beam

In radio astronomy, the term beam refers to the portion of the celestial sphere that the radio telescope is receiving signal from. It is not a simple disk shape: it includes the results of diffraction, the same mechanism that produces Airy disks in optical telescope images. The term beam pattern indicates the pattern in the sky from which signal is being collected, generally a characteristic of the antenna, reflecting dish, and receiver. The strongest contributer is in the direction that the antenna or reflecting dish faces, which is termed the primary beam or main lobe. Other contributors are concentric circles (if the antenna is round), termed side lobes. The beam width is the (primary) beam's diameter, and the smaller the diameter, the better angular resolution of the resulting data. An antenna with a narrow effective beam is termed a high gain antenna (HGA), and conversely, an antenna with a very wide beam (e.g., omnidirectional) is termed a low gain antenna (LGA). The signal received within the beam is not "all or nothing", but typically strongest at the center of the beam, and attenuated more outward from that point. One means of quantifying the beam width is treating the circle where the signal is half-attenuated as the edge of the beam (essentially using the full width at half maximum).

The term beamforming is used regarding a kind of processing of the signal from arrays of receivers: summing signals using a particular delay from each receiver (to their signals in the same phase) so as to emphasize the signal from a particular direction.

The term beam is used differently in discussions of radiative transfer: it indicates a (directed) line along which EMR is passing, used in discussions of the effects of particles that the EMR passes close to on all the EMR passing along that line. This is useful in describing the physics of a star's outward transfer of energy and the EMR it emits.