Reflectarrays combines key features of large reflectors and properly phased array elements to generate a collimated beam as required in high gain antennas. In reflectarrays, a large flat reflecting surface with many resonant patch elements is illuminated by a feed (or by a feed/reflector in dual reflector configuration). In many space applications, reflectarrays applications can be advantageous because their large surface can be folded or rolled as a part of spacecraft payload before being deployed. Consequently, a reflectarray can significantly reduce both volume and mass requirements in space deployment. To optimize reflectarray antenna performance, a phase correction mechanism must be applied to its individual array elements some of which are well documented in the literatures. In this study, we attempt to extend analytical techniques in reflector analysis to reflectarray antennas by introducing a number of approaches to estimate the reflectarray antenna performance independent of its phase compensation mechanism. In one approach, a Physical Optics (PO) current will be assigned to the surface of individual reflectarray elements upon which a phase correction will be applied. In the second approach, a transmit/receive (TX/RX) radiation characteristics will be assigned to individual reflectarray elements from which coupling coefficients will be calculated and assigned as the excitation coefficient of individual elements. PO approach is modeled in the UCLA reflector code while TX/RX method is implemented in both UCLA code as well as TICRA (GRASP) software. Results are presented for single and dual configurations with the main reflector as a reflectarray. The approach described is used to design a 3-m Cassegrain offset-fed configuration for dual X/Ka-bands application.

Wolfgang Menzel1, Ralf Leberer2 1Microwave Techniques, University of Ulm, PO Box, D89069 Ulm, Germany, E-Mail: wolfgang.menzel@ieee.org 2 EADS, Microwave Factory, Wörthstr. 85, 89077 Ulm, Germany, E-Mail: ralf.leberer@eads. com - Please use attached PDF file -

Reflectarray antennas combine the simplicity of reflector antennas and the capacities of active phased arrays. Thales Airborne systems has developed a reflectarray design based on a waveguide structure associated to a motherboard distributing DC bias and command signals to the diode phase shifters. This kind of electronic beam scanning antenna has proven to be a potentially interesting device suited for space applications like Synthetic Aperture Radars and Telemetry subsystems on satellites. To fit this concept to spatial constraints, developments have been conducted with CNES, French Space Agency, in order to obtain a low mass and reduced power consumption antenna. The low-mass technology is based on metallized thermoplastics, RF printed circuit and DC multilayer printed circuit. These studies led to the realization of an engineering model in X-band frequency (see figures below).

X-band reflectarray panel (288 cells)


Elementary cell section

Reflectarrays have attracted substantial interest in recent years as low-profile replacements for reflector antennas, particularly in space applications. Tunability adds a new dimension to reflectarray applications, enabling them to perform motion tracking, adaptive beamforming, diversity combining, and so on. The authors have previously demonstrated an electronically tunable reflectarray that experimentally exhibits excellent beam forming and reconfigurability.

The reflectarray elements in the original reflectarray design are based on varactor tuning diodes. The integration of the diode with the microstrip patch structure, shown in Figure 1, allows the elements to be tuned over a broad phase range. However, array element implementations based on varactor diodes suffer from a number of shortcomings. The first is a reflection loss resulting from the diode resistance, which becomes pronounced at the centre of the tuning range and usually amounts to 3-4 dB. The second is susceptibility to nonlinear effects due to modulation of the varactor capacitance at the carrier frequency. In transmit applications, this effect can lead to significant intermodulation and harmonic distortion.

Figures 1 and 2 illustrate the experimental performance of a typical varactor-based reflectarray cell. The cell used was fabricated on a 1.5mm thick substrate with a dielectric constant of 3.02, with the following dimensions: L = 17mm, W = 14mm, and g = 1mm. The scattered field amplitude and phase are plotted in Figure 1. Losses are clearly visible when the varactor bias is 10V. This is caused by a small but significant series resistance in the varactor diode. The measured third order intermodulation of a the varactor based cell is shown in Figure 2, and is a strong function of bias voltage. When the cell is biased at 10V, the intermodulation distortion is the worst due to the large amount of PM-to-AM conversion from the structure. This limits the application of tunable reflectarrays based on this technology to receiver antennas.

