Low cost bandpass filters (less than $100) at microwave frequencies cannot be purchased commercially. However, such filters are essential in the design of RF circuits in communications and radar equipment. Reliable microstrip band pass filters which provide an accurate filter response at microwave frequencies can be easily fabricated with low cost. Equations concerning the design of coupled microstrips and microstrip filters are published in the literature and were implemented in a design procedure f
or maximally flat microstrip band pass filters.
The published equations were theoretical and had not been extensively compared with experimental data. Thus, this work established an enhanced microstrip filter design procedure based on experimental data, for a wide range of frequencies and dielectric substrates.
The result of this work is an enhanced design procedure for microstrip band pass filters. The new procedure includes a correction factor for the length of the filter resonators which which controls the center frequency of the filter. This correction factor has been found from the measured responses of over 60 filters, which were designed with two different circuit board materials, three different substrate thicknesses, and frequencies ranging between 0.9 and 6 GHz. The experimentally determined length correction factor decreases the error in center frequency from ±5.9% down to ±L7% of the desired design frequency for a wide range of filter designs. The improved procedure has been implemented in a personal computer (PC) program which calculates all dimensions necessary to fabricate microstrip band pass filters in the low microwave frequency range. The maximally flat response obtained is accurate and requires very little tuning. Low cost microstrip band pass filters can now be designed and fabricated easily and with greater accuracy at microwave frequencies. This thesis describes the development of the enhanced design procedure and the results of the filters designed with the new procedure.
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