Circuit Modeling for UWB-MIMO Antenna Array Baskaran Kasi 1 , Yanti Erana Jalil 2 , Chandan Kumar Chakrabarty 3 Department of Electrical & Electronics Engineering 1 Universiti Infrastruktur Kuala Lumpur Kajang, Selangor 43000, Malaysia baskaran@iukl.edu.my Department of Electronics & Communication Engineering 2,3 Universiti Tenaga Nasional Kajang, Selangor 43000, Malaysia yanti@uniten.edu.my, chandan@uniten.edu.my Abstract—In this article a simple analytical model for a UWB- MIMO antenna is proposed. Lumped equivalent circuit elements are used to construct the proposed MIMO array. Parameters of the circuit elements are calculated from the geometry of the designed antenna. Using frequency transformation technique causes the antenna to have wider impedance bandwidth and compact size. Mutual coupling effect is also taken into consideration for better circuit modeling. The proposed model has been developed using simulations and measurements based techniques. The calculated data are compared with the experimental results and good agreement is noted. Keywords—circuit modeling; equivalent circuit; lumped element; multiple-input multiple output; ultra wideband antenna I. INTRODUCTION Ultra-wideband (UWB) antennas have attracted researcher attention in recent years since Federal Communication Commission (FCC) released its report in 2002 to allow the using of license free frequency band from 3.1 to 10.6 GHz for commercial applications [1]. To satisfy the UWB radio system, the designing of compact, low cost and broadband antennas are highly desirable. One of the key issues in UWB communication systems is the co-design of UWB antenna/circuit interface. For microwave circuit designer who intends to do co- simulation of the antennas with the UWB transmitter and receiver, it is important to be aware that circuit simulation is usually carried out using a time-domain tool such as SPICE. This limitation might be ease if engineers could reliably predict an equivalent circuit model of the UWB antenna. Various researchers have published their work in literatures regarding the methodologies to model the UWB antennas [2– 4]. In [2], the author offered a circuit modeling technique for UWB omnidirectional small antennas on both the transmitting and receiving sides. The proposed technique can be used to model the antenna/circuit interface and the relevant transfer functions by circuit simulators. Additionally Wang et al. have described a circuit refinement method which consists of a narrow band model augmented with a macro model [3]. Another structure is also developed for rectangular microstrip broadband antennas using optimization technique in [4]. Multiple-Input-Multiple-Output (MIMO) technology has been adapted to the UWB system to further increase data throughput and to combat multipath fading. In order to meet such requirements, system level design approaches are not sufficient for since accurate description of linear and nonlinear interactions effects among sub-systems is essential [5–7]. Equivalent circuit models are preferable for antenna array design because the exact electromagnetic simulation usually requires complex setting and heavy computation. It is worth noting that the lumped circuit models can be considered to replace the microstrip lines due to smaller size, lower cost, and wider impedance bandwidth [8]. This model will be suitable for monolithic microwave integrated circuit (MMIC) where small size requirement are of main criteria. In the recently published papers [9–11], it has been reported that the cavity patch model might be followed to obtain equivalent circuit models for UWB antennas. To date, no known studies of equivalent circuit model of UWB-MIMO array has been performed. In this paper, a planar UWB patch antenna element with defected ground plane is modeled using basic lumped elements. The lumped circuit model is derived by treating antenna structure into different stages of microstrip lines such as patch, step and feed line. Applying a frequency transformation to above lumped-element antenna model, relatively broadband characteristics can be obtained. Next, a two-element MIMO array has been illustrated using lumped element approach. To show the validity of the proposed model, experimental results for the fabricated MIMO array are also shown II. SINGLE UWB ANTENNA MODELING In this study, a planar UWB antenna is considered as the radiating element. The antenna and its optimized parameters are shown in Fig. 1. It is printed on a standard Taconic TLC- 30 substrate with a dielectric constant of 3, thickness of 1.575 mm, and a loss tangent of 0.003. The antenna is excited by a 50-Ω microstrip line printed on a partially grounded substrate. It is possible to model the element antenna in terms of lumped circuit elements by treating the antenna as a cascade connection of several individual line segments. Each segment is then replaced by its equivalent lumped circuit model. 2013 IEEE International RF and Microwave Conference (RFM2013), December 09-11, 2013 - Penang, Malaysia 978-1-4799-2214-7/13/$31.00 ©2013 IEEE 66