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A compact meta-atom loaded asymmetric coplanar strip-fed monopole antenna for multiband operation
Accepted Manuscript Regular paper A Compact Meta-atom Loaded Asymmetric Coplanar Strip-Fed Monopole Antenna for Multiband operation Rajeshkumar Venkatesan, Rajkumar Rengasamy, Praveen Vummadisetty. Naidu, Arvind Kumar PII: DOI: Reference:
S1434-8411(18)31192-0 https://doi.org/10.1016/j.aeue.2018.10.011 AEUE 52538
To appear in:
International Journal of Electronics and Communications
Received Date: Revised Date: Accepted Date:
9 May 2018 23 September 2018 9 October 2018
Please cite this article as: R. Venkatesan, R. Rengasamy, P.V. Naidu, A. Kumar, A Compact Meta-atom Loaded Asymmetric Coplanar Strip-Fed Monopole Antenna for Multiband operation, International Journal of Electronics and Communications (2018), doi: https://doi.org/10.1016/j.aeue.2018.10.011
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A Compact Meta-atom Loaded Asymmetric Coplanar Strip-Fed Monopole Antenna for Multiband operation Rajeshkumar Venkatesan1, Rajkumar Rengasamy 2, Praveen Vummadisetty. Naidu 1
2
and Arvind Kumar 3
School of Electronics Engineering, Vellore Institute of Technology (VIT), Tamilnadu, India, 2V. R. S. E. C, Vijayawada520007, India. 3K.I.T.E , Jaipur - 302022, India.
Email: [email protected], [email protected], [email protected] [email protected],
ABSTRACT–In this paper, a compact asymmetric coplanar strip (ACS)-fed monopole antenna is presented. The proposed antenna consists of a tapered shaped radiating element and a meta-atom (complementary split ring resonator-CSRR), loaded in the ground plane which helps in obtaining multiband characteristics with proper impedance matching performance. Antenna with dimensions of 25 × 12.2 × 1.6 mm3 has been designed, fabricated and tested. The experimental result that exhibits of -10 dB impedance bandwidth by the proposed antenna at the center frequency of 2.88 GHz (1100 MHz), 5.78 GHz (1940 MHz) and 7.64 GHz (660 MHz). It covers wireless applications namely wireless local area network (WLAN) 2.4/5.2/5.8 GHz, long-term evolution (LTE) 2500 GHz, public safety applications 4.9 GHz, worldwide interoperability for microwave access (WiMAX) 5.5 GHz, wireless access for vehicular environments (WAVE) 5.9 GHz and X-band downlink frequency band 7.5 GHz. The antenna has good radiation characteristics in both E-plane and H-plane for all the operating frequencies. The proposed antenna exhibits a better performance compared to the previously reported designs to the existing antenna designs which are discussed in the literature. Moreover, the antenna possesses compact size, the total size occupied by the design is 0.20λ0 ⅹ 0.1λ0. Keywords: multiband antenna; ACS-fed antenna; complementary split ring resonator (CSRR); WLAN; WiMAX
1. INTRODUCTION Currently, the antennas have been assuming the role of a vital part in the manufacturing of wireless communication devices. Among the various design approaches, the multiband antenna technique is an essential method which can integrate different wireless standards in a single antenna design and also it caters to the modern communication demands with ease [1-3]. Antennas with different geometric shapes like F, L, U and T and multiple radiating branches [4-14] have been proposed for multiband operations, especially for WLAN/WiMAX/WAVE applications. The designs mentioned above are known for effective cover of wireless applications but suffer many drawbacks such as larger antenna dimensions, complex structures and cover fewer applications which make the tedious implementation process. Printed antenna designs use a different feeding technique such as microstrip, coplanar waveguide (CPW) and asymmetric coplanar strip (ACS). Among these, ACS-fed technique helps achievement of a compact design owing to various advantages like very less space utilization, low loss compared to microstrip designs and has the features of the CPW feeding method. Though size reduction is the significant fact in most of the antenna designs, achievement of wider bandwidth is also vital. Since the bandwidth is very narrow in the existing antennas, it requires the implementation of additional antennas which make the device bulky. The metamaterial is an artificial material that has unique properties of negative permittivity and negative permeability. These properties can be used for the enhancement of the antenna performance such as improving bandwidth, creating notch bands, size reduction and gain enhancement [15-19].
