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Purpose: Theoretical and experimental studies of the active antenna – an element of the low-frequency radio telescope antenna array for the future observatory on the farside of the Moon. Design/methodology/approach: To study the active antenna, consisting of a complex-shaped dipole and a low-noise amplifier, we used its mathematical model in the form of a two-port network, whose electrical parameters are set by the scattering matrix, the noise parameters being set by the covariance matrix of the spectral densities of noise waves. This model allows ma[1]king the correct analysis of the signal-to-noise ratio at the active antenna output with account for the external and internal noise sources. The modelling results were compared with those of experimental measurements of antenna characteristics. A series of radio astronomy observations were made with the developed antenna under the Earth environmental conditions. Findings: A numerical analysis of the radio telescope active antenna parameters has been made in a wide frequency range of 4–40 MHz. Two versions of the low-noise amplifier were developed to operate in the active antenna under the space and Earth environmental conditions. Under the Earth conditions, it has been experimentally proven that the range of problems, which such radio telescopes can effectively solve at low frequencies, is quite wide – from the solar research to the search for cosmological effects. Conclusions: The results of numerical simulations and experimental measurements obtained in this work have shown a satisfactory agreement between them for the most of the frequency range. The results of this work can be useful in the research and development of active antennas designed for operation at the decameter and hectometer wavelength ranges, particularly those intended for using under the space environmental conditions. Keywords: active antenna, Moon, radio astronomy observations, sensitivity

The new Giant Ukrainian Radio Telescope (GURT) intended for operation in 8–80 MHz range is now under construction. It employs an active phased array antenna composed of many dipole subarrays, several of which have been implemented and now are used for radio astronomy observations. In this two-part paper, the electrical and noise parameters of the subarray are studied by numerical and experimental ways. In this part, a theoretical model of the subarray based on the wave theory of noisy multiport networks that takes into account the mutual coupling in the dipole array placed over imperfect ground and the correlation between all existing noise sources is proposed. It is assumed that the initial parameters for this model should be obtained using the correct electromagnetic and circuit simulation. A technique for using of this model to calculate the effective area, radiation efficiency, internal and external noise temperatures, as well as sensitivity of the subarray in terms of system equivalent flux density (SEFD) for all possible beam directions in the frequency band of the GURT radio telescope is described.

This second part of two articles’ sequence presents the detailed numerical analysis of electrical and noise parameters of a subarray of the Giant Ukrainian Radio Telescope (GURT) designed for operation in the frequency range of 8–80 MHz. The theoretical model of the subarray and calculation technique of its parameters developed in the first part is used. Frequency behaviors of the absorption area, radiation efficiency, and system noise temperature budget of the subarray are studied. Special attention is paid to the subarray sensitivity given in terms of sky noise dominance (SND) and system-equivalent flux density (SEFD). The calculation results show that strong mutual coupling between subarray elements enables a significant increase in the subarray SND and enhancing overall its performance. The frequency dependences of the system noise temperature for 23 beams of the subarray, obtained by in situ measurements, are presented. A good agreement between the measured and calculated results well enough validates the subarray theoretical model.

We report about observations of the solar U+III bursts on 5 June of 2020 by means of a new active antenna designed to receive radiation in 4-70 MHz. This instrument can serve as a prototype of the ultra-long-wavelength radiotelescope for observations on the farside of the Moon. Our analysis of experimental data is based on simultaneous records obtained with the antenna arrays GURT and NDA in high frequency and time resolution, e-Callisto network as well as by using the space-based observatories STEREO and WIND. The results from this observational study confirm the model of Reid and Kontar (2017)., Comment: 10 pages, 6 figures, 2 tables

