The first problem which is essential in the study of the EBG (Electromagnetic Band Gap) structures is the identification of the frequential band in which the electromagnetic wave propagation is forbidden for an exciting incident wave with well defined space direction.

The basic EBG modeling methodologies are : Direct Transmission Method, Dispersion Diagram Method, and Reflection Phase Method (RPM). This article presents this last approach based on the Reflection Phase Diagram. The procedure consists to plot the diagram of phase of the wave reflected by EBG structures excited by an incident wave, in the simplest case, plane and normal upon the structure. The forbidden band corresponds to the frequential band where the phase of the reflected wave is equal to 0°±45°, criterion of Sievenpiper [1], or to 90°±45°, criterion of Rahamat-Sami [2]. In the filtered frequential band, the structure behaves as a magnetic conductor : a High-impedence surface. The EBG structure studied and designed is a Mushroom-like [1]. The performances of this structure are modelled using the HFSS code.

The design of this EBG structure is for WiFi (Wireless Fidelity) antenna application, within VIP (Vehicular Internet Protocol) project. VIP Project has as a principal objective to define, specify and tried a service of connectivity to Internet aboard TGV (High-speed Train). The antenna must answer to the needs for WiFi under the constraint of the mobile ground connection at 300 km/h speed. We propose to use an antenna named Glasses-antenna. Under a classical substrate and functioning around 2,3Ghz, this antenna is characterized by a directivity of about 10dB but it has a thickness not miniaturized [20mm]. The EBG structure designed around 2,3Ghz aims to delete this disadvantage.

According to the two criterions for identification of the forbidden band and the thicknesses of the substrate used, we have designed four EBG structures meeting this objective. The results of simulations show that these structures enable to reduce the thickness of the antenna more than 60%. In objective to validate these results, measurements are under development.


Index terms
EBG Structures, Reflection Phase Diagram, magnetic conductor, High-impedence surface (HIS).

References
[1] D. Sievenpiper, L. Zhang, R. F. J. Broas, N. G. Alexópolous, and E. Yablanovitch, "High-Impedance Electromagnetic Surfaces with a Forbidden Frequency Band," IEEE Transactions on Microwave Theory and Techniques, Vol.47, No.11, November 1999, pp. 2059-2074.

[2] Fan Yang, Member, IEEE, and Yahya Rahmat-Sami, Fellow, IEEE, "Reflection Phase Characterizations of the EBG Ground Plane for Low Profile Wire Antenna Applications", IEEE Transactions on Antennas and Propagation, Vol.51, No.10, October 2003, pp. 2691-2703.

High dielectric EBG substrates can be used in planar antennas designs to obtain high efficiency and compact configurations. Placing a patch antenna over a ZrSnTiO₃ (ε_r = 40) substrate and surrounding it by a 2D EBG substrate aperture efficiency values around 60% can be achieved. This antenna is feed by aperture coupling method, using a slot in the ground plane. A cavity is created under the patch, where the defect of the EBG is included (no EBG hole under the antenna); therefore all the power is confined in this cavity. No Electromagnetic power is allowed to travel trough the substrate. At the same time, the dimensions of the antenna are reduced in a factor larger than 4 (comparing this substrate with an ε_r = 2.2 material) due to the high dielectric constant used as antenna substrate. These high dielectric EBG antennas are ideal for integrated circuits due to their reduced size.

The use of the EBG also prevents the undesired effect of radiated power coming from the substrate; therefore no interferences will be observed in the radiation patterns. Symmetric pattern are obtained.

In this paper a circular patch antenna on a high dielectric constant substrate (ZrSnTiO₃ ), feed by an aperture coupling slot has been designed, fabricated and measured in Ku band (see figures 1 and 2). The feeding part of the design has been done in Rogers 3010 material (ε_r = 10.2). Good agreement between simulations and measurements has been obtained. Symmetrical radiation patterns have been measured.

Figure 1: Fabricated patch antenna over a high dielectric constant EBG substrate

Figure 2: Setup used in the High dielectric constant EBG substrate measurements

The aim of this paper is to demonstrate that it is possible to perform a waveguide using parallel plates and metamaterials. The lack of lateral metallic walls is replaced by the effect of these metamaterials. We are going to develop a AMC-PEC-AMC structure. The importance of this structure is due to the planar architecture.

To prove that the structure described above works properly, we simulate it with CST 5.0. Therefore, we are able to obtain the field distribution when exciting with a port at the edge working at 12.65GHz and 12.95 GHz. We notice that at other frequencies different from the working ones (Eg.12GHz) the wave does not propagate.

In order to validate the theoretical and simulated results, we build one prototype.

Finally, we are going to use the structure above as if it were the feeding part of a planar slot antenna, in order to demonstrate the chance of using this kind of structures in this kind of antennas.

More and complete results will be presented in EuCAP 2006.

The representation of a frequency selective surface (FSS) using an equivalent circuit composed of lumped elements is useful in the design stage, where it is desirable to know if the FSS is able to give a fixed frequency response.

Our design methodology consists of obtaining a heuristic model that allows us to go from the full-wave-simulation domain FSS to a circuit-simulation domain and vice versa.

