Antennas couple propagating electromagnetic radiation to electrical currents in wires,electromagnetic fields in waveguides or aperture fields. This coupling is a phenomenoncommon to both transmitting and receiving antennas. For some transmitting antennas,source currents in a wire produce electromagnetic waves that carrying power in alldirections. Sometimes an antenna provides a means for a guided electromagnetic waveon a transmission line to transition to free-space waves such as a waveguide feeding adish antennas. For receiving antennas, electromagnetic fields can induce currents inwires to generate signals to be then amplified and passed on to a detector.
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