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ISSN 0010 9525, Cosmic Research, 2014, Vol. 52, No. 5, pp. 403­409. © Pleiades Publishing, Ltd., 2014. Original Russian Text © M.S. Turygin, 2014, published in Kosmicheskie Issledovaniya, 2014, Vol. 52, No. 5, pp. 440­446.

Antenna Feed Unit for the RadioAstron Project
M. S. Turygin
Special Design Bureau, Institute of Radio Engineering and Electronics, Russian Academy of Sciences, Fryazino, Moscow oblast, Russia e mail: tur@sdbireras.ru
Received December 16, 2013

Abstract--The design and parameters of the antenna feed unit in the ranges of 6, 18, and 92 cm are described. The unit was designed and manufactured for the RadioAstron space telescope with a diameter of 10 m. The parameters and test results are presented. DOI: 10.1134/S0010952514050104

An antenna feed unit (AFU) is intended for oper ation in the composition of the space radio telescope and single dish antenna of the RadioAstron Spektr R spacecraft. AFU receives noise signals with the con tinuum spectrum, reflected and focused by a parabo loidal reflector, and separates them with respect to two circular polarizations. The design had to meet the following require ments: the absence of unscreened insulators, high mechanical strength and vibrational stability, com pactness, 120­350 K range of working temperatures, 155­200 K regular working temperature, simulta neous reception of two orthogonal circular polariza tions, combined reception in several frequency bands, and the coincidence of beam pattern (BP) maximums in all frequency bands. Unscreened insulators should be absent because high sensitivity low noise amplifiers (LNAs) in four wavelength ranges should be at the AFU output. The 1.35 cm range in the composition of AFU is not con
Table 1 Working frequency bands: 92 cm 18 cm 6 cm BP width (­10 dB) Level of the first and second minor lobes Field polarization

sidered in this paper. Corona discharge, originating in insulators in outer space, is a powerful source of noise and can damage LNA. Mechanical strength and compactness require ments are related to the space equipment specificity. LNA and the feed are located on a radiationally cooled platform at a temperature of 155­200 K. The require ments imposed on the technical parameters are pre sented in Table 1. It is most difficult to provide for the mechanical strength without unscreened insulators. As a rule, these constructions are as a rule of a waveguide type; however, their size and mass would be unacceptable for space equipment, since working frequencies are rela tively low. Since AFUs should operate at a regular tempera ture of 155 K, additional restrictions are imposed on the construction and design procedure. Since it is obviously impossible to tune a feed at such a tempera

324 ± 8 MHz 1664 ± 60 MHz 4832 ± 60 MHz 120° ­18 dB Elliptic (simultaneous reception for two orthogonal polarizations) 0.9 Ellipticity Efficiency 98% BP axial symmetry <0.1 dB VSWR value at LNA input <1.2 Decoupling between polarization channels of one frequency 23 dB band Decoupling between frequency bands 15 dB 403


404
0° P=3 =4

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90° =5 180° =6 270°

P=1

P=2

Fig. 1. Scheme of a circular polarization divider.

Fig. 2. Receiving unit of a 6 cm feed without the upper screen. COSMIC RESEARCH Vol. 52 No. 5 2014


ANTENNA FEED UNIT FOR THE RADIOASTRON PROJECT

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Fig. 4. Circular polarization divider in the 6 cm range; thickness 0.5 mm; scan.

