# A Compact Penta-Band Low-SAR Antenna Loaded with Split-Ring Resonator for Mobile Applications

Authors: M. Manikandan; S. Karthigai Lakshmi
Journal: International Journal of Antennas and Propagation
 (2022)

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## Abstract

A compact rectangular patch with dual-ring SRR (split-ring resonator) is presented in this article. An antenna is designed on FR4 substrate with an overall footprint size of 26 mm × 30 mm × 1.6 mm. The antenna presented operates in five bands from 2.95 to 3.06 GHz, 3.79 to 3.87 GHz, 4.11 to 4.19 GHz, 5.39 to 5.51 GHz, and 5.97 to 6.11 GHz. Mobile and fixed voice communication, WiMAX (Worldwide Interoperability for Microwave Access), 5G (5th generation), WLAN (Wireless Local Area Network), and ISM (Industrial Scientific and Medical) are some applications that utilized the above resonating bands. The penta-band operation is due to the inclusion of dual-ring SRR. The optimum values of the critical parameter of the SRR are identified using parametric analysis, and the results are presented. The antenna is also analyzed for the SAR (specific absorption rate) values, and it was found to be less than 2 W/kg for 10 g volume of tissue. The designed antenna is fabricated and tested, and the presented results show that there is good agreement between the simulated and measured results. Penta-band operation with simple structure, stable radiation pattern, and low SAR makes this antenna more intelligent and suitable for the mobile application.


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## Body

## 1. Introduction

The major requirement of modern wireless personal communication devices like smart phones in the past two decades is antenna, which exhibits multiband frequency of operation, omnidirectional radiation pattern, and low SAR. Bluetooth, WLAN, Satellite Communication, LTE (Long-Term Evolution), 4G (4th generation), 5G (5th generation), WiMAX, RFID (Radio Frequency Identification), etc. are some of the wireless standards [1] which are the essential frequency ranges needed to be incorporated in modern wireless personal communication devices like mobile phones. These devices allow only a limited space for antenna integration. This leads to another challenge for the antenna researchers to achieve the compactness of the structure. There are a variety of antenna techniques that support multiband characteristics, and the primary advantage of using the multiband antenna is that it reduces the size and complexity of the system, as a single antenna is capable of satisfying all the applications. It also eliminates the filters in the system, which results in reduced complexity, fabrication difficulty, cost, and installation requirements. The microstrip patch antenna can easily achieve such requirements [2–5] due to its low-profile characteristics.Another essential parameter needed to be reduced in mobile device is SAR. The value of SAR in mobile phone antenna must be within the limit. According to ICNIRP (International Commission on Non-Ionizing Radiation Protection) guidelines, maximum SAR value should be 2 W/kg averaged over any 10 g volume of tissue [6], and according to the guidelines given by US FCC (Federal Communication Commission), it should be 1.6 W/kg averaged over any 1 g volume of tissue [7]. The high SAR mobile phone causes many biological effects on human body such as irreversible infertility, DNA damage, brain tumor, and so on. There are many antenna structures adopted ever since the formation of the first generation of mobile communication to reduce the SAR value. Along with main antenna structures, supplementary elements like conducting materials [8], reflector [9], ferrite shielding [10], directional antennas [11], and resistive cards [12] are added to reduce the SAR value. Even though the above supplementary elements reduce SAR at greater level, they will improve product cost and size. The other essential parameters, such as gain, radiation efficiency was not up to the required level for above structures and also lagging in resonating with multiple frequencies. By considering these limitations, many researchers recently focused to modify the main antenna structure instead of adding supplementary elements. The AMC (artificial magnetic conductor) structure [13, 14] will reduce the SAR value and improve gain by using high impedance surface property during operating frequency. But it requires additional space and does not support multiband of operation. The SAR value can also be reduced by optimizing electric [15] and magnetic field [16]. But there is no evidence of improving gain and increasing operating bands by using this technique. The EBG (electromagnetic band gap) structure [17–19] will suppress the surface waves which will improve radiation efficiency and gain. While exhibiting stop band characteristics, it behaves like AMC structure that redirects all the radiation opposite to human head which in turn reduces the SAR value. The structure can be further optimized by reducing the size and improving return loss values. Reducing SAR without affecting antenna parameters is a challenging task.Metamaterials are termed materials which have several distinct properties like negative permittivity and negative permeability [20–22]. These distinct properties play a major role in designing high performance antenna especially for exhibiting multiband operation [23–25] and reduce SAR at greater level [26]. The most common metamaterial structures widely used are SRRs (split-ring resonators) [27, 28] and CSRRs (complementary split-ring resonators) [29–31]. SRR may consist of two concentric split rings which act as resonators facing in opposite direction. The shape of the rings is not restricted to ring or circle structure [32], but it can also be implemented in square [33], triangular [34], and hexagonal [35] shapes. SRR acts as a resonating element due to the existence of inductor and capacitor in metal ring. The resonant frequency can be optimized by increasing the following factors such as split gap, spacing between two adjacent rings, side length, and metal width and by introducing multiple split gaps [36]. By introducing stop band at desired band of frequency, SAR can be reduced by using SRR structure. Recently, many researchers presented a SRR-inspired antenna structure that aims to achieve multiple frequency bands [37], to cut down the size [38], and to reduce SAR value [39–42] for mobile application. However, all the antennas presented in the literature have any one of the following pullbacks: unidirectional radiation pattern, extended in size, minimum number of operating bands, and high SAR, which are not suitable for modern wireless personal communication device like mobile phone.In this paper, a penta-band simple microstrip patch antenna is presented. The antenna has a rectangular patch with dual-ring SRR as the radiating element. The antenna presented satisfies the requirement of the antenna for the mobile device. In Section2, the evolution of the antenna is presented, in Section 3, the parametric analysis of the proposed antenna is presented, and in Section 4, the results are discussed. In Section 5, the SAR analysis is presented, followed by a conclusion.


