LC filters are used in a wide frequency band. With increasing frequency the parasitic effects of the reactive components become more pronounced and the quality demands continue to rise. Hence, in the GHz frequency range transmission line filters become an increasingly attractive alternative to classical LC filters. Due to reduced wavelength at higher frequencies transmission line filters offer compact physical dimensions.
From transmission line theory (see Section 3.1.8) we know that open-ended and short-ended loss-less transmission lines show reactive behaviour and can therefore be used to mimic reactive elements. An open-ended transmission line that is shorter than a quarter of a wavelength ( < λ/4) has a capacitive input impedance. The same line with a length in the range of λ/2 > > λ/4 is inductive. For 3λ/4 > > λ/2 the line is capacitive again, and so forth. So, a transmission line shows periodic behaviour.