The interpretation of solar radio bursts observed by Parker Solar Probe (PSP) in the encounter phase plays a key role in understanding intrinsic properties of the emission mechanism in the solar corona. Lower time-frequency resolution of the PSP receiver can be overcome by simultaneous ground-based observations using more advanced antennas and receivers. In this paper we present such observations for which the active active region 12765, begetter of type III, J, and U solar bursts, was within sight of ground-based instruments and behind the solar limb of the PSP spacecraft. We used a subarray of the Giant Ukrainian Radio Telescope (GURT) to get the spectral properties of radio bursts at the frequency range of 8-80 MHz, as well as the PSP radio instruments with a bandwidth of 10.5 kHz - 19.2 MHz, during solar observations on June 5, 2020. We directly detected the radio events initiated by the active region behind the solar limb of the PSP spacecraft, using special conditions in the solar corona, due to the absence of active regions from the PSP side. Following the generation mechanism of solar radio emission, we refined the density model for the solar corona above the active region 12765 responsible for the radio bursts. Based on the PSP spacecraft position near the Sun and delays of radio waves between space- and ground-based records, we found the corresponding radio responses on the PSP spectrogram. The absence of sunspots from the PSP side contributes to the propagation of radio waves from a dense loop of the Sun to quiet regions with low densities, through which PSP instruments can detect the radiation., Comment: 8 pages, 6 figures, 2 tables, accepted to be published in Astronomy and Astrophysics 2021

Purpose: Theoretical and experimental investigations of noise temperature of a subarray being a part of the active phased array for the GURT – a low frequency radio telescope of new generation. Design/methodology/approach: A mathematical model of the active phased array is developed in the form of a cascade connection of two noisy multiport networks, one of which is associated with the dipole array antenna placed over imperfect ground, and the other with the beam-forming network. The electrical parameters of these multiport networks are described by the scattering matrices, and the noise parameters – by the covariance matrix of the spectral densities of noise waves. The calculation expressions are obtained which allow analyzing the GURT subarray noise temperature with correct account for all internal noise sources and their mutual correlation which is caused by interaction of dipoles in the array. Findings: Numerical and experimental studies of the noise temperature at the subarray output of the GURT active phased antenna array have been performed. These studies made it possible to estimate the relation between the external and internal noise temperatures in a wide frequency range from 8 to 80 MHz when scanning the subarray beam in the upper hemisphere. It is shown that the external noise temperature at the subarray output is more than 6 dB higher than the internal one in the bandwidth of about 65 MHz. Good agreement between the results of calculation and experiment is obtained, that validates the developed model of the GURT subarray and the effectiveness of the proposed technique for numerical analysis of its parameters. Conclusions: The studies described here confirm the possibility of effective use of this subarray as the base cell of a large phased antenna array for the low frequency radio telescope, as well as the standalone antenna of the radio telescope when radio astronomical observations do not require high angular resolution. The results of this work can be useful in the development and studies of active phased antenna arrays for the decameter and meter wave ranges.

Ukraine has scientific and technical potential to construct a spacecraft and payloads for exploration of the Moon in collaboration with other interested countries. In this paper we propose a double mission that consists of two parts: (1) orbiter exploration from an elongated orbit with a pericenter over the north pole (100 km above the surface) and the apocenter over the south pole (altitude about 3000 km), and (2) exploration with a lander located on the lunar farside near the south pole in the surroundings of the crater Braude. The lander carries five antennas for radio astronomy observations from hundreds of KHz to 40 MHz. The farside landing is necessary to provide effective shielding electromagnetic noises from the Earth, including aurora and lighting radiation. The lander communicates with the relay satellite in a high portion of the orbit; the orbiter is equipped with scientific payloads for investigation of the lunar surface. In the mode of a radio spectrometer, the radio antennas may study solar flares of various types, coronal mass ejections, Jupiter radio emission (L and S bursts), and Saturn (lightning and kilometer radio emission). Overlapping frequencies in the range of 10–40 MHz will allow coordination with terrestrial radio telescopes of IRA NAS, Ukraine (UTR-2, GURT), whose operation is limited by the Earth ionosphere. The absence of terrestrial noise allows us to approach the solution of the problem of searching for cosmological effects associated with the line of neutral hydrogen at Z = 50 … 100. The lander panoramic camera equipped with color and polarization filters provides important observations of horizon glow caused by the effect of electrostatic levitation of the lunar dust. For the orbital module we suggest remote-sensing instruments, which had not previously been used in space exploration of the Moon. The expected spatial resolution of the data will be about 100 m for the northern hemisphere. The equipment should include instruments, prototypes of which are already available in science organizations of Ukraine. These include an infrared spectrometer to map the abundance of OH/H2O compounds in the lunar soil. A HiRes camera operating in two spectral bands is suggested for mapping structural and mineralogical characteristics of young surface formations. The 3-mm radar, working in a squint mode, will not only map the radio brightness of the surface, which characterizes its roughness, but also improve the lunar topographic model with a spatial resolution of 100 m.