Further, resonant type composite right left-handed (CRLH) transmission line inspired metamaterial antennas [20-22] contribute remarkably to the development of novel antennas with significant performance enhancement. The metamaterial elements are effectively utilized with multimode operations in [23-25], to perform controlled directive emissions with low polarization cross-talking, broadband diffusive scatterings and in multi-beam transmit arrays. Metamaterial-based structures produce better performance compared to the existing multiband antennas [26-30]. Conductor backed (CB) metamaterial loaded CPW-fed antenna designs have been proposed for achieving a compact, broad bandwidth and multiband performances in [31-32]. However, the above antenna design produces desired applications but requires a large space for implementation. A CB ACS-fed design was proposed for multiband operation in [33], but it is covering only WLAN frequency band. In [34-35], a compact ACS-fed multiband antenna is presented by etching SRR in the radiating element of the antenna which covers WLAN/WiMAX applications, but it had large antenna size and failed to cover higher frequency applications like 4.9 GHz safety application and X-band downlink frequency 7.5 GHz. A CB ACS-fed antenna with CSRR is proposed for multiband operation for overcoming the abovementioned drawbacks. The properties of CSRR structure helps in achieving broad bandwidth and proper impedance matching. The proposed antenna design covers wireless local area network (WLAN) 2.4 GHz (2.40–2.48 GHz)/5.2 GHz (5.15–5.35 GHz)/5.8 GHz (5.725–5.825 GHz), worldwide interoperability for microwave access (WiMAX) 2.5 GHz (2.5–2.69 GHz)/ (5.28–5.85) GHz, LTE 2500 GHz, 4.9 GHz (4.94– 4.99 GHz) public safety applications, 5.9 GHz (WAVE) and X-band downlink frequency (7.4 GHz) application. It has a compact structure and better radiation characteristics at the corresponding frequency bands. 2. PROPOSED ANTENNA DESIGN
The stages in the proposed antenna design evolution are shown in Figure 1, and its geometry is exhibited in Figure 2. Cost effective substrate FR-4 with a dielectric constant of 4.4 and height of 1.6 mm has been used. The antenna has a compact structure of 25 × 12.2 × 1.6 mm3 and is fed by ACS-fed line with a signal width (Fw) of 3.10 mm and gap (G) of 0.3 mm with 50Ω impedance.
Figure 1.Evolution stages of the proposed multiband antenna.
Figure 2. Schematic of the proposed antenna. (a) Radiator. (b) CSRR unit cell. (c) Partial ground plane with CSRRs. (d) Fabricated antenna prototype.
The characteristic impedance of the ACS-fed line has been calculated using the equations indicated below [36]
60 K (k ) eff K (k 1 )
Z0
K
a b
k1 1 k 2
and
(1)
(2)
(3)
K (k ) is the elliptical integral of first kind which is given by K (k 1 )
1 ln 2(1 k ) 1 k1 ) K (k ) K (k 1 ) 1 2(1 k ) ln 1 k)
eff 1
0k
1 2 (4)
1 k 1 2
r 1
(5)
2
The proposed radiating element with a ground plane helps achievement of the first (2.5 GHz) and third (7.4 GHz) resonant frequency. The CSRR is etched on the CB ground plane which realizes the second resonant frequency (5 GHz). Also, it enhances the impedance matching over center frequency region. Table 1.The proposed antenna parameters and their values
Parameter Value Parameter Value
W 12.2 RW 7.7
L 25 RL 7.7
FL 12.5 RT 0.7
GL 7.5 RG 0.3
GW 8.6 GL1 14.5
FW 3.1 L1 13.4
G 0.3 W1 8
L2 3.5 -
The values of the proposed antenna parameters are tabulated in Table 1. The parameters are optimized by parametric studies. The fabricated prototype of the proposed antenna is shown in Figure 2 (d). 3. RESULTS AND DISCUSSION The return loss characteristic at different stages of the proposed antenna is exhibited in Figure 3. Antenna #1 consists of a conventional square-shaped radiating patch which produces four resonance frequency bands. Though the antenna appears to have multiple resonant frequencies, it successfully covers only two useful resonant bands at 2.4 and 7.5 GHz. To enhance the antenna performance, the square-shaped patch of Antenna #1 is was tapered to 45˚ degree. Antenna #2 which helps in attaining additional frequency at 5.2 GHz. Figure 3 shows that Antenna #2 does not cover all the desired frequencies. Then, the antenna radiating element was further tapered for enhancing the impedance bandwidth. As shown in Antenna #3, bandwidth performance increases around 5 GHz frequency band. There is also a shift in the first and second resonance frequency band. The inference is that the Antenna #3 successfully covers multiple application such as WLAN (2.4/5.2/5.8 GHz), WiMAX (5.5 GHz), WAVE (5.9 GHz), safety application (4.9
GHz)
and
X-band
downlink
frequency
band
(7.4
GHz)
simultaneously.
Figure 3. Return loss characteristics of design evolution stages of the proposed antenna
The return loss characteristics of the proposed antenna with and without CSRR structure are shown in Figure 4. When the antenna was simulated without CSRR structure, it generated two resonance frequencies at 2.4 and 7.4 GHz but fails to cover 5 GHz frequency band.
Figure 4. Return loss of the proposed antenna with and without CSRR structure
Enhancement of impedance matching and achievement of bandwidth performance was obtained by etching of metamaterial based CSRR structure into CB ground plane. Impedance matching was improved following the loading of CSRR into the antenna. Hence the antenna achieved a broad bandwidth of 1940 MHz at 5 GHz frequency band. The antenna loaded with CSRR gives improved bandwidth which covered the desired applications when compared to the antenna without CSRR structure as shown in Figure 4. A comparison of simulated and measured return loss characteristics of the proposed antenna is shown in Figure 5. Both simulated and measured return loss results were seemed resulting in agreement. A small
deviation in measured and simulated result is observed, which may be attributed to soldering of SMA connectors and losses in the substrate.
Figure 5. Simulated and measured return loss of the proposed antenna.
