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UWB semi-elliptical printed monopole antenna with subband rejection filter
Int. J. Electron. Commun. (AEÜ) 64 (2010) 133 – 141 www.elsevier.de/aeue
UWB semi-elliptical printed monopole antenna with subband rejection filter Raha Eshtiaghi∗ , Reza Zaker, Javad Nouronia, Changiz Ghobadi Electrical Engineering Department, Urmia University, 165, Urmia 57153, Iran Received 10 July 2008; accepted 8 December 2008
Abstract A modified band-notched compact printed half-elliptical monopole antenna is presented for ultra-wideband (UWB) applications. Wider impedance bandwidth can be achieved by carving two sectors on top-side of the semi-ellipse-shaped patch. Two connected arc-shaped slots with a variable angle between them are inserted in the patch to act as a filter structure. The proposed antenna is etched on a FR4 substrate with the size of 26 × 26 × 0.8 mm3 and optimized to operate over the frequency band between 2.5 and 15 GHz for VSWR < 2, omitting the undesired frequency band of 5.1–5.9 GHz. 䉷 2009 Elsevier GmbH. All rights reserved. Keywords: Rejection filter; Elliptical monopole antenna; Ultra-wideband
1. Introduction With the definition and acceptance of ultra-wideband (UWB) systems, there is an increasing demand for antennas capable of operating at an extremely wide frequency band. Due to the attractive features, such as low profile, ease of fabrication, omnidirectional radiation pattern and wide frequency bandwidth, printed monopole antennas are currently under consideration for use in UWB systems. Since circular and elliptical planar monopole antennas show basic characteristics of the planar UWB antennas, they must widely be studied. To increase the impedance bandwidth in these antennas, techniques such as adding steps to the lower edge of the patch [1], increasing the ellipticity ratio of ellipse-shaped patch [2], the insertion of additional stub to the one side of circular patch [3] and adding of the slit on one side of the radiating element [4], have been reported. ∗ Corresponding author.
E-mail addresses: [email protected] (R. Eshtiaghi), [email protected] (R. Zaker), [email protected] (J. Nouronia), [email protected] (C. Ghobadi). 1434-8411/$ - see front matter 䉷 2009 Elsevier GmbH. All rights reserved. doi:10.1016/j.aeue.2008.12.007
In addition, UWB systems have been collocated to the bandwidth from 3.1 to 10.6 GHz, approved by the FCC [5]. However, over the designated UWB frequency band, there are existing wireless local area network WLAN bands from 5.15 to 5.85 GHz, which may cause interference with UWB operations. To overcome this problem, structures with bandrejection characteristic such as U-slot [6], inverted U-slot [3], small strip bar [1], H-shaped conductor-backed plane [7], rectangle-shaped plane [8], arc-shaped slot [9] and parasitic coplanar elliptical patch [10], are proposed to integrate with the former antennas. In this article, a modified compact planar monopole antenna with variable filter performance is presented. Firstly, using the previous studies [2], a brief discussion is done to formulate the proposed monopole antenna performance in the low in-band frequencies. The antenna structure is evolved from a semi-elliptical patch antenna by carving two sectors on the top-side of the patch to provide wider bandwidth with more design flexibility and also two connected arc-shaped slots with variable separated angle, to prevent interference with WLAN systems. Simulated and measured results of the proposed antenna are presented.
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Fig. 3. Simulated VSWR with and without carved sectors.
Fig. 1. Geometry of the proposed monopole antenna (unit: mm).
3 M=0deg M=20deg M=30deg M=40deg
2.5
cylindrical monopole antenna can be used with suitable modification. If L is the height of the planar monopole antenna in cm, which is taken same as that of an equivalent cylindrical monopole, and r in cm is the effective radius of the equivalent cylindrical monopole antenna, which is determined by equating area of the planar and cylindrical monopole antennas, and then the lower band-edge frequency is given as [2]
VSWR
fL =
(1)
where p is the length of the 50 feed line (gap distance) in cm. Unlike the planar disk monopole antennas, the printed configuration has dielectric layer on one side of the monopole. This dielectric martial increase the effective dimensions of the monopole, leading to a reduction in the lower band-edge frequency. Hence, more appropriate equation for the lower band-edge frequency is given as
2
1.5
1
7.2 c = GHz L +r + p
2
4
6
8
10
12
Freq (GHz)
Fig. 2. Simulated VSWR for various angles M.
