IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 53, NO. 6, JUNE 2005 2111
Bandwidth Enhancement of a Printed Wide-Slot Antenna With a Rotated Slot
Jen-Yea Jan and Jia-Wei Su
Abstract—In this paper, a printed wide-slot antenna fed by a microstrip line with a rotated slot for bandwidth enhancement is proposed and exper-imentally studied. Impedance, radiation, and gain characteristics of this antenna are presented and discussed. From experimental results, the mea-sured impedance bandwidth, defined by 10 dB return loss, can reach an operatingbandwidth of 2.2 GHz at operatingfrequencies around 4.5 GHz, which is about four times that of a conventional microstrip-line-fed printed wide-slot antenna. Also, the antenna gain within the operating band is mea-sured and studied, and a 2-dB gain bandwidth of at least 1 GHz is achieved. Index Terms—Bandwidth enhancement, microstrip-line-fed antennas, printed slot antennas.
I. INTRODUCTION
Printed slot antennas are attractive because their operating bands usually have wide impedance bandwidths. In addition, they are completely uniplanar and are easily integrated with active devices or MMICs. Therefore a great interest in various slot antennas with different feed methods can be seen in the literature [1]–[3].
In recent years, there has been a growing research activity on many microstrip-line-fed printed slot antennas, especially printed wide-slot antennas [4], [5] because of their favorable impedance characteristics. A wide-slot is a slot with an aspect-radio significantly smaller than that of the usual narrow slots, sometimes quite close to 1. Characteristics of printed wide-slot antennas fed by a microstrip line with different tuning stubs have also been widely studied, so that impedance char-acteristics [4], [5] and circularly polarized radiation charchar-acteristics [6], [7] have been reported. In the reported literature [5], a printed wide-slot antenna fed by a microstrip line with a fork-like tuning stub is good for bandwidth enhancement. However, it makes the configuration of the wide-slot antenna more complicated in the design on the feed line. In this paper, a new design of microstrip-line-fed printed wide-slot an-tennas with a simply rotated slot for bandwidth enhancement is pro-posed and investigated. The rotated slot used in this new design is dif-ferent from the various microstrip feed lines used in previous research [5]. By choosing a proper rotation angle with respect to the center of square wide slot, it can be expected that the other resonant mode oper-ating near one of the conventional wide-slot antenna can be obtained. Hence within the operating bandwidth, two resonant modes having similar slot radiation patterns and the same polarization planes makes significant bandwidth enhancement of the proposed wide-slot antenna possible. From the experimental results, the obtained impedance band-width (determined from 10 dB return loss) of the proposed antenna can reach about four times that of a conventional microstrip-line-fed printed wide-slot antenna.
II. ANTENNACONFIGURATION
Fig. 1 shows the geometry and dimensions of the proposed mi-crostrip-line-fed wide-slot antenna. The printed wide slot is chosen to be a square in order to excite two modes with close resonant frequencies. For exciting the operating frequencies at around 4.5 GHz, this printed square slot rotated with a angle has dimensions of
Manuscript received February 27, 2004; revised October 24, 2004. The authors are with the Department of Electronic Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 807, Taiwan, R.O.C.
Digital Object Identifier 10.1109/TAP.2005.848518
Fig. 1. Geometry and dimensions of the microstrip-line-fed printed square slot antenna with a rotated slot.
24.72 24.7 mm2 and is printed on an FR4 substrate of thickness 1.6 mm and relative permittivity 4.4. The ground plane is also chosen to be square with a side length of 70 mm. This square slot is fed by a 50- microstrip line with a simple tuning stub having a straight length ofL mm, which is printed on the opposite side of the microwave sub-strate. For design simplicity, the width of the tuning stub is chosen to be the same as that of the 50- microstrip line. Simulated results show that square slot antennas with various rotated angles need different tuning-stub length (L in Fig. 1) to be matched. The correct values can be obtained using a simulation tool such as HFSS.
III. EXPERIMENTALRESULTS ANDDISCUSSION
In this study, experimental results of impedance, radiation, and gain characteristics of the proposed antenna are measured and presented. The HP 8753E vector network analyzer is used for measurements of impedance characteristics. The NSI model 800F-10 far-field antenna measurement system that can supply full complement of standard gain horns for gain measurement from 3400 to 5600 MHz is used for mea-surements of radiation and gain characteristics.
A. Impedance Characteristics of the Proposed Antenna
The proposed microstrip-line-fed square slot antenna has been con-structed and experimentally studied. By varying the parameters of andL in Fig. 1, the measured return loss results of several design ex-amples are shown in Fig. 2. For comparison, the design parameters and corresponding measured data are listed in Table I.
From the obtained results, it can be seen that the proposed antenna with = 45andL = 31:5 mm has an impedance bandwidth as large as 2200 MHz, which is about four times that (543 MHz) of the proposed antenna with = 0andL = 33 mm. The measured impedance on a Smith chart for this antenna is presented in Fig. 3. The measured and simulated return loss results of this antenna are shown in Fig. 4. It is seen that other antennas have larger bandwidths than the proposed antenna with = 0andL = 33 mm. It is found that the biggest bandwidth can be obtained when the rotation angle is nearly 45. This bandwidth is enhanced because the other resonant mode in the vicinity of the fundamental mode can be excited by the rotated square wide slot. When the rotation angle of the square slot varies with a suitable value, the impedance curve can have more different loops on the Smith chart. 0018-926X/$20.00 © 2005 IEEE
2114 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 53, NO. 6, JUNE 2005
Fig. 7. Measured peak antenna gain against frequency for the antenna with = 45 and L = 31:5 mm.