Micro-electro-mechanical systems (MEMS) components hold significant potential for reflectarray applications. Since MEMS capacitors exhibit a very high quality factor, employing MEMS capacitors in place of semiconductor tuning diodes is expected to reduce reflection losses substantially. Being a mechanical device, the MEMS devices are also immune to capacitance modulation effects at the carrier frequency, virtually eliminating nonlinear distortion and allowing the reflectarray to be employed as a transmit antenna. Furthermore, a reflectarray element based on a MEMS capacitors has the added advantage of being monolithicaly integrated in a single process.

This paper presents an integrated MEMS reflectarray cell based on tunable MEMS capacitors. MEMS capacitors are subsituted for varactor diodes and integrated monolithically with the patch antenna in a single process. The capacitor is based on a bridge structure and fabricated with the patch on a fused silica substrate as shown in Figure 3. Experimental results illustrating the tuning capability as well as the linearity of this cell will be presented in the full paper and the conference.

Double square rings of variable length, printed on a conductor backed substrate, are proposed as the cell elements for a reflectarray in order to broaden the bandwidth of these antenna structures. Using this technique, a single layer reflectarray is designed, fabricated, and tested, exhibiting a wider bandwidth compared to conventional single layer reflectarrays was designed, fabricated and tested.

The lengths of double square ring cell elements are variable along both x- and y- directions. Top views of the reflectarray and the cell element are shown in Fig. 1. Varying the length of the square loops changes the impedance of the double square loop and therefore the phase of the reflected wave. The analysis was carried out using Ansoft HFSS subject to periodic boundary conditions. An extensive parameter study was performed to investigate the impact of the dimensions of the structural parameters of the cell element on the performance of the reflectarray. This parameter study, which entails design guidelines for reflectarray composed of double square rings, will be presented in the full paper

Subsequently, a reflectarray was designed and fabricated based on the parameter study of the cell element. A reflectarray of F/D=0.9 was etched on a 15cm 15cm substrate material of er = 3 of F/D=0.9 and a linearly-polarized pyramidal horn was used as the feed.

The reflectarray was tested in the far-field antenna chamber. The measured gain at the centre frequency of 22GHz was 27.5 dB, resulting in an aperture efficiency of 52%. Fig. 2 shows the measured gain versus frequency performance of this reflectarray. As shown in this figure, a 1-dB gain bandwidth of 10% was achieved for this reflectarray. This is much wider than what has been previously reported for conventional single layer reflectarrays.

Fig. 1- Top view of the reflectarray and an exploded view of the cell element.

Fig. 2- Gain versus frequency plot of the reflectarray.

Abstract - A rigorous method-of-moment (MoM) technique is presented for the accurate analysis of single- and multi-layer reflectarrays including patch elements of more general shape. For the efficient computation, the adaptive integral method (AIM) is applied. Loop-tree basis functions achieve accurate results also for small sized structures like additional phase shifting line segments. Green’s functions are evaluated by the complex image method (CIM) for single-layer structures; for multi-layer structures, Felsen’s equivalent line technique has turned out being more adequate. Large single- and triple-layer reflectarray application examples demonstrate the flexibility and versatility of the presented method also for contoured-beam applications. DUE to its attractive features, such as planarity of structure and ease of manufacturing, reflectarrays have found considerable interest in the recent past. For bandwidth improvements, multi-layer structures are utilized. Reflectarrays have hitherto been calculated mainly by more or less approximate techniques. This is certainly due to the high numerical effort to be expected from standard MoM techniques when immediately applied to usual reflectarrays. The application of rigorous but efficient methods, accurately considering all significant influences, like finite number of patch elements, their different shape and couplings, is highly desirable for reliably predicting all interesting parameters, like e.g. the cross-polarisation. Fast integral solvers based on the adaptive integral method (AIM) have proven being well appropriate for the solution of large-scale problems. The method retains the advantages of the conjugate gradient fast Fourier transform (CGFFT) technique allowing matrix vector products being effected in O(N logN) operations as well as flexible modeling techniques concerning adequate basis functions. Applications of the AIM have mainly been restricted to radar cross-section scattering problems and microstrip antennas, so far. An AIM analysis of single-layer reflectarrays with rectangular patches has been introduced by the authors more recently. This paper presents an efficient electric field integral equation (EFIE) AIM solution for multi-layer reflectarrays taking into account also patch elements of more general shape. Figs. 1, 2 show the results of single- and three-layer application examples. Fig.1: 30 x 30 single-layer reflectarray with twisted patch elements having phase shifting lines Fig.2: 64 x 64 three-layer reflectarray. Contoured beam application. Cross-polarization pattern.

