A Reconfigurable Planar Antenna Array (RPAA) with Back Lobe Reduction.

M.T. Ali1, T. A. Rahman1, M.R. Kamarudin1, R. Sauleau2, M. N. Md Tan1 and M.F.Jamlos1

1Wireless Communication Center (WCC) Universiti Teknologi Malaysia,81310 UTM Skudai, Johor, Malaysia.

mizi732002@yahoo.com , tharek@fke.utm.my,ramlee@fke.utm.my, mnor1408@yahoo.com, and faizal_hero@yahoo.com.sg

2 Institut d’Electronique et de Télécommunications de Rennes (IETR)

UMR CNRS 6164 University of Rennes 1, France. Ronan.Sauleau@univ-rennes1.fr

ABSTRACT: A novel structure of Reconfigurable Planar Antenna Array (RPAA) added with a microwave absorber is designed. This antenna is consists of two elements structure, which are the 16-element microstrip rectangular patch antenna structures (top structure) and the separated feed line structure (bottom structure). The unique property of this antenna design is that instead of fabricating all together in the same plane, the antenna’s feeding network is separated from the antenna radiating elements (the patches) by an air gap distance. This allows reducing spurious effects from the feed line. This antenna is integrated with RF switches to produce the scanning beam pattern to desired direction. This can be done by controlling the switch state to either on or off mode. The additional material was proposed involves placing a microwave absorber material such as ECCOSORB@ (Emersion & Cuming Microwave Products, Inc., Randolph, and Mass) at the back of the top substrate. The ECCOSORB is useful for suppression of unwanted radiated field and thus improved the back lobe pattern at an average of 10 dB. The simulated results are presented to demonstrate the excellent performance of this antenna.

INTRODUCTION

Reconfigurable antenna will be an attractive feature in the modern wireless communication system because it enables to provide a single antenna to be used for multiple systems. In the reconfigurable antenna, the structure of the antenna can be changed by integrated with Radio Frequency (RF) switches [1]. Most of these approaches were able to alter the fundamental characteristics such as operating frequency, polarization and radiation pattern [2]. The reconfigurable antennas have attracted much attention in wireless communication systems such as cellular-radio system, smart weapons protection and point to point propagation. In [3], the authors presented reconfigurable antennas, which were radiated at different beam patterns by adjusting the apertures and maintaining their operating frequencies. In array application, mutual coupling effect is often considered undesirable, since it reduce the antenna gain, and raises the sidelobe level. However, in the case of microstrip antenna, the mutual coupling between the driven elements and the parasitic elements can be used to direct the beam so that its peak can be tilted to the desired direction. For example, the Yagi antenna uses parasitic elements in combination with at active element to control the direction of a beam [4-6]. The amount of mutual coupling depends on the separation between antenna elements, and it will increase if the antennas are closer to each other. ANTENNA STRUCTURE The RPAA structure is constructed by comprising four sub-arrays of 4-elements with the size of 109 mm x 128 mm as show in Fig. 1. The size of each patch is 16.8 mm x 11.3 mm and their inter-element spacing is approximately λo/2. The spacing between sub-arrays in x-axis (T) direction is about 55 mm and y-axis (Y) direction of 51.9 mm. This antenna has been designed and simulated on a FR-4 substrate board with a dielectric constant of εr=4.7 and thickness hd=1.6mm. In this paper, the comparison results between RPAA structures integrated with and without microwave absorber were investigated. The simulation results are presented to demonstrate the excellent performance of this

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antenna and to show that the improvement in term of back lobe pattern. The proposed antenna is then simulated by using Computer Simulation Technology (CST) Studio Suite 2008. Reconfigurable Planar Antenna Array (RPAA).

The 16-element array comprises four sub-arrays of 4-elements with separated feed line by air gap between the patches elements and feed line is presented. The input ports on the antenna board are labeled as P1 to P4 (Fig. 1). The top antennas are fed by a vertical coaxial probe (SMA connector) connecting the feed network to each sub-array. The air gap thickness (ha) has been optimized to achieve a good return loss at 5.8GHz as shown in Fig. 2(b). The unique property of this antenna design is that instead of fabricating all together in the same plane, the antenna’s feeding network is separated from the antenna radiating elements (the patches) by an air gap distance as shown in Fig. 2(a). This allows reducing spurious effects from the feed line. There are three variations of the RPAA will be studied, that are CD-mode, RD-mode and LD-mode. The first configuration is to set the switches in on-mode for all the switches, Center Direction (CD). The second and third configuration requires only two sets of switches, S1 and S3 (RD, Right Direction) or S2 and S4, (LD, Left Direction) in off mode conditions. For example, RD-mode means that the switch S1 and S3 in off-mode and the switch S2 and S3 is in on-mode. This means that the current will be distributed only from input port (Pin) towards port 2 (P2) and port 4 (P4) at antenna element. In this condition, only Sub-Array 2 and Sub-Array 4 will be generated the signal and others elements acted as passive elements.