## 2. Evolution and Geometry of Low-SAR Penta-Band Microstrip Patch Antenna

The proposed antenna has four design stages, starting with a simple rectangular patch antenna. The seed rectangular patch antenna is fed with a 50-ohm microstrip line. The hexagonal ring is added in the second stage, followed by another ring with the same thickness in the third stage. Finally, in the fourth stage, the split is introduced near the right side of the feed line, which converts the dual closed ring resonator into dual-ring SRR. In Figure1, the evolution of the proposed low-SAR penta-band microstrip patch antenna is presented.Figure 1
Evolution of the proposed low-SAR penta-band microstrip patch antenna. (a) Ant w. (b) Ant x. (c) Ant y. (d) Ant z.
(a)(b)(c)(d)The proposed antenna is designed on a substrate with an effective dielectric constant of 4.4 and designed using CST software. The antenna is designed on a substrate with a width of 26 mm and 30 mm. First, the antenna is designed with a rectangular patch seed element presented as Ant w in Figure1. Ant w is designed to operate from 3.09 to 4.85 GHz, and it has a frequency of resonance at 3.54 GHz. Then, the first closed ring resonator is introduced in Ant w to design Ant x. The closed ring resonator changes the current direction and shifts the operating frequency. Ant x is operating from 5.51 to 6.28 GHz. Ant y is designed by introducing another closed ring resonator of the same thickness as the first ring. The spacing between the first and second closed ring resonator is 1 mm. Further introduction of the second closed ring resonator changes the current direction, increasing the frequency of operation from 6.69 GHz to 6.91 GHz.Finally, Ant Z is the proposed low-SAR penta-band microstrip patch antenna that is designed by cutting the part of closed rings at the right hand side of the feed line. The antenna has five resonant frequencies at 3.01 GHz, 3.83 GHz, 4.14 GHz, 5.46 GHz, and 6.06 GHz. Interpretation from the four evolution stages of the proposed structure is that the rectangular patch along with 2 hexagonal SRRs can resonate at five different frequencies. The final proposed antenna with its design parameters is depicted in Figure2. The parameter values are presented in Table 1. The antenna is designed using the below equations. The width, length, effective permittivity, and increase in electrical length of an antenna can be calculated from equations (1)–(4), respectively.(1)W=c2fr2εr+1,(2)L=c2frεeff−2∆L,(3)εeff=εr+12+εr−121+12hW−1/2,(4)∆L=0.412hεeff+0.3εeff−0.258W/h+0.264W/h+0.8,where c, fr, h, and εeff are the speed of light, resonant frequency, substrate height, and effective dielectric constant, respectively.Figure 2
Final proposed low-SAR penta-band microstrip patch antenna with its design parameters. (a) Front view. (b) Back view.
(a)(b)Table 1
Parameter values of the proposed low-SAR penta-band microstrip patch antenna (values are in mm).
wlwflfabcde26301.85115.2451076fgw1w2SS1g1th42111310.1510.00351.6The comparison between various antennas evolved during the design of the proposed low-SAR penta-band microstrip patch antenna is presented in Table2. In Figure 3, the return loss of the antennas is compared and depicted. The figure shows that after introduction of SRR along with the presence of rectangular patch, the proposed structure is operating at five different bands.Table 2
Ant w vs. Ant x vs. Ant y vs. Ant z.
AntennaOperating frequency (GHz)Bandwidth (MHz)Resonant frequency (GHz)Return loss (dB)Ant w3.09 to 4.8517573.54−17.28Ant x5.51 to 6.287735.81−12.81Ant y6.69 to 6.912136.78−12.2Ant z2.95 to 3.08953.01−17.763.79 to 3.87743.83−22.734.11 to 4.19794.14−12.335.39 to 5.511225.46−17.995.97 to 6.111496.04−15.06Figure 3
S11 comparison plot of antennas w, x, y, and z.