2. Antenna design 2.1. Design formulations for printed semi-ellipse monopole antenna
fL =
(2)
where the factor k can be thought of as having similar sig√ nificance as e f f [2]. For commonly used FR4 substrate with e f f = 4.4 and h = 0.08 cm, the empirical value of k ( = 1.18) estimates the lower band-edge frequency within 10%. As shown in Fig. 1, if A is the semi-major axis, B is the semi-minor axis and S is the total area of the modified semi-ellipse-shaped radiation patch, then r=
Although the prototype design is based on full-wave high frequency structure simulator (HFSS) [11] simulations, simple analytic formulas can be used to provide an initial design and insight. To estimate the lower band-edge frequency of monopole antennas, f L , the standard formulation given for
7.2 c = GHz (L + r + p) × k
S , 2××L
L=A
(3)
Considering (2) and (3), values of A and B should be set in the maximum value to increase the effective electrical length and decrease the lower resonant frequency [1]. The antenna size (Lsub × W sub) that is estimated to ensure the
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Fig. 4. Simulated VSWR for various length (a) L g , when Dg = 3.59 mmand (b) Dg , when L g = 6.45 mm.
Fig. 5. Simulated VSWR for various (a) length L s , when N = 87◦ and (b) angle N , when L s = 18.48 mm.
required values for A and B is 26 mm×26 mm. It is noted that Lsub= A+ p + Lg and W sub=2B. So the other parameters will be L =1.896 cm, r =0.325 cm and p=0.0535 cm and so f L = 2.68 GHz, which is suitable for initial design of UWB antenna. The value of p is chosen empirically.
2.2. Full-band UWB antenna with band-notched function design The geometry of the proposed monopole antenna is illustrated in Fig. 1. It is etched on commonly used 0.8 mm-FR4 substrate with relative permittivity of 4.4 and has the dimensions of 26 × 26 mm2 . The antenna is fed by a 50 microstrip line of width W f (=1.5 mm) that is partially backed by a rounded ground plane. To achieve a good impedance matching, a parametric study is done on the two main pa-
rameters of the ground plane, L g and Dg . In this present design, two sectors are carved by the variable angle M on the top-side of the semi-ellipse patch to make the current path more flexible and so excite the higher resonant modes of the antenna. Hence, by properly tuning the angle M, it is expected that the different number of resonant frequencies associated with the additional resonant modes would appear. As shown in Fig. 1, in order to achieve the required band-notched filtering property, two arc-shaped slots are connected in the point O, separated by the angle N and inserted on the patch, symmetrically. The total length of obtained structure is L S and its width is W S (=0.5 mm). At the present design, the surface current is concentrated around the edges of the slots and flows back to the feeding part at the notch frequency. This causes the antenna to operate in a transmission-line like mode which transforms the high impedance at the top of the slots to nearly zero at the
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Fig. 6. Simulated VSWR for various gap width g. Fig. 8. Measured VSWR characteristics with and without filter structure.
Table 1. Optimized parameters of the proposed antenna.
Fig. 7. Photograph of the realized proposed antenna.
antenna feed point [12]. This in turn leads slots to act as a resonant structure. The resonant frequency varies by changing the total length of the slots, while the slot width has negligible effect on it [3]. On the other hand, as the angle N is changed, the impedance bandwidth of the band-notch operation can be controlled. Early studies have shown that the current distribution of the elliptical monopole antenna at all frequencies is mostly concentrated on its periphery, with very low current density toward its center [13]. By decreasing the angle N , two arms of the filter structure moves toward the center of the patch. Thus the current density around the slots and also the coupling value between the outer and
L g (mm)
Dg (mm)
L s (mm)
g (mm)
M (deg)
N (deg)
6.5
4.7
19
1
26
87
inner edges of the slots is decreased. Then the filter performance is attenuated and so the notch frequency bandwidth is reduced. Moreover, changing the position of slots, which is denoted by g, causes slots to move backward or toward the center of the radiating patch. By increasing g, it is expected that the bandwidth of the band rejection performance would decrease due to the reduction of the current density of the edges of the slots and vice versa. On the other hand, the slots have an inductive loading effect on the electrical length of the antenna. Due to the inductive loading effect, for a given physical length of an antenna, if the density of magnetic field (or current distribution) at the section of high electrical current (near of the feed point in our design or point O) is increased, the electric length of the antenna increases also [14]. Thus, it is expected to decrease the lower band-edge frequency.
3. Results and discussion Simulation results are obtained using Ansoft HFSS. Parametric studies of the effect of angle Mon the VSWR are shown in Fig. 2, while the ground plane dimensions are fixed in Lg = 5.8 mm and Dg = 3.9 mm. It is observed from Fig. 2 that when M = 20◦ , the third resonant mode excites and so an additional resonant frequency appears. It is shown
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Fig. 9. Simulated current distributions at: (a) 3.5 GHz, (b) 5.5 GHz (notch frequency), and (c) 9.5 GHz.
