Integration ofpin Diodes with Slot Embedded Patch Elements for Active Reflectarray Antenna Design

Transcription

1 1st IEEE International Symposium on Telecommunication Technologies Integration ofpin Diodes with Slot Embedded Patch Elements for Active Reflectarray Antenna Design M. Inam * and M. Y. Ismail Wireless and Radio Science Centre (WARAS), University Tun Hussein Onn Malaysia, Batu Pahat, Johor, Malaysia muhammad_inamabbasi@yahoo.com, yusofi@uthm.edu.my Abstract-This work provides an investigation on the slot embedded patch configurations to be used as elements for the active reflectarray design. Rectangular slots have been introduced in the centre of the rectangular patch elements designed in the X-band frequency range. Waveguide scattering parameter measurements has been performed for the proposed slotted elements and a close agreement has been shown between the simulated and measured results in terms of resonant frequency, reflection loss and reflection phase. The use of PIN diodes integrated with the slotted patch elements has been successfully demonstrated for the design of active reflectarrays. It has been shown that frequency tunability from 9.64 GHz to 8.88 GHz can be achieved by using different states of PIN diode with O.5W slotted patch element. Moreover the individual lumped component properties of PIN diodes have been investigated thoroughly for the optimum design of active reflectarrays. Keywords-active reflectarray antenna; scattering parameter measurements; PIN diodes;frequency tunability I. INTRODUCTION A low profile printed reflectarray is considered as a promising alternative to the bulky parabolic reflectors and expensive phased array antennas for radar and long distance communications. It was introduced by D.G. Berry, R.G. Malech and W.A. Kennedy in 1963 [1]. However the low profile printed reflectarray is a fairly new concept which combines some ofthe best features ofparabolic reflector and phased array antennas. A feed antenna placed at a particular distance illuminates the array whose individual elements are designed to scatter the incident field with proper phase values required to form a planar phase surface in front of the aperture [2]. For proper phase requirements, different techniques such as, identical patches of variable-length stubs [3], square patches of variable sizes [4], and identical planar elements of variable rotation [5] are used. The significant advantages of the reflectarray antennas are of small size, low cost and easy deployability in any situation as compared to parabolic reflectors and phase array antennas. Although reflectarrays offer many advantages, the major shortcoming occurs when the bandwidth performance is compared with the parabolic reflector [6-9]. The bandwidth of the reflectarray can be improved by a number of single layer and dual layer techniques reported by various researchers [10-13]. In active reflectarrays, the reflected phase from each of the resonant element can be controlled. Hence the reflected beam can be directed in the desired direction which makes a reflectarray capable ofachieving a wide-angle electronic beam scanning. Such beam forming approach can have many advantages over traditional tunable antenna array architectures, including a major reduction in hardware required per element and increased efficiency [14]. There have been a considerable research in beam steering antennas such as the use of non-linear dielectric materials [15-17], the integration of Radio Frequency Micro Electro Mechanical Systems (RF MEMS) as switches in order to control the contributed phase from each element [18, 19], loading varactor diodes with the patch elements and varying the varactor capacitance by using various biasing [20, 21] and using aperture coupled elements where the tuning circuit can be located on the non resonating surface ofthe element [22]. In this work, a reflectarray element based on the use of a rectangular patch with an embedded rectangular slot is proposed. Two patch unit cells with different type of slots embedded into the patch elements have been designed using Rogers RT/Duroid 5880 dielectric substrate and the scattering parameters have been measured using waveguide simulator technique. Analytical model of integrated reflectarray elements based on Finite Integral Method (FIM) is verified experimentally. Additionally the proposed reflectarray design is also incorporated with PIN diodes in order to design a tunable antenna configuration. II. PROPOSED DESIGN In this work, unit cells with rectangular slot embedded in the patch element has been used for the design ofreflectarrays in the X-band frequency range. Apart from providing the space for the integration ofactive elements the introduction of slot in the patch elements also provides a number of performance improvements. The slot embedded design modifies the surface current distribution on the patch element and alters the electric field intensity inside the substrate. For this reason the progressive phase distribution in a reflectarray can be achieved by varying the width of the slot. This eliminates the need of varying the patch dimensions for frequency sweep and provides a constant inter-element spacing, hence reduces the chances of mutual coupling. Fig. 1 shows the current distribution on the surface of a single reflectarray element using rectangular patch with and without slot. This research work is fully funded by Best Project of Fundamental Research Grant Scheme (FRGS, VOT 0983) and Prototype Research Grant Scheme (PRGS VOT 0904) awarded by Ministry of Higher education, Malaysia. *Corresponding Author /12/$ IEEE 151

