A Dual-Frequency Metallic Waveguide System

Abstract

Conventional metallic waveguides have several major advantages, including low propagation losses and high power transmissions in the microwave frequency range. One disadvantage of metallic waveguides is that the propagation frequency band is limited at frequencies above the cutoff frequency fc. The usable frequency range is therefore restricted to fc < f < 2fc, because the TE20 mode can exist in rectangular metallic waveguides at frequencies higher than 2 fc. A ridge waveguide (or a double-ridge waveguide) (Cohn, 1947) has the advantage that it can extend the propagating frequency range by reducing the cutoff frequency for the TE10 mode. However, it has disadvantages in that it has a small and complex structure and a high attenuation constant. We investigated a new type of waveguide in which single-mode propagation is possible at frequencies higher than 2 fc using two arrays of dielectric rods with a dielectric constant of between 20 and 30 (Shibano et al., 2006 ; Kokubo et al., 2007a). This chapter introduces a system that uses a dual-frequency band waveguide. Firstly, we present the fundamental principles of this dual-frequency band waveguide in which a dual in-line dielectric array is installed. Since the electromagnetic reflection coefficient of a periodic array of dielectric rods is frequency dependent, at low frequencies electromagnetic waves may pass through the dual in-line dielectric rods located near the sidewalls, while at high frequencies electromagnetic waves may be reflected between them. Higher modes are suppressed in the waveguide when the space between two dielectric arrays is narrower than the space between the metal sidewalls. Secondly, if this type of waveguide contains a corner bend and a straight portion with even symmetry (including the power source), then the dielectric rods are only required in the corner bends of the waveguide, because odd-symmetry modes (such as TE20) are not excited (Kokubo et al., 2007b). As an example, a 90° bend waveguide is considered and its Sparameters are calculated. Another thick dielectric rod is located at the boundary between regions with and without dielectric rods, as reflection occurs there. This dielectric rod functions as a dielectric lens, concentrating electromagnetic waves at the center of the waveguide. Thirdly, we introduce a frequency multiplexer or demultiplexer that mixes or separates electromagnetic waves with wide band frequencies (Kokubo & Kawai, 2008). A coaxial waveguide converter is usually used for introducing electromagnetic waves into a waveguide. However, there is no converter that can match the impedance over such a wide 16