PACS number: 84.40.Ba Purpose: theoretical and experimental investigations of sensitivity by the signal-to-noise ratio criterion active antenna used as a phased array element of the GURT radio telescope of new generation. Design/methodology/approach: A mathematical model of active antenna is proposed, the technique of its application for calculation of the active phased array element sensitivity being described. Findings: Numerical and experimental studies of the temperatures of external and internal noises at the active phased array element output of the GURT radio telescope are carried out, as well as element sensitivity estimated over a wide frequency range from 10 to 80 MHz. Conclusions: The obtained agreement between the results of computation and experiment points to correctness of the proposed technique of calculating the sensitivity of active antennas, and the results of studies of the phased array element confirm the possibility of its effective use in construction of the antenna system for the GURT radio telescope. Key words: radio telescope, phased antenna array, active antenna, signal-to-noise ratio Manuscript submitted 15.02.2016 Radio phys. radio astron. 2016, 21(1): 48-57 REFERENCES 1. KRAUS, J. D., 1966. Radio Astronomy . New York, USA: McGraw-Hill. 2. DE VOS, M., GUNST, A. W. and NIJBOER, R., 2009. The LOFAR Telescope: System Architecture and Signal Processing. IEEE Proc . vol. 97, is. 8, pp. 1431–1437. DOI: https://doi.org/10.1109/JPROC.2009.2020509 3. ELLINGSON, S.,W., CLARKE, T. E., COHEN, A., CRAIG, J., KASSIM, N. E., PIHLSTROM, Y., RICKARD, L. J. and TAYLOR, G. B., 2009. The Long Wavelength Array. IEEE Proc . vol. 97, is. 8, pp. 1421–1430. DOI: https://doi.org/10.1109/JPROC.2009.2015683 4. ZARKA, P., TAGGER, M., DENIS, L., GIRARD, J. N., KONOVALENKO, A., ATEMKENG, M., ARNAUD, M., AZARIAN, S., BARSUGLIA, M., BONAFEDE, A., BOONE, F., BOSMA, A., BOYER, R., BRANCHESI, M., BRIAND, C., CECCONI, B., CELESTIN, S., CHARRIER, D., CHASSANDE-MOTTIN, E., COFFRE, A., COGNARD, I., COMBES, F., CORBEL, S., COURTE, C., DABBECH, A.,. DAIBOO, S, DALLIER, R., DUMEZ-VIOU, C., EL KORSO, M. N., FALGARONE, E., FALKOVYCH, I., FERRARI, A., FERRARI, C., FERRIERE, K., FEVOTTE, C., FIALKOV, A., FULLEKRUG, M., GERARD E., GRIEsMEIER, J.-M., GUIDERDONI, B., GUILLEMOT, L., HESSELS, J.,. KOOPMANS, L, KONDRATIEV, V., LAMY, L., LANZ, T., LARZABAL, P., LEHNERT, M., LEVRIER, F., LOH, A., MACARIO, G., MAINTOUX, J.-J., MARTIN, L., MARY, D., MASSON, S., MIVILLE-DESCHENES, M.-A., OBEROI, D., PANCHENKO, M., PANDEY-POMMIER, M., PETITEAU, A., PINCON, J.