Progress In Electromagnetics Research Symposium Proceedings, Suzhou, China, Sept.12–16, 2011 619 Simulation of a Conformal Reconfigurable Fractal Tree Antenna with Adaptive Multi Beam and Frequency Characteristics Huseyin Altun 1 , Erdal Korkmaz 1 , and Bahattin Turetken 2 1 Department of Electrical and Electronics Engineering, Fatih University, Istanbul, Turkey 2 The National Research Institute of Electronics and Cryptology (UEKAE), Tubitak, Gebze, Turkey Abstract— This paper presents the design and the analysis of a conformal fractal tree re- configurable antenna having adaptive multi beam radiation patterns and adaptive operation frequency characteristics. The proposed antenna covers some service bands such as: WiMAX (2.400–2.483) GHz, m-WiMAX (3.4–3.6) GHz and WLAN (5.15–5.825) GHz and operates some other frequencies between 2.24 GHz to 9.87 GHz. The designed antenna is reconfigured by using PIN diode switches. For biasing the diodes, influences on the antenna characteristics is presented in the results. The optimization in biasing and integration of these switches into the antenna is also discussed. 1. INTRODUCTION In radar and modern communication systems the demand on multi-functional antennas is increas- ing. The requirements for these antennas are the abilities to have multi radiation patterns, adapting the operation frequency and polarization, keeping the physical dimensions and positioning unal- tered. Reconfigurable antennas with switching capability used as a multiple input multiple output (MIMO) system have been used in recent years to fulfill these requirements. By means of switches with compatible antenna elements the antenna and its feed structure can be physically reconfig- ured to provide radiation pattern, frequency band and polarization diversity so they have more advantage to compare with conventional antennas [1]. The most prevalent implementation about reconfigurable antenna is related to the operation frequency [2] since it might be the easiest feature to alter. Polarization and pattern reconfigurable antennas are also attractive since they can provide diversity features which leads to an increased signal to noise ratio and therefore a higher quality of service of whole systems [3–5]. PIN diodes are generally used more than transistors and switches as switching devices for RF and microwave front-end communication systems since they have several crucial properties such as low insertion loss, good isolation, low power handling and low cost [6]. Although a reconfigurable antenna can take many shapes we will focus on fractal tree antennas in this work. In terminology, fractal means broken or irregular fragments which were originally entitled by Mandelbrot [7] to describe a family of complex shapes that possess an inherent self-similarity in their geometric structure. As a result of small investigation in the environment a lot of example for fractal shapes can be seen as trees, clouds, galaxies, leaves, snowflakes and much more. Fractal tree structures can be applied into antenna design to produce multiband characteristics [8–10]. A conformal antenna is an antenna that conforms to something or it conforms to prescribed shape. The shape can be some part of a train, airplane or other vehicle. The purpose in conformal antenna is to make surface matched structure so that it becomes integrated with the structure and does not extra drag. Since they have very low profile and can be applied on flexible substrates they can behave “hidden” antennas [11]. The target in this paper is showing the design of a new shape of fractal tree antenna for multiband and multi radiation pattern applications. PIN diodes are used as switches for multi frequency and multi beam reconfiguration. By switching PIN diodes, the resonant frequency and radiation pattern variation simulation by using CST is presented. 2. ANTENNA DESIGN A designed reconfigurable fractal tree antenna schematic is presented from its top view in Figure 1(a) and perspective view in Figure 1(b). Antenna is designed on a substrate with a Rogers RT5880 (lossy) material which has 2.2 relative permittivity. The height of the substrate is 70 mm and the width is 64.7 mm and with a 1.6 mm thickness. The ground has been chosen from PEC material with a thickness of 0.2 mm. Substrate and ground made conformal on the cylinder with a radius of 50 mm. The antenna is fed by a 50 ohm round coax. The inner conductor of the coax has diameter 620 PIERS Proceedings, Suzhou, China, September 12–16, 2011 of 0.689 mm with a material PEC and the