To avoid the high computational burden in the full-wave-simulation domain, a Regression Support Vector Machine (SVR) has been employed to obtain the equivalent circuit parameters. This model just depends on a subset of the training data, because the cost function for building the model does not take into account any training data that is close (within a threshold) to the model prediction. This kind of machines take its advantage in the weaker constrain imposed to the size of the training set comparing to other learning machines as Neural Networks. It is crucial in the present application as far as the data available for the analysis of the FSS is not very big in size.

By means of using the SVR model in the analysis process the equivalent circuit parameters can be obtain in a faster way. The SVR has been set up by using a multi-fold cross validation scheme, and using Gaussian kernels.

A periodic structure of metallic ring resonators on a dielectric substrate is used to analyze the method. This structure has a resonant behaviour, so its equivalent circuit is a tank LC circuit. The results are shown in the figure below.

MEMS devices have proven their usefulness in microwave applications with their reduced cost, improved performance, and miniaturized dimensions feasible for batch fabrication.

Since RF MEMS components have tunable characteristics, the integration of these components with radiators may yield several advantages in terms of reconfigurability in polarization, frequency, and radiation pattern. This paper presents a frequency tunable dual-frequency slot antenna integrated with fixed-fixed beam type RF MEMS capacitors. Figure 1 (a) shows the general view of the structure. The loading section of the antenna is implemented on a short circuited stub inserted opposite to the feeding transmission line on which MEMS bridges are placed periodically to dynamically tune the resonant frequency. The capacitors of the structure are fixed-fixed beam type with the anchors on the openings on the antenna which is designed to overcome the difficulties in the processing of long cantilevers of the previous structure presented in [1]. It is shown in [1] that the resonant frequency of a rectangular slot antenna can be changed by variable cantilever type capacitors placed on the stub. The cantilevers are required to be long to provide sufficient capacitance to tune the resonant frequency. However, the implemented cantilevers are curled which results in discrepancies between simulation and measurement results. To overcome this difficulty, fixed-fixed type capacitors as shown in Figure 1 (b) is used in this design. According to the simulation results of Ansoft HFSS, the resonant frequencies of the structure when the bridge is at 2 µm occur at 8.58 GHz (10 dB BW: 4.2 %) and 10.53 GHz (10 dB BW: 10 %). By applying electrostatic actuation, the heights of the beams are reduced to 1.4 µm, and the resonant frequencies shifts down to 7.3 GHz (10 dB BW: 1.6 %) and 10.2 GHz (10 dB BW: 11.7 %). The measurement results shown in Figure 1 (c) are performed on the structure which is fabricated using the standard surface micromachining process developed at Microelectronics Facilities of Middle East Technical University.

According to the measurement results, the resonant frequencies can be shifted from 8.7 GHz to 7.7 GHz, and from 10.57 GHz to 10.22 GHz. The results show a good agreement compared to the simulation results. This structure proves that MEMS capacitors can be used to tune the resonant frequency of the antenna.

Moreover, fixed-fixed type implementation is a proper solution to eliminate the discrepancies of the cantilever type implementation that might occur due to the stress on the structural layer.

Figure 1. (a) Reconfigurable slot antenna structure (b) cross-sectional view of the loading section (c) measurement results for different actuation voltages.

[1] E. Erdil, K. Topalli, O. Aydin Civi, and T. Akin, "Reconfigurable CPW-fed dual frequeny rectangular slot antenna," 2005 AP-S International Symposium and USNC/URSI National Radio Science Meeting, Washington D.C., vol.2A, pp.392-395, 3-8 July 2005.

This paper is dedicated to the phase centre study of the Electromagnetic Band Gap (EBG) antennas. Such antennas can be used to feed a reflector and they represent a new passive technology to realize a multibeam coverage [1] at an attractive cost compared to usual systems with beam forming networks. However, no study on the phase centre of the EBG antenna has been made before. The position of this phase centre must be known precisely to locate the feed at the focus of the reflector in order to reduce phase aberrations and to improve the efficiency of the reflector antenna. For example, a 3 wavelength focus error involves a 3dB increase of the radiation pattern side lobes and a 2% loss on the illumination efficiency with a 2.15 focal to length diameter ratio.

The first part of the article consists in a study of the EBG antenna phase centre position. Compared to classical feeds for reflector, such as horns, the results show that the phase centre is typically located behind the EBG antenna ground plane. This position depends on both the directivity of the antenna and the material. The study shows that a metallic grid is better than dielectric slabs. For example, a 23dB directivity EBG antenna has a phase centre located at 4 wavelength behind the ground plane with a metallic grid, whereas with dielectric slabs, the phase centre moves back until 18 wavelength.

The second part of the study deals with the stability of the phase centre of an EBG antenna over a wide frequency range (500 MHz between 29.5 and 30 GHz). This work is necessary because of a typical behaviour of the EBG antenna radiation patterns [2]. They are not Gaussian and vary over a wide frequency range. As a result, the phase centre, which is calculated thanks to the far field radiation patterns, varies. To explain this phenomenon, a parametric study has been made on the metallic EBG antenna. It consists in studying the grid shape influence, the EBG resonator quality factor and the excitation feed chosen. The results show that the evolution of the phase centre depends not only on the variation of the EBG antenna directivity but also on the choice of the material and the excitation feed.

At last, some solutions to reduce the variation of the phase centre position will be evaluated. For example, a patch array which replaces the classical patch used as excitation feed allows a quite good stability of the phase centre over 500 MHz.