that the new resonance increases with M, but while M becomes greater than 35◦ , the impedance matching gets worse. For the proposed antenna, the optimal value of the angle M is between 20◦ and 35◦ , where the maximum bandwidth and good impedance matching can be achieved. Fig. 3 comprises antenna VSWR curves with and without the carving sectors. It is obvious that the new resonant frequency is generated and the total impedance bandwidth is increased, which is impossible in the simple semi-elliptical patch due to the fixed current path. The simulation results show that the optimal value for M is 26◦ , to achieve the maximum impedance bandwidth. On the other hand, varying the carving angle M from 0◦ to 40◦ , decreases the total area of the radiating patch from 360 to 245 mm2 and so, according to Eqs. (2) and (3), the lower band-edge frequency increases slightly from 2.35 to 2.75 GHz. Generally, the modified ground plane acts as an impedance matching circuit [8]. As shown in Fig. 4(a), the small change in the height of the ground plane, L g , has a great effect on the bandwidth of the antenna in the upper frequency band. Also, the changes in the Dg affect the impedance matching especially in the lower frequency band, as illustrated in Fig. 4(b). From the simulation results, the optimal values of L g and Dg are 6.5 and 4.7 mm, respectively. Fig. 5 shows that two connected arc-shaped slots play a role in the filter performance. In Fig. 5(a), it is observed that the shorter total length of the slots L s leads to the higher notch resonant frequency. The optimized value of
L s is 19 mm which is approximately equals 0.34, where corresponds to the notched frequency (5.5 GHz). The effect of the variation of the separated angle between the slots, N , is illustrated in Fig. 5(b). Increasing the angle N leads to an increase in the bandwidth of the notched-frequency band. Parametric analyses show that the optimal value for N is 87◦ . As shown in Fig. 6, by choosing the proper value for g, which is the gap width between the feed point and slots, the desired filtering performance can be achieved. Moreover, due to the inductive loading effect, by decreasing g, the lower band-edge frequency decreases from 2.75 to 2.25 GHz which compensates the increasing of the lower band-edge frequency, produced by carving the sectors on top-side of the radiating patch. Fig. 7 presents a photograph of the realized printed monopole antenna on an FR4 substrate with its soldered SMA connector. For comparison, measured VSWR curves of the antenna without carving sectors, with optimal carving angle (M = 26◦ ) and with the optimized filter structure, are shown in Fig. 8. It is obvious from Fig. 8 that without the carving sectors, the impedance bandwidth for VSWR < 2 is from 2.35 to 9.15 GHz. After carving the sectors the frequency range will be from 2.5 to 15 GHz. It is also observed that the band-notch characteristic is obtained near the desired frequency of 5.5 GHz, when the two connected arc-shaped slots are inserted in the radiating element. The optimized
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Fig. 10. Measured y .z plane radiation pattern for the proposed antenna at: (a) 3.5 GHz, (b) 7.5 GHz, and (c) 10.5 GHz.
parameters are identified in Table 1. Using these identified values: L = 1.896 cm, r = 0.249 cm, p = 0.0535 cm. So f L results 2.6 GHz which is approximately equal to the measured result ( f L = 2.5 GHz). Fig. 9 shows the current distributions in three in-band frequencies. As shown in Fig. 9(b), the current density in the edges of the arc-shaped slots is stronger than any other area at the notch frequency of 5.5 GHz. The measured radiation patterns of the proposed antenna in y.z (H-plane) and x.z (E-plane) planes at 3.5, 7.5 and 10.5 GHz are plotted in Figs. 10 and 11, referring to the xyz coordinate system attached to the antenna structure in Fig. 1. The results contain both co-polarization and crosspolarization. Fig. 10 shows that the y.z plane pattern of the
antenna is approximately omni-directional for all in-band frequencies from 2.5 to 8 GHz. However, for frequencies beyond 8.0 GHz, the broadside is no longer the maximum. Also, in the y.z plane, the cross-polarization of the antenna increases with frequency and becomes comparable with the co-polarization at about 8.0 GHz. Concerning the x.z plane pattern, Fig. 11 shows that it is a figure-of-eight for low in-band frequencies up to 8.0 GHz, but its maximum tilts away from the broadside direction at high in-band frequencies, i.e. greater than 8.0 GHz. Measurement results show a cross-polarization in the x.z plane of less than −20 dB for all frequencies. However, degradation of the radiation pattern becomes significant from 8.0 GHz and above, but
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Fig. 11. Measured x .z plane radiation pattern for the proposed antenna at: (a) 3.5 GHz, (b) 7.5 GHz, and (c) 10.5 GHz.
the semi-elliptical monopole antenna can be said to show an acceptable radiation pattern variation in its entire operational bandwidth. Fig. 12 shows the measured gain of the proposed antenna with and without the filter structure. A sharp decrease of antenna gain is observed in the notch frequency (5.5 GHz). For other in-band frequencies, the antenna gain is nearly flat and is similar to those without filter structure.
4. Conclusion A small band-notched printed monopole antenna has been proposed for UWB applications. It is demonstrated
that in the carved semi-elliptical patch, the additional resonant modes can be excited due to the changes in the current paths. So the presented antenna exhibits a broad impedance bandwidth and good impedance matching from 2.5 to 15 GHz, using carved semi-ellipse-shaped radiating element and modified ground plane, simultaneously. To realize a sharp rejection frequency band from 5.1 to 5.85 GHz, two connected arc-shaped slots with variable angle are inserted in the radiating patch. It is verified that varying the defined angle or the position of the slots affects the bandrejection performance. Good radiation patterns and acceptable maximum gain are obtained over whole frequency band.
