Thin AMC Structure for Radar Cross-Section Reduction

3630 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 55, NO. 12, DECEMBER 2007 Thin AMC Structure for Radar Cross-Section Reduction Maurice Paquay, Member, IEEE, Juan-Carlos Iriarte, Iñigo Ederra, Ramon Gonzalo, Member, IEEE, and Peter de Maagt, Senior Member, IEEE Abstract—A thin artificial magnetic conductor (AMC) structure is designed and breadboarded for radar cross-section (RCS) Reduction applications. The design presented in this paper shows the advantage of geometrical simplicity while simultaneously reducing 16). The design is the overall thickness (for the current design very pragmatic and is based on a combination of AMC and perfect electric conductor (PEC) cells in a chessboard like configuration. An array of Sievenpiper’s mushrooms constitutes the AMC part, while the PEC part is formed by full metallic patches. Around the operational frequency of the AMC-elements, the reflection of the AMC and PEC have opposite phase, so for any normal incident plane wave the reflections cancel out, thus reducing the RCS. The same applies to specular reflections for off-normal incidence angles. A simple basic model has been implemented in order to verify the behavior of this structure, while Ansoft-HFSS software has been used to provide a more thorough analysis. Both bistatic and monostatic measurements have been performed to validate the approach. Index Terms—Artificial magnetic conductor (AMC), electromagnetic band gap structures, radar cross-section. I. INTRODUCTION R ADAR CROSS section considerations are generally associated with military platforms, especially since it became fashionably known as STEALTH techniques. However, it also finds its application in antenna and RCS measurement ranges. It is obvious that for the RCS measurements, it is recommended that the RCS of the support structure is less than the RCS of the target under test. But also for antenna measurements, the reflection of the positioner is of concern because it can cause a standing wave. This is known as range coupling. Especially in compact test ranges, with inherent low free space loss, the range coupling effect can be significantly reduced. RCS reduction can in general be obtained in two ways: application of radar absorbing materials (RAM) or coatings and shaping of the object. RAM transforms the electromagnetic energy into heat and generally carbon loaded material is used for this purpose. For broadband RAM, a smooth dielectric transition from air to RAM is required. In most cases, this is realized by using porous base material; the pyramidal foam absorbers Manuscript received September 11, 2006; revised May 25, 2007. This work was supported by Spanish project TEC2006-13248-C04-03/TCM. M. Paquay and P. de Maagt are with the Electromagnetics and Space Environments Division, European Space Agency TEC-EE, NL 2200 AG, Noordwijk, The Netherlands (e-mail: [email protected]). J.-C. Iriarte, I. Ederra, and R. Gonzalo are with the Electrical and Electronic Engineering Department, Public University of Navarra, Campus Arrosadia, E-31006 Pamplona, Spain (e-mail: [email protected]). Digital Object Identifier 10.1109/TAP.2007.910306 are well known examples. The mechanical properties of these materials are often conflicting with other requirements. For example, the aerodynamic properties of porous RAM are not very good and for low frequencies, the RAM needs to be impractically thick. On an antenna range positioner, there is only limited space available for applying RAM, so in many cases, too small absorbers are used and concessions are made to the quality of the measurements. Radar absorbing coatings or screens are based on a cancellation of multiple reflections, e.g., at the front and back of the layer. To create a phase difference between the various reflections, the layer has to have a well defined thickness. Inherently, the absorbing performance of these coatings and screens is narrowbanded. One implementation of RAM coating is the so-called Salisbury screen [1]. The geometry consists of a lossy resistive dielectric sheet placed a quarter of a wavelength above a perfectly conducting plane. By well matching the parameters of the dielectric sheet as well as the quarter wavelength spacer, the total reflection can be made zero. The disadvantages of the Salisbury screens are the overall thickness and the frequency and angular dependence. In their theoretical treatise, Fante and McCormack [2] showed that the overall thickness could be reduced by applying the sheet on a magnetic surface. In