The proposed CB ACS-fed antenna covered 2.33–3.43 GHz, 4.66–6.90 GHz and 7.31–7.97 GHz. It is mainly used in wireless applications such as 2.4/5.2/5.8 GHz (IEEE 802.11 a/b/g) WLAN, 2.5 GHz (IEEE 802.16e) WiMAX, 4.9 GHz public safety applications, 5.9 GHz (IEEE802.11p) WAVE applications and 7.4 GHz X-band downlink frequency. 4. PARAMETRIC STUDY OF THE PROPOSED ANTENNA In the proposed antenna design, some parameters have a larger effect on return loss characteristics which were analyzed and discussed in detail. Antenna parameters such as ground length (GL), ground width (Gw), conductor backed ground length (GL1), length between CSRR and ground (L2) and spacing between feed line and ground (G) were analyzed and plotted in Figures 6-10. There was a variation in the proposed antenna ground plane length (GL) from 3.5 mm to 11.5 mm as shown in Figure 6. For GL = 11.5 mm, antenna covered only two resonance frequencies of 6.2 GHz and 7.6 GHz, but it failed to achieve lower desired resonant frequencies. When G L was reduced to 9.5 mm, antenna resonance frequency tended to shift towards a lower frequency of 5.7 GHz and 7.4 GHz. Additionally, the level of impedance matching has been improved at in all three resonant frequencies, but the antenna bandwidth was narrow at 5 and 7.4 GHz frequency bands.
Figure 6. Return loss for various ground length ‘G L’ of the proposed antenna
When antenna ground length was further decreased to 7.5 mm, a shift in the resonance frequency towards the lower frequency was observed, and improved impedance matching performance covering applications at 2.4, 5 and 7.4 GHz. When GL is 3.5 mm, the performance of return loss characteristics degrades which was due to an increased level in the capacitive value. It results in a shift in resonance frequency which in turn brought poor impedance matching. The above analysis shows antenna ground length (GL) having a significant impact on improving the performance of impedance matching. Hence ground length G L was fixed as 7.5 mm for the proposed antenna.
Similarly, variance in ground width (Gw) were made from 4.6 to 8.6 mm and its return loss plot is shown in Figure 7. The lower resonant (2.4 GHz) and middle (5.5 GHz) frequency bands exhibited similar return loss characteristics except for the level of impedance matching. When G w was 8.6 mm, the antenna covered the entire 7.4 GHz frequency band along with other lower frequency applications (2.4 and 5 GHz). Ground widths (Gw) with dimensions of 7.6, 6.6 and 4.6 mm did not cover the entire frequency band due to smaller inductance values. Figures of above analysis provided inference of G w of 8.6 mm providing better performance compared to other ground width and was fixed as the proposed antenna ground width.
Figure 7. Return loss for various ground width ‘GW of the proposed antenna
Variations in the conductor backed ground plane length (GL1) were made from 12.5 to 18.5 mm, and their return loss characteristic is shown in Figure 8. When G L1 was 12.5 mm, the proposed antenna produced three resonance frequency bands of 2.8, 5 and 7.6 GHz where the 5 GHz frequency bandwidth was partially covered. For GL1 = 14.5 mm, antenna design covered the entire second resonant frequency band range from 4.66 to 6.9 GHz along with the first (2.8 GHz) and third (7.4 GHz) resonant frequencies. When GL1 was 16.5 mm and 18.5 mm, the proposed antenna did not cover the required frequency band and had very poor impedance matching due to changes in the inductance value of the antenna. Among the various conductor backed ground lengths (GL1), 14.5 mm was fixed as the proposed antenna ground length in the view of its capability to cover the desired frequency band effectively.
Figure 8. Return loss for various conductor backed ground length ‘GL1’ of the proposed antenna
The position of the CSRR structure loaded on CB ground plane of the proposed antenna was optimized by varying the parameter values of L2 from 5.5 to 1.5 mm. Details of its return loss performances are shown in Figure 9. When L2 was 5.5 mm, the antenna covered the first and third resonant frequencies completely, but not the entire second resonant bandwidth. With L2 equal to 4.5 mm, the proposed antenna produced three resonant frequency bands in which it partially covered the third (7.4 GHz) frequency band. When L2 is 3.5 mm, the antenna covered all the three resonant frequency (2.8, 5.5 and 7.4 GHz) bands successfully. When L2 was 1.5 mm, it comprises only the first and third resonant frequencies entirely and partially covered the second resonant frequency. The conclusion from the above analysis is that L2 of 3.5 mm has a proper impedance level and covers all the desired applications simultaneously.
Figure 9. Return loss for various CSRR position ‘L2’of the proposed antenna
Variation in the spacing between feed line and ground plane (G) were made from 0.10 mm to 0.30 mm as shown in Figure 10. For G = 0.10 mm, antenna resonates only at two frequencies (2.9 GHz and 7.4 GHz) with poor impedance matching.
Figure 10. Return loss for various spacing between feed line and the ground plane ‘G’ of the proposed antenna
When G was 0.15 mm and 0.20 mm, the antenna covered lower (2.5 GHz), middle (5 GHz) operating frequencies but failed to cover 7.5 GHz operating band. For G = 0.30 mm it covered the desired frequency bands with good return loss characteristics compared to other spacing gaps. The inferences from the above analysis are that the spacing between feed line and the ground plane (G) has a significant impact on return loss characteristics.
Table.2 presents details of the comparison of various antenna performances. The proposed antenna is seen producing better performances in various parameters aspects referenced to the existing antennas in the literature.