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[10] Chen HJ, Liu QZ, Li JF, Guo JL. A novel band-notched elliptical ring monopole antenna with a coplanar parasitic elliptical patch for uwb applications. J Electromagnetic Waves Appl 2008;22. [11] Ansoft high frequency structure simulation (hfss), ver. 10. Ansoft Corporation, 2005. [12] Kerkhoff, A, Hao Ling. A parametric study of bandnotched UWB planar monopole antennas. Appllied Research Laboratory Texas University Austin, TX, USA [This paper appears in: Antennas and propagation society international symposium, 2004. IEEE Publication Date: 20–25 June 2004, Vol. 2, on page(s): 1768–1771. [13] Azenui N, Yang H. A printed crescent patch antenna for ultrawideband applications. IEEE Antennas Wireless Propag Lett 2007;6. [14] Wang H, Zheng M. Triple wireless local area network monopole antenna. IET Microwave Antennas Propag 2008;2:367–72. Fig. 12. Maximum gain with and without filter structure.
Acknowledgments The authors are thankful to Iran Telecommunication Research Center (ITRC) for financial support and its Antenna Lab where the proposed antenna has been tested.
References [1] Kim K, Park S. Analysis of the small band-rejected antenna with the parasitic strip for uwb. IEEE Trans Antennas Propag 2006;54. [2] Ray K, Ranga Y. Ultrawideband printed elliptical monopole antennas. IEEE Trans Antennas Propag 2007;55. [3] Choi J, Chung K, Roh Y. Parametric analysis of a bandrejected antenna for uwb applications. Microwave Opt Technol Lett 2005;47. [4] Wang J, Sun X, Okada K. Uwb circular monopole omni-directional antenna with a slot for radiation pattern improvement. IEEE conference on Ultra-Wideband 2007; 478–82. [5] FCC. First report and order on ultra-wideband technology. Technical Report; 2002. [6] Cho Y, Kim K, Hyuk D, Lee S, Park S. A miniature uwb planar monopole antenna with 5-GHz band-rejection filter and the time-domain characteristics. IEEE Trans Antennas Propag 2006;54:1453–60. [7] Zaker R, Ghobadi C, Nourinia J. Novel modified uwb planar monopole antenna with variable frequency band-notched function. IEEE Antennas Wireless Propag Lett 2008;7: 112–4. [8] Chen Wen-Shan, Yang Kai-Cheng. CPW-fed planar ultrawideband antenna having a frequency band-rejected function. Southern Taiwan University Tainan [This paper appears in: TENCON 2007 — 2007 IEEE region 10 conference, publication date: October 30, 2007–November 2, 2007 on page(s): 1–3, location: Taipei]. [9] Khan S, Xiong J, He S. Low profile and small size frequency notched planar monopole antenna from 3.5 to 23.64 GHz. Microwave Opt Technol Lett 2008;50.
Raha Eshtiaghi was born on 11 January, 1984, in Iran. She received the B.S. degree in Electrical and Communication Engineering from Tabriz University, Tabriz, Iran, in 2006. She is currently working toward the M.S. degree in Electrical and Telecommunication Engineering in the Urmia University, Urmia, Iran. Her current research interests include UWB antennas and numerical methods in electromagnetic. Reza Zaker was born in Tabriz, Iran, in 1982. He received the B.Sc. degree in Electrical and Electronic Engineering from Azad University, Tabriz, Iran and M.Sc. degree in Electrical and Telecommunication Engineering from Urmia University, Urmia, Iran, in 2008. His current research interests include printed ultra-wideband (UWB) antenna designs and bandwidth enhancement and miniaturization techniques, mobile small antennas, RFID, microstrip filters and numerical methods in electromagnetic. Javad Nourinia was born on 1970 in Iran. He received the B.S. degree in Electrical and Electronic Engineering from Shiraz University and M.S. degree in Electrical and Telecommunication Engineering from Iran University of Science and Technology, and Ph.D. degree in Electrical and Telecommunication from University of Science and Technology, Tehran, Iran, in 2000. From 2000 he was an Assistant Professor and now is Associated Professor in the Department of Electrical Engineering, of Urmia University, Urmia, Iran. His primary research interests are in antenna design, numerical methods in electromagnetic, microwave circuits, filters and signal processing.
R. Eshtiaghi et al. / Int. J. Electron. Commun. (AEÜ) 64 (2010) 133 – 141
Changiz Ghobadi was born on 1 June, 1960 in Iran. He received the B.S. degree in Electrical and Electronic Engineering and M.S. degree in Electrical and Telecommunication Engineering from Isfahan University of Technology, Isfahan, Iran and Ph.D. degree in Electrical-Telecommunication from University of Bath, Bath, UK in 1998.
