Editing Transmission line (section)

===Stub filters===
{{see also|Distributed-element filter#Stub band-pass filters}}
If a short-circuited or open-circuited transmission line is wired in parallel with a line used to transfer signals from point A to point B, then it will function as a filter. The method for making stubs is similar to the method for using Lecher lines for crude frequency measurement, but it is 'working backwards'. One method recommended in the [[Radio Society of Great Britain|RSGB]]'s radiocommunication handbook is to take an open-circuited length of transmission line wired in parallel with the [[feed line|feeder]] delivering signals from an aerial. By cutting the free end of the transmission line, a minimum in the strength of the signal observed at a receiver can be found. At this stage the stub filter will reject this frequency and the odd harmonics, but if the free end of the stub is shorted then the stub will become a filter rejecting the even harmonics.

Wideband filters can be achieved using multiple stubs.  However, this is a somewhat dated technique.  Much more compact filters can be made with other methods such as parallel-line resonators.