that case, the quarter wavelength spacing is not required anymore. An implementation was proposed by Engheta [3] who presented a sketch of an idea for application of metamaterial surfaces. The concept still resembles a Salisbury screen but the overall thickness is considerably reduced by using the perfect magnetic conductor behavior of the metamaterial. While an ordinary metal groundplane will act as a low impedance surface with reflec, a properly designed artificial magnetic contivity ductor (AMC) will have a very high impedance and reflectivity . This means that effectively it acts as a magnetic conductor. One application of this phenomenon is low profile antennas because the image current will be in phase with the antenna current [4]. Of course, the same effect can be used in the design of thin absorbers. A further thickness reduction was proposed by Kerr [5] and another implementation was presented in [6]. The principle of RCS reduction by shaping is to reflect the energy away from the source, i.e., to avoid reflection in the direction of the incoming wave. Shaping aspects involve checking the mutual orientation of all surface parts, not only for double reflections (dihedrals), but also for multiple reflections and for all angles of incidence. The resulting shape is quite often conflicting with other operational requirements. For measurement purposes, special RCS target supports, like ogive shaped pylons 0018-926X/$25.00 © 2007 IEEE PAQUAY et al.: THIN AMC STRUCTURE FOR RADAR CROSS-SECTION REDUCTION 3631 Fig. 2. Schematic model used for analyzing the chessboard structure. Fig. 1. Chessboard structure. The black squares represent the PEC elements and the white squares the AMC elements. The inset shows the unit cell. have been designed, but their load capacity is limited. Many problems could be avoided by suppressing the normal and specular reflection. The design proposed in this paper is based on a combination of AMC and PEC cells. Its principle of operation is based on the cancellation effect of both contributions. This approach has some relation with the one presented in by Walser et al. [7], who proposed a parquet surface of soft and hard electromagnetic cells. For the proposed configuration in this paper the PEC cells are implemented with a ground plane meanwhile the AMC cells are based on Sievenpiper [8] mushrooms connected to ground plane through vias. In the absence of any lossy components, the energy is not absorbed, but scattered in offset directions, much like it is accomplished with shaping. As a consequence specular reflection is considerably reduced as it will be shown within the paper. Fig. 3. Radiation features of the configuration presented in Fig. 2. Continuous line is the and planes and the dotted line is the 45 plane. YZ XZ them are in phase and the other 2 in counter-phase. This is indicated as follows; with and with within Fig. 2. If the radiation features of this configuration are analyzed, the radiating field of each single elementary antenna in Fig. 2 is represented by II. PRINCIPLE OF OPERATION The main idea is based on the design of a surface that reflects the impinging incident wave in phase and counter-phase at the same time. This can be achieved by metallic cells, reflecting incident waves with a 180 change of phase, and AMC cells introducing no phase change to the reflected wave at the working frequency. Combination of those two contributions leads to destructive interference, achieving a null in boresight direction. The power will be reflected in other directions depending on the design. The combination of these cells is arranged in such a way that any element, PEC or AMC is surrounded by the other type of element, see Fig. 1. A unit cell will be composed of 4 elements; two with PEC and two with AMC properties, see inset of Fig. 1. The idea behind the concept can easily be understood by turning to standard array theory. The unit cell can be modelled as a 2 2 antenna array formed by 4 elementary antennas. As a first approximation the four elementary antennas are assumed to radiate the same amount of power. However, two of The total field radiated by the array is then simply given by where the array factors are described by the following: Plotting the radiation characteristics into the , and the 45 planes when and , and , the results are well known and given for reference in Fig. 3. As described theoretically by this model, under normal incidence the power will mainly be scattered in the 45 plane ( and 135 ) at , so the RCS value will 3632 