Ultrawideband Strip-Loaded Circular Slot Antenna With Improved ...

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IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 55, NO. 11, NOVEMBER 2007

REFERENCES [1] K. Finkenzeller, RFID Handbook, Second Edition, 2nd ed. U.K.: Wiley, 2003. [2] W.-S. Chen, C.-K. Wu, and K.-L. Wong, “Compact circularly-polarized circular microstrip antenna with cross-slot and peripheral cuts,” Electron. Lett., vol. 34, pp. 1040–1041, May 1998. [3] W.-S. Chen, C.-K. Wu, and K.-L. Wong, “Compact circularly-polarized microstrip antenna with bent slots,” Electron. Lett., vol. 34, pp. 1278–1279, Jun. 1998. [4] C. Carrender, P. S. Drazaic, R. Martin, P. K. Gregory, and J. M. Price, “Switching Patch Antenna,” U.S. 20050200528, Sep. 15, 2005. [5] S. D. Targonshi and D. M. Pozar, “Design of wideband circularly polarized aperture microstrip antennas,” IEEE Trans. Antennas Propag., vol. 41, pp. 214–200, Feb. 1993. [6] K. H. Lu and T. N. Chang, “Circularly polarized array with corporate-feed network and series-feed elements,” IEEE Trans. Antennas Propag., vol. 53, pp. 3288–3292, Oct. 2005. [7] H. Kim, B. M. Lee, and Y. J. Yoon, “A single-feeding circularly polarized microstrip antenna with the effect of hybrid feeding,” IEEE Antennas Wireless Propag. Lett., vol. 2, pp. 74–77, Apr. 2003. [8] H.-T. Chen, K.-L. Wong, and T.-W. Chiou, “PIFA with a meandered and folded patch for the dual-band mobile phone application,” IEEE Trans. Antennas Propag., vol. 51, pp. 2468–2471, Sep. 2003.

Ultrawideband Strip-Loaded Circular Slot Antenna With Improved Radiation Patterns Shi-Wei Qu, Jia-Lin Li, Jian-Xin Chen, and Quan Xue

Abstract—An ultrawideband strip-loaded circular slot antenna is proposed in this Communication, which presents a measured impedance band2), much stable radiation patterns and width of 144.8% (VSWR small dimensions. The loading strip is used for redistributing the electric field within the slot to achieve stable radiation patterns across a wide frequency band, and it is optimized to influence on the impedance bandwidth as slightly as possible. For clarity, the simulated three-dimension (3-D) radiation patterns are given and compared with those of a conventional circular slot antenna without strip loading. The measured gain of the proposed antenna in the broadside direction is over 0 dBi from 2.4 to 11 GHz. The measured and simulated return losses and radiation patterns of the proposed antenna are given, and finally, parametric studies are performed for practical applications. Index Terms—Broadband antenna, pattern improvement, slot antenna.

