Dual-Band Low-Profile Crossed Asymmetric Dipole Antenna on Dual ...

IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 13, 2014

587

Dual-Band Low-Profile Crossed Asymmetric Dipole Antenna on Dual-Band AMC Surface Son Xuat Ta and Ikmo Park

Abstract—A dual-band, low-profile, circularly polarized antenna on an artificial magnetic conductor (AMC) is introduced in this letter. The antenna employs a single feed and two crossed asymmetric dipoles as the primary radiating elements. In order to achieve a low profile and broadband characteristics in terms of impedance matching and 3-dB axial-ratio (AR) bandwidths at both bands, a dual-band AMC is utilized as a reflector of the antenna. The AMC structure utilizes four T-shaped slits in a unit-cell patch to significantly reduce its first and second resonant frequency ratio, and consequently its first and second resonances are easily adjusted for the desired operating frequencies. For performance verification, an antenna prototype was fabricated with an overall 2.4-GHz frequency size of mm . The measurements resulted in impedance 10 dB bandwidths of 2.20–2.60 and 4.90–5.50 GHz for and 3-dB AR bandwidths of 2.30–2.50 and 5.05–5.35 GHz. Additionally, the antenna yields right-hand circular polarization and high antenna efficiency at both bands. Index Terms—Artificial magnetic conductor, asymmetric dipole, circular polarization, dual-band operation, second resonance, T-shaped slit.

I. INTRODUCTION

A

RTIFICAL magnetic conductor (AMC)-based antennas have been intensely developed for enhancing their performance, including bandwidth and unidirectional radiation pattern with a miniaturized profile. The AMC surface, also known as a high-impedance surface (HIS) [1], an electromagnetic band-gap (EBG) surface [2], or a reactive impedance surface [3], can mimic a perfect magnetic conductor (PMC) within a certain frequency range. Therefore, the AMC surface allows for the placement of the antenna in close proximity with good impedance matching and highly efficient radiation. In many applications, an antenna works at two different frequencies simultaneously; consequently, the AMC structure requires dual-band operation that can match the antenna’s working frequencies. Many linearly polarized (LP) antennas on AMC surface [5]–[7] have been presented for dual-band operation, but there are still a limited number of studies on circularly polarized (CP) antennas loaded with the dual-band

Manuscript received January 20, 2014; revised March 05, 2014; accepted March 17, 2014. Date of publication March 20, 2014; date of current version April 07, 2014. The authors are with the Department of Electrical and Computer Engineering, Ajou University, Suwon 443-749, Korea (e-mail: [email protected]; [email protected]). 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.2014.2312950

Fig. 1. Geometry of the proposed antenna: (a) cross-sectional view and (b) top view of a crossed asymmetric dipole radiator.

AMC structures. Recently, a dual-band LP dipole was designed on a dual-band polarization-dependent EBG surface to realize a dual-band CP antenna [8]. However, its axial-ratio (AR) bandwidth is insufficient for some applications. A CP-crossed dipole antenna on an AMC surface has been presented for 2.4/5.2/5.8-GHz WLAN applications [9]. However, its profile is still large since its AMC surface exhibits as a perfect electric conductor (PEC) in the upper bands. In the present study, a dual-band CP radiator of a crossed asymmetric dipole [10] was incorporated with a dual-band AMC surface [11] for a low-profile and broadband characteristic in both operating bands. The AMC structure was designed with four T-shaped slits in a unit-cell patch to reduce its second resonant frequency, and consequently its first and second resonant frequencies are easily adjusted for the desired dual band. The resulting antenna was characterized first with the ANSYS-Ansoft High Frequency Structure Simulator (HFSS); its simulated performance was then verified by the measurements. II. GEOMETRY OF THE PROPOSED STRUCTURE Fig. 1 shows the geometry of the dual-band, crossed, asymmetric dipole antenna on the dual-band AMC surface. The

1536-1225 © 2014 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.

588


Fig. 3. Simulated values of the crossed asymmetric dipole elements radiating in different configurations.

