For extremely narrowband bandpass and bandstop filtering applications, high Q dielectrically loaded waveguide filters can be used. Dielectrically loaded waveguides have much higher Q's than combline structures and are much smaller than conventional waveguides. With standard combline technology, a filter of the same size will have a passband insertion loss of approximately 3 times that of a dielectrically loaded waveguide filter. Unloaded Q's of 25,000 or more are achievable in the 800 MHz band. This technique is particularly useful in cellular and PCS systems where highly selective preselectors are required to prevent interference from closely spaced service providers. The resulting low passband insertion loss also helps to minimize the noise figure in receive systems.
In transmit systems, the high Q's achieved reduce the filter's dissipative loss and can decrease the amplifier output requirement. Freedom Microwave has researched the application of dielectrically loaded waveguide filters for high power transmit systems and has developed a design techniques which provide for high power and temperature stable performance.
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