Aperture efficiency is an important parameter to be considered especially when, like in space application, room is limited and the antenna placement is a problematic issue. In this paper all regular lattices capable of to completely filling an aperture are analyzed [1]. A hexagonal lattice is analyzed and its properties are discussed and compared with the ones of conventional lattices such as the rectangular and triangular ones. Formulas giving the grating lobes positions, known for the other regular lattices, are here deduced and demonstrated for the hexagonal one. The three lattices are then compared in case of fixed inter-element distance and when the radiators have the same physical area. Finally, examples of hexagonal lattice usage are given.

Fig.1 Hexagonal lattice Fig.2 Grating lobe in blue and attenuated grating lobes in black

Reference

[1] H. S. M. Coxeter, Regular polytopes, 3rd ed. New York: Dover Publications, Inc.,1973.

Short dipole antennas are often characterized with a simple model description. They are widely used for operation at longer wavelengths at modern spacecrafts (s/c) like "WIND", "Cluster", "Ulysses" and "Cassini". Problems of "direction finding" can be handled by a multiple set of (orthogonal) dipoles. However, the satellite has to hoist more constructions (magnetometer boom, long-wire antennas, etc.) on its bus which cause scattering. The sizes of the latter obstacles can well exceed effective lengths of the dipoles.

Although special efforts are applied to avoid mutual interference, sometimes it is not effective completely. In this paper some non-compensated effects of interferences are studied by means of an example of the mutual impact of two different length orthogonal antennas (15 m dipole Y-antenna and 100 m S-antenna) placed in the equatorial plane of the WIND spacecraft of NASA. These dipoles are connected to a receiver for the WAVES experiment with WIND. A powerful Earth-based transmitter did send signals towards WIND (the SURA transmit facility near Nizhny Novgorod, Russia). It has been shown that particular peculiarities are shown in data received on-board by the WIND/WAVES receiver RAD2 of sounding signals, related to the scattering and mutual influence of above mentioned structure.

WIND is a slow spinning spacecraft. With the spin-axis in the z-direction, we have also dipoles in the equatorial plane in the x and y-direction, called X and Y dipoles respectively. Polar plots of the Y antenna output power P~V2 responses were studied for frequencies 4525, 5475, 8075 and 8925 kHz as a function of the antenna azimuth angle A. To obtain such plots the experimental sessions were selected for the case when the spacecraft was located in the ecliptic plane (the polar angle z of the spacecraft between the k wave direction and s/c spin-axis was near 90o). Each polar plot has been drawn by the "epoch folding" technique for intervals equal to 20 ¡V 30 s/c rotation periods (T ƒ° 3 sec).

The observed antenna responses differ in proportionality according to a function. The difference is most remarkable at frequencies 5475 and 8075 kHz, where narrow side lobes in the pattern have appeared; most relevant to a function is the response at 4525 kHz. Mostly probable reason of the differences, in our opinion, could be an impact of the WIND s/c environment, and in the first turn of the 100 m dipole loaded by the RAD-1 receiver, as well as 12 m boom supporting the magnetometer of MFI instrument.
To check both hypotheses we used a computer simulation with one of the commercial version simulating software. It has been studied a system: "Y dipole (15 m) + dielectric magnetometer¡'s boom + X dipole (100 m) short and open variants" by a finite difference time domain (FDTD) analysis saving calculation time.

Variation of the X (100 m) dipole's material conductivity in limits from aluminum conductivity value up to perfect one (PEC) did not reveal any influence to the azimuth (£p = A) cuts (£c = 90°) of directivity in our simulations, though it seems an obvious result. Teflon- like of the MFI boom stuff with losses (tg £_ = 0.0004, taken from software material list) does not lead to a shift or a forming of the lobes of the Y antenna pattern, but slightly changes it angular width., when 100 m dipole is absent. The boom impact to the Y dipole response is negligible in presence of the 100 m dipole. A taking into account of revealed interference could be interesting and useful for antennas development in future planned space missions.

A closed-form expression is presented for the exact evaluation of the directivity of planar arrays with cos^q(theta) scalar element patterns. Such element pattern typology finds useful application in the preliminary design and analysis of multibeam antennas based on Array Fed Reflector (AFRs) [1] or Direct Radiating Arrays (DRAs) [2] architectures. The expression is generally valid for linear and planar arrays with periodic and/or aperiodic element positions. The expression includes the classical result of linear isotropic elements [3-4] as well as known results for other element patterns [5] as particular cases. Comparisons between different linear and planar array layouts, number of elements, inter-element distance and element pattern directivities (i.e. the q exponent) are presented in a graphical form.