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 55, NO. 12, DECEMBER 2007 Fig. 4. Predicted bistatic reflectivity of chessboard for diagonal plane. be minimized under normal incidence. The scattering directions correspond to the grating lobes of the periodic structure. The and planes are zero comes from the reason that the fact that the array factor is zero for those planes. For incidences other than normal a phase difference between the array cells must be included in the analysis. Also in this case, the specular reflection at the structure under plane wave incidence for the model consisting of alternating regions of AMC and PEC separated by lambda is zero irrespective of the incident angle. incidence, the obtained In the diagonal plane, i.e., results are presented in Fig. 4. In this case the model is equivalent to two AMC (or PEC) cells apart with 2 PEC (or AMC) cells in between them. In the horizontal axis the incidence-angle is depicted, meanwhile the vertical axis represents the scattered angle. The (lower left to upper right) diagonal plane corresponds to the specular reflection, which is zero due to the cancellation effect explained above. The ridges are created by the grating lobes generated as a result of the periodicity larger than . These gratings lobes appears at angles different to the specular reflection irrespective of the incident angle. III. STUDY OF FEASIBILITY To verify the idea, a chessboard layout as described previously has been realized. This structure consists on the combination of two kinds of elements, on a square basis, i.e., PECs and AMCs. The distribution is similar to a “chessboard,” where the “black” cells are fully metalized parts and the “white” cells are filled with an AMC structure. Both cells have the same square area. The AMC mushroom structure as described by Sievenpiper [8] was selected as the AMC part of the chessboard in the array configuration. A full analysis of this configuration was carried out by means of the commercial software Ansoft-HFSS. The dimensions of the mushroom structure were determined to have 0 phase reflection at 15 GHz. The structure was manufactured on the selected substrate ROGERS 3010 with a thickness of 1.27 mm and a dielectric Fig. 5. (a) Manufactured chessboard structure, (b) detail of the cells, (c) detail of the AMC-Sievenpiper structure. Fig. 6. Unit cell chessboard structure. constant of 10.1. The dimensions obtained for the AMC part were rectangular squares of 1 mm width with a period of 1.4 mm. The radius of the pin is 0.2 mm Once the AMC structure has been defined as stand-alone, the whole chessboard structure was analyzed. Element dimensions . A 14 14 AMC were fixed to 19.55 19.55 array was used to fill in the AMC area. The manufactured chessboard structure can be seen in Fig. 5 together with some close-ups. The total size of the chessboard is 431.2 mm by 274.4 mm which corresponds with a rectangular structure formed by 22 14 elements. As the complete chessboard structure is too computationally intensive to be fully modelled, a unit cell configuration based on a 2 2 array as depicted in Fig. 6 was analyzed applying appropriate boundary conditions. The unit cell was surrounded by symmetry conditions on the side faces simulating an infinite board. A top radiation boundary condition was fixed in order to calculate the scattered power produced by the reflected field under a normal plane wave incidence. PAQUAY et al.: THIN AMC STRUCTURE FOR RADAR CROSS-SECTION REDUCTION 3633 Fig. 9. Measurement set-up. Fig. 7. Frequency dependence of the directivity value at boresight direction of the scattered pattern for the proposed chessboard and for the PEC-alone structures. Fig. 10. H and V polarization reflectivity as function of the frequency for normal incidence. Fig. 8. Scattered pattern from a 5 soft-HFSS. 2 5 chessboard structure obtained with An- By extracting the directivity value in the boresight direction from the scattered pattern as function of the frequency it is possible to obtain the operational bandwidth of the structure as presented in Fig. 7. It can be seen that the directivity of the reflected pattern has a minimum value at 15.32 GHz, which corresponds with the operational frequency, i.e., no reflected wave. The slight displacement with respect to the 0 reflected phase frequency, 15 GHz, can be attributed to the effect of the finite periodicity of the mushroom cell. It is also clear from that figure that the current behavior of the proposed configuration is narrow band. In order to prove the validity of the proposed argument a comparison