Scattering parameters for a matched antenna.
Antennas are reciprocal two port devices.
Port one is connected to the transmitter or receiver, and port two is connected to free space.
If an antenna is a lossless reciprocal two-port network, then s12 = s21, and s11 = s22.
If the antenna is matched to the transmitter source, there is no reflected wave back into the transmitter, so s11 must be zero.
It follows that s22 must be zero also, and therefore there is no reflected power from the antenna/receiver combination when external power is incident on the antenna. The wave incident on the receiver impedance is completely absorbed, as this impedance is
If the antenna is mismatched, there will be some reflected power :
s11 = s22 = 0.
Note that the antenna can only be matched over a limited bandwidth.
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