REFERENCES [1] Y. Rahmat-Samii, P. Cramer, K. Woo, S. W. Lee, "Realizable feed-element patterns for multibeam reflector antenna analysis", IEEE Trans. Antennas Propagat., Vol. 29, pp. 961-963, Nov. 1981
[2] P. Angeletti, M. Lisi, "New technologies and system approaches for future mobile satellite missions", Proceedings of the 19th AIAA International Communications Satellite Systems Conference (ICSSC 2001), Tolouse, France, 2001
[3] B. J. Forman, "A novel directivity expression for planar antenna arrays", Radio Science, Vol 5, pp. 1077-1083, July 1970
[4] H. L. Van Trees, Detection, Estimation, and Modulation Theory, Part IV, Optimum Array Processing, Paragraph 4.1.1.2, Directivity of a planar array, Wiley, 2002
[5] Y.T. Lo, "Array Theory", Chapter 11 in, Y.T. Lo, S.W. Lee, (Editors), Antenna Handbook, Vol. 2, Antenna Theory, Chapman & Hall, 1993

In this work, a specific type of UWB monopole antenna is presented. This type of antenna makes use of a dielectric coating for the monopole which is used both as a radome and to configure the radiation characteristics of the antenna itself. Making use of a dielectric material with a relatively high dielectric constant but easy to work with (for manufacturing purposes), fields radiated from the monopole can be focused as in the case of a lens. In order to satisfy previously defined criteria, the profile of this dielectric coating must be conformed so that fields are focused in a proper way.

To make this possible, a global optimization of the profile of the antenna using the Simulated Annealing algorithm and the Finite Element Method as the analysis technique is performed.

The problem with this scheme is that any modification in a relatively high permitivity dielectric situated close to the monopole affect not only its radiation properties but its input impedance, therefore changing the S11 of the antenna and probably eliminating the UWB behaviour. Hence, not only the profile of the dielectric coating but also the profile of the monopole itself would be subject of the global optimization. The cost function for the optimization would change as well to take into account adaptation.

With this methodology, various antennas have been designed. The main objective for this designs was the achievement of stable radiation patterns across the 2-11GHz band. This stability was defined as the minimum gain variation for a chosen direction which is specificated. Along with this goal, minimum return losses were intended also in all the frequency band.

Results from the optimization step show that very stable patterns were obtained while maintaining better than 15dB adaptation results. Different designs with different specification of the stable radiation direction prove that with this technique direction of maximum radiation of a monopole can be modified and forced to be stable with the use of a conformal dielectric surrounding it.

In paper properties of the synthetic aperture radar with continuous pseudonoise signal (PNS) are considered. Despite of traditional pulse signals continuous PNS allow to use small transmitters with low peak power level and high-stability oscillators. Moreover, such type of signal allows due to continuous nature increase unambiguity measurements intervals.

Characteristics of synthetic aperture radar as equivalent antennas with appropriate phase characteristics analysed. Methods of pseudonoise signal selection proposed and analysed.

Modeling results which shows characteristics of the proposed SAR system with continuous signals presented and analysed.

Wireless digital or analog communications are nowadays widely used in miscellaneous fields of applications. When wireless systems are used for short distance communications, the absence of cables does simplify the setup of some equipment, and is necessary for mobility. But the absence of cables also presents a strong interest for cryogenic systems. Indeed, hold times of cryostats are partly limited by heat loads due to thermal conduction between the room temperature environment and the cryogenic part. The conduction of cables that are used to transmit signals in and out of the cryogenic systems can represent a non negligible part of the thermal load for instruments that require many parallel channels. In case of cryogenerators, this load needs to be compensated by more power to run them, which puts some constraints on the instruments, especially if they need to be embarked on satellites. These constraints hold true for scientific payloads since future high frequency heterodyne or bolometric imagers for radioastronomy or Earth atmosphere studies do require several parallel data channels in the GHz to THz range. To this regard, it is valuable, for complex cryogenic systems requiring many communications channels like imagers, to feed or extract data through wireless means. In this work, we have chosen to use bow-tie antennas placed in near-field configuration to transmit data in the 8-12 GHz band. This frequency range corresponds to a typical intermediate frequency range used for Superconductor-Insulated-Superconductor (SIS) mixers cooled at 4K for radioastronomy applications. We will present results obtained in terms of transmission coefficient for elements composed of an emitting and a receiving antennas respectively placed across from each other on different thermal shields of a cryostat. We obtained transmission losses lower than 3 dB over a 1.5 GHz bandwidth. The sensitivity to slight displacements of one antenna with respect to the other one has been studied as well. Finally, we will show results concerning crosstalk between different elements composing an array of such microwave antennas.

