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Questions about devices which collect and couple electromagnetic signals (communications, radar or other electromagnetic measurements) directly between free space, and conductors (e.g. waveguides or other transmission lines).

Antennas come in a very wide range of forms, shapes and sizes, and generally couple electromagnetic radiation in space wtih electrical conductors such as wires, cables, or waveguides passively, so a photodetector or a photoemitter would not be an antenna because they require conversion between valence and conduction electrons in a semiconductor by absorption or emission whereas in a dish antenna or dipole for example, the electromagnetic waves couple directly with electrons in the conductors.

So for example antennas can be used for communication of data, or for transmitting or receiving radar signals, or even blackbody emission of radio or microwaves for temperature measurements, or even to receive signals from pulsars for time/position determination,