Table 2. Comparison of various antenna performances with the proposed ACS-fed antenna.
Size Refere nces
Antenna size (mm2)
Total size occupied by antenna
Frequency bands covered by antenna (GHz)
Antenna Type
Average gain (dBi)
575
2.4/5.2/5.5/5.8
Dual-band
2.3
[1]
23 × 25
(0×0)) @ lowest Frequency 0.19 λ0 × 0.2 λ0
[2]
22 × 29
0.18 λ0 × 0.23 λ0
638
2.4/3.5/5.2/5.8
Tri-band
1.9
[3]
28 × 26
0.19 λ0 × 0.2 λ0
728
2.4/5.2/5.8
Tri-band
2.4
[4]
28 × 30
0.17 λ0 × 0.18 λ0
840
1.8/2.1/2.4/5.2/5.5/5.8
Multi-band
2.1
[5]
31 × 15
0.25 λ0 × 0.12 λ0
465
2.4/5.2/5.8
Tri-band
3.07
[6]
21 × 19
0.17 λ0 × 0.15 λ0
399
2.4/5.2
Dual band
1.8
[8]
26.5 × 12
0.21 λ0 × 0.01 λ0
318
2.4/3.5/5.8
Tri-band
2.01
[9]
30×17.2
0.35 λ0 × 0.20 λ0
516
3.5/5.2/5/8/5.9
Tri-band
3.0
[10]
27.5×13
0.22 λ0 × 0.10 λ0
357.3
2.4/3.5/5.2/5.5/5.8
Tri-band
1.67
[11]
35 × 19
0.28 λ0 × 0.15 λ0
665
2.4/3.5/5.2/5.5/5.8
Tri-band
2.9
[13]
35 × 15
0.28 λ0 × 0.12 λ0
525
2.4/5.2/5.5/5.8
Tri-band
1.7
[14]
26 × 15
0.22 λ0 × 0.12 λ0
390
2.5/4.9/5.2/5.5/5.8/5.9
Tri-band
3.3
[31]
31 ×26.27
0.36 λ0 × 0.30 λ0
814.37
3.5/5.2/7.4/9.5
Multiband
3.5
[32]
31 ×24.57
0.54 λ0 × 0.43 λ0
761.67
5.2/5.8
Single band
2.51
[33]
32 × 12
0.26 λ0 × 0.01 λ0
384
Tri-band
3.15
[34]
25 × 17.5
0.20 λ0 × 0.14 λ0
437.5
2.4/5.2/5.5/5.8
Dual band
1.67
[35]
22 × 16.08
0.18λ0 × 0.13 λ0
353.76
2.4/5.2/5.5/5.8/ 8.2
Multiband
1.29
2.4/2.5/4.9/5.2/5.5/5.8/ 7.4
Multiband
This Work
25× 12.2
0.20 λ0 × 0.1 λ0
305
2.4/2.5/3.5/5.2/5.5/5.8
2.27
5. CURRENT DISTRIBUTIONS AND RADIATION PATTERN The focus of the current distribution was on both the tapered shaped radiating element and the CB ground plane of the antenna leading to the reference of the responsibility of both planes for the achievement of 2.4 GHz frequency band was shown in Figure 11 (a). At the frequencies 5.2, 5.5, 5.8 and 7.4 GHz, the focus of the current distribution is mainly on the CSRR structure, and concentrations of a slight degree were seen on the tapered shaped radiating element of the antenna as shown in Figures 11 (b), (c), (d) and (e). It confirming the excitement of these frequencies 5.2, 5.5, 5.8 and 5.8 GHz by loading CSRR, and the overall performance enhancement was due to the combined effects of radiating structure and CSRR of the proposed antenna.
(a)
(b)
(c)
(d)
(e)
Figure 11. Simulated surface current distributions of the proposed antenna at (a) 2.4 GHz, (b) 5.2 GHz, (c) 5.5 GHz, (d) 5.8 GHz and (e) 7.4 GHz.
The radiation characteristic of the proposed antenna was measured using an anechoic chamber. The proposed ACS-fed antenna was having good radiation characteristics in both E-plane and H-plane as shown in Figure 12. It can be observed that antenna exhibits bidirectional and omni-directional radiation patterns for E-plane and H-plane respectively.
(a)
(b) Figure 12. The radiation characteristics of the proposed antenna for H-plane and E-plane at 2.4, 5.2, 5.8 and 7.4 GHz. (a) Simulated and (b) Measured
At a higher frequency (7.4 GHz), deterioration in antenna radiation pattern shape was observed. This is due to the existence of the higher order modes and dielectric losses. The compact ACS antenna had good gain of 4.98, 1.06, 0.88 and 2.17 dBi at 2.4, 5.2, 5.8 and 7.4 GHz respectively, and an average gain of 2.27 dBi.
6. CONCLUSION A compact metamaterial loaded monopole antenna for WLAN/WiMAX/WAVE/LTE 2500 applications has been proposed. Bandwidth improvement and impedance matching were achieved with loading metamaterial structure. The proposed tri-band antenna covered the frequency range of 2.33–3.43 GHz, 4.66–6.90 GHz and 7.31–7.97 GHz with a corresponding fractional bandwidth of 38.19, 38.75 and 8.63 %. Besides, the antenna possesses a small size with good radiation characteristics at the operating frequency bands. The characteristics mentioned above denote that the proposed antenna is well suited for future compact wireless communication devices.
