The Friis transmission equation is a fundamental formula in wireless communications that describes the power received by an antenna under free space conditions. It relates the transmitted power, gain of the transmitting and receiving antennas, the distance between them, and the wavelength of the transmitted signal.
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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 by the receiver's antenna effective area . A receiving impedance mismatch multiplies the available received power by . Wavelength scaling must state which gains, apertures and impedances are held fixed.