Fig. 3. Section of the receiving unit of a 6 cm feed.

ture, the construction should have a stable tempera ture of parameters and no tuning elements, or the parameter drift should be exactly predictable. It is most difficult to provide isolation of polariza tion channels. A rather compact and broadband divider of circular polarizations could be designed based on lumped elements (the so called "quadrifi lars"); however, these devices have high losses (about 2 dB), which is unacceptable for low noise receivers. The feed in each of the considered ranges can be conditionally divided into two parts: a receiving unit and a divider of circular polarizations. Since BP maxi mums should coincide, the coaxial location of receiv ing units is the most suitable variant. The quarter wavelength section of a circular coaxial waveguide was selected as a receiving unit in the 6 and 18 cm ranges, since this section provides practically necessary BP and is consistent with space in a rather broad fre quency band.
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The branch bridge, the scheme of which is pre sented in Fig. 1, is used as a polarization divider. All elements on the scheme are quarter wavelength trans mission lines of corresponding impedances. The transmission coefficient phases at ports 3­6 are given relative to port 1 and are contra lateral with respect to port 2. The transmission coefficient power is 1/4 between port 1 and ports 3­6; the same power is typi cal of port 2 (owing to symmetry). We have corre sponding circular polarizations (left and right hand) at ports 1 and 2 when such a divider is connected to a ring type radiator (including a coaxial waveguide). The main problem in designing a receiving unit consists in the creation of a coaxial waveguide rigid fastening without using insulators, which would not affect emission parameters. Four metal insulators, which are shorted quarter wavelength screened stubs, are used as such fastening. During optimization, these stubs were transformed into the construction shown in Fig. 2. Metal insulator fastening points are simultaneously the points where a polarization divider is connected. An internal opening is the 1.35 cm feed input. The 6 cm feed receiving unit is completely shown in Fig. 3. It is necessary to screen metal insulators in order to neutralize their effect on feed BP. An outer cylinder is used to decrease minor lobes and is simultaneously the inner part of the 18 cm feed. The 18 cm feed receiv ing unit is designed similarly. The receiving unit in the 92 cm range is a normal ring radiator, since it can be rather rigid without metal insulators due to the long wavelength. A circular polarization divider (Fig. 1) is based on symmetric airstrip lines. For 6 and 18 cm feeds, the distance between screens was selected so that strips would be wide and thick enough to maintain the nec essary rigidity, parametric stability, and low loss. In a real polarization divider, the wavelengths in Fig. 1 gen erally differ from the 1/4 wavelength due to the com pensation of inhomogeneities. The following 6 cm


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Fig. 5. Polarization divider in the 6 cm range; the outer screen is removed.

Fig. 6. Assembled AFU.

VSWR, rel. units 1.20 1.16

Decoupling, dB ­15

­20 1.12 1.10 1.08 ­25 1.04

1.00 4750

4800

4900 4850 Frequency, MHz

4950

­30 4750

4800

4900 4850 Frequency, MHz

4950

Fig. 7. 4832 MHz feed VSWR in the 6 cm range.

Fig. 8. Decoupling of a 4832 MHz feed in the 6 cm range.

polarization divider shape was obtained during the optimization of the bandwidth and voltage standing wave ratio (VSWR) (Fig. 4). Strip structures are placed between two cylindrical shields (Fig. 5) and are fas tened at nodes using small dielectric loadings. Since loadings are rather far from outer space, as well as screened on almost all sides and small, the probability of corona discharge origination on these loadings is vanishing. For the 92 cm polarization divider, a similar con struction cannot be implemented due to space restric

tions; therefore, for this range, a polarization divider was designed as a three layer screened plane. The construction of the assembled AFU is shown in Fig. 6. Since the feed receiving unit and polarization dividers dock in the plane of single mode transmission lines, it is reasonable to model these units indepen dently and to calculate the common parameters (VSWR and decoupling between polarization chan nels) by joining corresponding S matrices. The receiv ing unit was modeled using the CST Microwave Studio program and the method of finite integration in the
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ANTENNA FEED UNIT FOR THE RADIOASTRON PROJECT Table 2 Working frequency bands: 92 cm 18 cm 6 cm BP width (­10 dB) Level of the first and second minor lobes Ellipticity AFU efficiency in frequency bands, MHz: 324 ± 8 1664 ± 60 4832 ± 60 from 18 000 to 26 000 BP axial symmetry VSWR at AFU output at frequencies, MHz: 324 ± 8 1664 ± 60 4832 ± 60 from 18 000 to 26 000 inclusive Decoupling between polarization channels of one frequency band in a low voltage switching system, MHz: 324 ± 8 1664 ± 60 4832 ± 60 from 18 000 to 26 000 Decoupling between frequency channels 14 dB 22 dB 22 dB 22 dB 22 dB 1.6 1.3 1.3 1.4 0.78 0.8 0.8 0.8 <1 dB 324 ± 8 MHz

