The maximum reflection at an open end of a standard rectangular waveguide is about −10 dB in its operating frequency range. It is often used without matching. For critical applications, it is desirable to further reduce the reflection coefficient. In this paper, a new technique is presented for the broadband impedance matching of an open-ended rectangular waveguide. The proposed technique employs three thin capacitive matching elements placed at proper intervals via a low-loss dielectric material. The capacitance of, and distance between, the matching elements are optimized for broadband impedance matching using a simulation tool. Based on the proposed technique, two design examples are presented for the matching of a WR75 waveguide radiator. A reflection coefficient of less than −16 dB and −20 dB has been achieved over a ratio bandwidth of 2.13:1 and 1.62:1, respectively.

This Communication presents a computational design approach for a broadband in-line coaxial-to-rectangular waveguide transition having a 2.83:1 bandwidth. Existing designs have a bandwidth ranging from 1.36:1 to 2:1. The proposed transition consists of a four-step ridge transformer and three sections of the rectangular waveguide. The proposed design approach extends the bandwidth of the transition by a theory-based design of the width and height of the three-section waveguides and by a computer-aided optimization of the stepped-ridge transformer. The dimensions of the three waveguide sections are determined such that the transition operates from very close to the TE₁₀-mode cutoff to slightly less than the TE₃₀-mode cutoff. A four-step ridge transformer was computer-optimized for a low reflection coefficient over a broad bandwidth. The proposed design approach was applied to the design of a coaxial-to-WR75 transition whose reflection coefficient is less than −22.6 dB at 8.14–23.00 GHz (2.83:1 bandwidth). The dimensions of the proposed transition were obtained using an electromagnetic simulation tool. The increased bandwidth of the transition can be very useful, especially in measurement applications.