stable radiation patterns, and constant gain in desired directions is required for the smallest degradation of the radiated pulses. Planar wide slot antenna (WSA), with bi-directional radiation patterns and medium gain, is one of the most attractive candidates for UWB antennas. The WSAs were investigated extensively for larger impedance BW in the past several years. It is noted that an electrically larger slot size of the WSA can produce a wider impedance bandwidth by proper excitation with a microstrip line or a coplanar waveguide (CPW) line [2], [3]. Thus, many WSAs, with different slot shapes such as rectangular, circular, arc-shape, and various feed shapes such as T, cross, fork–like, bowtie, radial stub, pi, double-T, circular, and rectangular, were presented [4]–[10]. Moreover, some special techniques are also applied for larger operating BW, such as adding round corners [11], [12], rotating the slot [13], and the impedance BW is enhanced to more than 160%. However, the radiation patterns of the WSA are distorted in the upper frequency band due to the unequal phase distribution and the larger magnitude of higher modes caused by the electrically large dimensions of the slot. In [14] and [15], the locations of the maximum gain versus frequency are investigated extensively, and the gain in the broadside direction is also given for easy comparisons, where it is shown that the WSA with a circular slot has wider impedance BW of 131.6% but more degraded radiation patterns than the one with a rectangular slot due to the larger electrical length along the electrical field direction. Another WSA with a circular slot [16] shows an impedance BW of 143.2% but still distorted pattern. Recently, [17] presents a few novel designs of planar elliptical/circular slot antennas exhibiting good UWB characteristics as well as an empirical formula. In this Communication, the antennas are fabricated on the high loss substrate of FR4, and fed by either microstrip lines or CPWs with U-shaped tuning stub, which provides alternatives for practical applications. For the cases with circular slots in [17], a maximum impedance BW of 117.2% is achieved, and a minimum electrical dimension of 0:495010 dB 0:575010 dB (010 dB is the free-space wavelength at the lower edge frequency of the 010 dB operating band). Based on the above analysis, a strip-loaded WSA (SWSA) is presented in this Communication, which has a reduced ground plane and a new-moon-shaped strip within the circular slot to stabilize the radiation patterns across the operating BW. The proposed SWSA is fed by a circular patch with a microstrip line and investigated numerically. The measurements of the fabricated prototype show the SWSA has a VSWR 2 : 1 impedance BW of 144.8%, ranging from 2.4 to 15 GHz, and a BW of 128.3% from 2.4 to 11 GHz for over 0 dBi gain in the broadside direction. Finally, parametric studies are performed for practical applications.

I. INTRODUCTION

II. GEOMETRY OF THE STRIP-LOADED WIDE SLOT ANTENNA

Ultrawideband (UWB) systems are attracting more and more attentions in a wide range of applications, including ground penetrating radars, high data rate short range wireless local area networks and communication systems for military purposes etc., due to their fine spatial resolution, extraction of target feature characteristics, and low probability of interception and non-interfering signal waveform [1]. As an important component of the UWB system, the UWB antenna with simple structure, wide impedance bandwidth (BW), linear phase delay,

Fig. 1 shows the configuration of the SWSA with a circular slot and reduced ground plane, and it is fed by a microstrip line, where a substrate with a relative permittivity of 2.33 and thickness of 0.787 mm is used. On one side of the substrate, a 50 microstrip feed line and a circular patch of radius RP are etched. On the other side a circular slot is formed by cutting a circular shape away from a rectangular ground plane with half-circle shaped top edge. The half-circular top edge holds a radius of 26 mm and its center is exactly at the origin of the coordination. The circular slot with a radius of RS is centered at the point (0; YS ), and a gap between the slot and the feed patch is set to be h. A new-moon shaped strip is located within the slot and connected to the ground plane at its two narrow tips; consequently the circular slot is divided into two parts, denoted by slot I and II. It should be mentioned that a proper connection between the strip and the ground is important, which obviously influences the radiation patterns and the return loss.

Manuscript received June 28, 2006; revised May 16, 2007. The authors are with the Department of Electronic Engineering, City University of Hong Kong, Kowloon, Hong Kong (e-mail: [email protected]). Color versions of one or more of the figures in this Communication are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TAP.2007.908847

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Fig. 1. Geometries of the proposed SWSA. (a) Side view, (b) top view, (c) geometry of the strip.

Fig. 2. Measured and simulated return losses of the SWSA compared with the simulated result of the RWSA. W = 2:3 mm, h = 0:4 mm, R = 25 mm, R = 8 mm, S = 21 mm, Y = 4; Y = 27; Y = 12, and AR = S=L = 0:9.

