A compact printed ultrawideband (UWB) diversity antenna with a size of 30？mm × 36？mm operating at a frequency range of 3.1–10.6？GHz is proposed. The antenna is composed of two semielliptical monopoles fed by two microstrip lines. Two semicircular slots, two rectangular slots, and one stub are introduced in the ground plane to adjust the impedance bandwidth of the antenna and improve the isolation between two feeding ports. The simulated and measured results show that impedance bandwidth of the proposed antenna can cover the whole UWB band with a good isolation of < ？15？dB. The radiation patterns, peak antenna gain, and envelope correlation coefficient are also measured and discussed. The measured results show that the proposed antenna can be a good candidate for some portable MIMO/diversity UWB applications. 1. Introduction Ultrawideband (UWB) communication systems attract great attention in the wireless world because of their advantages, including high speed data rate, extremely low spectral power density, high precision ranging, low cost, and low complexity, since the Federal Communication Commission (FCC) allowed 3.1–10.6？GHz unlicensed band for UWB communication [1]. Multiple input and output (MIMO) antennas used in UWB systems can improve systems’ performance such as providing increased data rates and making increased range available through beam forming [2]. Antenna diversity is a well-known technique to enhance the performance of MIMO systems by mitigating the multipath fading and cochannel interference [3–5]. Thus, a MIMO/diversity antenna covering UWB band and having good radiation patterns and diversity performance has become the focus of wireless communications [6–10]. However, the operating frequency of the antennas in [6–10] cannot cover the whole band of 3.1–10.6？GHz and the size of the antenna in [6–8] is relatively large. Recently, an UWB diversity antenna covering the whole UWB and having a small size has been presented in [11]. The proposed antenna has a size of , and a good isolation better than ？16？dB has been achieved through a tree-like structure introduced in the ground plane. However, the size of the antenna in [11] is still relatively large and a complicate tree-like structure should be designed to weaken the mutual coupling between two monopoles due to the close distance between them. In this paper, we propose a simple MIMO/diversity antenna covering the whole UWB band and having a small size of which is only 75 percent of the antenna in [11]. The proposed antenna is designed by using two semielliptical monopoles. By properly
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