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Instruments and Methods

129

Multi-element MMIC focal array for radio telescope
V.B. Khaikin1, E.K. Ma jorova1 , Yu.N. Parijskij1, M.D. Parnes2 , R.G. Shifman2 , V.A. Dobrov2 , V.A. Volkov2, V.D. Korolkov2, S.D. Uman2 , N.A. Esepkina3 , and S.K. Kruglov3
The Special Astrophysical Observatory of Russian Academy of Sciences Karachai-Cherkessia, N.Arkhyz, 357147, Russia, E-mail: vkh@brown.nord.nw.ru Svetlana", Rezonance", Ascor", 194156, Engels pr. 27, St.Petersburg, Russia St.Petersburg State Technical University, 195220, Polytechnicheskaya 21, Russia
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Focal receiver arrays seem to be an unavoidable solution for the existing and the next generation re ector radio telescopes where high sensitive or high speed mapping is the main goal Parijskij et al., 1993. The signi cant progress in MMIC array technologies in MM band Weinreb, 1998 gives us a chance to realize an important RATAN-600 advantage: the wide aberrationless focal zone Khaikin et al., 1999. A multi-element feed array placed at the focal plane may signi cantly increase sensitivity and the eld of view of any radio telescope.

Figure 1: Terraced" 7 8 element MMIC array architecture For the array substrate we use Rogers Corp. ceramic lled composite materials with 0.0013 loss tangent and 3.02 dielectric constant. Microstrip radiators of each level are fed by microstrip lines in the plane of radiating sheet. In the rst MMIC array prototype at 26 30 GHz radiators receive the signal of Y polarisation, X Y linear or circular polarisation will be available with the next prototypes as well. Mutual radiator coupling is provided at ,30 dB level, VSWR 1.35 in the range 26.5 30.5 GHz. Rather wide bandwidth for the microstrip radiator is reached by an air cavity under the dielectric substrate Khaikin et al., 1999.


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Figure 2: 4 element MMIC sub-array prototype at 26-30 GHz In the rst 8-element front-end prototype we used MMIC ampli ers of Litton SSD NF=2.5 dB which give us direct RF ampli cation 60 dB in receiving channels in the "total power" mode. With UMS MMIC ampli ers NF=1.8 dB we expect System temperature about 200 K for the array directed to Zenith. Input-output and mutual channel coupling is provided at a low enough level with the help of a cut-o waveguide covering eachchannel up to the detector Fig.1. The microstrip bandpass lters put before detectors limit channel bandwidth to 4 GHz in agreement with the input radiator bandwidth. A communal input channel calibration is produced through a special loaded 50 Ohm microstrip line connected with a loaded LMA-422 used as a noise oscillator in the same frequency range. Mutual coupling of the microstrip line with radiators is about ,40 dB. Super low noise HP Schottky square low detectors complete VHF parts of array. Low noise high precision AD FET monolithic operational ampli ers are applied in the wideband multi-channel back-end. Four-element sub-array prototype is shown in Fig.2.


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Figure 3: 3 8 element array prototype at 26-30 GHza, beam pattern measurements in HUT anechoic chamber b Beam patterns of a microstrip radiator in a 38-element array prototype at 26 30 GHz Fig.3a measured in HUT anechoic chamber are close to expected Fig.3b. The measured System temperature of the 8-element MMIC array prototype is 300 K. We expect 10 15 mK sensitivityper second in a channel in the "total power" receiving mode. To reduce 1 F noise and DG G contribution into sensitivitywe are testing now a modi ed radiometric "total power" scheme with a monochromatic "compensating" signal that can give us a factor 2 3 in sensitivity. Gann oscillator at 28 GHz with relative amplitude instability of 4 10,6 per second has been manufactured and tested for this aim. Calculations show that up to 70 78 element feed sub-arrays may be installed along the focal plane of the largest RATAN-600 secondary mirror so that a total number of RATAN-600 beams can exceed 3000 Khaikin et al., 1999. The prototype of Multi-Channel Data Acquisition System MCDAS for multi-element MMIC Array with the use of ADSP-2181 was developed, manufactured and tested in St.Petersburg State Technical University. A block diagram of MCDAS is shown in Fig.4. The described array technology can be used at RATAN-600 or other radio telescopes for di erent radio astronomy applications. It can give us new possibilities to study CMBA at sub-degree scales with high integrated sensitivity in a wide eld of view Parijskij et al.,


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Figure 4: Block-diagram of Multi-channel Data Acquisition System 1997 . The search of Synaev-Zeldovich e ect at RATAN-600 Parijskij et al., 1997 is among other possible applications. Mapping of the Sun and studying of the fast-variable solar events will be available. We plan to use it for the moment holography of radio telescope surface as well.
This work was partially supported by INTAS 97-1192.

References
Yu. Parijskij. RATAN-600 Word's Biggest Re ector at the Cross Road. IEEE AP Magazine, v.35, N.4, pp.7-12, 1993 Yu. Parijskij, G. Pinchuk, E. Ma jorova, D. Shannikov. Multi-beam Operational Mode at RATAN-600 Radio Telescope. IEEE AP Magazine, v.35, N.5, pp.18-27, 1993 S. Weinreb. Noise Temperature Estimations for a Next Generation Very Large Microwave Array. Square-Kilometer Array Workshop, Green Bank, WV, October, 1998 V.B. Khaikin, E.K. Ma jorova, R.G. Shifman, M.D. Parnes, V.A. Dobrov, V.A. Volkov, V.D. Korolkov and S.D. Uman. 78 element MMIC Array at 26 30 GHz for Radio Astronomy Applications. Proceedings of International Conference "Perspective on Radio Astronomy: Technologies for Large Antenna Arrays", Dwingeloo, The Netherlands, April 1999, in press Yu. Parijskij et al. "Dark Ages" of the Universe. NATO ASI series, vol.511, pp.443-446, 1998