-L., REVENU, B., RIBLE, F., RICHARD, C., RUCKER, H. O., SALOME, P., SEMELIN, B., SERYLAK, M., SMIRNOV, O., STAPPERS, B., TAFFOUREAU, C., TASSE, C., THEUREAU, G., TOKARSKY, P., TORCHINSKY S., ULYANOV, O., VAN DRIEL, W., VASYLIEVA, I., VAUBAILLON, J., VAZZA, F., VERGANI, S., WAS M., WEBER, R. and ZAKHARENKO V., 2015. NenuFAR: Instrument Description and Science Case. In: 10th International Conference on Antenna Theory and Techniques Proceedings . 21– 24 April 2015,Kharkiv,Ukraine, pp. 13–18. DOI: https://doi.org/10.1109/ICATT.2015.7136773 5. KONOVALENKO A. A., FALKOVICH I. S., GRIDIN A. A., TOKARSKY P. L. and YERIN S. N., 2012. UWB Active Antenna Array for Low Frequency Radio Astronomy. In: 6th International Conference on Ultrawideband and Ultrashort Impulse Signals Conference Proceedings . 17–21 Sept. 2012, Sevastopol, Ukraine, pp. 39–43. DOI: https://doi.org/10.1109/UWBUSIS.2012.6379725 6. IVASHINA M. V., MAASKANT R. and WOESTENBURG B., 2008. Equivalent System Representation to Model the Beam Sensitivity of Receiving Antenna Arrays. IEEE Antennas Wireless Propag. Lett . vol. 7, pp. 733–737. DOI: https://doi.org/10.1109/LAWP.2008.2006917 7. WIJNHOLDS S. J. and VAN CAPPELLEN W. A., 2011. Insitu antenna performance evaluation of the LOFAR phased array radio telescope. IEEE Trans. Antennas Propag . vol. 59, no. 6, pp. 1981–1989. DOI: https://doi.org/10.1109/TAP.2011.2122225 8. ELLINGSON, S. W., 2011. Sensitivity of Antenna Arrays for Long-Wavelength Radio Astronomy. IEEE Trans. Antennas Propag . vol. 59, no. 6, pp. 1855–1863. DOI: https://doi.org/10.1109/TAP.2011.2122230 9.SAZONOV, D. M., 2015. Multielement antenna systems. The matrix approach . Moscow: Radiotechnika-Press Publ. (in Russian). 10. TOKARSKY, P. L., 2006. Matrix Model of a Dissipative Antenna Array. Radiotekhnika. All-Ukr. Sci. Interdep. Mag . is. 146, pp. 156–170 (in Russian). 11. RAZEVIG, B. D. (ed.), POTAPOV, Yu. V. and KURUSHIN, A. A., 2003. Design of microwave devices using Microwave Office . Moskow: SOLON-Press Publ. (in Russian). 12. BABAK, L. I., 1980. Determination of microwave circuits noise characteristics. Radiotekhnika i Elektronika . vol. 25, no. 11, pp. 2380–2384 (in Russian). 13. KRYMKIN, V. V., 1971. The spectrum of background lowfrequencyradio emission. Radiophysics and Quantum Electronics . vol. 14, is. 2, pp. 161–164. DOI: https://doi.org/10.1007/BF01031395 14. MARKOV, G. T. and SAZONOV, D. M., 1975. Aerials , 2nd ed., Moscow: Energiya Publ. (in Russian). 15. 4nec2 – NEC based antenna modeler and optimizer by Arie Voors [online]. Available from: http://www.qsl.net/4nec2/

