= Friis transmission equation
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{title2=$P_r/P_t=G_tG_r[\lambda/(4\pi d)]^2$}
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The <Friis transmission equation> describes a far-field free-space antenna link with aligned directions, matched polarizations and negligible additional loss. It follows by multiplying the transmitter's axial <energy flux> $P_tG_t/(4\pi d^2)$ by the receiver's <antenna effective area> $\lambda^2G_r/(4\pi)$. A receiving impedance mismatch multiplies the available received power by $1-|\Gamma_L|^2$. Wavelength scaling must state which gains, apertures and impedances are held fixed.
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