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Fig. 1: The diagram of RPAA structures. (a) Feed line (bottom element). (b) Antenna (Top element).

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Fig. 2: (a) Diagram of RPAA structures. (b) Return loss computed for different air gap heights.

Reconfigurable Planar Antenna Array (RPAA) with Microwave Absorber.

In this section will be investigated and analyzed the reconfigurable planar array antenna (RPAA) incorporating with microwave absorber to improve a back lobe pattern. The RPAA structure has be studied and discussed detail previous section. This geometry uses two parallel substrates separated by a ground plane and air gap technique. A microstrip feed line on the bottom substrate is coupled through an air gap with the thickness of 19 mm to a top microstrip patch. One problem that arises using this type of antenna design is that the radiation occurs along the microstrip line from the feed line structure will be produced unwanted electric field to the radiating elements. The feed microstrip line resides below the ground plane and is coupled to each of the patch through an air gap. There is some level of RF signal leakage, either through the air or through the substrate material. The leakage through the substrate is caused by undesired surface wave propagation. This coupling effect between the two antenna arrays lowers antenna gain and reduces performance of the antenna. Presently, several techniques are used to improve the mutual coupling and the back lobe of radiation pattern generated from the separated feeding line. The new structure of the antenna was proposed involves placing the microwave absorbing materials such as ECCOSORB@ (Emerson & Cuming Microwave Products, Inc., Randolph, Mass.). This technique involves placing an absorbing material at the back of the top substrate. Fig. 3 shows the proposed antenna structure with the microwave absorbing materials (ECCOSORB-SF). The ECCOSORB is useful for suppression of radiation waves and thus improve isolation between the antennas. While using a absorbing material such as this will improve isolation between the two antennas, it has several limitations. The addition of a new material in the antenna consumes additional space and high in cost implementation.

Fig. 3: The diagram of RPAA with RF absorber.

RESULT AND DISCUSSION

The effect of placing absorber material with various thicknesses (habs) below the substrate is illustrated in Fig. 4. Several optimizations were done with various thicknesses, ranges 1.5mm to 2.8 mm in order to obtain the best back lobe level at resonant frequency. It is apparent from the curve that increasing the thicknesses induces a decreasing of the back lobe level. The best back lobe produced at 5.8 GHz is obtained at height of 2.5 mm with the same size as the antenna board. Hence, this height has been chosen for RPAA design.

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Fig. 4: The optimized results of radiation pattern at various absorber thicknesses (habs). (a) Cartisien graph. (b) Polar plot

Fig. 5, illustrates the comparison radiation patterns of two RPAA antenna structures, which are with and without ECCOSORB-SF absorber. All the simulated results are shown the improvement of the back lobe level in RPAA by an average of 10 dB after placing with ECCOSORB-SF absorber. The simulated radiation pattern steered to three different directions, that are +180, 00 and -170, respectively at frequencies across the entire 5.8GHz.

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Fig. 5: The comparison of simulated radiation pattern at differents configuration mode. (a) LD-mode. (b) CD-mode. (c) RD-mode.

CONCLUSION The RPAA with microwave absorber concept is studied in this paper with the objective to improve the back lobe level. The unique property of this antenna design is that instead of fabricating all together in the same plane, the antenna’s feeding network is separated from the antenna radiating elements (the patches) by an air gap distance. This allows reducing spurious effects from the feed line. The experimental radiation patterns show very good agreement which the back lobe level was suppressed in averages of 10 dB. The advantage of this design is that the radiation arising from the feeding line cannot interfere with the main radiation pattern generated by the antenna and the back lobe pattern was improved by placing microwave absorber. This antenna is also suggested for reconfigurable antenna which integrated with RF switching at feeding line applications. Finally, in order to proof the validity of the antenna design, the simulation results will compare with measurements. REREFENCES [1] S. Zhang, G. H. Huff, J. Feng, and J. T. Bernhard, "A pattern reconfigurable microstrip parasitic array,"

Antennas and Propagation, IEEE Transactions on, vol. 52, pp. 2773-2776, 2004. [2] B. A. Cetiner, H. Jafarkhani, Q. Jiang-Yuan, Y. Hui Jae, A. Grau, and F. De Flaviis, "Multifunctional

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[5] D. Gray, L. Jun Wei, and D. V. Thiel, "Electronically steerable Yagi-Uda microstrip patch antenna array," Antennas and Propagation, IEEE Transactions on, vol. 46, pp. 605-608, 1998.

[6] K. Mori, K. Uchida, and H. Arai, "Active antenna using parasitic elements," in Antennas and Propagation Society International Symposium, 1998. IEEE, 1998, pp. 1636-1639 vol.3.