A Printed Single-Layer UWB Monopole Antenna With ... - IEEE Xplore

IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 12, 2013

237

A Printed Single-Layer UWB Monopole Antenna With Extended Ground Plane Stubs Kuiwen Xu, Zhongbo Zhu, Huan Li, Jiangtao Huangfu, Changzhi Li, Member, IEEE, and Lixin Ran

Abstract—In this letter, a compact coplanar-waveguide-fed single-layer printed antenna with an ultrawideband (UWB) rectangular monopole radiator etched with a half-elliptical slot is presented. Collaborating with the UWB radiator, two symmetrical open-circuit stubs are extended from the ground plane to jointly achieve an ultrawideband impedance match with a compact size. The proposed antenna is fabricated and tested in an anechoic chamber, showing an ultrawide operating frequency range from 3.7 to 10.1 GHz with a quasi-omnidirectional gain from 2.0 to 7.3 dBi. With the advantages of a reduced size, an improved voltage standing wave ratio (VSWR), and a monolayer configuration without any back ground plane, the proposed antenna can be used in a wide range of UWB applications. Index Terms—Ground plane, monopole antenna, radiation pattern, ultrawideband (UWB).

I. INTRODUCTION

U

LTRAWIDEBAND (UWB) applications have been attracting tremendous interests. Compact-size planar antennas have played key roles in achieving optimal performance in portable and mobile applications [1], [2]. Among them, various coplanar waveguide (CPW)-fed monopoles with differently shaped radiators are drawing more and more attention due to their unique characteristics of compact structure, omnidirectional radiation pattern, and compatibility with the printed circuit board (PCB) technique. U-shaped, circular, elliptical, and triangular patches have been investigated to achieve a UWB bandwidth [3]–[13]. In all these antennas, the UWB radiations were obtained by monopole radiators with various shapes, where the resonant surface current can possess different electrical lengths at different frequencies. However, since the sizes of these antennas are primarily determined by the longest electrical length of the surface currents at the lowest frequency, further reduction of the antenna size would face a challenge. In this letter, apart from designing a CPW-fed single-layer UWB monopole radiator etched with a half-elliptical slot,

Manuscript received January 05, 2013; accepted February 11, 2013. Date of publication February 15, 2013; date of current version March 14, 2013. This work was supported by the NSFC under Grants 61071063 and 61131002 and the National Key Laboratory Foundation of China under Grant 9140C530404120C53014. K. Xu, H. Li, J. Huangfu, and L. Ran are with the Laboratory of Applied Research on Electromagnetics (ARE), Zhejiang University, Hangzhou 310027, China (e-mail: [email protected]). Z. Zhu is with the Science and Technology on Space Microwave Laboratory, Xi’an 710100, China. C. Li is with the Department of Electrical and Computer Engineering, Texas Tech University, Lubbock, TX 79409 USA (e-mail: [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.2013.2247555

Fig. 1. Configuration of the proposed antenna. (a) Dimensions. (b) Fabricated antenna. (c) Transient surface current distribution at 4.2 GHz.

two symmetrical open-circuit stubs extended from the CPW’s ground plane are introduced to implement an ultrawideband radiation with a compact size. We show that the two open-circuit stubs not only expand the original small-sized ground plane of the CPW, but also help resonance at lower frequencies. Different from just relying on the specifically designed UWB radiators in previous literature, the open-circuit stubs help to improve the low-frequency performance and provide an effective way to further decrease the antenna size. With a reduced size as small as mm , the fabricated antenna is measured to have a broad operating bandwidth from 3.7 to 10.1 GHz and a quasi-omnidirectional gain from 2.0 to 7.3 dBi. The measured results agree well with simulations. Full-wave simulations also imply that the proposed antenna has a satisfactory impulse response (IR) performance. The compact size, improved voltage standing wave ratio (VSWR), and monolayer configuration without any back ground plane make it suitable for a wide range of UWB applications. II. ANTENNA CONFIGURATION AND ANALYSIS The structure of the proposed antenna is shown in Fig. 1. The antenna consists of a rectangular patch etched with a half-elliptical slot and a short 50- CPW feedline with two I-shaped open-circuit stubs extended symmetrically from its ground plane. The proposed antenna is symmetrical in the -axis and is printed on a 1-mm-thick FR4 substrate with a dielectric constant of 4.6 and a loss tangent around 0.02. This

