Beams and Sidelobes

A fan beam formed by the MOST at meridian distance is an arc of a small circle making an angle () to due west. The beam has a width in MD of sec() and a width in the tilt coordinate of (FWHM). The principal sidelobes of the MOST are grating lobes arising from the periodic bay structure. They are also arcs of small circles spaced at equal intervals of sin() i.e. cosec(). The grating lobes are largely suppressed as they lie near the nulls of the bay responses. The MOST is designed to receive right-hand circular (IEEE) polarisation. However, it has some sensitivity to left-hand circular polarisation, particularly for monochromatic interference for which there is no delay decorrelation. If the MOST is set to meridian distance , the corresponding left-hand polarisation beam is at meridian distance , and it too has an associated set of grating lobes.

For distant interfering terrestrial transmitters, the MOST is likely to have the greatest response at azimuths where the small circles defining the beams and gratings intersect the horizon. However, for local transmitters the MOST will be out of focus to some extent, and the gain of the beams and gratings will be correspondingly reduced. For example, for sources at a range of  cos(azimuth)km the curvature of the incoming wavefront reduces the gain of the fan beams by 10dB. For transmitters much closer than this, recognisable fan beams are not formed. The sidelobe response is then quasi-random, relatively small, and at any given azimuth, varies rapidly with the MD setting of the telescope.

During a synthesis observation, this complicated sidelobe response structure sweeps across any fixed interfering transmitter, producing a signal in the fan beams which fluctuates at a rate dependent on the rate of change of MD. These fluctuations will be superimposed on those due to the transmitter modulation and propagation effects.

© Copyright Astronomical Society of Australia 1997