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From 1998 he was an Assistant Professor and now is Associated Professor in the Department of Electrical Engineering, of Urmia University, Urmia, Iran. His primary research interests are in antenna design, radar and adaptive filters.
UWB semi-elliptical printed monopole antenna with subband rejection filter Raha Eshtiaghi∗ , Reza Zaker, Javad Nouronia, Changiz Ghobadi Electrical Engineering Department, Urmia University, 165, Urmia 57153, Iran Received 10 July 2008; accepted 8 December 2008
Abstract A modified band-notched compact printed half-elliptical monopole antenna is presented for ultra-wideband (UWB) applications. Wider impedance bandwidth can be achieved by carving two sectors on top-side of the semi-ellipse-shaped patch. Two connected arc-shaped slots with a variable angle between them are inserted in the patch to act as a filter structure. The proposed antenna is etched on a FR4 substrate with the size of 26 × 26 × 0.8 mm3 and optimized to operate over the frequency band between 2.5 and 15 GHz for VSWR < 2, omitting the undesired frequency band of 5.1–5.9 GHz. 䉷 2009 Elsevier GmbH. All rights reserved. Keywords: Rejection filter; Elliptical monopole antenna; Ultra-wideband
1. Introduction With the definition and acceptance of ultra-wideband (UWB) systems, there is an increasing demand for antennas capable of operating at an extremely wide frequency band. Due to the attractive features, such as low profile, ease of fabrication, omnidirectional radiation pattern and wide frequency bandwidth, printed monopole antennas are currently under consideration for use in UWB systems. Since circular and elliptical planar monopole antennas show basic characteristics of the planar UWB antennas, they must widely be studied. To increase the impedance bandwidth in these antennas, techniques such as adding steps to the lower edge of the patch [1], increasing the ellipticity ratio of ellipse-shaped patch [2], the insertion of additional stub to the one side of circular patch [3] and adding of the slit on one side of the radiating element [4], have been reported. ∗ Corresponding author.
E-mail addresses: [email protected] (R. Eshtiaghi), [email protected] (R. Zaker), [email protected] (J. Nouronia), [email protected] (C. Ghobadi). 1434-8411/$ - see front matter 䉷 2009 Elsevier GmbH. All rights reserved. doi:10.1016/j.aeue.2008.12.007
In addition, UWB systems have been collocated to the bandwidth from 3.1 to 10.6 GHz, approved by the FCC [5]. However, over the designated UWB frequency band, there are existing wireless local area network WLAN bands from 5.15 to 5.85 GHz, which may cause interference with UWB operations. To overcome this problem, structures with bandrejection characteristic such as U-slot [6], inverted U-slot [3], small strip bar [1], H-shaped conductor-backed plane [7], rectangle-shaped plane [8], arc-shaped slot [9] and parasitic coplanar elliptical patch [10], are proposed to integrate with the former antennas. In this article, a modified compact planar monopole antenna with variable filter performance is presented. Firstly, using the previous studies [2], a brief discussion is done to formulate the proposed monopole antenna performance in the low in-band frequencies. The antenna structure is evolved from a semi-elliptical patch antenna by carving two sectors on the top-side of the patch to provide wider bandwidth with more design flexibility and also two connected arc-shaped slots with variable separated angle, to prevent interference with WLAN systems. Simulated and measured results of the proposed antenna are presented.
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Fig. 3. Simulated VSWR with and without carved sectors.
Fig. 1. Geometry of the proposed monopole antenna (unit: mm).
3 M=0deg M=20deg M=30deg M=40deg
2.5
cylindrical monopole antenna can be used with suitable modification. If L is the height of the planar monopole antenna in cm, which is taken same as that of an equivalent cylindrical monopole, and r in cm is the effective radius of the equivalent cylindrical monopole antenna, which is determined by equating area of the planar and cylindrical monopole antennas, and then the lower band-edge frequency is given as [2]
VSWR
fL =
(1)
where p is the length of the 50 feed line (gap distance) in cm. Unlike the planar disk monopole antennas, the printed configuration has dielectric layer on one side of the monopole. This dielectric martial increase the effective dimensions of the monopole, leading to a reduction in the lower band-edge frequency. Hence, more appropriate equation for the lower band-edge frequency is given as
2
1.5
1
7.2 c = GHz L +r + p
2
4
6
8
10
12
Freq (GHz)
Fig. 2. Simulated VSWR for various angles M.
2. Antenna design 2.1. Design formulations for printed semi-ellipse monopole antenna
fL =
(2)
where the factor k can be thought of as having similar sig√ nificance as e f f [2]. For commonly used FR4 substrate with e f f = 4.4 and h = 0.08 cm, the empirical value of k ( = 1.18) estimates the lower band-edge frequency within 10%. As shown in Fig. 1, if A is the semi-major axis, B is the semi-minor axis and S is the total area of the modified semi-ellipse-shaped radiation patch, then r=
Although the prototype design is based on full-wave high frequency structure simulator (HFSS) [11] simulations, simple analytic formulas can be used to provide an initial design and insight. To estimate the lower band-edge frequency of monopole antennas, f L , the standard formulation given for
7.2 c = GHz (L + r + p) × k
S , 2××L
L=A
(3)
Considering (2) and (3), values of A and B should be set in the maximum value to increase the effective electrical length and decrease the lower resonant frequency [1]. The antenna size (Lsub × W sub) that is estimated to ensure the
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135
Fig. 4. Simulated VSWR for various length (a) L g , when Dg = 3.59 mmand (b) Dg , when L g = 6.45 mm.