References

_{[1] R. Chantalat, T. Monediere, M. Thevenot, P. Dumon, B. Jecko, « Interlaced feeds design for a multibeam reflector antenna using an 1-D dielectric PBG resonator », IEEE Ant. Prop. Symposium, Columbus, June 2003.
[2] C. Menudier, R. Chantalat, L. Leger, T. Monediere, M. Thevenot, P. Dumon, B. Jecko, « Optimisation of the reflector antenna illumination law thanks to an electromagnetic band gap strucuture », 28th ESA Workshop on Space Ant. and Technologies, June 2005.}

Multibeam antenna aboard satellite for multimedia application (for example KA band) using one feed per beam concept with only one aperture suffers from the well known problems of spillover losses or beam crossover losses. To avoid these limits, bulky and costly systems as multiple aperture antenna architecture or beam forming network are actually used. Some studies [1,2] have demonstrated that a multisources EBG resonator antenna can achieve passively on its surface directive and overlapped focal feeds permitting to realize a multispot coverage with only one aperture. This paper deals with the design of a multisources metallic EBG antenna in Ka band dedicated to a Side Front Offset Cassegrain Antenna for Europe multispots coverage.

This reflector antenna which has a large focal to length diameter ratio limiting the defocusing effects, requires high focal feeds directivity. The drawback of an EBG antenna is the weak radiation bandwidth when the directivity reached is important. Consequently, the main difficulty has been to improve the radiation bandwidth of an EBG antenna. The second goal has been to design an EBG antenna which presents radiations patterns with Gaussian shape and very low side lobes to maximize the reflector antenna efficiency. These two critical points have been resolved by using a particular excitation source never used before which improves the performances of the EBG antenna. Indeed, the characteristics of the EBG antenna don’t depend only on the material properties but also on the source choice. The replacement of the classical patch by 12dB radiating aperture allows to increase the radiation bandwidth (30%) and the radiations patterns quality of the EBG antenna.

A 24 dB metallic EBG antenna with only one excitation feed has been realised at 30 GHz. The directivity varies of 3 dB on the 1.5 % frequency band, the side lobes measured of this antenna are lower than -30 dB and the intrinsic losses are estimated to 0.2 dB. The efficiency of the SFOCA illuminated by this EBG prototype is near to 70 % on the total frequency band 29.5-30 GHz (PROFIL software simulations). After this successful validation procedure with one source, we are actually working on the multisources EBG concept.

References

[1] R. Chantalat, T. Monediere, M. Thevenot, P. Dumon, B. Jecko, « Multibeam reflector antenna with interlaced focal feed by using an 1D EBG resonator antenna », JINA Novembre 2004, p. 147.

[2] S.L.K Castiglioni, G. Toso, C. Mangenot " Multibeam antenna based on a single aperture using overlapped feeds" , JINA Novembre 2004, p. 170

Directive antennas are necessary to main actual or future communication systems, on terrestrial or satellite systems. The two usefully directive antenna concepts are reflector or array antennas. The reflector ones have the advantages to be simple to use in large frequency bands, easy to fabricate and to have low losses thanks to the waveguide feeding and metallic manufacturing. On the other hand, the radiation pattern cannot be easily controlled. The array antennas allow to manage radiation pattern at the cost of complex and expensive feeding networks and high losses. We propose to combine the advantages of the reflector waveguide feeding with the flexibility of the electronically command of the array. The proposed antenna is composed by an Electromagnetic Band Gap material which acts as active reflector and a waveguide primary feed including sub-reflector at the EBG center. The active control of the reflection coefficient phase, all over the reflector surface, allows to obtain the desired radiation pattern: classic one like parabolic antenna, scanned, sectoral or shaped pattern.

The first step of the study has been to design a passive 8 zones L/4 Fresnel centered reflectarray based on High Impedance Surface [1]. The elementary mushroom cells were designed with IE3D, the whole antenna was simulated with SR3D [2]. The comparison experiments-simulations is excellent between 5.0 and 6.0 GHz and we present on figures 2 and 3 respectively the measured and the simulated radiation patterns at 5.25 GHz (blue and green lines are inverted between the two patterns).

So, we decided to design the active elementary cell in order to replace the variable size of the mushroom cell that was depending on its position by introducing a variable capacitance between each identical mushroom [1]. Like the passive one, the active elementary cell was designed with IE3D including lumped impedances depending on the radius in order to obtain the radiation pattern of an equivalent parabolic reflector. To be able to simulate this new 3D structure, the lumped RLC components were implemented in SR3D.
Before the manufacturing of the whole active structure, we have decided to design a strip portion (36 x 8 cells) of the circular reflector and to evaluate the reconfigurable radiation pattern capabilities of this strip reflectarray. In the figures 4 and 5, we present the simulated radiation pattern of a 5 zones “Fresnel strip” (28x8 cells), respectively in passive and active version at 5.25 GHz (the strip is in the E plane). We can see that the variable capacitance of the varactors allow to focus the EM field like the passive variable size cells.

