Waveguide bandpass filters consist of half wavelength resonant cavities coupled by capacitive or inductive irises. These extremely high Q structures provide very low passband loss and steep attenuation skirts. Freedom Microwave designs and manufactures waveguide filters in the frequency range 2 to
20 GHz, with peak power level ratings as high as 50 kW. The TE10 mode, the dominant mode in rectangular waveguide, is utilized in most designs. Bandpass filters can be realized with percent bandwidths ranging between .01 to 20 percent. For very narrow band, low loss requirements, circular waveguide can be used. The dominant mode for circular waveguide is the TE11 mode. Additionally, circular waveguide is utilized for dual mode designs.
Depending upon the bandwidth, either inductive irises or inductive posts are used to control the coupling between each cavity. Pseudo elliptic designs can be used to reduce the size and insertion loss or to equalize the group delay. Waveguide multiplexers can be produced by connecting several waveguide filters to a common port. Waveguide bandstop filters can be achieved by placing several short circuited cavities at intervals of three quarters wavelength along the waveguide body.
By using several adjacent high and low impedance sections, a corrugated waveguide lowpass filter can be designed to have a wide passband and a wide bandstop for power in the TE10 mode. Suppression of higher order modes can be achieved by using a waffle iron structure. Waffle iron lowpass filters can be designed to suppress up to the 5th harmonic and beyond.
Material selection for a waveguide filter depends upon the specific requirements. Copper is generally used, but for wide temperature ranges and narrowband applications invar is used to guarantee frequency stability. Aluminum can also be used for lightweight applications. All Freedom Microwave waveguide filters are silver plated to minimize the insertion loss. Waveguide filters are available with either waveguide flanges or coaxial connectors. Sealed versions are available for pressurized applications.
Return to Filter Topologies.