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1664 ± 60 MHz 4832 ± 60 MHz 120° 18 dB 0.9

time domain. The same program was used to calculate BP. The receiving unit was optimized with respect to the emission efficiency and BP width. The current surface density was one of the intermediate calculation stages. The feeds in the 6 and 18 cm ranges were modeled above an infinite perfectly conducting plane. The 92 cm feed was modeled in completely open space and the effect of a container with equipment was taken into account.
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The polarization divider was modeled and opti mized using the AWR Microwave Office program and the method of joining matrices of symmetric strip lines and inhomogeneities. The method of finite integration in the time domain in the CST Microwave Studio pro gram was used to model inhomogeneities and clamp ing elements. The polarization divider was optimized with respect to VSWR and decoupling between polar ization channels.


408 VSWR, rel. units 1.20 1.16

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­20 1.12 1.10 1.08 ­25 1.04

1.00 1550

1600

1700 1650 Frequency, MHz

1750

­30 1550

1600

1700 1650 Frequency, MHz

1750

Fig. 9. 1664 MHz feed VSWR in the 18 cm range.

Fig. 10. Decoupling of a 1664 MHz feed in the 18 cm range.

VSWR, rel. units 1.20 1.16 1.12 1.10 1.08

Decoupling, dB 0

­5

­10 1.04

1.00 305

310

315

325 330 320 Frequency, MHz

335

340

­15 305

310

315

325 320 330 Frequency, MHz

335

340

Fig. 11. 324 MHz feed VSWR in the 92 cm range.

Fig. 12. Decoupling of a 324 MHz feed in the 92 cm range.

Table 2 illustrates the measurements of the AFU parameters. Figures 7­12 present measured VSWR and decou pling between AFU polarization channels for the 6 , 18 , and 92 cm ranges at temperatures of 293 K (a solid curve) and 158 K (a dotted curve). Figures 13­15 show the calculated and normalized AFU BPs for the ranges of 6, 18, and 92 cm, respec

tively. BPs are presented for the axial sections, and the BP maximum is considered to be at 0 dB. Thus, the AFU technological sample was designed based on the performed calculations. The sample underwent mechanical, thermal, and radiotechnical tests. The second designed AFU sample was installed on the flight model of the space radio telescope.
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ANTENNA FEED UNIT FOR THE RADIOASTRON PROJECT BP, dB 0 ­2 ­4 ­6 ­8 ­10 ­12 ­14 ­16 ­80 ­60 ­40 ­20 0 20 Angle of view, deg 40 60 80 BP, dB 0 ­2 ­4 ­6 ­8 ­10 ­12 ­14 ­16 ­80 ­60 ­40 ­20 0 20 Angle of view, deg 40 60

409

80

Fig. 13. 6 cm feed BP.

Fig. 14. 18 cm feed BP.

BP, dB 0 ­2 ­4 ­6 ­8 ­10 ­12 ­14 ­16 ­80 ­60 ­40 20 ­20 0 Angle of view, deg 40 60 80

Fig. 15. 92 cm feed BP.

ACKNOWLEDGMENTS The RadioAstron project was supported by the Astrospace Center, Lebedev Physical Institute, Rus sian Academy of Sciences, and by the Lavochkin Association in the scope of the contract between the

Russian Space Agency and many Russian and foreign scientific and technical organizations. Translated by Yu. Safronov

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