The optimization of the whole strip of 36x8 cells is in progress. This structure includes 448 varactors controlled by 112 independent DC voltages. The manufacturing will begin soon in order to test the tunable strip in term of EM performances and evaluate its reconfigurable capabilities to manage several types of radiation patterns. [1] D.F. Sievenpiper, J.H. Schaffner, H. Jea Song, R.Y. Loo, G. Tangonan, "Two-Dimensional Beam Steering Using an Electrically Tunable Impedance Surface", IEEE trans. AP, pp 2713-2722, vol 51, n°10, Oct 2003. [2] R. Bills, P. Brachat, C. Dedeban, P. Ratajczak, "SR3D: Recent developments for Large EM problems", Proc. JINA 2002, Nice, Nov. 2002.

Recently, a growing interest has been devoted to the application of frequency selective surfaces (FSS) for obtaining specific electromagnetic properties, such as artificial surfaces, artificial magnetic conductors, enhanced directivity, electromagnetic band-gap properties. Another field of interest is the integration of FSSs with phased array antennas, with the purpose of improving the antenna performance (widening of scan region, scan-blindness removal, wide angle impedance matching).

In this work, a method is proposed for the analysis of multilayered FSS integrated with phased array antennas. The radiating elements are rectangular waveguides. The structure can include an arbitrary number of dielectric layers and FSSs. The method is based on the assumption that the structures are periodic, and large with respect to the wavelength. The array excitation may be weakly varying (e.g. tapered excitation) with respect to the periodicity. With these hypothesis, the entire structure can be considered as infinite and periodic; this allows to expand the electromagnetic quantities (fields and currents) in terms of Floquet wave (FW) expansion, thus, reducing the analysis to a single periodic cell.

The problem is solved by a full-wave analysis of each individual periodic interface (array or FSS) with the pertinent Green's function. From the method of moment matrix, an equivalent multimode impedance or admittance matrix of the periodic surface is derived. which permits to obtain a multimode equivalent network of the entire structure. The blocks interconnection is realized using conventional circuit methods. The order of the multimode equivalent network is determined by the number of accessible modes, that are those Floquet modes which are responsible of the interaction between two adjacent periodic interfaces. This approach drastically reduces the numerical effort and provides a physical insight into the mechanism of the interaction between the aforementioned structures. The developed software tool has been used to design integrated FSSs covers for phased array antennas, with the primary goal of achieving an out-of-band frequency rejection for EMI reduction in complex environments. Additional benefits that can be reached with these structures and will be investigated are wide angle impedance matching and surface wave suppression (for widening of the array blindness-free scan region).

Contoured beam reflectarrays for DBS applications have been demonstrated in [1] and [2] using one and three layers of varying size patches, respectively. Bandwidth is very limited in reflectarrays when a single layer of patches is used. To overcome the frequency band limitation in reflectarrays, the phase delay can be compensated on each element with the phase of the reflection coefficient at different frequencies. However this compensation of phase delay is not possible with a single layer of patches, because more degrees of freedom are required. This technique was applied in [2] by using three layers of varying size patches, and optimising their dimensions to match the required phase-shift at central and extreme frequencies. Using this technique, a 10% bandwidth was achieved for a 1-m breadboard, but it was demonstrated that larger bandwidth was not achievable by matching the phase at only three frequencies.

In the present work, a one-meter reflectarray has been designed to illuminate the European region shown in the figure with 28dBi in the frequency band 10.95-12.75 GHz (15%). The reflectarray is made of three stacked layers of printed arrays with rectangular patches, so that the phase of the reflected field at several frequencies and for each polarisation is controlled at each element by adjusting the dimensions of the staked patches.

A phase-only synthesis technique based on the Intersection Approach has been applied to obtain the required phase-shift on the reflectarray surface at several frequencies for the European coverage. The electrical design of the three-layer reflectarray has been made by optimising the patch dimensions to match the required phase-shift at five frequencies in the required working band for dual linear polarisation. The radiation patterns fulfil the contour requirements for both H- and V-polarisation in the required frequency band, which is a 15% bandwidth. A cross polarisation of -30dB below the maximum is achieved in the frequency band from 10.95 GHz to 12.3 GHz.

The achievement of the contour requirements in a 15% bandwidth is noticeable, since it is well known that the most restrictive limitation in reflectarrays is its narrowband behaviour, particularly for large size reflectarrays. The design in a 15% bandwidth was possible because the dimensions of the patches were optimised to match the required phase-shift at 5 frequencies in the working band. Provided that the required frequency band in Telecomm missions can be achieved with reflectarrays, they can be a valuable alternative to shaped reflectors in space applications.

References

[1] D. M. Pozar, S. D. Targonski, and R. Pokuls, "A shaped-beam microstrip patch reflectarray", IEEE Trans. Antennas and Propagation, vol. 47, No. 7, pp. 1167–1173, July 1999.

