= Antenna load-mismatch echo
{title2=$P_{\rm echo}/P_i=|\Gamma_L|^2\mathcal F^2$}
In an ideal matched-port <two-port network> for an antenna pair, the incident amplitude traverses the link, reflects from the receiver's load with amplitude coefficient $\Gamma_L$, then traverses the reciprocal link again. If each one-way power transfer factor is $\mathcal F$, the returned power fraction is $|\Gamma_L|^2\mathcal F^2$. With fixed gains, $\mathcal F\propto\lambda^2$ and this echo scales as $\lambda^4$. Actual antenna structural scattering or an existing input mismatch can add a coherent return not specified by the <Friis transmission equation> alone.
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