Fig. 5. Simulated VSWR for various (a) length L s , when N = 87◦ and (b) angle N , when L s = 18.48 mm.
required values for A and B is 26 mm×26 mm. It is noted that Lsub= A+ p + Lg and W sub=2B. So the other parameters will be L =1.896 cm, r =0.325 cm and p=0.0535 cm and so f L = 2.68 GHz, which is suitable for initial design of UWB antenna. The value of p is chosen empirically.
2.2. Full-band UWB antenna with band-notched function design The geometry of the proposed monopole antenna is illustrated in Fig. 1. It is etched on commonly used 0.8 mm-FR4 substrate with relative permittivity of 4.4 and has the dimensions of 26 × 26 mm2 . The antenna is fed by a 50 microstrip line of width W f (=1.5 mm) that is partially backed by a rounded ground plane. To achieve a good impedance matching, a parametric study is done on the two main pa-
rameters of the ground plane, L g and Dg . In this present design, two sectors are carved by the variable angle M on the top-side of the semi-ellipse patch to make the current path more flexible and so excite the higher resonant modes of the antenna. Hence, by properly tuning the angle M, it is expected that the different number of resonant frequencies associated with the additional resonant modes would appear. As shown in Fig. 1, in order to achieve the required band-notched filtering property, two arc-shaped slots are connected in the point O, separated by the angle N and inserted on the patch, symmetrically. The total length of obtained structure is L S and its width is W S (=0.5 mm). At the present design, the surface current is concentrated around the edges of the slots and flows back to the feeding part at the notch frequency. This causes the antenna to operate in a transmission-line like mode which transforms the high impedance at the top of the slots to nearly zero at the
136
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Fig. 6. Simulated VSWR for various gap width g. Fig. 8. Measured VSWR characteristics with and without filter structure.
Table 1. Optimized parameters of the proposed antenna.
Fig. 7. Photograph of the realized proposed antenna.
antenna feed point [12]. This in turn leads slots to act as a resonant structure. The resonant frequency varies by changing the total length of the slots, while the slot width has negligible effect on it [3]. On the other hand, as the angle N is changed, the impedance bandwidth of the band-notch operation can be controlled. Early studies have shown that the current distribution of the elliptical monopole antenna at all frequencies is mostly concentrated on its periphery, with very low current density toward its center [13]. By decreasing the angle N , two arms of the filter structure moves toward the center of the patch. Thus the current density around the slots and also the coupling value between the outer and
L g (mm)
Dg (mm)
L s (mm)
g (mm)
M (deg)
N (deg)
6.5
4.7
19
1
26
87
inner edges of the slots is decreased. Then the filter performance is attenuated and so the notch frequency bandwidth is reduced. Moreover, changing the position of slots, which is denoted by g, causes slots to move backward or toward the center of the radiating patch. By increasing g, it is expected that the bandwidth of the band rejection performance would decrease due to the reduction of the current density of the edges of the slots and vice versa. On the other hand, the slots have an inductive loading effect on the electrical length of the antenna. Due to the inductive loading effect, for a given physical length of an antenna, if the density of magnetic field (or current distribution) at the section of high electrical current (near of the feed point in our design or point O) is increased, the electric length of the antenna increases also [14]. Thus, it is expected to decrease the lower band-edge frequency.
3. Results and discussion Simulation results are obtained using Ansoft HFSS. Parametric studies of the effect of angle Mon the VSWR are shown in Fig. 2, while the ground plane dimensions are fixed in Lg = 5.8 mm and Dg = 3.9 mm. It is observed from Fig. 2 that when M = 20◦ , the third resonant mode excites and so an additional resonant frequency appears. It is shown
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Fig. 9. Simulated current distributions at: (a) 3.5 GHz, (b) 5.5 GHz (notch frequency), and (c) 9.5 GHz.