with a modified chessboard configuration where the AMC parts are removed is presented also in Fig. 7. As it can be seen, there is not cancellation effect at boresight proving that the improvement comes as a result of the cascade of zero phase shift and phase reversal cells (and not as a result of the grating lobes alone). The shape of the scattering pattern for the reflected field at 15.32 GHz is presented in Fig. 8. As was expected from the modelled configuration, it presents a minimum value at ( -direction) and the power is scattered at , and 135 . The and planes are not zero for any value of theta contrary to the predicted results in Fig. 3. This is due to the fact that the simple model is assuming a perfect phase cancellation. However, this does not hold for a real chessboard structure as it is shown in Fig. 8. However, the difference between the maximums of scattered power versus the and planes is still larger than 20 dB. This issue is explained in more detail in Section IV together with the measured results. IV. EXPERIMENTAL RESULTS To verify the theoretical results experimentally, a bistatic RCS set-up was to be realized (see Fig. 9). This was done by using a positioner of the type roll-over-elevation-over-linear slide-over Azimuth. By setting the elevation to 90 deg, the roll table becomes an upper azimuth stage. The chessboard sample was mounted in a vertical position on top of this upper azimuth stage. The linear slide on top of the lower azimuth stage is normally used to position the roll-over-elevation part in an offset-position to the azimuth axis in order to put the device under test (DUT) over this centre of rotation. Now, the receive antenna was mounted at the end of linear slide at a distance of about 1.8 m from the azimuth axis. The upper roll-over-elevation part was positioned in such a way that the axis of the upper azimuth stage (roll) was co-aligned with the lower azimuth axis. So the lower azimuth stage controlled the angle of the receive antenna while the upper azimuth stage controlled the orientation of the sample. The transmit antenna was at a fixed position. The first set of measurements corresponds to the study of the frequency dependence of the RCS for normal incidence. This 3634 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 55, NO. 12, DECEMBER 2007 Fig. 13. Monostatic results for H-polarization landscape orientation. Fig. 11. Normalized bistatic reflectivity of chessboard for landscape orientation. Fig. 14. Phase response of the AMC part as function of the angle of incidence. Fig. 12. H-polarization scattered wave at the resonant frequency as function of the angle. measurement was done for V-vertical and H-horizontal polarization. The results have been depicted in Fig. 10. It is observed that the resonant frequency, where the wave is scattered away from boresight is around 15.25 GHz and 15.5 GHz for vertical and horizontal polarizations respectively, very close to the predicted values. It is noticeable that in this case the V-polarization has a different resonance frequency than the H-polarization although the difference is below 2%. The most likely explanation for this deviation is, on one hand, due to geometrical imperfections; the Sievenpiper mushrooms are slightly rectangular instead of perfectly square, the grounding pins are slightly offset from the centre of the metal surface (see Fig. 2) and on the other hand, the overall fabricated chessboard is rectangular. The reflectivity has been measured for all combinations of incidence and scattered angle up to 60 deg off boresight at 15.4 GHz. There is no significant difference between the H and V polarizations results. The results for the H-polarization are shown in Fig. 11. The horizontal axis is the incidence-angle, and the vertical axis depicts the scattered angle. For the sake of reference, for each incidence angle the results have been normalized with respect to the specular reflection under the same incident angle obtained with an equal size metal plate. For a better understanding of this figure, a few special cases will be given extra attention. Along the diagonal line from lower left to upper right corner the scattered angle is equal to the incidence angle, i.e., the specular reflection. The central part of this diagonal corresponds to normal incidence. For incident angles close to it the cancellation works and the specular reflection is very low. For larger incidence angles the specular reflection grows, due to the non perfect cancellation. Clearly visible are the “ridges” parallel to the diagonal. These