2 III. SCATTERING PARAMETER MEASUREMENTS In order to carry out the scattering parameter measurements, various samples of the patch elements with rectangular slot configurations have been fabricated using Rogers RT/d 5880 as shown in Fig. 3. Recently, waveguide scattering parameter measurements technique for infinite reflectarrays has been demonstrated in [24]. Similar technique has been applied here for scattering parameter measurements of the fabricated reflectarray unit cells with slot embedded patch elements. Fig. 4 shows the measured and simulated reflection loss curves for different values of rectangular slots embedded in the patch. The width of the slot is varied from 0.3W to 0.5W and a change in resonant frequency is shown to occur from 9.8GHz to 8.3GHz while the reflectarray patch without slot resonates at 10GHz. The measured reflection loss also increased from 1.9dB to 4.1dB when the slot size is varied from 0.3W to 0.5W. The reason behind this trend is that the rectangular slots with variable width decreases the electrical dimensions of the patch element and lengthens the current distribution on the surface of the patch. Figure 1. Surface current distribution on a patch element without slot and a patch element with a rectangular slot A significant modification in the current distribution can be observed by the introduction of slot in the patch element. The modification of the surface current distribution on the patch element, shown in Fig. 1, is due to the fact that the effective area of the conducting material (copper) is reduced because of the extraction of slot from the patch element. This decreases the surface current density (J) which furthermore reduces the amount of current (I) according to Maxwell's equations [23]. The reduction of surface current density on the conducting material causes a decrease in the electric field intensity as well. This results in an increase in the electrical dimensions of the patch element and hence causes a decrease in resonant frequency. A detailed investigation of the effects of slot configurations on resonant frequency and reflection loss of reflectarray has been carried out using commercially available simulation tool of CST Microwave studio. The analysis is based on infinite reflectarray because of a well known fact that infinite reflectarray provides the best bases for the analysis of a very large array. For this purpose, a unit cell has been designed and proper boundary conditions are employed in order to represent and analyze an infinite reflectarray. Fig. 2 shows the setup of a single patch element with electric walls and magnetic walls around it created by the boundary conditions. This setup creates multiple images of a single patch element and hence investigates the behavior of an infinite reflectarray. Figure 4. Measured and simulated reflection loss curves for reflectarray with rectangular slots Figure 2. Setup of a single patch element unit cell with boundary conditions Figure 5. Measured and simulated reflection phase curves for reflectarray with rectangular slots Figure 3. Fabricated X-band unit cell slot embedded patch elements 152

3 The decreased dimensions cause a downward shift in the resonant frequency while the lengthened current distribution causes an increase in the loss performance. Moreover, despite the differences in the reflection losses due to the interconnection, the trend of the loss performance of both measured and simulated reflectarrays is identical. The variation in the reflection phase of the reflectarrays with different width of slots in the patch element is shown in Fig. 5. It can be observed from Fig. 5 that both the measured and simulated reflection phase curves are in good agreement with each other. The slight difference that can be observed in the measured and simulated reflection phases is due to the differences in the loss performance ofmeasured and simulated reflectarrays. Moreover it can be observed from Fig. 5 that the slope ofthe reflection phase curve is increasing as the width of the slot is increased, causing a loss in the bandwidth performance. This effect can be attributed to the increased reflection loss with increasing slot width as shown in Fig. 4. The results shown in Fig. 4 and Fig. 5 provide the viability for using these slot embedded patch elements for a periodic reflectarray design with progressive phase distribution as well as the design of active reflectarrays by integrating lumped components. In this work, the use of these proposed slot embedded patches has been demonstrated for the active reflectarray design by integrating with lumped components. The details of the design