0

0

0

The strip is constructed by two identical ellipses with a semi-major axis of L, a semi-minor axis of S , and different centers at (0; Yout ) and (0; Yin ), as shown in Fig. 1(c). The radius RS of the circular slot allows controlling the lower cutoff frequency of the antenna. The appropriate values of RP and h ensure the best matching between the feed line and the slot across a very wide band. The strip is employed in order to redistribute the electric field within the slot for stable radiation patterns, especially in the upper frequency band. Additionally, the total size of the antenna is only 52 mm 2 62 mm. For easy comparison, a WSA without the strip, which has a similar geometry to the one in [16] and holds a large ground size of 100 mm 2 100 mm, is used as a referenced WSA (RWSA), and its other parameters are identical with those of the proposed one. III. NUMERICAL AND EXPERIMENTAL RESULTS All simulations are done by Ansoft High Frequency Structure Simulation (HFSS). The prototype of the SWSA is designed and fabricated for demonstrating purposes. Fig. 2 shows the measured and simulated return losses along with the parametric values in the caption. The reasonable agreements are observed in the figure. The measured operating band for VSWR 2:1 ranges from 2.4 to 15 GHz. The slots’ electrical sizes of the SWSA and the RWSA are both about 0:4m (m is the free-space wavelength at the lower edge frequency fL of the operating band for VSWR 2 : 1, and the total electrical size of the SWSA is 0:416m 2 0:496m ), and it is not changed obviously compared to

Fig. 3. Simulated 3-D radiation patterns and 2-D ones in YZ plane of the proposed and the referenced antennas at 4 GHz. (a) Pattern of the proposed SWSA. (b) Patterns of the RWSA. (c) 2-D E-plane patterns of the SWSA and RWSA.

the WSAs with large ground sizes [14], [16]. Moreover, because the suspended strip is loaded in parallel with the slot and acts as a large reactance, it can be seen from Fig. 2 that the strip within the slot does not influence the impedance BW obviously.

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The simulated 3–D radiation patterns and two-dimension (2-D) ones in YZ plane of the SWSA and the RWSA at 4, 7, and 10 GHz are shown in Figs. 3–5. It can be seen that in the lower frequency band the two antennas have similar patterns, i.e., the strip slightly influences the electric field distributions in the slot in the lower frequency band, and the equivalent slot area is slightly changed compared with a total circular slot. However, as frequency increases, the effect of

the loading strip becomes obvious, consequently the effective slot area is decreased, and the equivalent electrical dimensions of the slot are almost constant due to the strip existence. In this case, the slot II can be considered to be dominant. For a WSA, the stable radiation patterns would be obtained in a frequency band where the electrical size of its slot is not too large. (For different slot shape, this limitation is also different. For the slot II and the circular


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Fig. 6. Measured E and H-plane radiation patterns of the proposed SWSA. Measured co-polarizatiaon, 3 Measured cross-polarization,—simulated co-polarization, - - - simulated cross-polarization.

Fig. 7. Investigation of the proposed SWSA’s gain. (a) Measured and simulated of the proposed antenna. (b) Simulated and of the SWSA and the RWSA.

slot, it is about , where is the free-space wave length.) Thus, here as long as the electrical size of the slot II along the Y-axis direction is lower than , its radiation patterns is almost identical with those in the lower frequency band, as shown in Figs. 4(a) and 5(a). However, in the upper band, when the dimensions of slot II is beyond along the Y-axis direction and becomes electrically larger as frequency increases, the SWSA shows the radiation pattern degradation in a similar manner to the RWSA [14], [16]. The obvious improvements of radiation patterns can also be known from the 2-D patterns in Figs. 3(c)–5(c). The main beams of the RWSA are closing to Y-direction from the broadside direction as frequency increases. Fig. 6 shows the simulated and the measured radiation patterns of the SWSA at 4, 7, and 10 GHz, respectively, where reasonable agreement is observed. The effects of the SMA connector and the coaxial feed line mainly cause the slight difference in the negative Y direction. In the YZ plane, the cross polarized (cr-pol) radiation about 020 dB is presented across the whole band, however, in the XZ plane it is become larger in the upper frequency band, for example at 7 and 10 GHz, the cr-pol shows almost the same level to the co-polarized radiation (co-pol). In many cases, the UWB antennas are used to receive/transmit electromagnetic pulses from/toward a fixed direction. Moreover, in order

