based on the implementation of a common junction three-port having almost nearly equal reflection coefficients at all ports. The exact location of each filter is then determined from the scattering coefficient of the common junction and those of the constituent filters, at their respective center frequencies. The