IV. CONCLUSION
A printed wide-slot antenna fed by a 50- microstrip line with a rotated square wide slot for bandwidth enhancement has been demon-strated. Several design examples have also been implemented. Exper-imental results show that the impedance bandwidth of a printed wide-slot antenna can significantly be improved by rotating a suitable angle of the square wide slot. For the parameters, and L, selected from = 40to 50andL = 31:5 to 32 mm in this study, the impedance
bandwidth determined by 10 dB return loss can reach nearly 2.2 GHz for the proposed antenna with designed operating frequencies around 4.5 GHz, which is about four times that of the corresponding conven-tional printed microstrip-line-fed wide-slot antenna. Within this wide impedance bandwidth, with gain variation less than 2 dBi, the operating bandwidth with usable or selectable broadside radiation patterns can be about 1100 MHz, or two times that of the corresponding conventional printed wide-slot antenna. In addition, the proposed printed wide-slot antenna shows a very wide 10-dB return-loss impedance bandwidth of about 2.2 GHz (about 3400–5600 MHz).
REFERENCES
[1] Y. Yoshimura, “A microstrip line slot antenna,” IEEE Trans. Microwave
Theory Tech., vol. 20, no. 11, pp. 760–762, Nov. 1972.
[2] A. Axelrod, M. Kisliuk, and J. Maoz, “Broadband microstrip-fed slot radiator,” Microwave J., pp. 81–94, Jun. 1989.
[3] W. Y. Tam, “Microstripline-fed cylindrical slot antenna,” IEEE Trans.
Antennas Propag., vol. 46, no. 10, pp. 1587–1589, Oct. 1998.
[4] M. Kahrizi, T. K. Sarkar, and Z. A. Maricevic, “Analysis of a wide ra-diating slot in the ground plane of a microstrip line,” IEEE Trans.
Mi-crowave Theory Tech., vol. 41, no. 1, pp. 29–37, Jan. 1993.
[5] J. Y. Sze and K. L. Wong, “Bandwidth enhancement of a microstrip-line-FED printed wide-slot antenna,” IEEE Trans. Antennas Propag., vol. 49, no. 7, pp. 1020–1024, Jul. 2001.
[6] K. L. Wong, J. Y. Wu, and C. K. Wu, “A circularly polarized patch-loaded square-slot antenna,” Microwave Opt. Technol. Lett., vol. 23, no. 6, pp. 363–365, Nov. 1999.
[7] M. Yamazaki, E. T. Rahardjo, and M. Haneishi, “Construction of a slot-coupled planar antenna for dual polarization,” Electron Lett., vol. 30, no. 22, pp. 1814–1815, Oct. 1994.
A V-Shaped Structure for Improvingthe Directional Properties of the Loop Antenna
Sivanand Krishnan, Le-Wei Li, and Mook-Seng Leong
Abstract—A V-shaped wire-loop antenna with a butterfly-like structure is proposed in this paper. The antenna consists of two identical half-loops inclined at an angle to each other and joined together at the feed point and the point at which they meet the ground plane. The performance of the structure was evaluated analytically for included angles 90 , 120 and 150 between the half-loops. At a frequency at which the lengths of the half-loops makingup the antenna are equal to a wavelength, the radiation characteristics were superior to that of a conventional half-loop. The re-sults were verified through measurements for the case where the included angle was 120 . The dominant radiation was in the direction perpendicular to the ground plane and the gain was better by 4.5 dB. The measured 3-dB beamwidth was also narrower, at 60 , compared to 135 in the case of the conventional half-loop.
Index Terms—Butterfly loop, circular loop antennas, half-loop antennas.
I. INTRODUCTION
Circular loop antennas with circumference of a wavelength or more are known to have directive radiation patterns and have found appli-cations as feeds, directors and reflectors in Yagi–Uda arrays [1]. So it is not surprising that these electrically large circular loop antennas are considered to be as important and fundamental as half-wavelength dipoles [2]. In the case of dipoles, the radiation patterns become more directional as the dimensions are increased, but when the length is greater than about one wavelength, the number of lobes increases and the antenna starts to lose its directional properties. To overcome this problem with long dipoles, an inclined two-element array structure in the form of the V antenna has traditionally been used as an alternative [2]. While moderate to large sized circular loops and loop arrays have been thoroughly investigated by several workers in the past [3]–[8], an analogous approach to the V antenna using loops does not appear to have been attempted. So in this paper we propose and investigate the performance of a V-shaped loop structure consisting of two circular loops that are inclined at an angle to each other and are fed by the same source.
The loop antenna, like the dipole, is a balanced structure and so re-quires the use of a balun when fed by a coaxial line. In most cases, a practical way of overcoming this problem is to use a half-loop config-uration that is fed through a ground plane. The ground plane also acts as a support for the wire loop. So, while the performance and analysis described in this paper are equally valid for full-loop configurations, for practical reasons, only the half-loop versions will be considered. It should be noted that unlike the dipole, the loop is a two-dimensional structure and so several different V-shaped configurations can be ob-tained using two inclined loops. Thus, due to its appearance and in order to differentiate the proposed structure from other possible V-shaped loop structures, it is referred to as the butterfly-loop antenna. The but-terfly-loop basically consists of two identical half-loops which are con-nected at an included angle(') to each other as shown in Fig. 1(a). At
Manuscript received April 8, 2004; revised December 6, 2004.
S. Krishnan is with the Department of Electrical and Computer Engineering, National University of Singapore, Singapore, 119260, Singapore and also with the Institute for Infocomm Research, Singapore, 117674, Singapore.
L.-W. Li and M.-S. Leong are with the Department of Electrical and Com-puter Engineering, National University of Singapore, Singapore, 119260, Sin-gapore (e-mail: [email protected]).
Digital Object Identifier 10.1109/TAP.2005.848519 0018-926X/$20.00 © 2005 IEEE
