Radio astronomy observations are usually performed using large, high-gain antennas or antenna arrays to provide the required angular resolution on the sky. Such antennas or arrays are commonly referred to as radio telescopes. With a typical beamwidth ranging from a few seconds of arc to a few degrees, the probability of a source of interference falling within the main beam generally need not be considered. Thus the concern is with interference entering through the sidelobes. A recommended model for the sidelobes of large parabolic antennas is given in Recommendation ITU-R SA.509, which is based on empirical data from a number of large antennas. It applies to antennas of diameter greater than 100 wavelengths, for frequencies between 2 GHz and 30 GHz, and for angle, α, measured from the axis of the main beam, greater than 1o. The model for the envelope of the gain (G) of the sidelobes is then given by
G = (32 – 25 log α ) dBi, 1o < α < 47.8o
G = -10 dBi, 47.8o < α < 180o (1)
Sidelobe levels of 0 dBi, i.e. gain equal to that of an isotropic radiator, occur at 19o from the main beam. In radio astronomy it is generally appropriate to calculate possible interference effects based on the 0 dBi sidelobe level (an exception is the case of geostationary satellites). It would be an unacceptable restriction on the area of sky available for astronomical observations if a source of interference precluded observations in an area of sky greater than 19o in radius. Also, at observatories in the northern hemisphere it is often necessary to observe at low angles of elevation to study sources near the center of our Galaxy, i.e. the beam must be close to the horizon, which is the direction from which much interference is incident.
The sidelobe model described above applies to symmetrical paraboloids which suffer from scattering of radiation by the focal support structure. Sidelobe levels for offset-feed reflectors with unblocked apertures are typically 10 to 15 dB lower than those given by the model. However, only a few antennas of the offset-feed type that are large enough for radio astronomy usage have been developed (from: “ITU-R Handbook on Radio Astronomy“, 1995, section 4.2.1).