are created by the periodicity of the structure as it was discussed in Section II. Fig. 12 is a vertical cut for a constant incidence angle of 0 , showing the scattering of a boresight illumination. This is equivalent to a central vertical cut of Fig. 11, with the difference that Fig. 12 shows both the curves of the chessboard and the reference plate while in Fig. 11 the difference between the two is depicted. The “ridges” of Fig. 11 appear here as sidelobes be. yond Fig. 13 shows a (upper left to lower right) diagonal cut of Fig. 11, i.e., . This is equivalent to the monostatic case in which the direction of observation is in the direction of the incident wave. High scatter lobes appear . These lobes are generated by the grating lobes of around the structures as was discussed previously. PAQUAY et al.: THIN AMC STRUCTURE FOR RADAR CROSS-SECTION REDUCTION 3635 (a) 2 Fig. 15. Reflected lobes from a 5 5 chessboard structure when the incident plane wave is (a) 10 deg., (b) 30 deg., and (c) 40 deg. Due to the measurement setup there can be some shadowing effect in this case and the values obtained in the monostatic case could be somewhat affected. However, the geometrical size of the chessboard structure should be larger than the shadow. Furthermore, the same shadowing effect should also be present in the case of the metallic reference plate. Therefore the ratio of both quantities is assumed to be unaffected. Comparing these results with the theoretical ones described in Section II based on the simple analysis, it is clear that this simple model is not able to explain all details. Theoretically no lobes should be produced in the monostatic case. However, the discrepancy can be explained by considering the phase reflection of an infinite AMC structure as a function of the angle of incidence [9] (see Fig. 14). The reflected phase value depends on the angle of incidence; the phase value increases from 0 to higher values when the angle of incidence is increased. This means that total cancellation will not be obtained for incident angles away from normal incidence. (b) Fig. 16. Predicted bistatic reflectivity of chessboard for (a) landscape and (b) diagonal orientations. Fig. 17. Bistatic reflectivity of the chessboard in the diagonal plane. A more thorough analysis with Ansoft-HFSS software has been carried out in order to provide a confirmation of these findings. Fig. 15 shows the reflected lobes from the chessboard 3636 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 55, NO. 12, DECEMBER 2007 Fig. 18. Scattered field in the chessboard 45 plane for the H-polarization. Background and the reference (metallic plate) are also plotted for clarity. , structure for several angles of incidence ( ) at working frequency. From this analysis it can be seen that there are some reflected lobes appearing in the plane when the angle of incidence is different from zero. Focusing on Fig. 15(b) it is possible to observe that a significant plane ( . amount of reflected power is in the and ), nevertheless these lobes were not visible in the measurements, since the measurements were performed and 45 cuts. Besides these, other lobes only in the plane, which correspond with the measured appear in the ones shown in Fig. 13. For a full numerical characterization of the chessboard structure, Fig. 16(a) shows the analysis of the chessboard scattering with a progressive variation of the phase value produced by the AMC part following the results derived in Fig. 14. Good agreement between simulations and measurements is obtained. The next set of measurements corresponds with the diagonal orientation. The chessboard structure has been tilted 45 and the scattered wave has been recorded for the two polarization cases, H and V. The reflectivity for this configuration for all combinations of incidence and scattered angle are shown in Fig. 17. In this case, only V polarization is presented as the results for both polarizations are the same. As previously, the results have been normalized for each incident angle with respect to the specular reflection of an equal size metal plate. If the same analysis as the one done for the landscape orientation is carried out for the diagonal incidence the results shown in Fig. 16(b) are obtained, in which the cancellation in the specular reflection is also lost for angles other than normal incidence. The agreement between simulations and measurements is pretty good. Fig. 18 corresponds to the vertical cut for a constant incidence angle of 0 . Also here, the figure shows the curves of the chessboard