and analysis are discussed in Section IV. Figure 7. Reflection loss for different states ofpin diodes with rectangular slots in centre of patch IV. ACTIVE REFELCTARAY CONFIGURATION Scattering parameter measurements of different slot embedded patch elements has provided the feasibility of utilizing the proposed configuration for the active reflectarray design. Therefore in this work, PIN diodes are proposed to be loaded with slotted patch elements in order to design an X band tunable reflectarray for beam steering applications. The ON and OFF states of a PIN diode provide different values of the lumped components and hence offers frequency tunability. A rectangular slot with 0.5W width has been used with an APD PIN diode. The APD PIN diode can operate at high frequencies and has low capacitance and low series resistance. Hence it provides lower losses and more tunability when used in tunable reflectarray design. Fig. 6 shows the design of frequency switchable reflectarray using a rectangular slot and a PIN diode. Figure 6. Proposed active reflectarray cell design configuration Figure 8. Reflection phase for different states ofpin diodes with rectangular slots in centre of patch The PIN diode has been used in its two different states and its effects on the reflection loss and resonant frequency has been observed. Fig. 7 and Fig. 8 show the reflection loss and reflection phase curves respectively. It can be observed from Fig. 7 that the introduction ofpin diode causes a change in the reflection loss and resonant frequency of reflectarray. The resonant frequency of the design was 8.63GHz with a reflection loss of -0.59dB without a diode, while in the OFF state of diode the resonant frequency was shifted to 9.64GHz with a loss of-0.37db. This change in resonant frequency and reflection loss is due to the additional capacitive affect added by the PIN diode when operated in the OFF state. On the other hand, in the ON state of diode the reflection loss increases up to -0.86dB with a resonant frequency of 8.88GHz. This increase in loss is due to the series resistance of the PIN diode. The change in the resonant frequency and loss by the introduction of the PIN diode is also visible in the reflection phase curves shown in Fig. 8. The reflection phase of an active reflectarray design can be characterized by dynamic phase range (LlqJd) which is 153

4 defined as the difference between the phases of two phase curves measured at the mean resonant frequency [10]. A dynamic phase range of has been shown in Fig. 8 which demonstrates the feasibility of realizing an active reflectarray using the proposed design. The proposed design of PIN diodes integrated with slotted patch elements can be used for the design of beam switchable reflectarray antennas. These antennas can be of vital importance in radar applications, radio astronomy, deep space communications and satellite tracking antenna applications. In order to further investigate the effect ofdiode properties on the loss and tunability performance of reflectarrays, diode resistance and diode capacitance have been studied separately using CST MWS. Fig. 9 shows the effect ofvarying PIN diode resistance on the reflection loss and resonant frequency of reflectarray with rectangular patch and a rectangular slot in the centre. It can be observed from Fig. 9 that the resonant frequency ofthe reflectarray does not change with the increase of diode resistance. However the reflection loss increases linearly from 0.67dB to 2.38dB with an increase in diode resistance from 1 ohm to 10 ohms. This is because the resonant frequency only depends on the inductance and capacitance of the design while the increase in loss is due to the increased resistance which adds up in the conductor loss ofthe design. Fig. 10 shows the effect of PIN diode capacitance variation on surface current density and electric field intensity of reflectarray unit cell. It can be observed that the surface current (I) decreases from 5805A/m to 170A/m as the capacitance is varied from 0.1pF to 1.0pF. On the other hand electric field intensity (E) decreases from 260kV1m to 48kV1m with an increase in capacitance from 0.1pF to 1.OpF. This effect on the surface current and electric field intensity affects the resonant frequency and reflection loss of reflectarray as shown in Fig. 11. It can be observed from Fig. 11 that the resonant frequency decreases from 9.64GHz to 8.91GHz and reflection loss increases from -0.37dB to -0.48dB as the PIN diode capacitance is increased from 0.1pF to 1.OpF. The decrease in resonant frequency with an increase in capacitor value is due to the inverse relationship of frequency and capacitance in an LC resonant tank. Figure 10. Effect of PIN diode capacitance variation on surface current density and electric field intensity of reflectarray unit cell Figure 11. Effect of PIN diode capacitance variation on the resonant frequency and reflection loss of reflectarray On the other hand, the loss increases with an increase in capacitance because the increased capacitance value has the same effect as the increased dielectric permittivity which increases the