This work presents the design of antennas formed by EBG (Electromagnetic Bandgap) structures that enhance the directivity of a simple aperture antenna (an open circular wave-guide). A ground plane is situated on the circular waveguide aperture flanges and the EBG periodic structures are over the metallic ground plane at specific distances. The design has been carried out employing MONURBS, [1].

The proposed antenna configuration is shown in Figure 1. It consists of a large metallic cylinder of length 113 mm and radius 17 mm, a small metallic cylinder of length 12.5 mm and radius 14 mm, a square ground plane whose dimensions are 80x80 mm and the EBG structure, which is comprised of two square grids of the same dimensions as the ground plane. The antenna operates at 9.1 GHz in TE11 mode. Each grid is composed by a metallic sheet with nine square holes. We have carried out a parametric analysis in order to find the dimensions shown in Fig. 2. The upper grid is situated 17.5 mm from the ground plane and the lower grid is situated 3.5 mm from the ground plane. These are the optimum physical dimensions in order to achieve desirable radiation characteristics. Figure 2 shows the main radiation pattern cuts for a linear excitation.

[1] I. Gonzalez, F. Saez de Adana, F. Cátedra: "Application of the Multilevel Fast Multipole Method to the Analysis of Conformed Multilayered Periodic Structures". IEEE AP-S International Symposium on Antennas and Propagation, Honolulu, Hawaii, USA, 10-15 June 2007.

The tolerancing of optical systems is usually performed semi-independently from the mechanical design process with a tolerance budget being submitted to the mechanical design team to which they must work. Most terrestrial antenna systems remain accessible throughout their working lives, and for such systems it is generally feasible and cost effective to design in some facility to fine tune the optics after installation. For satellites, terrestrial systems in remote and hostile environments, or future systems on other planets, this facility may be undesirable for reasons of cost, mass and risk of failure.

For the detailed design phase of high performance antenna fed systems we outline an integrated, non-interactive, tolerancing procedure driven by an automated control loop. The first element in the loop is a stochastic modelling engine that samples the feasible domain of all critical design parameters: on a component by component basis the dimensions, material properties, thermal and other loads. For each sample the process builds a finite element model of the opto-mechanical system from a parametric master model, then runs it. The output is a model of the optical system with all optical components displaced and deformed in an unpredictable but realistic way. Step three of the process is the generation of an optical model from a master model and the displaced nodes of the finite element. This is then run and the output - beam pattern, PSF, phase aberration - analysed. All input and output is recorded in a meta model.

When sufficient sample systems have been run the meta model is subject to statistical analysis to correlate optical performance to mechanical variables and the applicable, system-wide, tolerances derived. With adequate sampling the engineer can be confident that the system built will perform to specification on the working environment.

