DESIGN AND ANALYSIS OF MICROSTRIP FED SLOT ANTENNA FOR SMALL SATELLITE APPLICATIONS

Transcription

1 I J I T E ISSN: (1-2), 2012, pp DESIGN AND ANALYSIS OF MICROSTRIP FED SLOT ANTENNA FOR SMALL SATELLITE APPLICATIONS ELAMARAN P. 1 & ARUN V. 2 1 M.E-Communication systems, Anna University of Technology Madurai ( ela.tpaccet@gmail.com) 2 Assistant Professor, Dept of ECE, Anna University of technology Madurai. Abstract: In this proposed work, a cavity backed microstrip fed slot antenna is designed for satellite applications. Two rectangular narrow slot antennas are placed in series. The slot is backed by cavity to improve the antenna performance. When two serial slot antennas were placed close to each other, we found that one can achieve two resonances, and this suggests a dual band antenna design. The spacing between the two antennas can be adjusted to achieve an effective secondary resonance. The new resonance is found to be due to the mutual coupling between the two slot antennas. This simulated antenna operates at 5.2GHz and 10.7GHz which are frequencies of C band and X band respectively and these frequencies are used in small satellite communications. The proposed antenna can be integrated with solar panels to save surface real estate of small satellites, and to replace traditional deployed dipole antennas. Measured results show good return loss and radiation pattern at both frequencies. Index Terms: Microstrip, mutual coupling, slot antennas. 1. INTRODUCTION Slot antennas have appealing features such as low profile, low cost, and ease of integration on planar or non-planar surfaces. An example application is integrating slot antennas with solar panels of small satellites to save surface real estate [4]. While slot antennas are valuable for space applications and self-powered ground sensors [5], most designs are limited to single frequency operation. This communication presents a very simple design where one can achieve a dual band antenna by utilizing coupling between two adjacent slots. A slot antenna, when not backed by a cavity, radiates to both sides of the ground plane and can be a good substitute for a dipole antenna. In applications such as solar panel integration, it is important to limit the radiation to only the front side of the slot by utilizing a cavity backing. The cavity can be loaded with dielectrics to improve antenna performance, such as enhancing the impedance bandwidth. Due to the conformal nature and versatility in choosing the cavity geometry and material, cavity backed antennas have found popularity in both single element implementations and array configurations. When there is more than one slot element, it is necessary to study the coupling between elements. It has been found that cavity backed slot antennas have small mutual coupling. Further studies have been reported to compute the coupling using numerical techniques and analytical methods. Although there is abundant literature on cavity backed slot antennas and their coupling, most studies have focused on studying the slot elements in parallel alignments. When the slots are placed in series, the spacing between the elements is usually large enough (e.g., at least a half wavelength) for one to ignore the resonance due to coupling. But when two series slot antennas are placed close to each other, we found that one can achieve two resonances. This suggests a dual band antenna design. This communication presents the design method, analysis using an equivalent circuit modal, and a simulated dual band antenna. The antenna is studied using Ansoft s HFSS, and measured results agree well with the simulations, validating the dual band antenna design that can be potentially implemented on solar panels as communication links or sensor nodes.