The present state of solar wind investigations by IPS (interplanetary scintillations) observations at decameter wavelengths is discussed. Radio telescopes, equipment and methods which are used in these experiments are shown. We also describe some interesting results devoted to the long term monitoring of the solar wind beyond Earth’s orbit, the detection of the large scale disturbances associated with active processes at the Sun and the reconstructions of the solar wind stream structure. Emphasis is placed on perspectives of low frequency IPS investigations which are particularly connected with the creation of the Giant Ukrainian Radio Telescopes (GURT) and UTR-2 – URAN – GURT – LOFAR collaboration.

Earth-based observations are complicated by the opacity of Earth's ionosphere at very low frequencies and strong man-made radio frequency interference. This explains long standing interest in building a low frequency radio telescope on the farside of the Moon. Experience from ground-based observations near the ionospheric cutoff in dealing with the interference, ionosphere, and wide-field imaging/dynamic range problems provides crucial information for future radioastronomic experiments on the Moon. In this purpose we observed non-intensive solar bursts on the example of solar drift pairs (DP) at decameter-meter wavelengths with large and small arrays as well as by a single crossed active dipole. We used the large Ukrainian radio telescope UTR-2, the URAN-2 array, a subarray of the Giant Ukrainian radio telescope (GURT) and a single crossed active dipole to get the spectral properties of radio bursts at the frequency range of 8-80 MHz during solar observations on July 12, 2017. Statistical analysis of upper and lower frequencies, at which DPs are recorded, shows that the occurrence of forward DPs is more preferable at lower frequencies of the decameter range of observations in comparison with reverse DPs generated more likely at meter wavelengths. We conclude that DPs can be detected not only by antenna arrays, but even by a single crossed active dipole. Thus the latter antenna has a good potential for future low-frequency radio telescopes on the Moon., 9 pages, 9 figures, 1 table

We present the results of numerical analysis of two design versions of active antenna which were considered for using in phased antenna array of GURT radio telescope at the stage of preliminary design. Both antenna versions are supplied with the same preamplifier but their dipole shapes differ. The first active antenna version has linear horizontal dipole, the second one — inverted V-dipole. The differences of input impedances, efficiencies, and mismatch coefficients of dipoles with preamplifier as well as the effect of these parameters on noise temperatures and SNR of antennas are analyzed in detail.

© 2014, Springer Science+Business Media Dordrecht. The results of the first observations of solar sporadic radio emission within 10 – 70 MHz by the Giant Ukrainian Radio Telescope (GURT) are presented and discussed. Observations in such a wide range of frequencies considerably facilitate the registration of harmonic pairs. The solar U-burst harmonic pair observed on 8 August 2012 is analyzed. The burst key features were determined. Among them, the time delay between the fundamental and harmonic emissions was of special interest. The fundamental emission was delayed for 7 s with respect to the harmonic emission. A model for explaining the occurrence of such a delay is proposed, in which the emission source is located inside a magnetic loop containing plasma of increased density. In this case, the delay appears due to the difference in group velocities of electromagnetic waves at the fundamental and the harmonic frequencies. ispartof: Solar Physics vol:290 issue:1 pages:181-192 status: published