In the last decade, microstrip reflectarrays have attracted considerable interest for terrestrial and satellite communication system, since they represent a valid alternative to classical reflector antennas. The microstrip reflectarray antennas offer the advantages of being flat, low cost and low profile. Moreover, compared with conventional reflector antennas, they can be easily conformed to a specific physical size, implemented, and installed. Generally, a reflectarray consist of a planar array of microstrip patches illuminated by an electromagnetic source [1][2]. Several synthesis techniques have been adopted in order to satisfy pattern requirements by opportunely tuning some parameters such as patches dimensions, rotations or positions. In most cases, square and rectangular shapes are preferred, mainly due to their easier analysis. However, different geometries, such as crossed-dipoles or rings, are also adopted to integrate the antenna into other structures such as the solar panels for satellite applications, obtaining a significant mass, physical size, and cost reduction. The analysis and design of reflectarrays can be accurately performed by means of a rigorous full-wave method [3], that allows taking into account the coupling among the elements of the array. However such a procedure could be numerically expensive, especially when classical optimization schema based on the minimization of a suitable cost function is adopted in order to obtain desired radiation characteristics and at the same time optimizing the geometrical parameters. As a matter of fact the bottleneck of any optimization schema is the computation of the cost function. To reduce the computational burden of the design phase, approaches based on learning by examples (LBE) techniques [4] have been recently proposed for the on-line solution of some complex electromagnetic problems [5]. This work deals with the optimization of radiation characteristics as well as geometrical parameters of reflectarrays by means of a support vector machine (SVM). Exploiting this concept, the developed method is able to design and optimize the geometry of a reflectarray, in a reduced amount of time, obtaining the desired radiation characteristics, such as direction and amplitude of the main beam, and the desired secondary side lobe level. To assess the effectiveness of the proposed approach, selected numerical and experimental results related to different reflectarray antenna geometries are reported and discussed.
References
[1] D. G. Berry, R. G. Malech, and W. A. Kennedy, "The reflectarray antenna," IEEE Trans. Antennas and Propagat., vol. AP-11, pp. 645-651, Nov. 1963.
[2] D. M. Pozar, S. D. Targonski, and H. D. Syrigos, "Design of millimeter wave microstrip reflectarrays," IEEE Trans. Antennas and Propagat., vol. 45, no. 2, pp. 287-296, Feb. 1997.
[3] D. M. Pozar, T. A. Metzler, "Analysis of a reflectarray antenna using microstrip patches of variable size," Electronics Letters, vol. 29, pp. 657-658, April 1993.
[4] V. Vapnik, Statistical Learning Theory, Wiley, New York, 1998.
[5] A. Massa, A. Boni, and M. Donelli, "A Classification Approach Based on SVM for Electromagnetic Subsurface Sensing". IEEE Transactions on Geoscience and Remote Sensing, Vol. 43, no. 9, pp. 2084-2093, Sep. 2005.

Electromagnetic Band-Gap (EBG) metamaterials are periodic structures made of dielectric or metallic inclusions inside a host material and can be implemented in one, two, or three dimensions. The periodicity yields the material interesting properties, such as the existence of a bandgap, i.e., a frequency range within which the propagation of electromagnetic waves inside the crystal is inhibited. In recent years, an extensive use of EBG in antenna application, especially within the microwave region, has been carried out. This trend replies to the needs of high-gain, high-efficiency, and reduced-sidelobe antennas for satellite communication systems, high-precision Global Position System (GPS), wireless transmissions and mobile phones.

In this work, results obtained for a woodpile crystal are reported. The woodpile crystal has been characterized in terms of band-gap. Moreover, considering configurations where the periodicity is broken, the EBG acts as a spatial filter. The woodpile analysis has so led to the design of a high-performance antenna, thanks to the EBG employment. In particular, a low-directivity double-slot antenna has been turned into a highly-directive antenna, with features of low weight and low cost. The directivity enhancement is obtained by employing the woodpile cavity as cover for the antenna. The interaction between the antenna and the woodpile is explored, pointing out the role of the polarization of the incident field with respect to the woodpile orientation. The proposed EBG-covered antenna turns out to be an innovative radiator, which could be successfully employed in space applications.

The Support Vector Regression (SVR) framework is proposed as an efficient alternative for the development of accurate models of antenna arrays and their inclusion in appropriate synthesis algorithms accounting for the effect of complex environments. The SVR is a data-learning method able to obtain a model that relates the feeding values applied to a radiating structure and the corresponding radiation pattern, provided that some feeding/field data pairs are previously available so that the method is able to learn the behavior of the antenna. If those data pairs have been obtained/measured accounting for all the real properties of the radiating structure and its environment, the obtained model will also take them into account.

A proper representation of the antenna model can be done using a vector notation:

where g(Ω,Φ)=(g₀(Ω,Φ), g₁(Ω,Φ),..., g_M-1(Ω,Φ))^T; (.)^T denotes the transpose, and g_i(Ω,Φ) is a term that indicates the influence of the i-th radiating element in the direction {Ω,Φ}. A matrix G=(g(Ω₁,Φ₁), g(Ω₂,Φ₁),..., g(Ω_N,Φ_L)) can be defined, allowing to calculate the field distribution radiated by the structure in the directions Ω_i, i=1...N, Φ_j, j=1...L, when the voltage set is applied, as

where e is a vector containing samples of the radiation pattern at all the directions of interest.

