Design and performance of electrically small planar antennas with matching circuit at 2.4GHz band Yuta Nakamura, Haruichi Kanaya, Ramesh K. Pokharel, and Keiji Yoshida Department of Electronics, Kyushu University, JAPAN kanaya@ed.kyushu-u.ac.jp 1. Introduction In recent years, the miniaturization of antennas is essential in radio communication devices such as wireless LAN, RF-ID, and MIMO, and extensive studies are made of an electrically small antenna (ESA), i.e., the antenna whose dimension is much smaller than one-wavelength, towards further reduction of the antenna size[1]-[3]. It is widely known that small antenna has an extremely narrow bandwidth, low radiation resistance and large impedance mismatch. So, we must simultaneously implement a broadband impedance matching circuit. Moreover, the small antenna is sensitive to the conductor resistance because of its low radiation resistance, and the decrease of the radiation efficiency often makes serious problem. However, there are almost no reports about the details of the relations between the radiation resistance, realized gain and efficiency of the small antenna. In this paper, we design some ESAs with impedance matching circuit, which have various gains, and discuss about the relations between peak realized gains and s parameters. The substrate has dielectric constant ε r =4.25 and tanδ=0.015. The thickness of the substrate and copper top metal is 0.8mm and 18μm. By using the impedance matching circuit, we can reduce the total antenna size more than one-wavelength. So, we designed the ESAs at 2.45GHz and fractional bandwidth 3%. Also, we carried out experiments on the ESAs and compared experimental results and simulated results. 2. Design theory of the electrically small antenna (ESA) We design the ESAs with impedance matching circuit at 2.45GHz, whose antenna size are 0.09λ 0 *0.09λ 0 (ESA1) and 0.24λ 0 *0.16λ 0 (ESA2), respectively. Antenna ESA2 has a larger gain. Fig.1 shows the layout of ESA1 with impedance matching circuit. In Fig.1, the impedance matching circuit is composed of admittance (J)-inverter and the transmission line from the slot antenna[4]. The input impedance of the ESA is designed to be Z 0 =50 Ω for the experiment to be convenient. Fig.2 shows the input impedance 978-1-4244-2642-3/08/$25.00 ©2008 IEEE