Fig. 2. (a) Unit-cell geometry of the AMC and (b) its simulated reflection phases with and without slits.

crossed dipoles were suspended at a height from the top of the reflector. The primary radiator element, which is similar to the one presented in [10], was designed on both sides of an RT/Duroid 5880 piece with , , and mm. The shorter branch of the dipole arm did not contain a meander line in order to achieve a dual band with a large frequency ratio. To produce CP radiation at 2.4/5.2-GHz bands, the dipoles were crossed through a vacant-quarter printed ring, which had a length of approximately at the lower band and at the upper band ( being the guided wavelength at the center frequency) [9]. A 6 6 metal patch array forming an AMC surface was backed as a reflector of the proposed antenna. Fig. 2(a) shows the geometry of the unit cell of the AMC structure. A model based on simulating scattering parameters of a single-port air-filled waveguide with two PEC and two PMC walls was used for the design simulation [12]. The unit cell was printed on a grounded -mm ( at 2.4 GHz) RT/Duroid 6010 substrate with a relative permittivity of 10.2, a loss tangent of 0.0023, and a thickness of mm ( at 2.4 GHz). There was no via. Four T-shaped slits were symmetrically inserted into the square patch of the AMC unit cell. A two-dimensional metallic square patch without T-shaped slits printed periodically on the grounded substrate [3] was chosen for the initial design of the proposed AMC structure. As shown in Fig. 2(b), with an -mm patch, the AMC without slits yielded the first and second resonant frequencies at 2.75 and 9.1 GHz for a zero reflection phase, respectively. It is well known that slits on microstrip patch antennas help to reduce the antenna size because they change the current distribution on the patch [13]. This idea was also applied to reduce the resonances of the AMC design and consequently achieve a dual-band operation [2]. Accordingly, the proposed

Fig. 4. Simulated AR values of the crossed asymmetric dipole elements radimm, AR is ating in different configurations. For the PEC reflector with excluded from the lower band because the minimum value is larger than 9 dB.

AMC structure utilized four T-shaped slits on the metallic patch for two desired resonant frequencies [14]. The AMC structure was optimized via adjusting the slit shape to have the first and second resonances around 2.4 and 5.5 GHz, respectively. The optimized design parameters of the patch with T-shaped slits were as follows: mm, mm, mm, mm, mm, and mm. Fig. 2(b) also shows the simulated reflection phases of the AMC. For a unit cell without slits, the operating bandwidth was 2.60–3.05 and 9.05–9.35 GHz for the phase of the reflection coefficient. When the slits were incorporated, the frequency bands decreased to 2.30–2.50 and 5.30–5.75 GHz, respectively. III. PERFORMANCE COMPARISON OF CROSSED ASYMMETRIC DIPOLE ELEMENT RADIATING IN DIFFERENT CONFIGURATIONS The crossed asymmetric dipoles were first optimized in free space for 2.4/5.2-GHz bands with good CP radiation and then incorporated with PEC and AMC reflectors to render a unidirectional radiation pattern. Referring to Fig. 1(b), the optimized design parameters were as follows: mm, mm, mm, mm, mm, mm, mm, mm, mm, mm, mm, mm, and mm. As shown in Figs. 3 and 4, the antenna in free space yielded impedance matching bandwidths of 2.35–2.80 and 4.80–5.95 GHz for 10 dB, and 3-dB AR bandwidths of 2.38–2.46 and 5.10–5.41 GHz with two CP center frequencies at 2.42 GHz dB and 5.25 GHz dB , respectively.

TA AND PARK: DUAL-BAND LOW-PROFILE CROSSED ASYMMETRIC DIPOLE ANTENNA ON DUAL-BAND AMC SURFACE

Fig. 5. Simulated broadside gain values of the crossed asymmetric dipole elements radiating in different configurations.