and reference separately. As predicted with the model [see Fig. 16(b)], the scattered , power by this structure is concentrated around the , 135 , 225 and 315 angles as it can be seen in Fig. 19. Monostatic results for the diagonal plane. Fig. 18. The scattered wave produced by the reference metallic plate and the background are also plotted for comparison. Fig. 19 shows the monostatic reflection for the diagonal case. In this plot, the background and the reference case results are also depicted. The lobes around 20 are again created by the grating lobes of the structure. They can also be explained as constructive interference of the chessboard elements with either a spacing and opposite reflection coefficient (the single path length dif) or a spacing of and identical reflecference is then tion coefficient (single path length difference is ). The lobes around 45 can only be explained as constructive interference of identical chessboard elements spaced . As in the previous case, there is no difference between polarizations. In order to determine the frequency behavior another set of measurements was performed. The chessboard structure was measured under normal incidence and the scattered field was recorded as function of the frequency (see Fig. 20). In the lower half of the picture, the PEC behavior is clearly visible, which demonstrates itself as one large peak at boresight. Above 15 GHz, the AMC’s elements start exhibiting a 0 phase response. This can be seen in the figure as a splitting of the single PAQUAY et al.: THIN AMC STRUCTURE FOR RADAR CROSS-SECTION REDUCTION 3637 [6] S. A. Treyakov and S. I. Maslovsi, “Thin absorbing structure for all incidence angles based on the use of a high-impedance surface,” Microw. Opt. Tech. Lett., vol. 38, no. 3, pp. 175–178, Aug. 2003. [7] R. M. Walser, A. P. Valanju, W. Win, M. F. Becker, R. W. Bene, and B. Buckman, “New smart materials for adaptative microwave signature control,” in SPIE, 1993, vol. 1916, pp. 128–134. [8] D. Sievenpiper, D. L. Zhang, R. F. J. Broas, N. G. Alexopolous, and E. Yablonovitch, “High-impedance electromagnetic surfaces with a forbidden frequency band,” IEEE Trans. Microw. Theory Tech., vol. 47, no. 11, pp. 2059–2074, Nov. 1999. [9] C. R. Simovski, P. de Maagt, S. A. Tretyakov, M. Paquay, and A. A. Sochava, “Angular stabilisation of resonant frequency of artificial magnetic conductors for TE-incidence,” Electron. Lett., vol. 40, no. 2, pp. 92–93, Jan. 2004. Fig. 20. Boresight scattered field results under normal incidence as function of the frequency. peak into two peaks (upper half of the figure). The operational bandwidth of the structure can be inferred from this figure. Maurice Paquay received the M.S. degree in electrical engineering from Eindhoven University of Technology, The Netherlands in 1987. He started his career at the Radar Group of the TNO Physics and Electronics Laboratory, the Hague, The Netherlands, as a Radar Antenna Engineer where he later progressed to near field measurements. For two years, he was with Thales Netherlands as a Radar System Engineer. Currently, he is an Antenna Measurement Engineer at the European Space Agency’s Research and Technology Centre (ESA-ESTEC) in Noordwijk, The Netherlands. Mr. Paquay received the L. K. Wilson Award for his contributions to student activities in Europe in 1986. V. CONCLUSION The principle of using AMC structures for thin RCS reducing materials has been validated both theoretically and experimentally. The so-called chessboard structure is a thin and rigid material that scatters the incident wave in off-normal or off-specular directions. A full set of bistatic and monostatic measurements has been performed to verify the predictions. Reduction of the RCS value larger than 20 dB has been obtained with respect to a reference metallic plate for normal incidence. For other incidences the structure is also working but it is suffering from some limitations, non full cancellation, due to the angular dependent behavior of the AMC parts. Although the current structure is limited by a narrow bandwidth, new designs to improve the bandwidth are now in progress. REFERENCES [1] W. W. Salisbury, “Absorbent Body for Electromagnetic Waves,” U. S. Patent 2 599 944, Jun. 10, 1952. [2] R. L. Fante and M. T. McCormack, “Reflection properties of the Salisbury screen,” IEEE Trans. Antennas Propag., vol. 36, no. 10, pp. 1443–1454, Oct. 1988. [3] N. Engheta, “Thin absorbing screens using metamaterial surfaces,” in Proc. IEEE Antennas Propagation Societ Int. Symp., 2002, pp. 392–395. [4] J. M. Baracco, M. Paquay, and