Synthesis and Design of Novel Dual-Mode Microwave Filters
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This thesis describes the synthesis and design of two novel dual-mode microwave waveguide filter structures. The mode degeneracy is fundamental to this design technique. The first design introduces a structurally symmetric dual-mode cavity pseudo-elliptic band-pass filter, which has been implemented by only asymmetric structures so far. It is also used to shed light on the concepts of positive and negative coupling in dual-mode filters. A novel iris coupling structure is investigated allowing negative coupling coefficients required for certain polarizations of modes in dual- mode rectangular cavities. A 4th order dual-mode band-pass filter is designed at 11 GHz with a bandwidth of 200 MHz, an in-band return loss of 20dB and two transmission zeros at normalized frequencies of ±1.6. The filter makes use of the negative coupling property of the iris for inter-resonance coupling of the modes of horizontal orientation inside the cavity. The desired performance of the filter was achieved by the initial design without any optimization. The second design is especially convenient when temperature compensation is of prime concern. A direct design technique is developed and demonstrated with computer simulation and measurement in circular waveguide technology on a 4th order filter with a bandwidth of 50 MHz at 11 GHz, an in-band return loss of 20 dB and two normalized transmission zeros at ±1.5.