2 DUAL BAND ANTENNA ANALYSIS The configuration of the proposed dual band cavity backed slot antenna is shown in Figure 1. The antenna and the feed lines are composed of two circuit board substrates. Two radiating slots are etched on the top layer, which is a copper layer, of the first substrate. The feed lines are printed on the top layer of the second substrate. The bottom layer of the second substrate is the ground plane. The two substrates are then assembled together with antennas on the topmost layer and the feed lines sandwiched between the two substrates. Also, the antenna elements are designed and assembled to be orthogonal to the feed lines (Figure 1). It should be noted that one does not have to choose the same substrates to etch antennas and to print feed lines. Depending on applications and practical demands, the excitation method can be simple probe feed [1], coplanar waveguide (CPW) feed [2], or microstrip line feed [5]. This communication demonstrates Elamarab P. & Arun V. the microstrip line feed due to its simplicity and ease in matching the lines to the slot antenna by adjusting the position and length of the feed lines. After assembling the two substrates, the four side-walls of the substrates and the top plane (i.e., slot antenna and the metal plane) are shorted to the ground plane with either conductive pastes or conductive tapes. Table 1 Calculated Parameters of Slot Antenna Name of the Parameters Values Substrate 1 (Silicon, r = 11.9) thickness 3.38 mm Substrate 2 (Roger 4003C, r = 3.55) thickness.831 mm Substrate 1 and Substrate 2 Size 100 mmx 100 mm Ground Size 200 mmx 200 mm Slot Size 25.5 mmx 0.9 mm The feed design is a 50 microstrip line divided into two 100 lines to excite the two slot elements that resonate at the same frequency (Figure 2). We found that when the two elements are placed close, a new resonance appears due to the strong coupling between the two slots. The explanation for the second resonance is that the coupling between the two slots acts as if there were an equivalent slot antenna that is longer than the individual slot but shorter than the total length of the two slot elements. In Table 1, the value of the all parameters is given. Figure 1: Illustration of the Dual Band Antenna Figure 3: Illustration of the Location of Feed Lines and the Two Slot Antennas Figure 2: Final HFSS Model of the Dual Band Slot Antenna In order to analyze the mechanism of the dual band resonance and provide some insight for designing effective antennas for both frequencies, an approximate circuit model to study the input

3 Design and Analysis of Microstrip Fed Slot Antenna for Small Satellite 355 impedance of the slot antenna is established. The important parameters of the slot geometry are marked in Figure 3, where L e1 and L e2 are critical for matching the impedance of the two slots, and the spacing between the two slots (d in Figure 3) has been seen to affect the impedance of the equivalent slot. The approximate model is presented in Figure 4. It is derived by modifying Syahkal s circuit model [6] for a single slot antenna fed by a microstrip line. Each slot is modeled as two shortcircuited slot lines in parallel with a radiation conductance Gr that represents the radiated power from the slot. The parameters L e1 and L e2 (Figure 3) correspond to the length of the two short-circuited slot lines, and are marked on Figure 4 for ease of reading. The characteristic impedance of the slot Line is Z cs, and L e1, L e2 are inductance of the microstrip feed line and slot line, respectively. The mutual inductance M 1 represents the coupling between the microstrip line and slot line. The mutual inductance M 2 represents the coupling between two serial slots. Because of the coupling, there appears an equivalent slot that radiates at a frequency lower than the two slots. The circuit model for the equivalent slot is presented in Figure 5. Figure 4: Approximate Circuit Model of the Dual Band Slot Antenna Figure 5: Circuit model of the Equivalent Slot Antenna that is Due to the Coupling of the Two Original Slot Antennas The length of the equivalent slot is L eq, and is expected to be between (L e1 +L e2 ) and 2 (L e1 +L e2 ). The coupling between the two slots reflects as added impedance Z couple to the equivalent slot line (Figure 4). It is straightforward to expect that changing the spacing d (Figure 2) between the two slots will change M 2, and accordingly change Z couple, where Z couple is the dominant factor for the input impedance of the equivalent slot because the other factors (characteristic impedance Z cs, radiation conductance Ger, and the length L eqtot of the equivalent slot line) do not vary much with respect to d. Therefore, after matching the impedance of the two slots, one can adjust the spacing between the two slots to achieve a reasonable return loss for the equivalent slot antenna. Two methods can be employed to validate the model shown in Fig. 3.and then determine the values of Z cs, L 1, L 2, M 1, and M 2 following Syahkals. Then one can compute the input impedance of the equivalent slot before comparing the computed S 11 value of the equivalent slot at the input port with experiments. Alternatively, using the model as the guideline, one can perform parametric studies using simulation software and then validate the design with measurements. Ansoft s HF SS is u sed to perform the simulations for two serial slot antennas on substrates with different thickness and relative