[2] J. A. Encinar, et al., "Breadboard of a three-layer printed reflectarray for dual polarisation and dual coverage", 28th ESA Antenna Workshop, Noordwijk, The Netherlands, June 2005

This paper presents a microstrip slot coupled patch reflectarray where the phase of the scattered field from a slot coupled patch is controlled by changing the length of microstrip lines using RF MEMS switches. Microstrip reflectarray is a planar surface consisting of patches or dipoles that is designed to scatter the incident field to a desired direction. Reflectarray surface is illuminated by a source, and the direction of the reflected field can be controlled by the phase of each element. Compared to electronically scanned phased arrays, this approach eliminates the losses of a microstrip feed network which limits the performance of high-gain millimeter arrays [1]. In order to scatter the field with the desired phase, the idea of introducing a small shift in the resonant frequency of the element is employed. In the proposed design, this can be achieved by inserting variable length microstrip transmission line coupled to the patch by means of a slot in the ground plane as shown in Figure 1 (a) and (b). The length of the microstrip line is controlled by the RF MEMS series switches implemented on the line. The patch antenna is printed on a glass substrate (ε_r=4.6, h=500 µm) bonded to a high-resistivity silicon (ε_r=11.9, h=150 µm) on which microstrip line with RF MEMS switches is implemented. Multilayered structure reduces the interference effects on the scattered field due to tuning elements. The use of RF MEMS switches provides a dynamic reconfigurability on the direction of the reflected field. To design the scanning array, first the reflection phase curve of an infinite array of slot coupled patches as a function of the microstrip line length is obtained by HFSS simulations and plotted in Figure 1 (c). A phase swing of 285° is obtained with the variation of stub length. Based on this curve, a linear array of four slot coupled patches is designed to rotate the main beam to 25° and 56° by actuating the required RF MEMS switches on the microstrip lines. RF MEMS switches are placed on L= 1.1, 1.3, 1.47, 1.65 and 2.38 mm to provide required phase shifts. For instance, to rotate the beam to 25°, switch 1 on the 1^st patch, switches 1 and 2 on the 2^nd patch, switches 1 to 3 on the 3^rd patch and switches 1 to 5 on the 4^th patch are in down state. To rotate the beam to 56°, all the switches are in the upstate on the first patch whereas switches 1 to 3 on the 2^nd patch, switch 1 on the 3^rd patch, and switches 1 to 4 on the 4^th patch are actuated. Designed array is fabricated using the standard surface micromachining process developed at Microelectronics facilities of Middle East Technical University for the fabrication of various RF MEMS devices. The MEMS implementation of the structure, fabrication process, and measurements results will be presented at the conference.

Figure 1. The top view (a) and cross section (b) of the slot coupled patch antenna. (c) Phase design curve.

[1] Pozar D.M., Targonski S.D., Syrigos H.D., "Design of millimeter wave microstrip reflectarrays," Antennas and Propagation, IEEE Transactions on Antennas and Propagation, Vol. 45, No. 2, Feb. 1997 Page(s):287-296.

Printed reflectarrays (PRAs) are always a challenging theme of research in antennas, because of their attracting characteristics in space and terrestrial telecommunication applications: high directivity antennas with deployable lighweight flat structure, reduction of the losses with respect to the array feed system, combining the advantages of reflectors and arrays.

The most common PRAs have square (or rectangular) patches, and the phasing characteristics are obtained with well know techniques. Less investigation (although with some interesting results, as e.g. in [1]) has been carried out on PRAs consisting of non-square/rectangular shapes, as circular, annular, etc. In particular, the use of annular patches may be interesting because in the fundamental mode TM11 the resonant size is significantly lower than for the circular or rectangular patch, and also because of the additional degree of freedom of the aspect ratio (outer/inner radius).

In this paper the use of annular patches in PRAs will be discussed: the different phases of the reflection coefficient can be obtained by varying the size of the patch and in particular only the inner ring radius. The analysis of the structure and the computation of the phase curves needed for the design of the patches have been done with several techniques (among which those previously used in [2]), taking into account the interaction among the elements. Parametric analyses for different values of aspect ratio, thickness, dielectric constants have been carried out, to obtain suitable phase curves for the reflected field (see an example in the figure for the effects of thickness), with wide range and relatively low sensitivity to the tolerances. A 5x5 reflectarray sample has been designed and computed, and the results, that include a frequency analysis, have shown good pattern behaviour, with relatively low sidelobes and good pattern symmetry.

If the outer dimension of the patch remains the same, a more regular layout of the patches can be obtained, with constant distances among adjacent elements, allowing an easier treatment of the mutual coupling effects and a reduction of the element spacing, with less possibility of grating lobes.

In conclusion, the use of annular patches in reflectarrays opens interesting possibilities and options that are absent in rectangular patch shapes, as:
Phase control with inner radius only;
Wide range of variation of the reflection coefficient phase;
Smaller elements and constant outer size allow a more regular and dense lattice;
Behaviour independent on incident polarization allows the use of any polarization.

References

[1] Bozzi, M. et al., AWPL, Vol. 2, 2003, pp. 219 - 222
[2] Pirinoli, P. et al.: IEEE Trans. AP, Sept. 2004, pp. 2415- 2423

In the last few years, reflectarray antennas have received an increased attention because of their compactness, light weight, and low manufacturing cost. However, only a few examples have been published in literature employing active antennas in order to add amplifying functions or to dynamically change the phase of the scattered field, achieving beam steering capabilities. The most significant examples presented in literature show how to scan the main reflectarray beam by implanting low-bit and low-loss phase shifters into the printed elements or by placing miniatures motors under the patches when circular polarization is required. Even if good results can be achieved with the mentioned techniques, they significantly increase the complexity and the costs of the scanning system and they become not practicable when reflectarray of several hundreds of elements should be designed.