SVR makes use of the Structural Risk Minimization principle and is able to find the matrix model G from a reduced amount of data using multiple regressions, allowing then its inclusion in accurate synthesis schemes, some of them also based on SVR, that will be able to account for the influence of non-idealities, obstacles, metallic structures, etc., in the conformance of the global radiation pattern.

The circular waveguides with azimuthally magnetized ferrite, propagating normal TE₀₁ mode have been an object of investigation for nonreciprocal digital phase shifters [1-3]. Recently an iterative method for analysis of a coaxial ferrite configuration has been developed [3]. It allows to obtain the exact values of the elements A, B, C of a special column matrix T for all pairs of working points, corresponding to positive and negative structure magnetization. They are used to get the differential phase shift produced in normalized form by simple formulae [3]. The approach, however, is complex and very time consuming.

Here an approximate method for computation of A, B, C numbers is suggested which allows to avoid the complicated iterative techniques. First, a new function F is defined as difference of two products of complex Kummer and Tricomi confluent hypergeometric functions whose second parameters are positive integers, their variables are positive purely imaginary and the real part of their first parameters equals the half of the second ones. In each of the products the variable of first or second functions is multiplied by a factor, varying between zero and unity. The zeros of F coincide with the roots of characteristic equation of the coaxial structure considered. It is proved numerically that when the imaginary part of the first parameter of functions goes to minus infinity, the products of purely imaginary zeros of F by the modulus of the parameter mentioned and by the modulus of its imaginary part tend to the same finite positive real limit, called L number. The latter determine special envelope curves in the phase diagram of geometry at which the characteristics for negative magnetization terminate. Using the slight dependence of A, B, C numbers on certain structure parameters [3], it is suggested to compute their values only once at an arbitrary point at the envelope and at its relevant one at the corresponding characteristic for positive magnetization, instead for all pairs of points in the diagram. Simple formulae are derived, giving the approximate values of A, B, C numbers in terms of L ones at any pairs of point of the latter. Introducing the approximate values of A, B, C in any of the formulae for computation of the differential phase shift, gives the approximate values of the latter. The method is very simple and applicable in the whole range of phaser operation , introducing an error less than one percent. Simultaneously the employment of iterative techniques is kept at a minimum.

References
[1] P.J.B. Clarricoats and A.D. Olver, "Propagation in anisotropic radially stratified circular waveguides," Electron. Lett., vol. 2, no. 1, pp.37-38, Jan. 1966.
[2] G.N. Georgiev and M.N. Georgieva-Grosse, "An approximate method for analysis of azimuthally magnetized circular ferrite waveguide phase shifter," in Conf. Proc. 2006 1st Europ. Conf. Antennas Propagat. EUCAP 2006, Nice, France, in CD, Nov. 6–10, 2006.
[3] G.N. Georgiev and M.N. Georgieva-Grosse, "An iterative method for analysis of coaxial ferrite waveguide phase shifter with azimuthal magnetation", in Proc. 29th ESA Antenna Worksh. Multiple Beams Reconfigurable Antennas, ESA/ESTEC, Noordwijk, The Netherlands, Apr. 18-20, 2007, in CD.
[4] F.G. Tricomi, Funzioni Ipergeometriche Confluenti. Rome, Italy: Edizioni Cremonese, 1954.

Doppler shift of the down link signal of e.g. a LEO satellite is often a problem for the down conversion. Doppler shift can be compensated for on the transmitting side by varying the carrier frequency. But it is impractical, for vehicles not cruising at nearly the speed of light, to use this frequency variation for frequency scanning. Hence if future space vehicles would travel at nearly that speed, beam steering by means of frequency scanning will be a very advantageous choice.

In this paper it is mathematically shown that the Doppler shift frequency caused by the movement of a vehicle coincides with the frequency shift needed for frequency scanning, if the ratio of the element spacing over the additional line length equals the ratio of the vehicle speed over the speed of light. By choosing a high value for the permittivity of the substrate, this additional line length can still be reduced by e.g. a factor of ten, but this is insufficient for practical applications.

For LEO satellites hence this technique is inappropriate as it would require several hundreds of meters of additional line lengths per array element, but for future space vehicles traveling at nearly the speed of light, this technique has obvious advantages.

The drawback of the method is that with only one degree of freedom, namely the varying frequency, the beam can indeed only be steered with one degree of freedom. This is sufficient for a linear array, but a planar or any other array with dimension more than one needs an additional steering mechanism.