A waveguide filter is an electronic filter constructed with waveguide technology. Waveguides are hollow metal conduits inside which an electromagnetic wave may be transmitted. Filters are devices used to allow signals at some frequencies to pass (the passband), while others are rejected (the stopband). Filters are a basic component of electronic engineering designs and have numerous applications. These include selection of signals and limitation of noise. Waveguide filters are most useful in the microwave band of frequencies, where they are a convenient size and have low loss. Examples of microwave filter use are found in satellite communications, telephone networks, and television broadcasting.
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Waveguides are a low-loss medium. Losses in waveguides mostly come from ohmic dissipation caused by currents induced in the waveguide walls. Rectangular waveguide has lower loss than circular waveguide and is usually the preferred format, but the TE01 circular mode is very low loss and has applications in long-distance communications. Losses can be reduced by polishing the internal surfaces of the waveguide walls. In some applications which require rigorous filtering, the walls are plated with a thin layer of gold or silver to improve surface conductivity. An example of such requirements is satellite applications which require low loss, high selectivity, and linear group delay from their filters.[18]
The initial non-military application of waveguide filters was in the microwave links used by telecommunications companies to provide the backbone of their networks. These links were also used by other industries with large, fixed networks, notably television broadcasters. Such applications were part of large capital investment programs. They are now also used in satellite communications systems.[34]
The need for frequency-independent delay in satellite applications led to more research into the waveguide incarnation of cross-coupled filters. Previously, satellite communications systems used a separate component for delay equalisation. The additional degrees of freedom obtained from cross-coupled filters held out the possibility of designing a flat delay into a filter without compromising other performance parameters. A component that simultaneously functioned as both filter and equaliser would save valuable weight and space. The needs of satellite communication also drove research into the more exotic resonator modes in the 1970s. Of particular prominence in this respect is the work of E. L. Griffin and F. A. Young, who investigated better modes for the 12-14 GHz band when this began to be used for satellites in the mid-1970s.[35]
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