1536-1225/$31.00 © 2013 IEEE

238


simple structure only occupies an area of mm , which is the smallest monolayer UWB antenna to the best of our knowledge. The radiator is derived from a traditional CPW-fed rectangular monopole patch. The etching of the half-elliptical slot on the -axis of the original small-sized rectangular patch brings in continuously tunable electrical resonance lengths for different fed wavelengths, especially for the intermediate and higher frequencies in the operating bandwidth. In the meantime, the reduced radiation efficiency in the lower frequency band due to the small size of the radiator will be compensated by the introduced open-circuit stubs. The physics behind the stubs is that, at microwave frequencies, they act as two capacitor-fed monopoles in the low frequency band. Fig. 1(c) illustrates three successive snapshots of the transient surface current distribution in one cycle at 4.2 GHz. It is seen that in the beginning, the surface currents flow along the CPW line. After 23 ps, the surface currents flow across the gaps between the bottom edge of the radiator and the top edge of the ground plane (equivalent to two parasitic capacitors). After another 30 ps, the surface currents flow along the stubs, exhibiting the typical distribution of two monopoles. In the following, we will show that the combination of the above two structures results in simultaneous UWB radiation and impedance match, which will be illustrated with full-wave simulations and experimental results. III. SIMULATION AND MEASUREMENT Since both the stubs and the monopole radiator contribute to the UWB radiation, the antenna design mainly consists of three steps: 1) optimization of the stubs; 2) the monopole radiator design; and 3) optimization of the gaps between them. Since the stubs act as two monopoles, the initial value of the length can be set as one fourth of the effective wavelength , where , , and is the dielectric constant of the substrate. This length will be optimized by full-wave optimization for the whole antenna structure. In our design, its resonance frequency is chosen as 4 GHz, and its final length, i.e., , is optimized as 17 mm. The fact that the monopole radiator comes from a CPW-fed rectangular patch can be used to determine its initial size, and the bandwidth extension introduced by the half-elliptical slot can be optimized by full-wave simulation. In our design, the radiator is designed to work in the frequency range from 6 to 12 GHz, and its final dimensions are optimized to be mm, mm, mm, mm. Finally, the width of the gap is optimized as 0.8 mm by simulation. The rest of the dimensions can be determined as mm, mm, mm, mm, and mm. The proposed antenna was simulated with CST Microwave Studio and measured inside a microwave anechoic chamber [14]. The simulated and measured return loss is plotted in Fig. 2(a). The measured 10-dB bandwidth for the return loss ( ) is from 3.7 to 10.1 GHz, which agrees quite well with the simulation result from 3.8 to 10.8 GHz. Considering the inevitable fabrication error and deviation between the actual dielectric constant and the one used in the simulation, this slight bandwidth shift between simulation and measurement

Fig. 2. (a) Simulated and measured return loss of the proposed antenna. (b) Comparison between the antennas with and without stubs.

Fig. 3. Simulated and measured radiation patterns of the proposed antenna at (a) 4.2, (b) 7.5, and (c) 9.8 GHz.

results is acceptable. It is worth noting that the bandwidth with a measured return loss below 15 dB (corresponding to a VSWR of 1.5) is from 4.6 to 9.6 GHz, implying excellent impedance match in this 5-GHz-wide frequency range. To demonstrate how the stubs impact the antenna performance, Fig. 2(b) plots the simulated return loss with and without the stubs. It is seen that for the same monopole radiator, the introduction of the stubs significantly improves the impedance match in the low frequency band from 4 to 8 GHz. The measured (in solid lines) and the simulated (in dashed lines) normalized radiation patterns on the H-plane ( -plane) and E-plane ( -plane) at 4.2, 7.5, and 9.5 GHz are shown in Fig. 3. It is seen that for all the frequencies, the measured patterns are in accordance with the simulated results. All H-plane patterns exhibit quasi-omnidirectional properties, which complies with the expectation that the modified radiator still behaves as a monopole antenna. In the meantime, the E-plane radiations have different quasi-symmetrical dipole-like patterns with respect to different fed wavelengths, which are also in accordance with the monopole nature of the proposed antenna. It should be noted that mismatch of the E-plane pattern from 210 to 330 was caused by the influence of the SMA connector and test cable when measuring the antenna on the E-plane ( -plane). To understand the radiation characteristics of the proposed antenna, the corresponding surface current distribution on the radiator and the stubs are illustrated at the same frequencies of 4.2, 7.5, and 9.8 GHz, as shown in Fig. 4. As expected, we see that the lower frequency resonance is dominated by the

XU et al.: PRINTED SINGLE-LAYER UWB MONOPOLE ANTENNA WITH EXTENDED GROUND PLANE STUBS

239

Fig. 4. Simulated patterns of the surface current distribution at (a) 4.2, (b) 7.5, and (c) 9.8 GHz. Fig. 6. (a) Input pulse and (b) its normalized power spectrum for time-domain analysis.