to decrease the pulse degradation, a constant gain in the desired direction, for example the broadside direction, is expected. Thus, in this Communication, the gain in the broadside direction G0 of the SWSA as well as the maximum power gain Gmax is investigated. As a matter of fact, since the pattern degradation brings the decrease of G0 , the degree of the pattern degradation can be denoted by G0 indirectly in the next section for simplicity. From Fig. 7(a), the SWSA shows a measured G0 0:5 dBi band from 2.4 to 11 GHz. After the substrate and conductor losses are considered in the simulations and the SMA connector loss is removed experimentally, the measured and simulated curves have reasonable agreements. The slight difference between them is mainly caused by fabrication and measurement errors. As frequency increases, Gmax of both antennas increases due to larger electrical dimensions of the slot. Since the main beams are narrower than those of the SWSA, the RWSA presents a larger Gmax , as shown in Fig. 7(b). However, due to the radiation degradation and shifted main beams, the G0 fluctuation of the RWSA is larger, and the band for G0 0 dBi only covers from 2 to 7.5 GHz, compared with that from 2 to 11.8 GHz of the SWSA. This means that stable radiation patterns and the constant gain requirements of UWB antennas over a wide frequency band are obtained. Additionally, the slight decrease of G0 around 7.5 GHz can

G

G

G

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IV. PARAMETRIC STUDIES

G

Fig. 8. Simulated return losses and of the proposed antenna versus W (in millimeter). W = 2:3 mm, h = 0:4 mm, R = 25 mm, R = 8 mm, S = 22 mm, Y = 4; Y = 14, and AR = S=L = 0:9.

0

0

Since the WSA with a circular slot has been extensively investigated, the parametric studies of the loading strip are mainly performed in this section. When one parameter is studied, the others are shown in the caption of each figure. First, the strip width, illustrated by WS = Yout 0 Yin , is given in Fig. 8, and it can be seen that the larger WS is, the larger the BW for G0 0 dBi is, with slightly decrease of fL , but the worse the return loss around 4.5 GHz is, because the slot II becomes smaller. If WS is larger than 16 mm, the SWR will be over 2:1. Second, for the axis ratio AR = S=L, the larger AR brings slightly wider BW for G0 04 dBi but worse return loss to the antenna, as shown in Fig. 9. The parameter Yin or Yout (WS is fixed), which is not given in detail for brevity, determines the location of the strip within the slot and influences the antenna performances in a similar manner as WS . The third parameter studied is S , as shown in Fig. 10. It can be observed that the larger S brings slight increase of fL; but obvious decrease of the BW for G0 0 dBi due to the wider connecting section between the strip and the ground plane. As S = 18 mm, the extremely poor performance is suffered because the strip is not connected to the ground. Actually, the parameters need to be adjusted carefully to obtain the optimal gain BW and return loss in condition that the connection between the strip and the ground plane, which cannot be ensured according to the parametric values given in Figs. 8 as S changes. Thus, in practical designs, S can be adjusted finally after the other parameters are done. V. CONCLUSION

Fig. 9. Simulated return losses and G of the proposed antenna versus AR. W = 2:3 mm, h = 0:4 mm, R = 25 mm, R = 8 mm, S = 21 mm, = 4; Y = 27, and Y = 12. Y

0

0

A strip–loaded circular slot antenna is numerically investigated and confirmed by experiments in this Communication, which shows an impedance bandwidth of 144.8% for SWR 2 : 1 and relatively stable radiation patterns over a band of 128.3% illustrated by the gain in the broadside direction. For clarity, the 3-D radiation patterns of the proposed antenna are given compared with those of a conventional circular slot one. The reasons of the performance improvements due to the strip loading are explained based on the electric field distributions in the slot. In the end, the parametric studies are performed for practical applications.