An original approach, named the Scale Changing Technique (SCT), is proposed here for the electromagnetic modelling of MEMS-controlled Reflectarrays. In the design of such reflectarrays reported in [1,2] it has been observed that the use of existing electromagnetic software -based on the Method of Moments or the Finite Element Method- is very time consuming as the number of RF-MEMS switches increases. Moreover, convergence problems in the numerical results arise due to the wide diversity of scales from the fine geometry of the switches to the large size of the reflectarray. For handling the multi-scale nature of such planar structures and to avoid numerical problems due to pathological aspect ratios, the SCT has been recently proposed [3]. It consists in: (1) partitioning of the discontinuity plane in planar sub-domains with various scale levels (the Fig. 1 (b) displays all the chosen sub-domains for the MEMS-controlled cell shown in Fig. 1 (a)); (2) computing separately all the networks modelling the electromagnetic coupling between two successive scale levels (the Mode-Matching Technique may be advantageously used at this stage); (3) the derivation of the phase-shift variation generated by the MEMS-controlled reflectarrays from the simple cascade of networks, each network describing the electromagnetic coupling between two scale levels. The SCT is an approximate method : it requires to artificially close the sub-domains by boundary conditions that are assumed to not greatly perturb the electromagnetic field. However, by a proper partition of the discontinuity plane, successive scale levels can be chosen in order to avoid critical aspect ratios and consequently, for eliminating numerical problems related to the treatment of ill-conditioned matrices. Moreover, at each scale level, the electromagnetic field can be described as precisely as wished by taking an appropriate number of modes in the corresponding sub-domains. Finally, since the computation of all the scale changing networks can be performed separately a modification of the geometry at a given scale requires the recalculation of two networks only. In order to show the accuracy and performances in time costing of the SCT experimental validations and numerical results will be shown. Finally key advantages and limitations of the proposed technique will be discussed.

(a) (b) Figure:(a) The MEMS-controlled phase-shifter element of the reflectarray [2], (b) its scattered view with the sub-domains.

[1] D. Cadoret, A. Laisne, R. Gillard, H. Legay, New reflectarrays cell using coupled microstrip patches loaded with slots, Microwave and Optical Technological Letters, vol. 44, No. 3, pp. 270-273,February 2005
[2] H. Legay et al., "MEMS Controlled Phase-Shift Elements for a Linear Polarised Reflectarray," ESA Antenna Technology Workshop on Space Antenna Systems and Technologies, Noordwijk, pp. 443-448, 20-31 May 2005.
[3] E. Perret, H. Aubert, H. Legay "Scale changing technique for the computation of the input impedance of active patch antennas," submitted to IEEE Microwave Theory and Techniques, 2006.

