Microstrip matched filters for transmit/receive MIC antenna applications

Entirely planar microstrip bandpass filters are described for MIC (microwave integrated circuit) antenna applications. The technique uses almost 0-dB bandpass-type couplers. These filters have matched characteristics in the entire frequency range (i.e., both attenuation and passband). Passband flatness and sharpness are directly controlled by the filter length. Therefore, a long filter may be required to satisfy stringent performance requirements. At millimeter wavelengths (>30 GHz), the size of the filter is much smaller and it can easily be accommodated in the same transmit/receive (T/R) module with the patch antenna. For microwave applications, if the size of the filter constitutes a major problem, the filter can be constructed separately on a high dielectric constant substrate. A prototype filter was designed on alumina substrate with dielectric constant epsilon /sub r/=9.9 and thickness h=0.635 mm. The computed performance is shown.<>