dielectric loss ofthe reflectarray antenna. V. CONCLUSION Rectangular patch elements with rectangular slots have been proposed for an active reflectarray design. The feasibility of utilizing the proposed slot embedded patch element configurations for the active reflectarray design has been demonstrated through simulations and scattering parameter measurements. PIN diodes are proposed to be loaded with slotted patch elements in order to design tunable reflectarray for beam steering applications. The ON and OFF states of a PIN diode provides different values ofthe lumped components and hence offers frequency tunability. Moreover the investigation of different PIN diode properties provides important criteria for the selection of suitable active components for the active reflectarray design. Figure 9. Effect of PIN diode resistance variation on the resonant frequency and reflection loss of reflectarray 154

5 ACKNOWLEDGMENT The authors would like to thank the staff of Wireless and Radio Science Centre (WARAS) of University Tun Hussein Onn Malaysia (UTHM) for the technical support. REFERENCES [1] G. D. G. Berry, R. G. Malech, and W. A.Kennedy, "The reflectarray antenna", IEEE Trans. Antennas and Propagation, Vol. AP-ll, Nov [2] D. M. Pozar, S. D. Targonski, and H. D Syrigos, "Design of millimeter wave microstrip reflectarrays", IEEE Trans. Antennas and Propagation, Vol. 45, NO.2, pp , February [3] R. D javor, X. D. Wu, K. Chang, "Design and performane of microstrip reflectarray antenna", IEEE Trans. Antennas and Propagation, Vol. 43,No. 9 pp , Sep [4] S. D. Targonski and D. M. Pozar, "Analysis and design of a microstrip reflectarrayusing patches of variable size", IEEE AP-S/URSI Symposium, Seattle,Washington, pp , June [5] 1. Huang and R. 1. Pogorzelski,"Microstrip reflectarray with elements having variable rotation angle", IEEE AP-S Symposium Digest, pp , April [6] M. Y. Ismail and M. Inam, "Analysis of Design optimization of Bandwidth and Loss Performance of Reflectarray Antennas Based on Material Properties". Modern Applied Sci. J CCSE., Vol. 4, No.1, pp , [7] S. D. Targonski and D. M. Pozar, "Analysis and design of microstrip reflectarray using patches of variable size" IEEE AP-S/URSIInt Symp. Dig., Seattle, WA, pp ,1994. [8] M. E. Biallowski and 1. Encinar, "Reflectarray: Potential and Challenges" International Conference on Electromagnetics in Advanced Applications, pp , (ICEAA) [9] D. M..Pozar and S. D. Targonski, "A shaped-beam Microstrip patch reflectarray" IEEE Transactions on Antennas propogation, Vol. 47, No. 7, pp , [10] M. Y. Ismail and M. Inam, "Performance Improvement of Reflectarrays Based on Embedded Slots Configurations". Progress In Electromagnetics Research C, Vol. 14, pp ,2010. [11] 1. Huang and 1. Encinar, Reflectarray Antennas, Wiley, interscience, [12] K. Y. SZE and L. Shafal, "Analysis of phase variation due to varying patch length in amicrostrip reflectarray" IEEE Trans. Antennas and Propagation, Vol. 46, No.7, pp [13] 1. Huang, "Analysis of microstrip reflectarray antenna for microspacecraft applications" Spacecraft Telecommunications Equipment section, TDA Progress report , February 15,1995. [14] S. V. Hum, M. Okoniewski and R. 1. Davies, "Realizing an Electronically Tunable Reflectarray Using Varactor Diode-Tuned Elements". IEEE Microwave and Wireless Components Letters,Voi. 15, pp , [15] M. Y. Ismail, W. Hu, R. Cahill, V. F. Fusco, H. S. Gamble, D. Linton, R. Dickie, S. P. Rea and N. Grant, "Phase Agile Reflectarray Cells Based On Liquid Crystals". Proc. let Microw. Antennas Propag., Vol. 1, No.4, pp ,2007. [16] W. Hu, M. Y. Ismail, R. Cahill, H. S. Gamble, R. Dickie, V. F. Fusco, D. Linton, S. P. Rea and N. Grant, "Tunable Liquid Crystal Patch Element". letelectronic Letters, Vol 42, No 9, [17] A. Mossinger. R. Marin, S. Mueller, 1. Freese and R. Jakoby, "Electronically Reconfigurable Reflectarrays with Nematic Liquid Crystals". letelectronics Letters, Vol. 42, No. 16,2006. [18] H. Rajagopalan, Y. Rahmat and W. A. Imbriale, "RF MEMES Actuated Reconfigurable Reflectarray Patch-Slot Element". IEEE Trans. Antennas Propag., Vol. 56, No. 12,pp ,2008. [19] F. A. Tahir, H. Aubert and E. Girard, "Equivalent Electrical Circuit for Designing MEMS-Controlled Reflectarray Phase Shifters". Progress In Electromagnetics Research, Vol. 100, pp. 1-12, [20] L. Boccia, F. Venneri, G. Amendola and G. D. Massa, "Application of Varactor Diodes for Reflectarray Phase Control". IEEE International Symposium ofantennas and Propagation Society, Vol 4, pp , [21] S. V. Hum, M. Okoniewski and R. Davies, "Modeling and Design of Electronically Tunable Reflectarrays". IEEE Trans. Antennas Propag., Vol. 55, No.8, pp ,2007. [22] M. Riel and Laurin, "Design of an Electronically Beam Scanning Reflectarray Using Aperture-Coupled Elements". IEEE Transactions on Antennas and Propagation, VOL. 55, NO.5, pp ,2007. [23] D. M. Pozar, Microwave Engineering, 3 rd edition, John Wiley and Sons, [24] M. Inam and M. Y. Ismail, "Reflection Loss and Bandwidth Performance of X-Band Infinite Reflectarrays: Simulations and Measurements". Microwave and Optical Technology letters, Vol. 53, No.1, pp ,