Contributions to the Theory, Design and Optimization of Microwave Bandpass Filters

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Bekheit, Maged
Microwave Filters , Dual-Mode Filters , Dual-Band Filters , Space Mapping Optimization , Representation Theory , Eigen-Modes , Synthesis , Coupling Matrix , Maxwell's Equations , Boundary Conditions , Resonators , Resonant Modes
Bandpass microwave filters are often modeled as a set of coupled discrete and localized resonators. This model is adequate in the narrow-band case. It, however, fails to describe accurately compact structures where stray couplings can be strong. To address this problem, a new view is proposed in this thesis. Instead of basing the model on localized discrete resonances, we start by constructing a model that is based on the global resonances of the structure. These are the resonances that the ports see and emerge when the entire structure is treated as a single unit. The resulting circuit, the transversal circuit, is universal. It is valid for any coupled resonator filter. The circuit is used in optimization of compact and ultra wideband suspended stripline filters and excellent results were obtained. In order to relate the global-eigen modes model to the conventional model, the issue of representation of microwave filters is investigated in detail. It is shown that a microwave filter can be represented by an infinite number of similar coupling matrices by using different modes as basis. According to this new view, a similarity transformation in microwave coupled resonator filters is interpreted as a change of basis. Two circuits that are related by a similarity transformation represent the same filter structure by using different sets of modes as basis. These conclusions were exploited in establishing a design theory for filters with dual-mode cavities. The new theory leads to direct and accurate design techniques that need no, or minimal, optimization. No tuning is used in the CAD steps. Tuning may only be required to account for manufacturing tolerances. A new tuning configuration is described and validated by computer simulation. A novel dual-mode filter with improved quality factor and reduced sensitivity is developed and designed within the same approach. The filter is fabricated and measured and excellent results are achieved. The same design methodology was used to introduce a new class of dual-mode dual-band microwave filters with improved sensitivity. It is also shown that canonical dual-mode filters can be designed within the same view with minimal local optimization of the input cavity.
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