1 Frequency-Reconfigurable MIMO Monopole Antenna with Wide- Continuous Tuning Range Y. F. Cao * , S.W. Cheung, T.I. Yuk Department of Electrical and Electronic Engineering, the University of Hong Kong, Pokfulam Road, Hong Kong * yfcao@eee.hku.hk Abstract: The design of a frequency-reconfigurable (FR) multiple-input-multiple-output (MIMO) monopole antenna with a wide-continuous tuning range in handheld devices is presented. The antenna consists of two symmetrical FR monopole elements and a FR defected-ground-structure (DGS). The monopole elements have two sections, sections 1 and 2, connected together using a PIN diode. Section 1 alone generates a high-frequency band at 2.26 GHz. By turning ON the PIN diode, sections 1 and 2 will be connected together to generate a low-frequency band at 1.93 GHz. The DGS is used to achieve high isolation between the monopole elements and is reconfigurable to follow the low- or high- frequency operations of the FR-MIMO antenna. Simple and novel biasing circuits employing only 3 DC wires are designed to control these diodes. The methodology of using the design in other frequency bands is proposed. Results on S-parameter, radiation pattern, realized peak gain and envelope-correlation coefficient (ECC) for pattern diversity are presented. Measured results show that the antenna has a wide-frequency tuning range from 1.88 to 2.64 GHz, covering the LTE 1, 2, 23, 25, 33-37 and 39-40 bands with low mutual-coupling and ECC of less than -20 dB and 0.5, respectively. The effects of the feeding cable used in measurement are also studied. 1. Introduction Multiple-input-multiple-output (MIMO) technology has attracted much attention for its potential to increase the link quality and channel capacity of wireless systems without increasing the frequency spectrum [1]. Different MIMO antennas have been proposed for handheld devices in different communications systems such as the ultra-wideband (UWB) [2, 3], the global system for mobile communications (GSM) [4, 5], the long term evolution (LTE) [6, 7], and the wireless-local-area networks (WLANs) [8, 9]. With the developments of many different wireless communications standards, it is desirable to integrate as many standards as possible into a single wireless device. Multiband and wideband MIMO antenna have been proposed to serve this purpose [2-9]. However, frequency-reconfigurable (FR) MIMO antennas with the advantages of better out-of-band noise rejection [10] and more flexibility are more attractive to serve the same purpose. Different FR-MIMO antennas have been proposed such as those based on using monopole antennas [11-13], slot antennas [14], planar inverted-F antennas (PIFAs) [15] and loop antennas [16]. In [11-13], the FR of the MIMO monopole antennas were achieved by placing PIN diodes on the monopole elements to change the resonant frequencies. In [14], a FR MIMO slot antenna was designed by putting PIN diodes across the slot elements. In [15], a FR MIMO PIFA was designed by placing PIN diodes on both the planar inverted-F antenna (PIFA) radiators and the slits, so that the impedance bandwidth (IMBW) and isolation could be reconfigured simultaneously. In [16], a FR