P. de Maagt, “An electromagnetic bandgap curl antenna for phased array applications,” IEEE Trans. Antennas Propag., vol. 53, no. 1, pp. 173–180, Jan. 2005. [5] D. J. Kern and D. H. Werner, “A genetic algorithm approach to the design of ultra-thin electromagnetic bandgap absorbers,” Microw. Opt. Tech. Lett., vol. 38, no. 1, pp. 61–64, Jul. 2003. Juan Carlos Iriarte was born in Pamplona, Navarra, Spain, in 1978. He received the Ingeniero de Telecomunicación degree from the Universidad Pública de Navarra (UPNA), Pamplona, Spain, in 2002, where he is currently working toward the Ph.D. degree. Since July 2001, he has been with the Antennas Group at the Electrical and Electronic Engineering Department in UPNA. From September 2002 to March 2004, he was involved in the design of electromagnetic band gap antennas and, since March 2004, he has been a Research Assistant. His current areas of research are in the field of electromagnetic bandgap structures for microwave and millimeter wave antenna applications with emphasis on space antenna applications, design of arrays on EBG substrates using different kinds of EBG structures as metallic ones. Mr. Iriarte received a Grant from the UPNA. Iñigo Ederra was born in Isaba, Navarra, Spain in 1972. He received the Ingeniero de Telecomunicación and Ph.D. degrees from the Universidad Pública de Navarra, Pamplona, Spain, in 1996 and 2004, respectively. In 1997, he joined the Microwave and Millimetre Wave Group, Universidad Pública de Navarra, where he was involved in the study of high-power millimeter-wave components. From 1999 to 2000, he was with the European Space Research and Technology Centre (ESTEC), ESA, Noordwijk, The Netherlands, where he was working on electromagnetic bandgap materials and their applications in the field of antennas. Since 2001, he has been with the Antenna Group, Universidad Pública de Navarra. From June to October 2002, he was Visitor Scientist at the Rutherford Appleton Laboratory, Chilton, Didcot, U.K., participating in the Startiger project. His research interests are in the field of electromagnetic bandgap materials and metamaterials and their applications in microwave and millimeter wave components and antennas. 3638 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 55, NO. 12, DECEMBER 2007 Ramón Gonzalo (S’95–M’04) received the M.Sc. degree in telecommunication engineering (with honors) and the Ph.D. degree in telecommunications from the Public University of Navarra (UPNA), Navarra, Spain, in 1995 and 2000, respectively. Since October 1995, he has been with the Electrical and Electronic Engineering Department in UPNA. From September 1997 to December 1998, he was a Research Fellow in the Antenna Section at ESA-ESTEC, Noordwijk, The Netherlands, where he was involved in the modelling and design of EBG antennas at microwave and millimeter wave frequencies. He has been involved in more than 20 research project acting as coordinator in eight of them. He has supervised two Ph.D. and more than 18 M.Sc. dissertations. His current areas of research are in the field of photonic band structures for microwave and millimeter wave antenna applications with emphasis on space antenna applications and design of imaging arrays at submillimeter wave frequencies. Peter de Maagt (S’88–M’88–SM’02) was born in Pauluspolder, The Netherlands, in 1964. He received the M.Sc. and Ph.D. degrees from Eindhoven University of Technology, Eindhoven, The Netherlands, in 1988 and 1992, respectively, both in electrical engineering. In the period 1992/1993 he was Station Manager and Scientist for an INTELSAT propagation project in Surabaya, Indonesia. He is currently with the European Space Research and Technology Centre (ESTEC), European Space Agency, Noordwijk, The Netherlands. His research interests are in the area of millimeter and submillimeter-wave reflector and planar integrated antennas, quasioptics, electromagnetic bandgap antennas, and millimeter- and submillimeter-wave components. Dr. de Maagt was co-recipient of the H. A. Wheeler Award of the IEEE Antennas and Propagation Society for the Best Applications Paper of 2001. He was granted a European Space Agency Award for Innovation in 2002. He was co-recipient of Best Paper Awards at the Loughborough Antennas Propagation Conference (LAPC) 2006 and the International Workshop on Antenna Technology IWAT) 2007. He serves as an Associate Editor for the IEEE TRANSACTION ON ANTENNAS AND PROPAGATION and was co-Guest Editor of the November 2007 Special Issue on Optical and THz Antenna Technology.