Recently, a simpler method has been presented where the reflectarray beam is steered loading each patch with a varactor diode. In this case, the voltage controlled tuning varactor introduces at the open end of the antenna a variable capacitive reactance which modifies the electrical length of the patch. By electrically adjusting the capacitance of the diode, the resonant frequency of the antenna can be varied within a specified range. This capability is beneficial for reflectarray antennas because the small shift in the resonant frequency introduced by the tuning diodes changes the reflection phase of the single element, thus allowing a dynamic phase control. The main advantage of this technique is that it permits a considerable reduction of the system architecture complexity and costs. Furthermore, the phase of the single element is dynamically governed by using only a voltage control.

One of the main issues related to the design of varactor loaded reflectarrays is the characterization of the unit cell. The varactor, indeed, is quite difficult to model accurately and in most of the cases an experimental assessment is necessary before the reflectarray is designed. In this work the experimental measurement of the phase reflected by a varactor loaded reflectarray will be presented. In particular, a waveguide simulator will be used to optimize the varactor loaded patch configuration so that a wider reflection phase range can be obtained. Both simulated and measured results will be presented and discussed.

A new passive phasing technique for use in microstrip reflectarray antennas is proposed. The array antenna elements are identical slotted patches. The slots are loaded with a SMD capacitor to set the required phase shift needed for array implementation. The design procedure and critical parameters are discussed. Simulations show promising results. Mounting a SMD capacitor in such a configuration is the first step in capacitive loading on a slotted patch for active microstrip reflectarrays. It is shown that by adjusting the capacitance values it is possible to scan the beam.

Introduction-MICROSTRIP reflectarray antennas are low cost, low weight and very thin flat alternative to traditional reflector antennas. This paper outlines a new phase shift technique for microstrip elements using a slotted patch loaded with a SMD capacitor. The conventional methods, i.e., stubs, variable size, rotation require a geometrical difference between the patch elements in an array. The phase shift technique of particular interest is the slotted patch. The patch has a slot which variation in its length will change the electrical length of the antenna element. The passive option presented in this paper provides a unique approach using a capacitive load across the slot of the patch. Different values of the capacitance leads to a change in the reflected phase. This work is the preparation to design and measuring the reflected phased array antenna using active capacitive loading which authors are working on.

Single element and array performance-The geometry of the single element is shown in fig 1 a. The geometry consists of a metal slotted patch on a grounded substrate. The capacitive loading is provided using a SMD capacitor. The SMD is placed at the center of the slot. The antenna is illuminated by a linear horizontally polarized plane wave in the positive y-direction. Fig 1b shows the reflected phase of a single element as function of the frequency for different values of the capacitance. At 7.0 GHz the SMD values between 1.0-3.9 pF have a phase range of 200°. A wider range in SMD values especially in the lower region would increase this phase range. Using the phase diagram Fig 1.c shows far field radiation pattern of three 3x3 arrays scanning at different elevation angle. Figure 1 (a) Geometry of single element, (b) phase of a single element with a ideal SMD, (c) Far field simulation results for three different 3x3 arrays.

Measurement results- Measurement results and analysis for not ideal SMD will be provided in the final manuscript.

Conclusions- This paper presents a novel passive phase shifting technique for microstrip elements in a reflectarray antenna. The microstrip element is loaded with a capacitance. A design at 7.0 GHz using SMD capacitors is performed. Uses of commercially available SMD capacitors do not provide extensive array design possibilities however it is an interesting and unique method in creating phase-shift elements. By using geometrically identical arrays the beam scanning concept is accentuated. The results given in this paper is used to design, build and measure the active capacitive loading reflected phased array antenna. The work is in progres

Microstrip antennas are increasingly finding new application in many radar and communication systems, due to the relatively low cost, small size and conformal mounting capabilities. This paper presents a design approach for a planar microstrip reflectarray antenna operating at 32.5 GHz. The single array element is a rectangular patch. The progressive phasing necessary to scan the main beam off the broadside is achieved by loading each element with a variable length transmission line. To validate this technique, number of arrays are built and measured, showing good agreement with CAD predictions.

Phasing and Array design and measurements- Of primary concern is to establish the phase of the reflected field as a function of the stub length. Using the MOM based EM simulator FEKO the optimized patch dimensions are W=3.325 mm and L=2.518 mm. It is shown that mutual coupling is negligible in microstrip arrays where substrate thickness, dielectric constant and beam scan angle are not excessively large. Based on these assumptions an easy single cell analysis can efficiently be implemented. The single cell is built on a truncated ground plane and dielectric slab. Using FEKO the loaded patch has been simulated in a cell of 0.46 lambda0x0.65lambda0. Figure 1 shows the single cell geometry and reflected phase as function of the stubs length.

Figure 1 (a) single cell dimension (b) Phase of the reflected field from a single patch cell versus stub length. Three different reflectarrays have been designed, built and measured at International Research Centre for Telecommunications-transmission and Radar (IRCTR) in Delft. All the examples are designed at 32.5 GHz; using a RT/Duroid 5870 substrate material with height of 0.381 mm and dielectric constant of 2.3 to scan the beam in E-plane. Figure 2 shows the geometry of three different array.

Figure 2 7x7 and 9x9 reflectarray scanning at (a) 20°, (b) 10°, (c) 20°.
Figure 3 shows the measurement results.