The dual-band radiator was placed on a finite PEC reflector with a basic dimension of mm at a distance of mm for the unidirectional radiation patterns. As shown in Figs. 3 and 4, the performances of the crossed asymmetric dipole antenna on the PEC reflector were significantly degraded compared to that of the free space since mm ( at 2.4 GHz and at 5.2 GHz) was much smaller than a quarter-wavelength at both bands. The 10-dB bandwidths were 2.35–2.40 and 5.13–5.43 GHz, whereas it yielded a zero 3-dB AR bandwidth; the minimum AR values were greater than 9 and 3 dB at the lower and upper bands, respectively. Optimum performance of the PEC reflector antenna can be obtained at each band when the spacing between the radiator and reflector is approximately at the center frequency. However, in the proposed design, it was not possible to determine the appropriate value of for both operating bands, particularly since the frequency ratio between the lower and upper bands was greater than two [9]. In order to achieve a unidirectional radiation pattern, good impedance matching, and good low-profile CP radiation at both operating bands, a crossed asymmetric dipole antenna was incorporated with the dual-band AMC surface presented in Section II. Interactions between the radiator and the reflector, such as the air gap and finite size of the AMC surface, were meticulously considered for the optimization. The overall size of the final design was mm ( at 2.4 GHz), including the substrate thickness of the radiator. The simulated and AR values of the proposed antenna are also illustrated in Figs. 3 and 4, respectively. The AMC-based antenna yielded a broadband characteristic at both bands; the impedance bandwidths were 2.25–2.60 and 4.80–5.50 GHz for 10 dB, while 3-dB bandwidths were 2.30–2.46 and 5.04–5.36 GHz. Additionally, several extra resonances appeared in the high-frequency region of the profile (Fig. 3). These were caused by transverse electric (TE) surface waves propagating on the finite AMC surface. Analysis and modeling of this phenomenon for an LP dipole on a finite AMC surface were done in [4]. Unlike [4], the proposed antenna employed a CP radiator; thus, extra CP bands were also generated (Fig. 4). As shown in Fig. 5, without any reflector, the crossed asymmetric dipole elements radiated a bidirectional electromagnetic wave and had a broadside gain of only about 2 dBic. Fig. 5 also shows that gain was improved significantly with the PEC or AMC reflectors. These two cases showed gain higher than 5.0

589

Fig. 6. Top view of the fabricated antenna.

Fig. 7. Comparison between the simulated and measured (a) values versus frequency.

and (b) AR

dBic at both bands. These results indicate that the AMC reflector enhanced the performance of the crossed asymmetric dipole radiating elements in terms of the resulting broadband impedance, CP radiation characteristics, and unidirectional gain patterns. Similar to the LP radiator on the finite AMC surface [4], in the proposed design the grating lobes appeared in the radiation patterns at the higher modes of the TE surface wave resonances. Therefore, the broadside gain dropped in the high-frequency region of both bands as shown in Fig. 5, respectively. IV. MEASUREMENT RESULTS The crossed asymmetric dipole on the dual-band AMC surface was fabricated and measured. A sample of fabricated antenna is shown in Fig. 6. A comparison of the simulated and measured and AR values is given in Fig. 7. The measured impedance bandwidths for 10 dB were 2.20–2.48 and 4.80–5.45 GHz, values that agree closely with the simulated bandwidths of 2.25–2.60 and 4.80–5.50 GHz. The measured

590


V. CONCLUSION A crossed asymmetric dipole antenna over a dual-band AMC surface has been introduced. With the presence of the dual-band AMC surface, the proposed antenna yields a low profile, good impedance matching, good CP radiation, and an improvement in the 3-dB AR bandwidth at both operating bands. It has a broad bandwidth: 2.20–2.60 and 4.90–5.85 GHz for 10 dB, and 2.28–2.50 and 5.08–5.36 GHz for dB. It has a unidirectional radiation pattern as well as high efficiency. The AMC structure is presented with four T-shaped slits in the unitcell patch to tune its resonant frequencies, and consequently it is optimized for dual-band operation. Thanks to the presence of the dual-band AMC surface, a low profile and broadband CP radiation of the proposed antenna are obtained. This antenna, exhibiting many advantages, is a good candidate for dual-band wireless communications, such as WLAN, RFID, and WiMAX, as well as gap-filler applications.

Fig. 8. Radiation patterns of the antenna at (a) 2.4 and (b) 5.2 GHz.

TABLE I COMPARISON OF MEASURED PERFORMANCES OF DUAL-BAND CP ANTENNAS LOADED WITH DUAL-BAND AMC (L: LOWER BAND, U: UPPER BAND)