Fig. 5. Simulated and measured gain-frequency dependency of the proposed antenna.

extended open-circuit stubs, which play a key role in achieving the low frequency radiation and impedance match. On the other hand, the intermediate and higher frequency resonances are more determined by the elliptically edged small-size radiator, which is able to provide similar current distribution patterns with different, continuously tuned electrical resonance lengths. When tuning the performance of the antenna, the gap between the ground plane and the patch also significantly influences the impedance match due to the strong coupling between them. The surface current distribution discussed above clearly shows that the elliptical-edged radiator and the extended ground plane stabs collaboratively establish the UWB radiation. Different from only relying on the radiator to achieve the UWB radiation, the introduced open-circuit stubs contribute to the lower frequency resonance, which effectively reduces the difficulty when optimizing the monopole radiator. The surface current distribution can also explain the slight deviation of the H-plane patterns from the ideal circular shape. A small negative influence of introducing the open-circuit stubs is that the current distribution along the -direction is extended, giving the proposed antenna a slightly increased gain along the 0 and 180 directions, especially at higher frequencies. However, this deviation does not change the overall quasi-omnidiretional characteristic of the proposed antenna, which meets the omnidirectional specifications required in most practical applications. Finally, the simulated and measured gain-frequency dependency of the proposed antenna is illustrated in Fig. 5. In the operating frequency band, the gain ranges from 2 to 7.3 dBi. The simulated and experimental results are consistent with each other.

Fig. 7. Pulse transmission analysis for the face-to-face and side-to-side cases.

IV. IMPULSE RESPONSE ANALYSIS In order to investigate the time-domain impulse response of the proposed antenna, we performed a study based on full-wave simulation using CST Microwave Studio. The analysis followed the approaches outlined in [15]–[17]. In the simulation, we used a fourth-order Rayleigh pulse as the input pulse, which takes a form of

(1) With ps, the waveform of the pulse and its normalized Fourier transform are shown in Fig. 6. It is seen that most of the pulse energy is stored in the band of 3.5–10.5 GHz, which fits well with the operating band of our antenna. In the first simulation, we assumed the transmitting and receiving antennas are separated by a distance of 0.5 m. The pulse transmission for two extreme antenna orientations, namely “face-to-face” and “side-to-side,” were investigated, as shown in Fig. 7. To quantitatively evaluate the IR characteristics, we calculated the fidelity factors for the received pulses according to [17], i.e., (2)

240


simulation of the ultrawideband, flat group delay also supports this conclusion. V. CONCLUSION

Fig. 8. Pulse transmission analysis for different antenna orientations.

A compact CPW-fed UWB antenna with a half-elliptical-edged monopole radiator and two symmetrical open-circuit stubs extended from the ground plane is proposed and investigated. We show by both simulation and experiments that the proposed UWB monopole has a 6.4-GHz bandwidth, an average 4.5-dBi gain, and an omnidirectional H-plane radiation pattern. Assisted by the introduced open-circuit stubs, this monolayer printed antenna with a simple structure only occupies an area of mm , which is, to our knowledge, the smallest monolayer UWB antenna with similar operating bandwidth reported so far. Simulations show that the proposed antenna has a satisfactory IR performance, implying that it can be used in a wide range of portable and mobile UWB applications. REFERENCES

Fig. 9. Group delay analysis for the pulse transmission.

where and are the normalized transmitted and received pulses, respectively, and is the constant delay time. Using the raw waveform data in Fig. 7, the calculated fidelity factors for the “face-to-face” and “side-to-side” cases are 0.90 and 0.83, respectively. We also performed a simulation similar to that of [16], in which a configuration including a transmitting antenna and five virtual probes is established, as shown in Fig. 8. The probes are located in the -plane with the angle equal to 0 , 30 , 45 , 60 , and 90 , respectively. The distance between the transmitting antenna and the probes maintains at 0.5 m. The calculated fidelity factors for each angle are 0.868, 0.901, 0.924, 0.936, and 0.938, respectively. Fig. 9 plots the magnitude and phase of the pulse transmission coefficient ( ) in frequency domain and the group delay calculated from the data for both the “face-to-face” and “side-to-side” cases in Fig. 7. It is seen that while the magnitude of the pulse transmission has the same trend as the antenna gain with respect to frequency, the phase changes linearly with frequency. As a result, the group delay has an ultrawideband flat frequency response. The variation of the group delay is limited to less than 0.5 ns in the band from 3 to 12 GHz, except for a 1-ns dip at 8.8 GHz for the “side-to-side” case. Compared to the fidelity factors reported in [16] and [17], the IR performance of the proposed antenna is satisfactory. The