A beamforming device for a 5×5-element subarray of receiving phased antenna array of new GURT radio telescope designed to operate within 10 − 70 MHz is presented. The device is composed of six identical units. Each unit contains equal-arm 5:1 combiner with connected to its arms discrete phase shifters based on time delay lines switching principle. Five of these units combine the signals of array elements inside rows of the subarray, whereas the sixth one assembles the combined signals of subarray rows. Operation principle of beamforming unit is described, the results of its computer modeling and performance measurement results are given. The time-delay lines manufacturing errors influence on subarray power parameters is estimated, as well as their fabrication tolerance found. Key words: phase changer, beamformer, phased array, radio-telescope Manuscript submitted 24.04.2014 Radio phys. radio astron. 2014, 19(3): 240-248 REFERENCES 1. KONOVALENKO, A. A., 2005. Low-Frequency Radio Astronomy Prospects. Radio Phys. Radio Astron . vol. 10, special issue, pp. S86–S114 (in Russian) 2. MYLOSTNA, K. Y. and ZAKHARENKO, V. V., 2013.Search and Study of Storm Activity on Saturn and Other Planets of the Solar System. Radio Phys. Radio Astron . vol. 18, no. 1, pp. 12–25 (in Russian). 3. DE VOS, M., GUNST, A. W., and NIJBOER, R., 2009.The LOFAR Telescope: System Architecture and Signal Processing. IEEE Proc. vol. 97, Is. 8, pp. 1431–1437. DOI: https://doi.org/10.1109/JPROC.2009.2020509Source 4. ELLINGSON, S. W., CLARKE, T. E., COHEN, A., CRAIG, J., KASSIM, N. E., PIHLSTROM, Y., RICKARD, L. J., and TAYLOR, G. B., 2009. The Long Wavelength Array. IEEE Proc . vol. 97, is. 8, pp. 1421–1430. DOI: https://doi.org/10.1109/JPROC.2009.2015683 5. GIRARD, J. N., ZARKA, P., TAGGER, M., DENIS, L., CHARRIER, D.,KONOVALENKO, A. A., and BOONE, F., 2012. Antenna design and distribution of the LOFAR super station. Comptes Rendus Physique . vol. 13, Is. 1, pp. 33–37. 6. DEWDNEY, P. E., HALL, P. J., SCHILIZZI, R. T., and LAZIO, T. J. L. W., 2009. The Square Kilometre Array. IEEE Proc . vol. 97, is. 8, pp. 1482–1496 7. KONOVALENKO, A. A., FALKOVICH, I. S., GRIDIN, A. A., TOKARSKY, P. L., and YERIN, S. N., 2012. UWB active antenna array for low frequency radio astronomy. In: Proc. of the VI-thIntern. Conf. on Ultrawide band and Ultrashort Impulse Signals (UWBUSIS'12). Sevastopol (Ukraine). pp. 39–43. DOI: https://doi.org/10.1109/UWBUSIS.2012.6379725 8. BRUCK, Y. M. and INYUTIN, G. A., 1978. Binary digital delay line (phase shifters) for electrically controlled broadband antenna. In: Anteny . Moscow, USSR: Svyaz' Publ., no. 26, pp. 107–121 (in Russian). 9. BRAUDE, S. Y., MEGN, A. V. and SODIN, L. G., 1978. Decameter wave band radio telescope UTR-2. In: Anteny . Moscow, USSR: Svyaz' Publ. no. 26, pp. 3–15 (in Russian). 10. FALKOVICH, I. S., KONOVALENKO, A. A., GRIDIN,A. A., SODIN, L. G., BUBNOV, I. N., KALINI CHENKO, N. N., RASHKOVSKII, S. L., MUKHA, D. V.and TOKARSKY, P. L., 2011. Wide-band high linearity active dipole for low frequency radio astronomy. Exp. Astron .vol. 32, is. 2, pp. 127–145. DOI: https://doi.org/10.1007/s10686-011-9256-z 11. VOSKRESENSKII, D. I., STEPANENKO, V. S., and FILIPPOV, V. S. Microwave devices and antennas. Designing phased arrays . Moscow: Radiotekhnika Publ. (in Russian). 12. Surface-mounting, 1-GHz-Band / 3-GHz-Band, Miniature, DPDT, High-frequency Relay datasheet [online]. Available from:www.omron.com/ecb/products/pdf/en-g6k_2f_rf.pdf. 13. Surface Mount Power Splitter/Combiner SCP-5-1 datasheet [online]. Available from: www.minicircuits.com/pdfs/SCP-5-1.pdf. 14. SHIFRIN, Ya. S., 1970. Questions of statistical antenna theory. Moscow: Sov. radio. Publ. (in Russian). 15. INDENBOM, M. V., 2008. Fluctuations of the directivity pattern of the antenna array for output. Anteny , no. 4, pp. 40–46. 16. MERKULOV, V. V., 1966. By the estimation of emitters CPV system with phase and amplitude distortion. Radiotechnika i elektronika , vol. 11, no. 11, pp. 2066–2069. 17. STEBLIN, V. I. and KLASSEN, V. I., 1981. The method of analysis of statistical characteristics of CPV phased array antenna with random amplitudes and phases of the location of radiators. Radiotechnika i elektronika , vol. 26, no. 3, pp. 524–531. 18. SHIFRIN, Ya. S. and KORNIENKO, L. G., 2000. Statistics field antenna arrays. Anteny , no. 1, pp. 3-26. 19. SAZONOV, D. M., GRIDIN, A. N., and MISHUSTIN, B. A., 1981. Microwave devices . Moscow: Vyshchaya shkola Publ. (in Russian).

The results of radio emission observations for Cassiopeia A source at frequencies 35, 38, 40, 45, 50, 55, 60 and 65 MHz are presented. At these frequencies, the radio-emission flux density ratio between Cassiopeia A and Cygnus A for the epoch of 2014 is obtained. Brief information about the antenna facility and reception equipment used in the measurements is given. At frequencies 35−65 MHz, the radio emission flux of Cassiopeia A is found for the epoch of 2014 by comparison with the well-known flux density of Cygnus A. The intersection frequency of the low-frequency spectra for Cassiopeia A and Cygnus A is experimentally determined. The results obtained by other authors for the secular decrease of the flux density of Cassiopeia A radio emission at 38 MHz are confirmed.