ACKNOWLEDGEMENT The authors wish to thank editors and anonymous reviewers for their valuable comments and suggestions for improving the quality of manuscript.
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S1434-8411(18)31192-0 https://doi.org/10.1016/j.aeue.2018.10.011 AEUE 52538
To appear in:
International Journal of Electronics and Communications
Received Date: Revised Date: Accepted Date:
9 May 2018 23 September 2018 9 October 2018
Please cite this article as: R. Venkatesan, R. Rengasamy, P.V. Naidu, A. Kumar, A Compact Meta-atom Loaded Asymmetric Coplanar Strip-Fed Monopole Antenna for Multiband operation, International Journal of Electronics and Communications (2018), doi: https://doi.org/10.1016/j.aeue.2018.10.011
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A Compact Meta-atom Loaded Asymmetric Coplanar Strip-Fed Monopole Antenna for Multiband operation Rajeshkumar Venkatesan1, Rajkumar Rengasamy 2, Praveen Vummadisetty. Naidu 1
2
and Arvind Kumar 3
School of Electronics Engineering, Vellore Institute of Technology (VIT), Tamilnadu, India, 2V. R. S. E. C, Vijayawada520007, India. 3K.I.T.E , Jaipur - 302022, India.
Email: [email protected], [email protected], [email protected] [email protected],
ABSTRACT–In this paper, a compact asymmetric coplanar strip (ACS)-fed monopole antenna is presented. The proposed antenna consists of a tapered shaped radiating element and a meta-atom (complementary split ring resonator-CSRR), loaded in the ground plane which helps in obtaining multiband characteristics with proper impedance matching performance. Antenna with dimensions of 25 × 12.2 × 1.6 mm3 has been designed, fabricated and tested. The experimental result that exhibits of -10 dB impedance bandwidth by the proposed antenna at the center frequency of 2.88 GHz (1100 MHz), 5.78 GHz (1940 MHz) and 7.64 GHz (660 MHz). It covers wireless applications namely wireless local area network (WLAN) 2.4/5.2/5.8 GHz, long-term evolution (LTE) 2500 GHz, public safety applications 4.9 GHz, worldwide interoperability for microwave access (WiMAX) 5.5 GHz, wireless access for vehicular environments (WAVE) 5.9 GHz and X-band downlink frequency band 7.5 GHz. The antenna has good radiation characteristics in both E-plane and H-plane for all the operating frequencies. The proposed antenna exhibits a better performance compared to the previously reported designs to the existing antenna designs which are discussed in the literature. Moreover, the antenna possesses compact size, the total size occupied by the design is 0.20λ0 ⅹ 0.1λ0. Keywords: multiband antenna; ACS-fed antenna; complementary split ring resonator (CSRR); WLAN; WiMAX
1. INTRODUCTION Currently, the antennas have been assuming the role of a vital part in the manufacturing of wireless communication devices. Among the various design approaches, the multiband antenna technique is an essential method which can integrate different wireless standards in a single antenna design and also it caters to the modern communication demands with ease [1-3]. Antennas with different geometric shapes like F, L, U and T and multiple radiating branches [4-14] have been proposed for multiband operations, especially for WLAN/WiMAX/WAVE applications. The designs mentioned above are known for effective cover of wireless applications but suffer many drawbacks such as larger antenna dimensions, complex structures and cover fewer applications which make the tedious implementation process. Printed antenna designs use a different feeding technique such as microstrip, coplanar waveguide (CPW) and asymmetric coplanar strip (ACS). Among these, ACS-fed technique helps achievement of a compact design owing to various advantages like very less space utilization, low loss compared to microstrip designs and has the features of the CPW feeding method. Though size reduction is the significant fact in most of the antenna designs, achievement of wider bandwidth is also vital. Since the bandwidth is very narrow in the existing antennas, it requires the implementation of additional antennas which make the device bulky. The metamaterial is an artificial material that has unique properties of negative permittivity and negative permeability. These properties can be used for the enhancement of the antenna performance such as improving bandwidth, creating notch bands, size reduction and gain enhancement [15-19].
Further, resonant type composite right left-handed (CRLH) transmission line inspired metamaterial antennas [20-22] contribute remarkably to the development of novel antennas with significant performance enhancement. The metamaterial elements are effectively utilized with multimode operations in [23-25], to perform controlled directive emissions with low polarization cross-talking, broadband diffusive scatterings and in multi-beam transmit arrays. Metamaterial-based structures produce better performance compared to the existing multiband antennas [26-30]. Conductor backed (CB) metamaterial loaded CPW-fed antenna designs have been proposed for achieving a compact, broad bandwidth and multiband performances in [31-32]. However, the above antenna design produces desired applications but requires a large space for implementation. A CB ACS-fed design was proposed for multiband operation in [33], but it is covering only WLAN frequency band. In [34-35], a compact ACS-fed multiband antenna is presented by etching SRR in the radiating element of the antenna which covers WLAN/WiMAX applications, but it had large antenna size and failed to cover higher frequency applications like 4.9 GHz safety application and X-band downlink frequency 7.5 GHz. A CB ACS-fed antenna with CSRR is proposed for multiband operation for overcoming the abovementioned drawbacks. The properties of CSRR structure helps in achieving broad bandwidth and proper impedance matching. The proposed antenna design covers wireless local area network (WLAN) 2.4 GHz (2.40–2.48 GHz)/5.2 GHz (5.15–5.35 GHz)/5.8 GHz (5.725–5.825 GHz), worldwide interoperability for microwave access (WiMAX) 2.5 GHz (2.5–2.69 GHz)/ (5.28–5.85) GHz, LTE 2500 GHz, 4.9 GHz (4.94– 4.99 GHz) public safety applications, 5.9 GHz (WAVE) and X-band downlink frequency (7.4 GHz) application. It has a compact structure and better radiation characteristics at the corresponding frequency bands. 2. PROPOSED ANTENNA DESIGN
The stages in the proposed antenna design evolution are shown in Figure 1, and its geometry is exhibited in Figure 2. Cost effective substrate FR-4 with a dielectric constant of 4.4 and height of 1.6 mm has been used. The antenna has a compact structure of 25 × 12.2 × 1.6 mm3 and is fed by ACS-fed line with a signal width (Fw) of 3.10 mm and gap (G) of 0.3 mm with 50Ω impedance.
