The Feldtkeller Equation

Given that passive filters work on the principle of frequency dependent mismatch between a source (generator) and a load we should be able to find a link between the mismatch properties of a filter (a passive network) and its transmission properties. If you understand scattering parameters (S-parameters) then it is the link between |S11| and |S21|. This link is often refered to as ‘Feldtkeller’ Equation’.

delim{|}S21{|} = 1-delim{|}rho{|}^2 = 1-delim{|}S11{|}^2