In this paper we show a possibility of applying the technique of radio astronomical calibration of antenna arrays of low-frequency radio telescopes using pulsar radio emission. A placement of two low-frequency radio telescopes UTR-2 and GURT at one observatory gives us a unique possibility of joint simultaneous observations in the same ionospheric conditions. In the paper results of B0809+74 pulsar observations are used to illustrate the proposed technique.

International audience; The current status of the large decameter radio telescope UTR-2 (Ukrainian T-shaped Radio telescope) together with its VLBI system called URAN is described in detail. By modernization of these instruments through implementation of novel versatile analog and digital devices as well as new observation techniques, the observational capabilities of UTR-2 have been substantially enhanced. The total effective area of UTR-2 and URAN arrays reaches 200 000 m2, with 24 MHz observational bandwidth (within the 8-32 MHz frequency range), spectral and temporal resolutions down to 4 kHz and 0.5 msec in dynamic spectrum mode or virtually unlimited in waveform mode. Depending on the spectral and temporal resolutions and confusion effects, the sensitivity of UTR-2 varies from a few Jy to a few mJy, and the angular resolution ranges from ~ 30 arcminutes (with a single antenna array) to a few arcseconds (in VLBI mode). In the framework of national and international research projects conducted in recent years, many new results on Solar system objects, the Galaxy and Metagalaxy have been obtained. In order to extend the observation frequency range to 8-80 MHz and enlarge the dimensions of the UTR-2 array, a new instrument - GURT (Giant Ukrainian Radio Telescope) - is now under construction. The radio telescope systems described herein can be used in synergy with other existing low-frequency arrays such as LOFAR, LWA, NenuFAR, as well as provide ground-based support for space-based instruments.

International audience; Rapid progress currently takes place in the field of low-frequency radio astronomy in the meter-decameter-hectometer range of wavelengths. It is caused by a radical modernization of the existing radio telescopes, creation of a new generation of instruments, space-borne observations, and by the development of research on all classes of astrophysical objects, including the Solar System. On the other hand, a range of difficulties specific to low-frequency radio astronomy is known, which are caused by technical, methodological, and physical limitations. An effective strategy for overcoming these difficulties is based on synchronous observations using several radio telescopes separated by distances from a few to several thousand kilometers. This provides an opportunity to reduce and identify radio interference and the influence of the propagation media, to increase the sensitivity and resolution, and to solve many problems with higher efficiency. In recent years such simultaneous observations were carried out for the Sun, Jupiter, Saturn, interplanetary medium, pulsars, exoplanets, and transients using the radio telescopes UTR-2, URAN, GURT, NDA, NenuFAR, LOFAR and other. Parallel observations with the space missions WIND, STEREO, Cassini and Juno also facilitate improvement of the quality and reliability of low-frequency radio astronomical experiments.

This paper describes digital radio astronomical receivers used for decameter and meter wavelength observations. This paper describes digital radio astronomical receivers used for decameter and meter wavelength observations. Since 1998, digital receivers performing on-the-fly dynamic spectrum calculations or waveform data recording without data loss have been used at the UTR-2 radio telescope, the URAN VLBI system, and the GURT new generation radio telescope. Here we detail these receivers developed for operation in the strong interference environment that prevails in the decameter wavelength range. Data collected with these receivers allowed us to discover numerous radio astronomical objects and phenomena at low frequencies, a summary of which is also presented., 24 pages, 15 figures

We have developed an active dipole that is intended for use in new generation low frequency array applications. The preamplifier of the active dipole has very high linearity (input IP2 = 70 dBm, input IP3 = 31 dBm) and low noise temperature (100–360 K). The frequency dependence of the dipole impedance and the match between the dipole and preamplifier have been optimized to achieve Galactic noise limited operation. The ratio between the antenna temperature due to Galactic noise and the noise temperature of the preamplifier is 10 ± 1.5 dB over the whole 10 to 70 MHz range. The total cost of the active cross-dipole is 220 euro.