4 356 permittivity, and different ground plane size. It is found that after matching two identical serial slots to a resonant frequency f1, a secondary resonance f2 appears when the spacing between the two slots is less than 0.19 wavelengths. When the dimension of the slots is fixed, the input impedance of the equivalent slot is mainly determined by Z couple which is affected by the mutual inductance M 2 (Figure 4). It is also seen from Fig ure 4 that M 2 affects the i nput impedances of two serial slots, and therefore the spacing between them will affect S11 value at f1. Elamarab P. & Arun V. Figure 7: VSWR 3. MEASUREMENT RESULTS The normalized radiation patterns for both bands were simulated using Ansoft HFSS. The simulated antenna Return loss is -26 db at 5.2 GHz and -23 db at 10.7 GHz is shown in the Figure 6. The measured and VSWR (1.1<=) plot have a good agreement. The measured co-polarization and cross-polarization patterns at 5.2 GHz and 10.7 GHz are given in following figures. It can be seen that the agreement between E-plane patterns is good and the cross-polarization level is overall less than -24dB and -20 db at 5.2 GHz and 10.7 GHz respectively, and H-plane cross polarization is -24 db and -18 db respectively. In E-plane pattern Co-polarization level of 5.2 GHz and 10.7 GHz is 2.4 db and 2 db respectively and In H-plane Co-polarization level of 5.2 GHz and 10.7 GHz is 1 db and -11 db respectively. Measured Gain of 5.2 GHz 10.7 GHz is 2.2 db and -5 db respectively. Figure 6: Return loss Figure 8: E-Plane Cross Polarization (a) 5.2 GHz Cross Polarization (b) 10.7 GHz Cross Polarization

5 Design and Analysis of Microstrip Fed Slot Antenna for Small Satellite 357 Figure 9: H-Plane Cross Polarization (a) 5.2 GHz Cross Polarization (b) 10.7 GHz Cross-polarization Figsure 11: E-Plane Co-polarizations (a) 5.2 GHz Copolarization (b) 10.7 GHz Co-polarization Figure 10: E-Plane Co-polarizations (a) 5.2 GHz Co- (polarization (b) 10.7 GHz Co- polarization 4. CONCLUSION The proposed work represents alternative antenna geometry for small satellites, particularly CubeSats. The proposed antenna topology is based on the dual band cavity-backed slot antenna. The antenna was designed using two dielectric substrates and was fed by microstrip lines. The antenna can be designed to operate effectively at 5.2 GHz and 10.7 GHz (C band and X band frequency). The antenna can be integrated with solar panels to save surface real estate of small satellites, and to replace traditional deployed dipole antennas. The dual band operation is achieved simply by utilizing the mutual coupling between two closely placed series of slot antennas. The dual band resonance can be tuned by adjusting spacing between the two slots and by modifying the matching microstrip feed lines. In order to understand the mechanism of the dual band operation, an approximate circuit model was presented. The model was validated by parametric study from Ansoft s HFSS.

6 358 Elamarab P. & Arun V. References [1] D. Sievenpiper, H.-P. Hsu, and R. M. Riley Low-profile cavity-backed Crossed-slot Antenna with a Single- Probe Feed Designed for 2.34-GHz Satellite Radio Applications, IEEE Trans. Antennas Propagations, Vol. 47, No. 1, pp , Jan [2] S. Gai, Y.C. Jiao, Y. B. Yang, C. Y. Li and J. G. Gong Design of a Novel Microstrip-fed Dual-band Slot Antenna for WLAN Applications Progress in Electromagnetic Research Letters, Vol. 13, 75-81,2010. [3] S. Sierra-Garcia and J.-J. Laurin, Study of a CPW Inductively Coupled Slot Antenna, IEEE Trans. Antennas Propagations. Vol. 52, No. 3, pp , Mar [4] S. Vaccaro, C. Pereira, J. R. Mosig, and P. de Maagt, In-flight Experiment for Combined Planar Antennas and Solar Cells (SOLANT), IET Microw.Antennas Propagations, Vol. 3, No. 8, pp , [5] T. Wu, R. L. Li, and M. M. Tentzeris, A Mechanically Stable, Low Profile, Omni-directional Solar-cell Integrated Antenna for Outdoor Wireless Sensor Nodes, Presented at the IEEE Antenna and Propagations. International Symposium, Charleston, June [6] Mahmoud N. Mahmoud, Reyhan Baktur (2011), A Dual Band Microstrip-Fed Slot Antenna, IEEE Transactions on Antennas and Propagation, Vol. 59, No. 5, May [7] Y. Yoshimura, A Microstrip Slot Antenna, IEEE Trans. Microwave.Theory Tech., Vol. MTT-20, pp , Nov