RadioNet Engineering Forum Workshop: “New Trends in Receiver Developments”Medicina (Bologna), 30 May 2005

DESIGN OF A DUAL-FREQUENCY PRIMARY-FOCUS FEED SYSTEM FOR SRT

P. Bolli(1), S. Mariotti(1), R. Nesti(2), A. Orfei(1), G. Pelosi(3), C. Riminesi(4)

Istituto di RadioastronomiaIstituto Nazionale di Astrofisica

Via P. Gobetti, 101 - 40126 Bologna

Osservatorio Astrofisico di Arcetri Istituto Nazionale di Astrofisica

Largo E. Fermi, 5 - 50125 Firenze

Dipartimento di Elettronica e Telecomunicazioni Università di Firenze

Via C. Lombroso, 6/17 - 50134 Firenze

Istituto di Fisica Applicata "Nello Carrara" Consiglio Nazionale delle RicercheVia Panciatichi, 64 - 50127 Firenze

1.3 ÷ 1.8GHz / 310 ÷ 425MHz - simultaneousFrequency Band

≤ -35dBCross-polarizationIlluminatorsà uncooled ; LNA à cooledCooling

Best hoped for valuesSEFD≤ 0.2 ÷ 0.3dBInsertion loss≥ 26dBReturn loss

870 x 870 x 870 mmDimensionsPrimaryFocusTwo circular or linear polarizations for each frequencyPolarization

TECHNICAL GOALS

ABSTRACTIn this poster a study of a dual-frequency primary-focus feed system for the new Sardinia Radio Telescope is described. This feed will allow the simultaneous observation of two bands, the L-band and the P-band, for serving mainly Pulsar research and VLBI. For the L-band feed a circular waveguide will be adopted ensuring an optimal primary mirror illumination. For what regards the other band, which need to be allocated without interfere with the L-band feed, several configurations have been investigated. Among them two in particular seem to be very interesting: a coaxial horn and a symmetrical arrangement with wired antennas. For both configurations an optimization job has been done in order to find the best trade off between the aperture efficiency, reflection coefficient, cross-coupling all along the bandwidth. Advantages and drawbacks of both solutions are discussed.

L-BAND RECEIVER

WIRE SOLUTION

HORN SOLUTION

WHICH KIND OF P-BAND RECEIVER?

The L-Band receiver is needed mainly for VLBI, Pulsar Observations and Hydrogen Line Observation. It will be build following a consolidated topology based on cooled receivers. Both the LNA and the waveguide to coaxial transitions will be cooled. The cryogenic receiver will be fed by the received signal trough a 20 cm wide vacuum window. A circular waveguide aperture, fringed at least by one choke ring, makes the beam of this illuminator relatively insensitive to the closer P-Band illuminator regardless its morphology. Each polarization is collected by two opposite probes in order to rise the port isolation and to reduce high order modes excitation; of course they will be added along the receiver chain.We expect ourselves a System Noise Temperature around 13 K and an Antenna Efficiency around 55%.

Analysis performed by: (a) FEKO: MoM-based commercial software - http://www.feko.co.za; (b) HFSS: FEM-based commercial software - http://www.ansoft.com; (c) GRASP: PO/GO-based commercial software –http://www.ticra.com; (d) Other proprietary software packages developed at the Computational Electromagnetic Lab (University of Florence). Licenses: (a) and (b) at Computational Electromagnetic Lab, (c) at MECSA.

SCIENTIFIC GOALSSome observational activities, like Pulsar Survey, require an accurate knowledge of the ionosphere dispersion. To model this, a simultaneous observation at two different and far-between frequencies is required. Other scientific activities will require multi frequency observation in order to reduce observing time. Of course, two different, but coincident and collimated beams are required; this calls for a coaxial displacement of the illuminators. Due to the fact that the two illuminators are close and each one may disturb the other one, the study should take care for the efficiency maximization and for beam symmetrization. Usually, circular polarization may be requested; in this case a following hybrid combiner may be introduced.

COMPARISON

The horn-based approach P-Band illuminator consists on a circular-symmetric coaxial waveguide aperture. While the inner circular horn works in the L-Band, the outer coaxial one operates in the P-Band. Four dipole launchers give proper excitation of symmetrical field configurations thus reducing cross-coupling. Since the inner waveguide acts as a reactive load for the coaxial horn, great attention has been placed in order to have good input matching, low cross-polarization, high efficiency and wide band. In order to manage the two incoming linear polarizations, a power combiner is required for every couple of launchers. Few choke rings are placed around the illuminator in order to improve radiation features and to reduce surface currents and backscattering.

Edge taper (74°)Freq. [MHz]

-25.8dB-29.0dB425

-18.2dB-27.1dB365

-13.8dB-22.2dB305

HE

G=10.7dB

The P-Band illuminator based on dipole arrays consists in four dipoles placed as the sides of a square. The L-Band circular horn is surrounded by a ground metal sheet, acting as reflector for the P-Band. The entire system should be considered as a coaxial structure. In order to synthesize the appropriate pattern, each dipole (dipole length ≅ 0.4?cen) is coupled to an opposite one placed at 0.5?sup. Hence, four dipoles are needed to receive both linear polarizations. The four dipoles are very thick (L/D ≅15) in order to reach the largest bandwidth. More specifically, to reduce radial lobes the ground plane is placed at ?cen/8 (instead ?cen/4) and to cancel shadow the dipoles are outside the illumination region of the L-band receiver. The lightweight and simple dipole array has been used in Astron for LFFE for Westerborg Antenna and forseveral (73 and 49 cm) receivers of MPIfR – for Effelsberg Antenna.

It is well known that a cooled LNA gives noise temperature lower than an uncooled one. This assumption may be not completely true at metric wavelengths, where the noise temperature of uncooled LNA is very low too. A noise temperature budget study has been performed in order to choose the right topology. An uncooled LNA gives a noise temperature around 25K, whereas a cooled LNA (1K) preceded by stainless steel dewar cable (3K), preceded by 30 cm long coaxial cable (4K), gives a noise temperature around 8K. The noise temperature of both horn or thick dipoles is negligible. The cooled solution will be chosen. Wire

WIRE SOLUTION

RECEIVER FRONT-END

Horn

Cooled system Uncooled system

Edge taper (74°)Freq. [MHz]

-15.2dB-17.2dB420

-12.2dB-9.9dB350

-10.6dB-7.0dB305

HE

Radiating Unit :

Tsys = 29Kη = 62%

SEFD = 40 Jy

Tsys = 29Kη = 41%

SEFD = 61 Jy

@ 425MHz

Tsys = 46Kη = 60%

SEFD = 66 Jy

Tsys = 39Kη = 48%

SEFD = 70 Jy

@ 365MHz

Tsys = 60Kη = 68%

SEFD = 76 Jy

Tsys = 51Kη = 54%

SEFD = 81 Jy

@ 305MHz

Overall sensitivitiesWeight

-27 dB typical-10dB over 43MHz BW860 x 860 x 600Horn solution

-32 dB typical-10dB over 65MHz BWNegligible for

illuminators.Path to LNA:

0.14dB

800 x 800 x 605Wire solution

Cross-polarization

(feed+reflector)Return LossInsertion LossRFI

SensitivityCosts and

easy to buildDimension [mm]

L x W x H