The paper is devoted to comparison of a geomagnetic index Аp with solar wind parameters received from observations during 2003–2005. Some characteristic features of the solar wind geoeffective flows on the decline of the 23rd cycle of solar activity are determined. In particular, the speed of the solar wind geoeffective flows during the observation periods is shown to be on the average lower, and the spatial fluctuations spectrum index higher than that for the quasistationary high-speed solar wind.

In the abstract the results of transfer coefficient calculations of active antenna subarray of GURT radio telescope for all possible scan directions are presented. Also these results are supplied by measurements of signal changing with beam switching at subarray of GURT radio telescope which prove the correctness of the computations. The presented results could be useful for designing of new active antenna arrays and interpreting data obtained at present antenna arrays.

The purpose of the work is to develop testing and calibration technique of large phased antenna arrays using reference noise generator as a standard signal source. This technique is developed for calibration of the phased antenna array of the unique decameter wave radio telescope UTR-2, but they could be easy adapted for similar large arrays with analog and digital beam forming.

International audience; The low frequency radio astronomy (decameter-meter range, frequencies of 10-300 MHz) currently demonstrates rapid progress all over the world. New generations of large antennas-LOFAR, LWA, MWA and others – have been created in many countries. At the same time Ukrainian radio astronomical systems UTR-2 and URAN still remain the largest and most informative ones at the lowest frequency range available for the ground-based radio astronomy (below 33 MHz), especially after their radical modernization during the most recent years. A great number of top priority results have been obtained on the basis of these radio telescopes. The results prove a high significance of the low frequency radio astronomy for astrophysics. Substantial part of these results have been obtained in the course of many year cooperation between Ukraine on one side and France, Austria, Germany and other countries on the other. Creation of new low frequency instruments GURT (Ukraine) and LSS/NenuFAR (France) for the wide frequency range of 10-80 MHz opens up new possibilities for research and fruitful cooperation.

Advantages of the diamond dipole antenna as an active antenna are presented. Such an antenna is like an inverted bow-tie antenna, but the former has some advantages over the ordinary bow-tie antenna. It is shown that the diamond dipole antenna may be an effective element of a new antenna array for low-frequency radio astronomy as well as a communication antenna.

An effective wide-band (10 to 60 MHz) active antenna element has been developed. The cost of one short (3 m), thin dipole with built-in amplifier and metal construction is less than 45 euro. It was shown both theoretically and experimentally that the upper limiting frequency is at least 60 MHz, the dynamic range is 90 dB/μV and the share of the amplifier noise to the background antenna temperature is about 10%. The developed active dipole was tested by building a 30-element antenna array and comparing its parameters with one of the subpart of the UTR-2 radio telescope having passive dipoles of 8.6 m in length and 1.8 m in diameter. The 3C461 ionospheric scintillation spectra observed in the experiments show that the sensitivities and noise-immunities of both antennas are close. This proves the availability using of a short cheap active dipole in new generation giant radio telescopes.

According to the special Program of the National Academy of Sciences of Ukraine, creation of the new giant Ukrainian radio telescope (GURT) was started a few years ago on the UTR-2 radio telescope observatory. The main goal is to reach maximum band at the lowest frequencies (10-70 MHz), effective area (step-by-step up to 100,000 sq.m), and high interference immunity for resolving many astrophysical tasks when the sensitivity is less limited by the confusion effects. These tasks include stellar radio astronomy (the Sun, solar wind, flare stars, pulsars, transients) and planetary one (Jupiter, planetary lightnings, Earth ionosphere, the Moon, exoplanets). This array should be complementary to the LOFAR, E-LOFAR systems. The first stages of the GURT (6 x 25 cross-dipole active elements) and broad-band digital registration of the impulsive and sporadic events were tested in comparison with the existing largest decameter array UTR-2.

In this paper, test results from a 25-element active antenna prototype array operating in the frequency range of 10–70 MHz are presented. Observations of radio emission from different sources: solar sporadic radio emission and powerful cosmic radio sources including their ionosphere scintillation, demonstrate the high effectiveness of the system due to the Galactic background limited sensitivity and high dynamic range of the antenna amplifier (noise immunity). This demonstrates the capability of this 25-element active antenna array to engage in a wide range of unique wide band radio astronomical observations of solar system objects that do not require high sensitivity and angular resolution.