## 3. Parametric Analysis on SRR Ring

To find the optimum dimension for the critical parameters of the proposed antenna, the parametric analysis in CST is utilized. The width of the rectangular patch and ground length is analyzed parametrically but the parametric analysis of the split-ring resonator is elaborately discussed with results in this section, since it is the reason for the multiband operation of the proposed antenna. The critical parameters of the SRR, such as the width of the ring, separation between the ring, and split width in the rings, are analyzed. Then, the effect of adding the ring to the structure is also presented in this section. First, the split width is analyzed; it is increased from 3 mm to 7 mm in steps of 2 mm. The effect of increasing the split width on the return loss is presented in Figure4. From the figure, it is observed that as the split width increases, the capacitance value increases and the frequency is shifted. For the fabrication, the optimum value of 5 mm is chosen since it achieves good impedance matching compared to 3 mm and 7 mm. Then, the width of the ring is varied from 0.5 mm to 1.5 mm in steps of 0.5 mm. The results of the same are depicted in Figure 5.Figure 4
S11 comparison plot for various values of b.Figure 5
S11 comparison plot for various values of ring width.From the figure, it is observed that the width of 1 mm in both the rings can achieve good bandwidth in the resonating bands, and as a result, it is chosen for the final fabrication. Similarly, Figure6 shows the return loss plot for various separation values between the rings. It is observed from the figure that the separation of 1 mm between the two SRR rings achieves impedance bandwidth in all the resonating bands due to distributed capacitance effect between the rings. In Figure 7, the effect of the rings on the return loss is presented. As the rings increase, the number of operating bands increases. The figure shows that the antenna is operating over a wideband of frequency without a ring. With the introduction of the first ring, the antenna is operating at four different bands. With the addition of another ring, the electrical length increases so the resonant frequency increases to five. The additional resonance is obtained because SRR rings will act as parallel LC circuits.Figure 6
S11 comparison plot for various values of ring separation.Figure 7
S11 comparison plot for various number of rings.


