page_title: Waveguide filter
section_title: Waveguide stub filter
hierarchical_section_title: Waveguide filter / Waveguide stub filter
caption_attribution_description: Waveguide stub filter
context_page_description: 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, while others are rejected. 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.
  Waveguide filters were developed during World War II to meet the needs of radar and electronic countermeasures, but afterwards soon found civilian applications such as use in microwave links. Much of post-war development was concerned with reducing the bulk and weight of these filters, first by using new analysis techniques that led to elimination of unnecessary components, then by innovations such as dual-mode cavities and novel materials such as ceramic resonators.
context_section_description: A stub is a short length of waveguide connected to some point in the filter at one end and short-circuited at the other end.  Open-circuited stubs are also theoretically possible, but an implementation in waveguide is not practical because electromagnetic energy would be launched out of the open end of the stub, resulting in high losses.  Stubs are a kind of resonator, and the lumped element equivalent is an LC resonant circuit.  However, over a narrow band, stubs can be viewed as an impedance transformer.  The short-circuit is transformed into either an inductance or a capacitance depending on the stub length.
  A waveguide stub filter is made by placing one or more stubs along the length of a waveguide, usually λg/4 apart, as shown in figure 12.  The ends of the stubs are blanked off to short-circuit them.  When the short-circuited stubs are λg/4 long the filter will be a band-stop filter and the stubs will have a lumped-element approximate equivalent circuit of  parallel resonant circuits connected in series with the line.  When the stubs are λg/2 long, the filter will be a band-pass filter.  In this case the lumped-element equivalent is series LC resonant circuits in series with the line.
caption: Figure 12.  Waveguide stub filter consisting of three stub resonators
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