Figure 1.Evolution stages of the proposed multiband antenna.
Figure 2. Schematic of the proposed antenna. (a) Radiator. (b) CSRR unit cell. (c) Partial ground plane with CSRRs. (d) Fabricated antenna prototype.
The characteristic impedance of the ACS-fed line has been calculated using the equations indicated below [36]
60 K (k ) eff K (k 1 )
Z0
K
a b
k1 1 k 2
and
(1)
(2)
(3)
K (k ) is the elliptical integral of first kind which is given by K (k 1 )
1 ln 2(1 k ) 1 k1 ) K (k ) K (k 1 ) 1 2(1 k ) ln 1 k)
eff 1
0k
1 2 (4)
1 k 1 2
r 1
(5)
2
The proposed radiating element with a ground plane helps achievement of the first (2.5 GHz) and third (7.4 GHz) resonant frequency. The CSRR is etched on the CB ground plane which realizes the second resonant frequency (5 GHz). Also, it enhances the impedance matching over center frequency region. Table 1.The proposed antenna parameters and their values
Parameter Value Parameter Value
W 12.2 RW 7.7
L 25 RL 7.7
FL 12.5 RT 0.7
GL 7.5 RG 0.3
GW 8.6 GL1 14.5
FW 3.1 L1 13.4
G 0.3 W1 8
L2 3.5 -
The values of the proposed antenna parameters are tabulated in Table 1. The parameters are optimized by parametric studies. The fabricated prototype of the proposed antenna is shown in Figure 2 (d). 3. RESULTS AND DISCUSSION The return loss characteristic at different stages of the proposed antenna is exhibited in Figure 3. Antenna #1 consists of a conventional square-shaped radiating patch which produces four resonance frequency bands. Though the antenna appears to have multiple resonant frequencies, it successfully covers only two useful resonant bands at 2.4 and 7.5 GHz. To enhance the antenna performance, the square-shaped patch of Antenna #1 is was tapered to 45˚ degree. Antenna #2 which helps in attaining additional frequency at 5.2 GHz. Figure 3 shows that Antenna #2 does not cover all the desired frequencies. Then, the antenna radiating element was further tapered for enhancing the impedance bandwidth. As shown in Antenna #3, bandwidth performance increases around 5 GHz frequency band. There is also a shift in the first and second resonance frequency band. The inference is that the Antenna #3 successfully covers multiple application such as WLAN (2.4/5.2/5.8 GHz), WiMAX (5.5 GHz), WAVE (5.9 GHz), safety application (4.9
GHz)
and
X-band
downlink
frequency
band
(7.4
GHz)
simultaneously.
Figure 3. Return loss characteristics of design evolution stages of the proposed antenna
The return loss characteristics of the proposed antenna with and without CSRR structure are shown in Figure 4. When the antenna was simulated without CSRR structure, it generated two resonance frequencies at 2.4 and 7.4 GHz but fails to cover 5 GHz frequency band.
Figure 4. Return loss of the proposed antenna with and without CSRR structure
Enhancement of impedance matching and achievement of bandwidth performance was obtained by etching of metamaterial based CSRR structure into CB ground plane. Impedance matching was improved following the loading of CSRR into the antenna. Hence the antenna achieved a broad bandwidth of 1940 MHz at 5 GHz frequency band. The antenna loaded with CSRR gives improved bandwidth which covered the desired applications when compared to the antenna without CSRR structure as shown in Figure 4. A comparison of simulated and measured return loss characteristics of the proposed antenna is shown in Figure 5. Both simulated and measured return loss results were seemed resulting in agreement. A small
deviation in measured and simulated result is observed, which may be attributed to soldering of SMA connectors and losses in the substrate.
Figure 5. Simulated and measured return loss of the proposed antenna.
