Report at International Conference:"Perspectives on Radio Astronomy:Technologies for Large Antenna Arrays", Dwingeloo, the Netherlands, 12-14 April, 1999
 
7x8 ELEMENT MMIC ARRAY AT 26-30 GHZ FOR RADIO ASTRONOMY APPLICATIONS
 
V.B.Khaikin, E.K.Majorova, Yu.N.Parijskij
The Special Astrophysical Observatory of
Russian Academy of Sciences
Karachai-Cherkessia, N.Arkhyz, 357147, Russia
E-mail: vkh@ratan.sao.ru
M.D.Parnes, R.G.Shifman, V.A.Dobrov, V.A.Volkov, V.D.Korolkov and S.D.Uman
"Svetlana", "Rezonance", "Ascor", 194156, Engels pr. 27, St.Petersburg, 194156, Russia.
 
 

Focal receiver arrays seem to be an unavoidable solution for the existing and the next generation reflector radio telescopes where high sensitive (or high speed)  mapping is the main goal (Parijskij et all, 1993).

The significant progress in MMIC array technologies in MM band (Weinreb, 1998) gives us a chance to fully realize an important RATAN-600 radio telescope advantage - the wide aberrationless focal zone(Khaikin at al.,1998). A multi-element  feed array if placed along the focal plane may significantly increase RATAN-600 sensitivity and the field of view.
 
The "terraced" three  dimensional construction of 7x8 element MMIC focal array in 26-30 GHz range is shown in Fig.1.
 

 
c)
Fig.1."Terraced" 7x8 element MMIC array arhitecture (a,b), MMIC amplifier LMA422 of Litton SSD(c).
 

For the array substrate we used Rogers Corp. ceramic filled composite materials with 0.0013 loss tangent  and  3.02 dielectric constant. Microstrip radiators of each level are fed by microstrip lines lieing in the plane of radiating sheet.  In the  first array prototype radiators receive the signal of Y polarization, X/Y linear or circular polarization may be available with the next prototype as well. Microstrip front-ends with "warm" MMIC LMA-422 (Fig 1, c) of Litton SSD (NF=2.5 dB) give us direct RF amplification in receiving channels in the "total power" mode.  Mutual radiator coupling is provided at -30 dB level(Fig.2), 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 (Fig.2 b).
 

 
(a)
 
(b)
 
VSWR of the microstrip radiator for one and three dimensional arrays as function of  frequency is presented at Fig.3.
 
 
Fig.3.VSWR of microstrip radiator for one (1) and three (2) dimensional arrays.
 
Input-output and mutual channel coupling is provided at a low enough level with the help of a cut-off waveguide covering each channel up to the detector (Fig.1). The microstrip bandpass filters 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 (fig.4) connected with a loaded LMA-422  which is used as a noise oscillator in the same frequency range.  Mutual coupling of the microstrip line with radiators is provided at -40 dB level.

 

Fig.4.The loaded microstrip line for communal channel calibration
 

Super low noise HP Schottky square low detectors complete VHF parts of array. Low noise high precision AD FET monolithic operational amplifiers are applied in the wideband multi-channel back-end. 4 element sub-array prototype is shown in Fig.5.
 
 

               Fig.5. 4 element MMIC sub-array prototype at 26-30 GHz.
 
Beam patterns of  a  microstrip radiator in the one dimensional 8 element array (Fig. 6) give us good enough agreement with theory.
 
   Fig.6. Power beam pattern of  a single microstrip radiator (5.9x3.4 mm)  in 8 element array  in H-plane

Beam patterns of a microstrip radiator in a 3x8-element  array prototype at 26-30 GHz(Fig.8 a) measured in HUT anechoic chamber  are close to expected(Fig.7.). Fig.8 b. shows the measurement  process.

Fig 7. Power beam patterns of radiating element of 3x8 element array measured in HUT anechoic chamber.
 
 
 
              (a)
 

The measured system temperature of the 8-element MMIC sub-array prototype  is 300 K. We expect 10-15 mK sensitivity per second in a channel in the "total power" receiving mode. To reduce 1/F noise and deltaG/G contribution into sensitivity we are testing now a modified 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 4x10-6 per second has been  manufactured and tested for this aim.

Calculations show (Fig.10,11) that up to 70  7x8 element feed sub-arrays may be installed along the focal plane of  the largest RATAN-600 secondary mirror N5 (Fig.9 a) so that a total  number of RATAN-600 beams can exceed 3000 (Fig 11 c).
 

 (a)
(b)
Really, in some observation modes an aberrationless zone along the focal plane  may exceed 3 m (Fig.10.) for the largest RATAN-600 secondary mirror (12 m size).
 
 
Fig. 10. RATAN-600 power aberration curves along the focal plane
C-Zenith mode (H=90 deg.) with 400 meter aperture. 
B-Zenith mode (H=89 deg.) with 400 meter aperture. 
D-Radio-Schmidt mode with 100 meter aperture (H=45 deg). 
 
Cross direction of the focal plane is also available for the tight packed feed array. Vertical RATAN-600 beam patterns for  a 7x8 element flat focal array are shown in Fig.5 a.  A tight packed "terraced" array  gives us even a less (8-16%) fall of power beam efficiency than a flat array(0-24%) (Fig.5 a,b). Optimal two- dimensional  array architecture for RATAN-600 focal plane  is presented in Fig.11 c.
 
(a)
 
(b)
(c)
  In the Radio-Schmidt mode RATAN-600  can track cosmic sources with an unmovable main mirror and a  focal array during one hour. The RATAN-600 beam patterns in  Radio-Schmidt mode for different elevation angles and  azimuths
of cosmic source    are shown in Fig.12. For a shortened aperture we can use this mode up to 10 mm wavelength.
 
The described array technology can be used at RATAN-600 for different radio astronomy applications. It can give us new possibilities to study CMBA  at sub-degree scales with  high integrated sensitivity in a wide field of view (Parijskij et al, 1997 ). The search of Synaev-Zeldovich effect at  RATAN-600 (Parijskij et al., 1997) is among other possible applications. Using focal arrays we can study quick-variable cosmic objects like pulsars or Sun as well.

 This work was partially supported by INTAS 97-1192.