1382 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 8, 2009 Compact UWB Planar Monopole Antenna Mohammad Naser-Moghadasi, Hedayat Rousta, and Bal S. Virdee, Member, IEEE Abstract—In this letter, a novel ultrawideband (UWB) planar monopole antenna is presented that has variable band-notch fre- quency characteristic. The triangular-shaped antenna is electro- magnetically coupled to the feed microstrip-line on one side of the dielectric substrate, and this line also directly feeds, through a via- hole, a radiating element in the form of a triangular shape in the ground plane. By inserting an equilateral triangular slot in the ground plane, a much wider impedance bandwidth is realized. Fur- thermore, the dimensions of conductor plane radiating element and the truncated ground plane can be used to tune the antennas’ band-notch and bandwidth characteristics. Index Terms—Frequency band-notch, microstrip feed-line, planar monopole antenna, ultrawideband (UWB). I. INTRODUCTION T HE Federal Communications Commission (FCC) al- located the frequency band 3.1 to 10.6 GHz for ultra- wideband (UWB) systems [1], which provides an excellent opportunity for short-range high-speed indoor data commu- nication applications. The proposed next-generation UWB transceivers will transmit and receive in all 14 bands of the 3.1–10.6 GHz spectrum. This will satisfy the worldwide re- quirements for applications based on certified wireless USB and WiMedia [2]–[4]. Furthermore, recent advances in UWB radio are targeted for medical imaging and sensing applications using UWB radar [5], [6]. Existing wireless local area network (WLAN) standards such as IEEE 802.11a and HIPERLAN/2, operating in the bands 5.725–5.825 GHz and 5.15–5.35 GHz, respectively, overlap with the designated UWB spectrum. Due to the coexistence of the UWB system with frequency bands reserved for narrowband wireless technologies, there is a need in wideband devices to provide filtering in order to avoid interference from, or causing interference to, narrowband devices. Hence, discriminating against signals from such narrowband sources is an important requirement for UWB systems. Instead of using a stop-band filter, which will increase the dimensions of the UWB system, the strategy employed to extract the desired UWB signal in these overlapping bands is to use an antenna with intrinsic band-notch capability. In literature to date, such antennas are implemented by cutting out a slot within the radiating element Manuscript received October 06, 2009; revised November 22, 2009. First published December 22, 2009; current version published January 15, 2010. M. Naser-Moghadasi and H. Rousta are with the Faculty of Engineering, Sci- ence and Research Branch, Islamic Azad University, Tehran 1477893855, Iran (e-mail: mn.moghaddasi@srbiau.ac.ir; he.rousta@gmail.com). B. S. Virdee is with the Faculty of Computing, London Metropolitan Univer- sity, London N7 8DB, U.K. (e-mail: b.virdee@londonmet.ac.uk). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LAWP.2009.2039111 Fig. 1. Geometry of the proposed antenna: (a) dimensions of the proposed an- tenna; (b) photograph of the antenna (front and back). itself [7]–[9]. Unfortunately, these slots cause destructive in- terference for the excited surface currents in the antenna and consequently make it insensitive to certain frequencies, which depend on the slots dimensions and positions. In this letter, a novel microstrip-line-fed UWB planar monopole antenna is proposed, which exhibits the desired band-notch return-loss characteristic without mutilating the physical geometry of the antenna itself, i.e., either the radi- ating patch or feed-line, and making it insensitive to certain frequencies. The notched band, covering the UWB spectrum 3.1–10.6 GHz, is provided by using a triangular-shaped con- ductor and a triangular-shaped slot in the truncated ground plane of the dielectric substrate. This structure provides a much wider impedance bandwidth. II. ANTENNA DESIGN Investigations show that, at low frequencies, the performance of the antenna critically depends on the microstrip-line feed length. This length determines the lower edge of the antenna’s impedance bandwidth, and as a result defines its lateral size. However, in this letter, a novel feeding mechanism is used to miniaturize the antenna for minimum lateral length. Fig. 1(a) shows the configuration of the proposed UWB monopole antenna. In the design, the radiating element, which is in the form of an equilateral triangle, and truncated ground 1536-1225/$26.00 © 2010 IEEE