Performance data of radio telescopes with a multielement focal arrays are investigated. The problem of optimization of the array design, the elements of which are horizontally oriented strip radiators, are discussed with reference to RATAN-600 and a paraboloid. A computation of beam patterns of these telescopes operated in the mode of multibeam reception with the use of different modifications of focal arrays is performed. A "terrace-like" design of focal array for illumination of non-symmetric secondary mirror of RATAN-600 and symmetric mirrors (paraboloids) is proposed.

The multibeam operation of a radiotelescope can be implemented by placing a matrix of primary feeds in its focal plane. This kind of multielement receiving array at the focus of a reflector radio telescope widens essentially its field of view, improves the integral sensitivity, speeds up the imaging of extended sources, reduces the influence of the atmosphere.       A matrix of primary feeds is placed on the focal line of the secondary mirror of RATAN-600. The secondary mirror is a non-symmetric parabolic cylinder with a horizontal generant, on the focal line of which (X axis in Fig.1(b)) primary feed cabins are located, which enables simultaneous observations at several wavelengths. This design of the secondary mirror is convenient for the multibeam mode of operation of the radio telescope with the one-dimensional, two-dimensional and three-dimensional arrays.

FIg.1.  The section of the secondary mirror in the vertical (a) and horizontal (b) planes. 1 -- the parabolic mirror, 2 -- the primary feed.

The number of elements in the array is determined by the size of aberretion- free zone of the radio telescope, which, under the condition of two-mirror focusing system, depends on the elevation H of the sourse being observed. Fig.2 shows an computed aberration curves of RATAN-600. By the aberration curves we mean the relationships between the maximum of the telescope BP, Fmax, and the removal of the primary feed from the focus. Examples of a computed and experimental curves give in section "Experimental investigations of the beam pattern RATAN-600".

Fig.2.  The aberration curves at the wavelength 10 mm with the removal of the primary feed along the focal line of the secondary mirror (X axis) on the elevation H=0o, 30o, 60o, 90o. The regime of the two-mirror focusing system with the full aperture.

The one-dimensional array placed on the focal line of the secondary mirror is the most efficient when observing sources with elevation close to 90o, since under this condition the aberration-free zone of RATAN-600 is maximum. With decreasing elevation of the source, the aberration-free zone narrows rapidly. As to the spurious polarization, it is minimum at altitude close to zero and grows monotonically with increasing elevation of the sourse. For this reason, in polarization measurement the three-mirror focusing system of RATAN-600 with the periscopic flat mirror ("South+Flat" mode), in which the circular mirror is set vertically to the altitude H=0o, appears to be more preferable. The number of strip radiators in the one-dimensional focal array at the wavelength 10 mm in observing near-zenith sources may thus be 160 elements, whereas for effective reception of radiation from low elevation souoces their number should not be larger than 10.       To assess the possibility of using two- and three-dimensional arrays in focal region of RATAN-600, calcucations of the vertical BP were carried out with arbitrary removal of the primary feed from the focus F on the plane A (Fig.1(а)). Curves 1--9 in Fig.3 show the vertical BP at the wave of 10 mm with a removal of the primary feed from the focus by quantity 5 mm and locating it at points 1, 2, 3, 4, 5, 6, 7, 8 , 9 (Fig.3(b)) for H=0o, 10o, 30o, 50o и 85o. The beam patterns of the primary feeds and the real sizes of the focusing system of RATAN-600 were specified in the calculation. In each of the locations the maximum of the primary feed BP makes an angle =50o with the horizon.

Fig.4.   The schematic view of the "terrace" arrangement of the primary radiators in the focal region of the secondary mirror of RATAN-600 (vertical section) (from the left). 1 - the parabolic mirror, 2 - the fences of the primary radiators. The examples of "terrace" design of the multielement receiving array with microstrip radiators (from the right).

Fig.5.   The vertical BP of radio telescope RATAN-600 under the condition "South +Flat" at the wave of 10 mm with location in its focal region of the receiving array of "terrace" (a) and flat (b) design.

A "terrace" design of the thee-dimensional array consisting of linear strip arrays (LSA) of radiators is considered. These fences are parallel to the focal line of the sacondary mirror and shifted with respect to each other in the vertical plane by a value h, the distance between the radiators in the fences is set by the parameter l (Fig.4). Such a design is not only more simple in realization, but also turns out to be more optimum in energy characteristics. It is well seen from Fig.5 that with the use of the "terrace" design the drop in the maxima of the BP of individual beams is less than with the flat design. With the same separation of the elements and their number (N = 7) the drop in signal in the marginal elements of the array amounts to about 40% in the case of the flat array, while for the "terrace" array it is no more than 15%. Manual influence of the elements in the "terrace" design is less essentially comparing to the flat array [1].

Fig.6. The isophotes of the two-dimensional multibeam BP of the RATAN-600 with location in it focal region of array of "terrace-like" design at the wave 1 cm. The BP of the outermost and central elements of focal array are shown. A total number of beams in this mode can amound to 7x10 ( 7 - in the vertical and 10 - in the horizontal planes).

Рис.7.  The diagram of the "terrace" design of the array in the focal range of a paraboloid (circular alternative) in the radial section (a), in the frontal section (b). 1 - the paraboloid, 2 - the primary radiators.

A focal arrays of "terrace" design can be employed in paraboloids. Fig.7 displays examples of "terrace" design of the array, which take account of axial symmetry of the paraboloids (circular variant). The location of the radiators is shown in one of the radial sections of the paraboloid - in the plane that passes throught the radius of the circle, which is the aperture of the paraboloid and its axis of symmetry (Fig.7a), and in the frontal section - in the plane perpendicular to the paraboloid axis (Fig.7b). In this design the radiators are arranged in rings shifted with respect to each other in depth by the quantity h. Apart from the axially symmetric array construction, rectangular design shown in Fig.9 are possible. In the plane (X0Y) the LSA of the radiators are shifted by h, and location of the radiators in this plane is the same as in Fig.7a.

Fig.8.  The BP of a parabolic radio telescope in the radial planes at the 10 mm wave with a "terrace" array (circular alternative) placed in its focal region. N is the number of elements in the radial section.

The BP of the parabolic antenna in the focal plane of which strip radiators are placed, which form a "terrace" array are displayed in Fig.8 (circular alternative) and in Fig.10 (restangular alternative). N is the number of array elements in each of the radial section for circular alternative or in each of the LSA for restangular alternative. The beam patterns were calculated for optimum values of   h and l. Location of array relative to the paraboloid focus also were optimized.

Fig.9.  The schematic view of the "terrace" array (rectangular alternative) in the focal region of a paraboloid. 1 - the paraboloid, 2 - the primary radiators.

Fig.10.  The BP of a paraboloid at the 10 mm wavelength in the Y0Z with the "terrace" array (rectangular alternative) placed in its focal region.

The BP of a paraboloid in the plane X0Y (rectangular alternative of an array design) coincide with the BP of the axially symmetric array in its radial section, the shape of the multibeam BP in the plane Y0Z are displayed in Fig.10. The "terrace" design of the array is thus quite suitable for paraboloids and, in same cases, has even certain advantages over the flat array. With the number of elements 3х3 and 4х4 the "terrace" arrangement of the elements makes it possible to get a multibeam BP with the same level of signal in all beams, that is imposible to get with the flat array.