[1] T. Mitchell, “Broad is the way,” Inst. Elect. Eng. Rev., vol. 47, no. 1, pp. 35–39, Jan. 2001. [2] Z. N. Chen, M. J. Ammann, X. M. Qing, and X. H. Wu, “Planar antennas,” IEEE Microw. Mag., vol. 7, no. 6, pp. 63–73, Dec. 2006. [3] Y. J. Ren and K. Chang, “Ultra-wideband planar elliptical ring antenna,” Electron. Lett., vol. 42, no. 8, pp. 447–449, Apr. 2006. [4] I. K. Joon and J. Yong, “Design of ultra wideband coplanar waveguide-fed LI-shape planar monopole antennas,” IEEE Antennas Wireless Propag. Lett., vol. 6, pp. 383–387, 2007. [5] A. S. Abbosh and M. E. Bialkowski, “Design of ultra wideband planar monopole antennas of circular and elliptical shape,” IEEE Trans. Antennas Propag., vol. 56, no. 1, pp. 17–23, Jan. 2007. [6] M. Gopikrishna, D. D. Krishna, C. K. Aanandan, P. Mohanan, and K. Vasudevan, “Compact linear tapered slot antenna for UWB applications,” Electron. Lett., vol. 44, no. 20, pp. 1174–1175, Sep. 2008. [7] M. N. Moghadasi, H. Rousta, and B. S. Virdee, “Compact UWB planar monopole antenna,” IEEE Antennas Wireless Propag. Lett., vol. 8, no. 22, pp. 1382–1385, 2009. [8] O. Ahmed and A. R. Sebak, “A printed monopole antenna with two steps and a circular slot for UWB applications,” IEEE Antennas Wireless Propag. Lett., vol. 7, pp. 411–413, 2008. [9] C. Deng, Y. J. Xie, and P. Li, “CPW-fed planar printed monopole antenna with impedance bandwidth enhanced,” IEEE Antennas Wireless Propag. Lett., vol. 8, pp. 1394–1397, 2009. [10] J. Liang, L. Guo, C. C. Chiau, X. Chen, and C. G. Parini, “Study of CPW-fed circular disc monopole antenna for ultra wideband applications,” Inst. Elect. Eng. Proc., Microw. Antennas Propag., vol. 152, no. 6, pp. 520–526, Dec. 2005. [11] D. T. Nguyen, D. H. Lee, and H. C. Park, “Very compact printed triple band-notched UWB antenna with quarter-wavelength slots,” IEEE Antennas Wireless Propag. Lett., vol. 11, pp. 411–414, 2012. [12] E. S. Angelopoulos, A. Z. Anastopoulos, D. I. Kaklamani, A. A. Alexandridis, F. Lazarakis, and K. Dangakis, “Circular and elliptical CPW-fed slot and microstrip-fed antennas for ultrawideband applications,” IEEE Antennas Wireless Propag. Lett., vol. 5, pp. 294–297, 2006. [13] E. Ebrahimi, O. Litschke, R. Baggen, and P. S. Hall, “Isolation enhancement of planar disc antenna and ground plane in UWB applications,” Electron. Lett., vol. 46, no. 23, pp. 1539–1541, Nov. 2010. [14] CST, Framingham, MA, USA, “Computer Simulation Technolgy,” [Online]. Available: http://www.cst.com/ [15] Z. N. Chen, X. H. Wu, H. F. Li, N. Yang, and M. Y. W. Chia, “ Considerations for source pulses and antennas in UWB radio systems,” IEEE Trans. Antennas Propag., vol. 52, no. 7, pp. 1739–1748, Jul. 2004. [16] A. Mehdipour, K. M. Aghdam, R. F. Dana, and M. K. Khatib, “A novel coplanar waveguide-fed slot antenna for ultrawideband applications,” IEEE Trans. Antennas Propag., vol. 56, no. 12, pp. 3857–3962, Dec. 2008. [17] G. Quintero, J. F. Zurcher, and A. K. Skrivervik, “System fidelity factor: A new method for comparing UWB antennas,” IEEE Trans. Antennas Propag., vol. 59, no. 7, pp. 2502–2512, Jul. 2011.