that the new resonance increases with M, but while M becomes greater than 35◦ , the impedance matching gets worse. For the proposed antenna, the optimal value of the angle M is between 20◦ and 35◦ , where the maximum bandwidth and good impedance matching can be achieved. Fig. 3 comprises antenna VSWR curves with and without the carving sectors. It is obvious that the new resonant frequency is generated and the total impedance bandwidth is increased, which is impossible in the simple semi-elliptical patch due to the fixed current path. The simulation results show that the optimal value for M is 26◦ , to achieve the maximum impedance bandwidth. On the other hand, varying the carving angle M from 0◦ to 40◦ , decreases the total area of the radiating patch from 360 to 245 mm2 and so, according to Eqs. (2) and (3), the lower band-edge frequency increases slightly from 2.35 to 2.75 GHz. Generally, the modified ground plane acts as an impedance matching circuit [8]. As shown in Fig. 4(a), the small change in the height of the ground plane, L g , has a great effect on the bandwidth of the antenna in the upper frequency band. Also, the changes in the Dg affect the impedance matching especially in the lower frequency band, as illustrated in Fig. 4(b). From the simulation results, the optimal values of L g and Dg are 6.5 and 4.7 mm, respectively. Fig. 5 shows that two connected arc-shaped slots play a role in the filter performance. In Fig. 5(a), it is observed that the shorter total length of the slots L s leads to the higher notch resonant frequency. The optimized value of
L s is 19 mm which is approximately equals 0.34, where corresponds to the notched frequency (5.5 GHz). The effect of the variation of the separated angle between the slots, N , is illustrated in Fig. 5(b). Increasing the angle N leads to an increase in the bandwidth of the notched-frequency band. Parametric analyses show that the optimal value for N is 87◦ . As shown in Fig. 6, by choosing the proper value for g, which is the gap width between the feed point and slots, the desired filtering performance can be achieved. Moreover, due to the inductive loading effect, by decreasing g, the lower band-edge frequency decreases from 2.75 to 2.25 GHz which compensates the increasing of the lower band-edge frequency, produced by carving the sectors on top-side of the radiating patch. Fig. 7 presents a photograph of the realized printed monopole antenna on an FR4 substrate with its soldered SMA connector. For comparison, measured VSWR curves of the antenna without carving sectors, with optimal carving angle (M = 26◦ ) and with the optimized filter structure, are shown in Fig. 8. It is obvious from Fig. 8 that without the carving sectors, the impedance bandwidth for VSWR < 2 is from 2.35 to 9.15 GHz. After carving the sectors the frequency range will be from 2.5 to 15 GHz. It is also observed that the band-notch characteristic is obtained near the desired frequency of 5.5 GHz, when the two connected arc-shaped slots are inserted in the radiating element. The optimized
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Fig. 10. Measured y .z plane radiation pattern for the proposed antenna at: (a) 3.5 GHz, (b) 7.5 GHz, and (c) 10.5 GHz.
parameters are identified in Table 1. Using these identified values: L = 1.896 cm, r = 0.249 cm, p = 0.0535 cm. So f L results 2.6 GHz which is approximately equal to the measured result ( f L = 2.5 GHz). Fig. 9 shows the current distributions in three in-band frequencies. As shown in Fig. 9(b), the current density in the edges of the arc-shaped slots is stronger than any other area at the notch frequency of 5.5 GHz. The measured radiation patterns of the proposed antenna in y.z (H-plane) and x.z (E-plane) planes at 3.5, 7.5 and 10.5 GHz are plotted in Figs. 10 and 11, referring to the xyz coordinate system attached to the antenna structure in Fig. 1. The results contain both co-polarization and crosspolarization. Fig. 10 shows that the y.z plane pattern of the
antenna is approximately omni-directional for all in-band frequencies from 2.5 to 8 GHz. However, for frequencies beyond 8.0 GHz, the broadside is no longer the maximum. Also, in the y.z plane, the cross-polarization of the antenna increases with frequency and becomes comparable with the co-polarization at about 8.0 GHz. Concerning the x.z plane pattern, Fig. 11 shows that it is a figure-of-eight for low in-band frequencies up to 8.0 GHz, but its maximum tilts away from the broadside direction at high in-band frequencies, i.e. greater than 8.0 GHz. Measurement results show a cross-polarization in the x.z plane of less than −20 dB for all frequencies. However, degradation of the radiation pattern becomes significant from 8.0 GHz and above, but
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Fig. 11. Measured x .z plane radiation pattern for the proposed antenna at: (a) 3.5 GHz, (b) 7.5 GHz, and (c) 10.5 GHz.
the semi-elliptical monopole antenna can be said to show an acceptable radiation pattern variation in its entire operational bandwidth. Fig. 12 shows the measured gain of the proposed antenna with and without the filter structure. A sharp decrease of antenna gain is observed in the notch frequency (5.5 GHz). For other in-band frequencies, the antenna gain is nearly flat and is similar to those without filter structure.
4. Conclusion A small band-notched printed monopole antenna has been proposed for UWB applications. It is demonstrated
that in the carved semi-elliptical patch, the additional resonant modes can be excited due to the changes in the current paths. So the presented antenna exhibits a broad impedance bandwidth and good impedance matching from 2.5 to 15 GHz, using carved semi-ellipse-shaped radiating element and modified ground plane, simultaneously. To realize a sharp rejection frequency band from 5.1 to 5.85 GHz, two connected arc-shaped slots with variable angle are inserted in the radiating patch. It is verified that varying the defined angle or the position of the slots affects the bandrejection performance. Good radiation patterns and acceptable maximum gain are obtained over whole frequency band.