shield is 2.13 mm Rogers RT5880. The feeding line is 5 mm long from end to top. As shown in Figure 1(a) some parts at the antenna is numbered for the description. The numbered part are called trunk and they make the connection from source to branches. For switching, PIN diodes are used. The connection between feed line and trunks are established by the diodes namely, D1t, D2t, D3t, D4t while the connection between trunks and branches are established by diodes D1b, D2b, D3b, D4b. In our structure totally eight PIN diodes are deployed to alter the electrical length of the antenna to operate at difference resonant frequencies. Dimension of the trunks (part 1, 2, 3, 4) are 20 × 2 mm and branches are 10 × 2 mm with the rotation angle of 45 degree and with the rotation angle of 90 degree at the middle. Fractal structure has thickness of 0.1 mm. PIN diodes are 0.5 mm. The PIN diodes are modeled as a series RL circuits for ON state, and a combination of series- parallel RLC circuits for the OFF state which can be seen in Figure 2. As a result of searching for suitable PIN diodes, MACOM-MA4AGBLP 912 was chosen for the simulation since it has high switching cut off frequency, low series inductance and small forward resistance [12]. The equivalent circuit of the PIN diode, shown in Figure 2, is used in the simulation. The values of the elements of the equivalent circuits for simulation are given in Table 1. Lumped elements are used in modeling the PIN diode within CST Microwave Studio. (a) (b) Figure 1: Designed antenna schematic. (a) Top view and (b) perspective. (a) (b) Figure 2: The equivalent circuits of the pin diode. (a) ON state, (b) OFF state. Table 1: The values of the elements of the PIN diode simulation equivalent circuit. Element Value Serial Inductance (Ls) 0.5 nH Serial Resistance (Rs) 5 Ω Papallel Resistance (Rp) 1 kΩ Parallel Capacitance (Cp) 0.02 pF Progress In Electromagnetics Research Symposium Proceedings, Suzhou, China, Sept. 12–16, 2011 621 3. SIMULATION RESULTS The desired conformal reconfigurable fractal tree antenna is designed and simulated by using CST Microwave studio. By changing the diodes conditions, we alter on the characteristic of antenna and it results shifting in operating frequency and radiation pattern. By different combinations of diodes ON state or OFF state we have many different radiation patterns with different operating frequencies. When all eight diodes are ON state, antenna starts to operate at 2.89 GHz, 6.25 GHz and 8.17 GHz and return loss and radiation pattern (phi = 0, turning about theta) is shown in Figure 3. In Figure 4, the diodes on the vertical axis become ON state and horizontal axis become OFF state, then opposite condition of this is shown (theta = 90, turning about phi). In Figure 5, radiation patterns sequentially are shown while diodes are ON state between source to trunk and trunk to branch is showing separately for each trunk-branch couple. It can be seen that while the trunk is becoming ON or OFF state with branch, radiation direction is changing with 90 degree since there are 90 degree angles between trunks. In order to notice this situation we had a two- Table 2: Compilation of results. Mode ON State Diodes Resonant Frequency (GHz) Max. Peak Gain (dBi) I D1t, D1b, D3t, D3b 2.52, 5.89, 8.4, 9.3 6.64 II D3t, D3b, D4t, D4b 2.58, 5.7, 8.02, 9.5 4.51 III D1t, D2t, D3t, D4t 4.32, 4.63, 7.59, 8.2 9.91 IV D2t, D2b 2.29, 5.44, 8, 9.58 4.63 V D1t 3.49, 4.91, 7.5,8.33 6.09 VI ALL 2.89, 6.25,8.17 4.47 (a) (b) Figure 3: When all diodes are ON state. (a) Return loss and (b) radiation pattern at 2.9 GHz. (a) (b) Figure 4: (a) Vertical diodes are ON and others are OFF state. (b) Horizontal diodes are ON state at 2.4 GHz. 622 PIERS Proceedings, Suzhou, China, September 12–16, 2011 Figure 5: When diodes are ON between source to trunk and trunk to branch for each trunk-branch couple at 2.4 GHz. dimension result which has the theta = 45 angle by turning about phi. By using this specification the radiation pattern side can be easily controlled by diodes. This feature can work properly in WiMAX (2.400–2.483) GHz and WLAN (5.15–5.825) GHz bands. Also, a detailed compilation of the results by using different combination are shown in Table 2. 4. CONCLUSIONS In this paper a multiband frequency reconfiguration is studied by using of a conformal fractal tree antenna controlled by PIN diodes. The effect of the biasing lumped elements on the antenna per- formance is discussed based on the simulation results. 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