3-dB AR bandwidths were 2.30–2.45 and 5.05–5.35 GHz, while the simulated 3-dB AR bandwidths were 2.30–2.46 and 5.04–5.36 GHz. Fig. 8(a) and (b) shows the radiation patterns of the antenna at 2.4 and 5.2 GHz, respectively; there is good agreement between the measurement and simulation results. The radiation was right-handed CP (RHCP) with quite symmetric profiles at both bands. At 2.4 GHz, the measurements yielded a gain of 5.1 dBic, a front-to-back ratio of 27 dB, and half-power beamwidths (HPBWs) of 65 and 60 in the and -planes, respectively. At 5.2 GHz, the measurements yielded a gain of 6.2 dBic, a front-to-back ratio of 24 dB, and HPBWs of 98 and 82 in the - and -planes, respectively. The patterns were also measured for other frequencies (not shown) within the 3-dB AR bandwidth of the lower and upper bands and were very similar to the 2.4- and 5.2-GHz patterns. In addition, the measurements resulted in radiation efficiency of greater than 78% compared to the simulated values of greater than 80% within the 3-dB AR bandwidth. Table I shows the comparison of the measured performances of the proposed antenna and the dual-band CP antenna loaded with dual-band AMC structure [8]. The proposed antenna yielded lower profile and broader impedance matching and 3-dB AR bandwidths at both operating bands.

REFERENCES [1] S. Clavijo, R. E. Diaz, and W. E. McKinzie, “Design methodology for Sievenpiper high-impedance surfaces: An artificial magnetic conductor for positive gain electrically small antennas,” IEEE Trans. Antennas Propag., vol. 51, no. 10, pp. 2678–2690, Oct. 2003. [2] F. Yang and Y. Rahmat-Samii, “Reflection phase characterizations of the EBG ground plane for low profile wire antenna applications,” IEEE Trans. Antennas Propag., vol. 51, no. 10, pp. 2691–2703, Oct. 2003. [3] H. Mosallaei and K. Sarabandi, “Antenna miniaturization and bandwidth enhancement using a reactive impedance substrate,” IEEE Trans. Antennas Propag., vol. 52, no. 9, pp. 2403–2414, Sep. 2004. [4] F. Costa, O. Luukkonen, C. R. Simovski, A. Monorchio, S. A. Tretyakov, and P. M. Maagt, “TE surface wave resonances on high-impedance surface based antennas: Analysis and modeling,” IEEE Trans. Antennas Propag., vol. 59, no. 10, pp. 3588–3596, Oct. 2011. [5] S. Zhu and R. Langley, “Dual-band wearable textile antenna on an EBG substrate,” IEEE Trans. Antennas Propag., vol. 57, no. 4, pp. 926–935, Apr. 2009. [6] O. Folayan and R. Langley, “Dual frequency band antenna combined with a high impedance band gap surface,” Microw., Antennas Propag., vol. 3, no. 7, pp. 1118–1126, 2009. [7] N. A. Abbasi and R. J. Langley, “Multiband-integrated antenna/artificial magnetic conductor,” Microw., Antennas Propag., vol. 5, no. 6, pp. 711–717, 2011. [8] H. Yi and S. Qu, “A novel dual-band circularly polarized antenna based on electromagnetic band-gap structure,” IEEE Antennas Wireless Propag. Lett., vol. 12, pp. 1149–1252, 2013. [9] S. X. Ta, I. Park, and R. W. Ziolkowski, “Circularly polarized crossed dipole on an HIS for 2.4/5.2/5.8-GHz WLAN applications,” IEEE Antennas Wireless Propag. Lett., vol. 12, pp. 1464–1467, 2013. [10] S. X. Ta, I. Park, and R. W. Ziolkowski, “Dual-band wide-beam crossed asymmetric dipole antenna for GPS application,” Electron. Lett., vol. 48, no. 25, pp. 1580–1581, Dec. 2012. [11] S. X. Ta and I. Park, “Circularly polarized dual-band crossed dipole antenna on an artificial magnetic conductor reflector,” in Proc. Int. Congress Adv. Electromagn. Mater. Microw. Opt., Bordeaux, France, Sep. 2013. [12] A. Lamminen, A. Vimpari, and J. Saily, “UC-EBG on LTCC for 60-GHz frequency band antenna for applications,” IEEE Trans. Antennas Propag., vol. 57, no. 10, pp. 2904–2912, Oct. 2009. [13] X. X. Zhang and F. Yang, “The study of slit cut on the microstrip antenna and its applications,” Microw. Opt. Technol. Lett., vol. 18, no. 4, pp. 297–300, Apr. 1998. [14] S. X. Ta and I. Park, “Design of miniaturized dual-band artificial magnetic conductor with easy control of second/first resonant frequency ratio,” J. Electromagn. Eng. Sci., vol. 13, no. 2, pp. 113–119, Jun. 2013.