1536-1225 (c) 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/LAWP.2017.2682958, IEEE Antennas and Wireless Propagation Letters > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 1 Abstract—In this letter, an omnidirectional circularly polarized antenna is proposed. This new antenna has simple structure, which is composed of two dielectric substrates, four shorting pins and a feeding pin. A circular polarized (CP) radiation can be realized at the azimuth plane. This antenna can cover broad communication bands, such as GSM900, GSM1990, PCS and UMTS band. To validate the correctness of the simulation results, the omnidirectional CP antenna was fabricated and measured. With a profile of 0.066   (   is the free-wavelength at the lowest frequency during the operate bandwidth), the antenna has a -10dB impedance bandwidth of 25.6% (range from 1.7GHz to 2.2GHz) and a 3dB axial ratio bandwidth of 34.7% (range from 1.57GHz to 2.23GHz). At the center frequency 1.95GHz, the measured average axial ratio and right-hand circularly polarized (RHCP) gain are 2.69dB and -1.12dBic at the azimuthal plane, respectively. Index Terms—Broadband antenna, circularly polarization (CP), omnidirectional antenna I. INTRODUCTION ith the fast development of wireless communication system, simple linear polarization antenna has been difficult to meet the requirements of the communication system. More and more omnidirectional CP antennas were widely used in the omnidirectional coverage communication system, and the demand for circularly polarization (CP) antennas is growing rapidly [1]. Especially in the broadcasting, satellite communication, and amateur radio field, the demand for the omnidirectional circularly polarized antennas are gradually increasing [2]-[3]. Recently, a wideband omnidirectional circularly polarized antenna developed in [4] has an identical bandwidth of 40% (| 11 |<-10dB and AR<3dB). However, the high profile with the height of 0.4λ (λ is the free-space wavelength at the center frequency) limits its application in the communication system. Because of the dimension limit in the communication device, smaller size and wider bandwidth will be more suitable for application. An overall size of 0.288 0 × 0.288 0 × 0.016 0 was presented in [5]. However, palpable defect in this antenna was narrow impedance bandwidth, and only 0.541% -10dB impedance bandwidth achieved in this antenna, which limited its applications in the wireless system. And there were similar problems existing in other ZOR omnidirectional CP antennas [6]-[7]. In [6], a 0.58% -10dB impedance bandwidth and a good CP performance with AR<1.2dB around the azimuth plane was realized. There were also some other designs to achieve omnidirectional CP with a broadband. In [8], for instance, the All the authors are with the Northwestern polytechnical university, Xi;an, 710072, China (e-mail: nwpu8501@163.com, ). antenna consisted of four bended monopoles with a power divided feeding network structure. The vertical part and the horizontal part together would produce a vertical polarized and a horizontal polarized with a 90 0 phase difference at the azimuth plane. A good CP characteristic could be satisfied by adjusting the length of the vertical dipole and the horizontal dipole. An identical bandwidth of 3.56% ( | 11 | <-10dB and AR<3dB) was realized in this structure. However, the bandwidth of this structure was still narrow in the application of wireless communication system. Recently, wider bandwidths of omnidirectional CP antennas were proposed in [9]-[10]. A short dipole and a small current loop structure were designed to realize omnidirectional CP. Six metal pins around the circumference and a similar current loop structure was proposed to create an omnidirectional CP characteristic. An antenna with -10dB impedance bandwidth of 13.8% and a 3dB AR bandwidth of 17.5% was proposed in [9]. What’s more, antenna in [10] with a bandwidth of 16.6% in | 11 |<-10dB and 14.8% in AR<3dB could be achieved with sector patch connected with shorting vias structure. In this letter, a novel omnidirectional CP antenna with a profile of 0.067  0 is developed. This omnidirectional CP antenna consists of a top loaded monopole with four shorting pins and four arc dipoles which printed on a substrate. Each arc dipole is coupled with a power divided feeding network [11]. This antenna shows a -10dB impedance bandwidth of 25.6% and a 3dB axial ratio bandwidth of 34.7%. In Section II, the configuration of this novel omnidirectional CP antenna is presented and the influence of some main parameters is discussed. Simulated results and measured results are shown in section III. II. ANTENNA DESIGN AND PARAMETERS STUDIES A. Antenna Design Fig.1 shows the configuration of the proposed antenna. The proposed antenna consists of two parts. The four metallic arc dipoles are printed on the bottom layer of the lower substrate as the ground, which would connect with the outer conductor of a coaxial cable. Each arc dipole is about half of wavelength. Besides, a broadband power divider balun feeding network is printed on the top layer of the lower substrate, and the center of the feeding network would connect with the inner conductor of a coaxial cable [12]. Moreover, an impedance matching circuit is also used by controlling the width of the microstrip line (w1 and w2) in this feeding network to broaden the impedance bandwidth. In this antenna, four arc dipoles will be excited simultaneously with such a coupling feeding structure, and the four bowtie radiation patterns will be overlapped. Through properly adjusting antenna structure parameters, the arc dipoles Design of Omnidirectional Circularly Polarized Antenna Yihan Ma, Jianying Li, Member, IEEE, Rui Xu W