A planar reflectarray is a flat reflector consisting of an array of printed elements illuminated by a primary source [1,2]. Each element is designed to re-radiate the incident field with a phase shift defined to steer the main beam in a specified direction. The mutual coupling is usually held for responsible of a fluctuation of the field magnitude on the radiation aperture, which results in increase of sidelobes. This paper investigates an approach to analyse mutual coupling effects for reflectarray cells. Usually, the classical approach assumes that the unitary cell is extracted from an infinite periodic array. It results in fast simulations, as only a single cell must be analysed thanks to Floquet theorem. This approach also accounts for mutual coupling but gives only the behaviour of a reflectarray with identical cells. In [3], we proposed the so-called "surrounded-element" approach, which considers the actual mutual coupling for a realistic configuration with non identical cells. It is based on the FDTD analysis of a unitary cell surrounded by its closest neighbours. Consequently, the resultant radiated fields represent the response of all cells (the central one and the surrounding elements) when only the central one is excited. In this paper, a "disturbance criterion" is elaborated to evaluate the effects of mutual coupling in different cases depending on the array lattice, the frequency, the patch dimensions, and the type of substrate. Moreover, techniques to reduce the sensitivity of the reflectarray element to the mutual coupling are also investigated. Fig.1(a) shows a comparison of the "disturbance criterion" versus frequency for a patch element loaded with a slot. As expected, this example clearly shows that increasing the array lattice or using metallic cavities succeeds in enhancing the isolation. In addition, it demonstrates that the "disturbance criterion" is very fluctuant with frequency. This variation can be easily linked with the resonant mode of the structure. These issues will be discussed in the final paper. To validate these conclusions, several 437 elements phase arrays have been constructed. Fig.1(b) compares the radiation pattern of two reflectarrays, which are only different by the presence or not of metallic cavities (made via holes). Cavities succeed in reducing the sidelobes. A significant 5dB reduction is indeed obtained. To conclude, the "disturbance criterion" is a tool that provides a better understanding of the coupling mechanism. Thanks to this indicator, laws will be worked out to optimise the layout of reflectarray, taking into account the mutual coupling.

Fig.1 "disturbance indicator" versus frequency (a) Radiation patterns (in the E-plane) with or without cavities (b).

REFERENCES

1. Huang, J.: 'Microstrip reflectarray', AP-S. Digest, June 1991, 2, pp 612-615.

2. Encinar, J.A: `Design of two-layer printed reflectarrays using patches of variable size', IEEE Trans. on AP, 49, (10), Oct. 2001, pp. 1403-1410.

3. Milon, M.A and al.: `Comparison between the "Infinite Array" Approach and the "Surrounded-Element" Approach for the simulation of Reflectarray Antennas', APS Symposium, July 2006, accepted for publication.

The reflectarray is a potentially more efficient and economical alternative to directly-radiating phased array antennas. Implementing a practical scanning version has proven elusive. The ferroelectric reflectarray described herein involves phase shifters based on coupled microstrip patterned on BaxSr1-xTiO3 films, that are laser ablated onto LaAlO3 substrates. These devices outperform their semiconductor counterparts from X- through and K-band frequencies. There are special issues associated with the implementation of a scanning reflectarray antenna, especially one realized with thin film ferroelectric phase shifters. This paper will discuss these issues which include modulo 2ð effects and phase shifter transient effects on bit error rate, scattering from the ground plane, relevance of phase shifter loss and presentation of a novel hybrid ferroelectric/semiconductor phase shifter and other types under investigation, and the effect of mild radiation exposure on phase shifter performance. A photograph of the hybrid ferroelectric/semiconductor phase shifter is shown in figure 1. Measured insertion loss data is shown in figure 2. Maximum phase shift was about 320 degrees.

Microstrip reflect-array antennas are extensively applied in civil and military systems due to their attractive features: lower-profile, low-cost, lighten-weight, easy fabrication and conformability, etc. The microstrip reflect-array consists of a feed and a planar array of patch element, they reflect the incident field into phase-coherent beam in a desired direction. The phase compensation may be simply achieved by different sizes of square patches corresponding to different resonant frequencies, i.e. different reflection phases at operation frequency. In order to broaden the bandwidth and also extend the dynamic range of phase compensation, the element of two-layer and then three-layer patches had been employed by Encinar successively. On the other side, a scheme of polarisation transform was proposed by the authors for reducing the feed-blockage effect and then enhancing the aperture efficiency, based on two-layer rectangular patches with specific aspect.

In this article, the element of three-layer rectangular patches with similar geometry, thin substrates and air gaps, backed by a ground plane (Fig.1) is simulated. The reflection phase depends on the side-length parallel to the polarization, but is not sensitive to the aspect ratio t = a_y /a_x . The dynamic range of reflection phase approaches to 700^o in the case of normal incidence (Fig. 2). Furthermore, the phase difference between two orthogonal linear polarisation due to different side-lengths of rectangle, throughout the aspect ratio t, determines the resultant polarised state described by axial ratio (Fig. 3).

A sample of this type of reflect-array is designed and tested with improved performances.