Figure 3 Measured and calculated E-plane patterns at 32.5 GHz for 7x7, 9x9 reflectarray scanning at (a) 20°, (b) 10°, (c) 20°.
Conclusions- Due to their mechanical feature and low cost, microstrip reflectarrays antennas represent an attractive perspective for many applications in telecommunication and radar. In this paper an easy design procedure for planar microstrip reflectarray using passive stubs has been developed at the Ka-band. Number of arrays have been built and measured. There is good agreement between the measurements and simulations.

Reflectarrays are very advantageous at mmWave frequencies regarding to their low losses compared to classical printed line fed arrays. Nevertheless, the high blockage of the primary source may decrease their performances. This problem is usually overcome by using a primary source as small as possible, i.e the open-ended circular or rectangular waveguide which affects the side lobe levels. It is of particular importance to consider the development of multibeam or beamscanning applications.

In this paper we propose a new primary source in order to achieve the best compromise between compactness and radiation characteristics. Influence of the primary source can be seen as a signal processing window for the overall far-field radiation pattern of the reflectarray. It is well known that various windows filtering such as Hamming or Blackman can reduce the off-mainlobe level, i.e. the side lobes. Nevertheless, optimized functions for this application are prolate spheroidals as described by Slepian and Pollak [1]. They are used for astronomy in the coronography for circular apertures[2]. Some antenna applications were also investigated in order to decompose the radiated field onto a base of prolate functions [3].

A program based on ray tracing theory was developed to simulate the radiation pattern of reflectarrays taking into account the blockage, the amplitude and the phase distribution of the primary source. A 200mm diameter reflectarray was simulated using a open-ended waveguide and a source corresponding to a prolate spheroidal function. Results shown in fig. 1 validate the concept with a 17 dB sidelobe level reduction. The corresponding primary source has been designed and simulated with France Telecom software (SRSRD) at 94 GHz (fig. 2), designed for structures with symmetry of revolution. In order to be able to compare rigorous simulations with measurements, we had to replace the reflectarrays by a PVC lens antenna of 200 mm diameter and f/D of 1. Comparisons between simulations of lens+open waveguide and lens+"prolate antenna" are illustrated in figure 3. The improvement of the sidelobe level with the "prolate antenna" is close to 15 dB. Measurements show a same tendency (fig.4). Upcoming measurements will be conducted on reflectarrrays. Same results are obtained in E-plane.

References

[1] D.Slepian and H.O Pollak:"Prolate Spheroidal wave functions, Fourier analysis and uncertainty", Bell System Technical Journal, January 1961

[2] R. Soummer, C. Aime and P.E Falloon: "Prolate Apodized Coronography: Numerical Simulations for Circular Apertures", Astronomy with High Contrast Imaging, C. Aime and R. Soummer (eds) EAS Publications Series (8) pp. 93-105, 2003

[3] D.R RHODES: "The Optimum Line Source for the Best Mean-Square Approximation to a Given Radiation Pattern", IEEE Transactions on Antennas Propagat. Vol 11 Issue 4 pp.440-446, July 1963

The aim of the abstract below is to present a particular structure, working as if it were a double array lens (Transmitarray). The structure consists of a patch array in the reception part, a phase delay for each patch and another patch array in the transmission part. The idea in using this device is to place it in front of a particular antenna in order to obtain two important advantages:
-correct the phase error of the antenna (i.e. a horn antenna).
-configure a new radiation pattern.

The architecture we are going to use implies the use of patches with via feeding. To avoid undesired coupling between the reception and the transmission arrays, we are placing ground planes that isolate the reception array, the phase delay and the transmission array, respectively. We use the architecture below:

We have designed the structure for 12 GHz. First, we simulate half the structure with Ensemble, in order to achieve proper radiation and matching results.

At last, we design and simulate the phase delay line for each patch in order to correct the phase error of the feeding antenna (in future designs we will configure new radiation patterns). We have a prototype under construction whose results will be presented in the complete EuCAP 2006 paper.

Planar arrays are a very interesting option to substitute reflector antenna because of their well-known characteristics of low profile, potential low cost, reliability and flexibility in achieving contoured beams and multiple beams with a simple planar geometry. Suitable solutions for space applications have been proposed using reflect-arrays with countered beams and multibeam. Another proposed solutions are transmit arrays. In this case, the antenna acts as a lens. These consist in a periodic planar array having two patch antennas connected by a line. One element receives the signal from -z direction and the other transmits the signal in the +z direction. By a proper selection of the phase delay in the connection line, the phase distribution in the transmitting array can be set. In a equal output phase configuration the transmitting array behaviour would be similar to the obtained with a parabolic reflector. With the advantage of removing the fed blockage. Moreover, lenses are less sensitivity to the thermo electrical distortions. While transmit array are less volume and mass than conventional lenses. However the design of the proper and suitable radiating elements to obtain a transmit array is not trivial because many difficulties appear due to the fact of the radiating configuration in the positive and negative directions. In the literature several configurations have been proposed. One of them is based in two slot coupled-patches [1], however, when the authors try to design a transmit-array array using this configuration in the X-band, no suitable solutions were found because the parallel plate waveguide that results from the presence of the two ground planes of the patches.
In this paper two radiating element are proposed: one directly fed patch and one slot coupled patch. These two element drive to two different transmit-array configuration. The first one use as transmitting and receiving element the direct fed patch. And the other uses the direct fed patch and the slot coupled patch. In both of them, only one ground plane is found, having been removed the parallel plate waveguide.
The design, simulations and measurement of these elements for a X band transmit-array are presented in this paper. Other critical issues that appear in a practical design such us coupling between element, periodic structure, and parasitic radiations are evaluated. These elements are considered suitable for a practical transmit array design.
Figures show the element configuration for both antenna concepts and photographs of the designed elements.
D.M. Pozar, "Flat lens antenna concept using coupled microstrip patches", Electroncis Letters, 7th November, 1996. Vol. 32, No. 23, pp. 2109-2110.