REFERENCES

Fig. 10. Simulated return losses and G of the proposed antenna versus S . W = 2:3 mm, h = 0:4 mm, R = 25 mm, R = 8 mm, Y = 4; = 27; W = 15 mm, and AR = S=L = 0:9. Y

0

0

be improved by adding another strip in the slot parallel to the original one at the cost of the upper edge frequency decreases of the G0 0 dBi band [18].

[1] J. D. Taylor, Introduction to Ultra-Wideband Radar System. Boca Raton, FL: CRC Press, 1995. [2] M. K. Kim, K. Kim, Y. H. Suh, and I. Park, “A T-shaped microstripline-fed wide slot antenna,” in IEEE Antennas Propag. Soc. Int. Symp. Dig., 2000, vol. 3, pp. 16–21. [3] L. Zhu, R. Fu, and K.-L. Wu, “A novel broadband microstrip-fed wide slot antenna with double rejection zeros,” IEEE Antenna Wireless Propag. Lett., vol. 2, pp. 194–196, 2003. [4] Y.-F. Liu, K.-L. Lau, Q. Xue, and C.-H. Chan, “Experimental studies of printed wide-slot antenna for wide-band applications,” IEEE Antenna Wireless Propag. Lett., vol. 3, pp. 273–275, 2004. [5] J.-Y. Sze and K.-L. Wong, “Bandwidth enhancement of a microstripline-fed printed wide-slot antenna,” IEEE Trans. Antennas Propag., vol. 49, no. 7, pp. 1020–1024, Jul. 2001. [6] J.-Y. Chiou, J.-Y. Sze, and K.-L. Wong, “A broad-band CPW-fed striploaded square slot antenna,” IEEE Trans. Antennas Propag., vol. 51, no. 4, pp. 719–721, Apr. 2003. [7] H. D. Chen, “Broadband CPW-fed square slot antennas with a widened tuning stub,” IEEE Trans. Antennas Propag., vol. 51, no. 8, pp. 1982–1986, Aug. 2003. [8] X. C. Lin and L. T. Wang, “A broadband CPW-fed loop slot antenna with harmonic control,” IEEE Antenna Wireless Propag. Lett., vol. 2, pp. 323–325, 2003. [9] Y. W. Jang, “Broadband cross-shaped microstrip-fed slot antenna,” Electron. Lett., vol. 36, no. 25, pp. 2065–2057, Dec. 2000.


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[10] P. H. Rao, V. F. Fusco, and R. Cahill, “Linearly polarized radial stub fed high performance wide-band slot antenna,” Electron. Lett., vol. 37, no. 6, pp. 335–337, Mar. 2001. [11] H. L. Lee, H. J. Lee, J. G. Yook, and H. K. Park, “Broadband planar antenna having round corner rectangular wide slot,” in Proc. IEEE Antennas and Propagation Society Int. Symp., Jun. 2002, vol. 2, pp. 16–21. [12] S.-W. Qu, C. L. Ruan, and B.-Z. Wang, “Bandwidth enhancement of wide-slot antenna fed by CPW and microstrip line,” IEEE Antenna Wireless Propag. Lett., vol. 5, pp. 15–17, 2006. [13] J. Y. Jan and J. W. Su, “Bandwidth enhancement of a printed wide-slot antenna with a rotated slot,” IEEE Trans. Antennas Propag., vol. 53, no. 6, pp. 2111–2114, Jun. 2005. [14] G. Sorbello, F. Consoli, and S. Barbarino, “Numerical and experimental analysis of a circular slot antenna for UWB communications,” Microw. Opt. Tech. Lett., vol. 44, no. 5, pp. 465–470, Mar. 2005. [15] G. Sorbello, M. Pavone, and L. Russello, “Numerical and experimental study of a rectangular slot antenna for UWB communications,” Microw. Opt. Tech. Lett., vol. 46, no. 6, pp. 315–319, Aug. 2005. [16] T. A. Denidni and M. A. Habib, “Broadband printed CPW-fed circular slot antenna,” Electron. Lett., vol. 42, no. 3, pp. 135–136, Feb. 2006. [17] P. C. Li, J. X. Liang, and X. D. Chen, “Study of elliptical/circular slot antennas for ultrawideband applications,” IEEE Trans. Antennas Propag., vol. 54, no. 6, pp. 2111–2114, Jun. 2006. [18] S.-W. Qu, J.-L. Li, and Q. Xue, “Broadband microstrip-line-fed wide slot antenna with improved patterns,” Electron. Lett., vol. 42, no. 16, pp. 893–894, Aug. 2006.