The proposed CB ACS-fed antenna covered 2.33–3.43 GHz, 4.66–6.90 GHz and 7.31–7.97 GHz. It is mainly used in wireless applications such as 2.4/5.2/5.8 GHz (IEEE 802.11 a/b/g) WLAN, 2.5 GHz (IEEE 802.16e) WiMAX, 4.9 GHz public safety applications, 5.9 GHz (IEEE802.11p) WAVE applications and 7.4 GHz X-band downlink frequency. 4. PARAMETRIC STUDY OF THE PROPOSED ANTENNA In the proposed antenna design, some parameters have a larger effect on return loss characteristics which were analyzed and discussed in detail. Antenna parameters such as ground length (GL), ground width (Gw), conductor backed ground length (GL1), length between CSRR and ground (L2) and spacing between feed line and ground (G) were analyzed and plotted in Figures 6-10. There was a variation in the proposed antenna ground plane length (GL) from 3.5 mm to 11.5 mm as shown in Figure 6. For GL = 11.5 mm, antenna covered only two resonance frequencies of 6.2 GHz and 7.6 GHz, but it failed to achieve lower desired resonant frequencies. When G L was reduced to 9.5 mm, antenna resonance frequency tended to shift towards a lower frequency of 5.7 GHz and 7.4 GHz. Additionally, the level of impedance matching has been improved at in all three resonant frequencies, but the antenna bandwidth was narrow at 5 and 7.4 GHz frequency bands.
Figure 6. Return loss for various ground length ‘G L’ of the proposed antenna
When antenna ground length was further decreased to 7.5 mm, a shift in the resonance frequency towards the lower frequency was observed, and improved impedance matching performance covering applications at 2.4, 5 and 7.4 GHz. When GL is 3.5 mm, the performance of return loss characteristics degrades which was due to an increased level in the capacitive value. It results in a shift in resonance frequency which in turn brought poor impedance matching. The above analysis shows antenna ground length (GL) having a significant impact on improving the performance of impedance matching. Hence ground length G L was fixed as 7.5 mm for the proposed antenna.
Similarly, variance in ground width (Gw) were made from 4.6 to 8.6 mm and its return loss plot is shown in Figure 7. The lower resonant (2.4 GHz) and middle (5.5 GHz) frequency bands exhibited similar return loss characteristics except for the level of impedance matching. When G w was 8.6 mm, the antenna covered the entire 7.4 GHz frequency band along with other lower frequency applications (2.4 and 5 GHz). Ground widths (Gw) with dimensions of 7.6, 6.6 and 4.6 mm did not cover the entire frequency band due to smaller inductance values. Figures of above analysis provided inference of G w of 8.6 mm providing better performance compared to other ground width and was fixed as the proposed antenna ground width.
Figure 7. Return loss for various ground width ‘GW of the proposed antenna
Variations in the conductor backed ground plane length (GL1) were made from 12.5 to 18.5 mm, and their return loss characteristic is shown in Figure 8. When G L1 was 12.5 mm, the proposed antenna produced three resonance frequency bands of 2.8, 5 and 7.6 GHz where the 5 GHz frequency bandwidth was partially covered. For GL1 = 14.5 mm, antenna design covered the entire second resonant frequency band range from 4.66 to 6.9 GHz along with the first (2.8 GHz) and third (7.4 GHz) resonant frequencies. When GL1 was 16.5 mm and 18.5 mm, the proposed antenna did not cover the required frequency band and had very poor impedance matching due to changes in the inductance value of the antenna. Among the various conductor backed ground lengths (GL1), 14.5 mm was fixed as the proposed antenna ground length in the view of its capability to cover the desired frequency band effectively.
Figure 8. Return loss for various conductor backed ground length ‘GL1’ of the proposed antenna
The position of the CSRR structure loaded on CB ground plane of the proposed antenna was optimized by varying the parameter values of L2 from 5.5 to 1.5 mm. Details of its return loss performances are shown in Figure 9. When L2 was 5.5 mm, the antenna covered the first and third resonant frequencies completely, but not the entire second resonant bandwidth. With L2 equal to 4.5 mm, the proposed antenna produced three resonant frequency bands in which it partially covered the third (7.4 GHz) frequency band. When L2 is 3.5 mm, the antenna covered all the three resonant frequency (2.8, 5.5 and 7.4 GHz) bands successfully. When L2 was 1.5 mm, it comprises only the first and third resonant frequencies entirely and partially covered the second resonant frequency. The conclusion from the above analysis is that L2 of 3.5 mm has a proper impedance level and covers all the desired applications simultaneously.
Figure 9. Return loss for various CSRR position ‘L2’of the proposed antenna
Variation in the spacing between feed line and ground plane (G) were made from 0.10 mm to 0.30 mm as shown in Figure 10. For G = 0.10 mm, antenna resonates only at two frequencies (2.9 GHz and 7.4 GHz) with poor impedance matching.
Figure 10. Return loss for various spacing between feed line and the ground plane ‘G’ of the proposed antenna
When G was 0.15 mm and 0.20 mm, the antenna covered lower (2.5 GHz), middle (5 GHz) operating frequencies but failed to cover 7.5 GHz operating band. For G = 0.30 mm it covered the desired frequency bands with good return loss characteristics compared to other spacing gaps. The inferences from the above analysis are that the spacing between feed line and the ground plane (G) has a significant impact on return loss characteristics.
Table.2 presents details of the comparison of various antenna performances. The proposed antenna is seen producing better performances in various parameters aspects referenced to the existing antennas in the literature.
Table 2. Comparison of various antenna performances with the proposed ACS-fed antenna.
