Microwaves -- The New Horizons of Radio Astronomy

The development of radio astronomy is closely linked with progress in microwaves. Some of the first applications of new microwave devices were for radio observations of the universe. Radio astronomy which started at metre wavelengths has first moved to centimetre waves, is presently operating at millimetres and is now moving into the submillimetre range. As a result of this vast frequency span many different microwave techniques are used. Antenna construction (surface accuracy) had to keep pace with the desired frequency range. At the shortest wavelengths the development of carbon-fibre technology helped to make efficient antennas. At the highest frequencies (and at the lowest) space projects are under consideration to get away from atmospheric (ionospheric) propagation problems. The receivers in all frequency ranges must have the most sensitive state of art detectors. In the millimetre range Schottky-Barrier (S-B) mixer receivers are only slowly being replaced by the SIS (Superconductor-Insulator-Superconductor) detector. In centimetre and long millimetre waves the availability of FET and HEMT amplifiers has led to a number of multichannel projects. At submillimetre wavelengths, the last unexplored spectral range, new receiver developments are pushing the sensitivity further all the time. The use of microwaves has given us a new understanding of the universe. The opening up of new spectral ranges as a result of the development of microwaves gave us new horizons in radio astronomy.