Broadband Printed Dipole Antenna With a Step-Shaped Feed Gap for DTV Signal Reception

Fig. 1. (a) Configuration of the proposed broadband printed dipole antenna with a step-shaped feed gap for DTV signal reception. (b) Simulated excited surface currents at 530 and 730 MHz on arms 1 and 2 of the proposed antenna.

Yun-Wen Chi, Kin-Lu Wong, and Saou-Wen Su

Abstract—A broadband printed dipole antenna capable of generating a wide operating band for digital television signal reception in the 470–806 MHz band is presented. The antenna is of a rectangular shape of width 20 mm and length 227 mm ( ), and comprises two asymmetric radiating portions of arm 1 and arm 2 separated by a step-shaped feed gap with its one open end at the center of antenna’s one long side edge and the other open end at about /4 away from the center of the opposite long side edge. The antenna can generate two adjacent resonant modes to form a wide operating band of larger than 50% in 2.5:1 VSWR bandwidth, which is much wider than that of the corresponding conventional center-fed dipole antenna. Index Terms—Broadband dipole antennas, digital television (DTV) antennas, dipole antennas, printed antennas, printed dipole antennas.

I. INTRODUCTION Digital television (DTV) terrestrial broadcasting has already reached the implementing stage in many countries after more than a decade of intense research and development [1]. The DTV reception has thus become very attractive for applications in mobile communication devices Manuscript received June 28, 2006; revised May 16, 2007. Y.-W. Chi and K.-L. Wong are with the Department of Electrical Engineering, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan, R.O.C. S.-W. Su is with the Technology Research and Development Center, Lite-On Technology Corp., Taipei 114, Taiwan, R.O.C. Digital Object Identifier 10.1109/TAP.2007.908848

such as the laptop computers and mobile phones [2], [3]. Similarly, it is also very attractive for vehicle owners to have their vehicles equipped with a DTV signal reception device [4]. For these perspective applications, it is anticipated that a variety of mobile antennas for DTV signal reception will be increasingly required. In this Communication, we present a broadband planar dipole antenna with a step-shaped feed gap for DTV signal reception in the 470–806 MHz band [5]. Owing to the use of the step-shaped feed gap replacing the simple straight feed gap for the conventional center-fed dipole antenna, an additional resonant mode adjacent to the antenna’s fundamental (0.5-wavelength) resonant mode can be excited. These two resonant modes can be formed into a wide operating band of larger than 50% (2.5:1 VSWR) centered at about 600 MHz, which is much wider than that of the corresponding conventional center-fed dipole antenna. Over the obtained wide operating band, the antenna also shows stable radiation characteristics (similar to that of the conventional center-fed dipole antenna) with good radiation efficiency. Also note that the concept of using two adjacent resonant modes excited in the proposed dipole antenna for broadband operation are different from that of using two separate resonant modes excited in the reported dipole antenna with two U-slotted arms or two L-slotted arms for dual-band operation [6], [7]. The proposed antenna also has a planar configuration [8] and is easy to fabricate at low cost by printing on a dielectric substrate. II. DESIGN CONSIDERATIONS OF THE PROPOSED ANTENNA Fig. 1(a) shows the proposed dipole antenna. The antenna is printed on a 0.4 mm thick FR4 substrate and is of a rectangular shape of width

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