Size Refere nces
Antenna size (mm2)
Total size occupied by antenna
Frequency bands covered by antenna (GHz)
Antenna Type
Average gain (dBi)
575
2.4/5.2/5.5/5.8
Dual-band
2.3
[1]
23 × 25
(0×0)) @ lowest Frequency 0.19 λ0 × 0.2 λ0
[2]
22 × 29
0.18 λ0 × 0.23 λ0
638
2.4/3.5/5.2/5.8
Tri-band
1.9
[3]
28 × 26
0.19 λ0 × 0.2 λ0
728
2.4/5.2/5.8
Tri-band
2.4
[4]
28 × 30
0.17 λ0 × 0.18 λ0
840
1.8/2.1/2.4/5.2/5.5/5.8
Multi-band
2.1
[5]
31 × 15
0.25 λ0 × 0.12 λ0
465
2.4/5.2/5.8
Tri-band
3.07
[6]
21 × 19
0.17 λ0 × 0.15 λ0
399
2.4/5.2
Dual band
1.8
[8]
26.5 × 12
0.21 λ0 × 0.01 λ0
318
2.4/3.5/5.8
Tri-band
2.01
[9]
30×17.2
0.35 λ0 × 0.20 λ0
516
3.5/5.2/5/8/5.9
Tri-band
3.0
[10]
27.5×13
0.22 λ0 × 0.10 λ0
357.3
2.4/3.5/5.2/5.5/5.8
Tri-band
1.67
[11]
35 × 19
0.28 λ0 × 0.15 λ0
665
2.4/3.5/5.2/5.5/5.8
Tri-band
2.9
[13]
35 × 15
0.28 λ0 × 0.12 λ0
525
2.4/5.2/5.5/5.8
Tri-band
1.7
[14]
26 × 15
0.22 λ0 × 0.12 λ0
390
2.5/4.9/5.2/5.5/5.8/5.9
Tri-band
3.3
[31]
31 ×26.27
0.36 λ0 × 0.30 λ0
814.37
3.5/5.2/7.4/9.5
Multiband
3.5
[32]
31 ×24.57
0.54 λ0 × 0.43 λ0
761.67
5.2/5.8
Single band
2.51
[33]
32 × 12
0.26 λ0 × 0.01 λ0
384
Tri-band
3.15
[34]
25 × 17.5
0.20 λ0 × 0.14 λ0
437.5
2.4/5.2/5.5/5.8
Dual band
1.67
[35]
22 × 16.08
0.18λ0 × 0.13 λ0
353.76
2.4/5.2/5.5/5.8/ 8.2
Multiband
1.29
2.4/2.5/4.9/5.2/5.5/5.8/ 7.4
Multiband
This Work
25× 12.2
0.20 λ0 × 0.1 λ0
305
2.4/2.5/3.5/5.2/5.5/5.8
2.27
5. CURRENT DISTRIBUTIONS AND RADIATION PATTERN The focus of the current distribution was on both the tapered shaped radiating element and the CB ground plane of the antenna leading to the reference of the responsibility of both planes for the achievement of 2.4 GHz frequency band was shown in Figure 11 (a). At the frequencies 5.2, 5.5, 5.8 and 7.4 GHz, the focus of the current distribution is mainly on the CSRR structure, and concentrations of a slight degree were seen on the tapered shaped radiating element of the antenna as shown in Figures 11 (b), (c), (d) and (e). It confirming the excitement of these frequencies 5.2, 5.5, 5.8 and 5.8 GHz by loading CSRR, and the overall performance enhancement was due to the combined effects of radiating structure and CSRR of the proposed antenna.
(a)
(b)
(c)
(d)
(e)
Figure 11. Simulated surface current distributions of the proposed antenna at (a) 2.4 GHz, (b) 5.2 GHz, (c) 5.5 GHz, (d) 5.8 GHz and (e) 7.4 GHz.
The radiation characteristic of the proposed antenna was measured using an anechoic chamber. The proposed ACS-fed antenna was having good radiation characteristics in both E-plane and H-plane as shown in Figure 12. It can be observed that antenna exhibits bidirectional and omni-directional radiation patterns for E-plane and H-plane respectively.
(a)
(b) Figure 12. The radiation characteristics of the proposed antenna for H-plane and E-plane at 2.4, 5.2, 5.8 and 7.4 GHz. (a) Simulated and (b) Measured
At a higher frequency (7.4 GHz), deterioration in antenna radiation pattern shape was observed. This is due to the existence of the higher order modes and dielectric losses. The compact ACS antenna had good gain of 4.98, 1.06, 0.88 and 2.17 dBi at 2.4, 5.2, 5.8 and 7.4 GHz respectively, and an average gain of 2.27 dBi.
6. CONCLUSION A compact metamaterial loaded monopole antenna for WLAN/WiMAX/WAVE/LTE 2500 applications has been proposed. Bandwidth improvement and impedance matching were achieved with loading metamaterial structure. The proposed tri-band antenna covered the frequency range of 2.33–3.43 GHz, 4.66–6.90 GHz and 7.31–7.97 GHz with a corresponding fractional bandwidth of 38.19, 38.75 and 8.63 %. Besides, the antenna possesses a small size with good radiation characteristics at the operating frequency bands. The characteristics mentioned above denote that the proposed antenna is well suited for future compact wireless communication devices.
ACKNOWLEDGEMENT The authors wish to thank editors and anonymous reviewers for their valuable comments and suggestions for improving the quality of manuscript.
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