Radio interferometer
apparatus that is used to study heavenly bodies by receiving and analyzing electromagnetic radiation that these bodies emit or reflect in the radio wavelengths. A radio interferometer is a radio telescope (q.v.) consisting of two or more separate antennas receiving radio waves from the same source, joined to one receiver. The antennas may be placed close together or thousands of kilometres apart. The principle of a radio interferometer's operation is the same as for an optical interferometer (q.v.), but, because radio waves are much longer than light waves, the scale of the instrument is generally correspondingly greater. Parts of a radio wave reach the spaced antennas at different times. This time difference is compensated for by a variable-delay mechanism, and the waves can be made to interfere, much as in the optical interferometer. In another version, the spacing of the antennas can be changed in an attempt to make the waves interfere; the distance between them for interference depends on the wavelength and on the diameter of the source of the waves. The diameter can be calculated when the other quantities are known. If the diameter of the radio-wave source is not too small to be resolved by the interferometer, the radio signals will alternately reinforce and cancel each other in a manner analogous to the way fringes are produced in the optical interferometer.