The main limitation of microstrip reflectarray antennas is related to their narrow bandwidth, caused by two primary factors, the narrow bandwidth of the radiating elements and the differential phase delay. For small reflectarray aperture (as large as 500), the dominant factor limiting the antenna bandwidth is generally the frequency band of the single radiator, so the design of wideband elements is the first task to be satisfied. As well known, the microstrip patch element can generally achieve a bandwidth of only 3 percent, which causes a non-linear behaviour of the reflection phase versus the patch length. As a matter of fact, a high slope near resonance and very slow variations near the extremes can be observed in the phase design curve, so giving a pronounced sensitivity to frequency variations in the neighbourhood of resonance. The use of stacked patches has been considered in literature to improve the reflectarray bandwidth [1] . Recently, an aperture coupled reflectarray element with smooth and linear phase variations has been presented by the authors [2]. This structure significantly reduces interference effects on the scattered field due to phase tuning elements usually located on the patches side. As a matter of fact, the prescribed radiation pattern is obtained without changing the geometry of the reflecting surface. Moreover, the above configuration can be simply integrated with passive load elements or active circuitries for beam scanning capabilities. The aperture coupled configuration is optimized in this paper to improve bandwidth features by exploiting the coupling between patches as a way to obtain a reactive load on each radiator. A bandwidth of about 13% is achieved when properly acting on the coupling between array elements, as demonstrated by the smooth and similar behaviour of the phase curves reported in fig.1. They are obtained as result of full-wave MoM simulations performed at frequencies f=9, 9.4, 9.8, 10, 10.2 GHz on a rectangular patch having dimensions L=8.09mm, W=14.05mm, printed on a dielectric substrate with r=2.5 and thickness t=0.762mm. A rectangular slot of length Ls=4.8mm is used for coupling the patch to a microstrip line of width Wm=2.177mm, having a fixed part of length Lf=3.25mm and a phase tuning stub with 1mm ≤ Lm ≤ 12mm, printed on the same dielectric substrate. Figure 1. Phase design curves at different frequencies References 1. J. A. Encinar, J. A. Zornoza, "Broadband design of three-layer printed reflectarrays", IEEE Trans. on Antennas and Propagat., vol. 51, no. 7, 2003, pp. 1662-1664. 2. S. Costanzo, F. Venneri, G. Di Massa, "Transmission line model for slot-coupled microstrip reflectarray antennas", 2004 URSI, International Symposium on Electromagnetic Theory, May 23-27 Pisa (Italy).

A two-layer reflectarray is proposed as a central station antenna for Local Multipoint Distribution System (LMDS) in the band 24-26 GHz. The reflectarray is illuminated by three feeds, which are used to generate independent beams in azimuth, in order to cover adjacent 30° sectors, see Fig 1. The beams are shaped both in azimuth and in elevation. Usually, a far-field model of the feed-horn antenna is used in the design process [1]. However, in this case the reflectarray elements are placed in the Fresnel near-field region of the feed-horns and the real field radiated by the feeds must be taken into account in the design process.

The real near-field radiated by the feed-horn has been obtained on the reflectarray elements in two different ways. In the first one, the incident near-field on each element of the reflectarray has been directly measured using a X-Y scan at Universität Ulm (Germany). In the second method, the far-field generated by the horn has been measured at Universidad de Oviedo (Spain) both in amplitude and phase. Then, equivalent currents on the horn aperture have been retrieved from the far-field measurement, and the near-field distribution on the reflectarray surface is calculated through near-field integral equations. Results from both measurements have been compared, showing a reasonable good agreement.

The antenna geometry and the design process using the far-field model of the feed-horn are detailed in [1]. The reflectarray elements are made up of two stacked layers of rectangular patches whose dimensions are adjusted to produce the required phase-shift. When the near field data is used to compute the radiation patterns of the reflectarray designed with the far-field model of the feed some distortions are observed, and therefore the design must be updated.

In the present work, the near field data from equivalent currents reconstruction have been included in the design process of the reflectarray for the central and lateral beams. The amplitude and phase of the incident near-field on each element of the reflectarray has been used to update the phase distribution required to produce the beam shaping. The new phase distribution is used to optimise the patch dimensions of the two-layer configuration. Finally, the radiation patterns are computed for the three beams, showing a beam shaping close to the requirements.

Fig. 1. Reflectarray geometry

[1] Manuel Arrebola, Jose A. Encinar, "Two-layer printed reflectarray as a multi-beam central station antenna", XX Nac. Symp. URSI, Gandia (Valencia, Spain), Sept. 2005.