Finite impedance section of a transmission line ($\mathcal{T}=[1+({\alpha }^2-1{)}^2{\sin }^2(\omega \ell /v)/(4{\alpha }^2){]}^{-1}$)

A length $\ell$ with impedance ratio $\alpha$ between identical matched lossless lines has multiple internal reflections. Summing their amplitudes gives the displayed power transmission when the propagation speed is the same in all sections. It is unity at half-wave resonances even though either individual interface reflects. Matched remote terminations remove external echoes but not this frequency dependence.