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[10] Chen HJ, Liu QZ, Li JF, Guo JL. A novel band-notched elliptical ring monopole antenna with a coplanar parasitic elliptical patch for uwb applications. J Electromagnetic Waves Appl 2008;22. [11] Ansoft high frequency structure simulation (hfss), ver. 10. Ansoft Corporation, 2005. [12] Kerkhoff, A, Hao Ling. A parametric study of bandnotched UWB planar monopole antennas. Appllied Research Laboratory Texas University Austin, TX, USA [This paper appears in: Antennas and propagation society international symposium, 2004. IEEE Publication Date: 20–25 June 2004, Vol. 2, on page(s): 1768–1771. [13] Azenui N, Yang H. A printed crescent patch antenna for ultrawideband applications. IEEE Antennas Wireless Propag Lett 2007;6. [14] Wang H, Zheng M. Triple wireless local area network monopole antenna. IET Microwave Antennas Propag 2008;2:367–72. Fig. 12. Maximum gain with and without filter structure.
Acknowledgments The authors are thankful to Iran Telecommunication Research Center (ITRC) for financial support and its Antenna Lab where the proposed antenna has been tested.
References [1] Kim K, Park S. Analysis of the small band-rejected antenna with the parasitic strip for uwb. IEEE Trans Antennas Propag 2006;54. [2] Ray K, Ranga Y. Ultrawideband printed elliptical monopole antennas. IEEE Trans Antennas Propag 2007;55. [3] Choi J, Chung K, Roh Y. Parametric analysis of a bandrejected antenna for uwb applications. Microwave Opt Technol Lett 2005;47. [4] Wang J, Sun X, Okada K. Uwb circular monopole omni-directional antenna with a slot for radiation pattern improvement. IEEE conference on Ultra-Wideband 2007; 478–82. [5] FCC. First report and order on ultra-wideband technology. Technical Report; 2002. [6] Cho Y, Kim K, Hyuk D, Lee S, Park S. A miniature uwb planar monopole antenna with 5-GHz band-rejection filter and the time-domain characteristics. IEEE Trans Antennas Propag 2006;54:1453–60. [7] Zaker R, Ghobadi C, Nourinia J. Novel modified uwb planar monopole antenna with variable frequency band-notched function. IEEE Antennas Wireless Propag Lett 2008;7: 112–4. [8] Chen Wen-Shan, Yang Kai-Cheng. CPW-fed planar ultrawideband antenna having a frequency band-rejected function. Southern Taiwan University Tainan [This paper appears in: TENCON 2007 — 2007 IEEE region 10 conference, publication date: October 30, 2007–November 2, 2007 on page(s): 1–3, location: Taipei]. [9] Khan S, Xiong J, He S. Low profile and small size frequency notched planar monopole antenna from 3.5 to 23.64 GHz. Microwave Opt Technol Lett 2008;50.
Raha Eshtiaghi was born on 11 January, 1984, in Iran. She received the B.S. degree in Electrical and Communication Engineering from Tabriz University, Tabriz, Iran, in 2006. She is currently working toward the M.S. degree in Electrical and Telecommunication Engineering in the Urmia University, Urmia, Iran. Her current research interests include UWB antennas and numerical methods in electromagnetic. Reza Zaker was born in Tabriz, Iran, in 1982. He received the B.Sc. degree in Electrical and Electronic Engineering from Azad University, Tabriz, Iran and M.Sc. degree in Electrical and Telecommunication Engineering from Urmia University, Urmia, Iran, in 2008. His current research interests include printed ultra-wideband (UWB) antenna designs and bandwidth enhancement and miniaturization techniques, mobile small antennas, RFID, microstrip filters and numerical methods in electromagnetic. Javad Nourinia was born on 1970 in Iran. He received the B.S. degree in Electrical and Electronic Engineering from Shiraz University and M.S. degree in Electrical and Telecommunication Engineering from Iran University of Science and Technology, and Ph.D. degree in Electrical and Telecommunication from University of Science and Technology, Tehran, Iran, in 2000. From 2000 he was an Assistant Professor and now is Associated Professor in the Department of Electrical Engineering, of Urmia University, Urmia, Iran. His primary research interests are in antenna design, numerical methods in electromagnetic, microwave circuits, filters and signal processing.
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Changiz Ghobadi was born on 1 June, 1960 in Iran. He received the B.S. degree in Electrical and Electronic Engineering and M.S. degree in Electrical and Telecommunication Engineering from Isfahan University of Technology, Isfahan, Iran and Ph.D. degree in Electrical-Telecommunication from University of Bath, Bath, UK in 1998.
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From 1998 he was an Assistant Professor and now is Associated Professor in the Department of Electrical Engineering, of Urmia University, Urmia, Iran. His primary research interests are in antenna design, radar and adaptive filters.