Antennas operate as transducers interfacing the energy propagating in a waveguide or cable system to the radiant or Maxwellian energy in free space. The roles and properties of transmitting and receiving antennas are the dual of each other in respect of power gain, directivity, equivalent noise temperature and efficiency, which can be expressed as follows:

If a transmitter, matched to antenna 1, excites a given power into the input of a receiver, matched to antenna 2, then the same power will be excited into the receiver, if the transmitter and receiver are interchanged, provided that the matching conditions are maintained.