## 4. Result and Discussion

In Figure8, the comparison of the return loss plot with and without a ring is presented; from the figure, it is observed that the inclusion of SRR along with rectangular patch leads to multiple resonance.Figure 8
S11 comparison plot of the antenna with and without SRR.The resonant frequency of an individual hexagonal split ring has been estimated using [35](5)fr=1.8412×c2πSℇr.For outer hexagonal split ring having side length,S = 13 mm,(6)fr=1.8412×3×1082×3.14×13×10−3×4.4=3.22GHz.For inner hexagonal split ring having side length,S1 = 10.15 mm,(7)fr=1.8412×3×1082×3.14×10.15×10−3×4.4=4.13GHz.The effective dielectric constantεr is 4.4, from the above calculations; the side length of the hexagonal ring S determines the resonant frequency. The outer hexagonal split ring contributes 2.95 to 3.06 GHz band and inner hexagonal split ring contributes 4.11 to 4.19 GHz band. Both the resonant frequencies are shifted slightly due to coupling between two rings.In Figure9, the current distribution on the antenna’s surface at various resonant frequencies is presented. It is clearly observed that the current is maximum at different locations for different resonating bands. In all the resonating bands, the current is highly accumulated at the dual-ring SRR, and we can conclude that the rings have the greater effect on the resonating bands. In Figure 9(e), the surface current is highly accumulated next to SRR in rectangular patch at 6.04 GHz which leads to existence of 5.97 to 6.11 GHz band. In order to get additional resonance, the two closed hexagonal rings are converted into split rings by creating a gap in right side of feed line. The gathering of charges at the edge of split gap gives rise to capacitive behavior. Therefore, the presence of inductance and capacitance makes the two hexagonal SRRs to generate two additional resonances [36]. Therefore, two split gaps result in 3.79 to 3.87 GHz and 5.39 to 5.51 GHz band, respectively. The inductance LSRR and capacitance CSRR of the two hexagonal SRRs are calculated [35] using (8) and (9), respectively. The resonant frequency can be determined from (10), where l = 13 mm and 10.15 mm for outer and inner hexagonal SRR’s side length, respectively, N = 2 which is the number of hexagonal SRR, w = 1 mm which is the width of both hexagonal SRRs, s = 1 mm which is the distance between two SRRs, and Kk is the first kind of elliptical integral identity.(8)LSRR=4∗µ0∗l−N−1w+s∗ln0.98∗l−N−1w+sN−1w+s+1.84∗l−N−1w+sN−1w+s,(9)CSRR=N−12∗2l−2N−1w+s∗ɛ0∗K1−k2Kk,where k=s/2w+s.(10)F=12ΠLSRR∗CSRR.Figure 9
Distribution of surface current on the proposed low-SAR penta-band microstrip patch antenna. (a) 3.01 GHz. (b) 3.83 GHz. (c) 4.14 GHz. (d) 5.46 GHz. (e) 6.04 GHz.
(a)(b)(c)(d)(e)In Figure10, the radiation pattern of the proposed low-SAR penta-band antenna is presented; it is observed from the figure that the pattern is stable in both E and H planes. The E plane exhibits a dumbbell-shaped pattern while the H plane exhibits the omnidirectional pattern, which is the need for the personal wireless communication standard application. A perfect dumbbell-shaped pattern and omnidirectional pattern was achieved in both E plane and H plane, respectively, for 4.14 GHz. In Figure 11, the fabricated antenna is presented. The comparison between the simulated and measured results of S11 is presented in Figure 12. In Figure 13, the simulated and measured gain was presented concerning the operating frequency.Figure 10
E plane and H plane of the proposed low-SAR penta-band microstrip patch antenna. (a) 3.01 GHz. (b) 3.83 GHz. (c) 4.14 GHz. (d) 5.46 GHz. (e) 6.04 GHz.
(a)(b)(c)(d)(e)Figure 11
Front view and back view of fabricated antenna. (a) Front view. (b) Back view.
(a)(b)Figure 12
Simulated vs. measured results ofS11.Figure 13
Simulated vs. measured results of gain.The proposed antenna exhibits gain above 2 dBi in all the resonating bands, and the maximum gain of the proposed antenna is 6.2 dBi.


## 5. SAR Analyses

We used three layers of bespoke body tissue in this study. The model is made up of a 10 mm thick muscle layer, a 6 mm thick fat layer, and a 0.5 mm thick skin layer. The personalized body tissue was used to assess the SAR, which defines the amount of energy absorbed by body tissues, and explore the influence of the human body on wearable antenna performance. Because the human body is a frequency-dispersive material, the higher frequency bands will be influenced more when the antenna touches the skin. On the other hand, adding spacing will enhance impedance bandwidth and impedance matching.In Figure14, the proposed antenna with the fragmented human phantom model in the CST environment is presented. In Figure 15, the effect of the human body in the return loss and frequency of operation is presented. Due to the robustness of the antenna, the centre frequency of the supported applications was not detuned. There was a good agreement between the results, as shown in Figure 15. Because the human body is a complex medium with high permittivity, placing the antenna closer to the body’s tissues significantly reduced the gain and efficiency of the antenna. IEEE/IEC 62704-1 averaged on 10 g was used to calculate the SAR value of the biological tissues. Mathematically, SAR is calculated by σ [E2/ρ], where σ is the tissue conductivity, E is the electric field, and ρ is the mass density of the tissue. The maximum limit of SAR for 10 g volume of tissue is 2 W/kg. In Figure 16, the simulated SAR is presented, from which at all the resonant frequencies, the proposed structure exhibits SAR < 2 W/kg. So, the proposed antenna could be used in personal communication devices. In Table 3, the numerical values of the SAR are presented against the resonating frequencies.Figure 14
Proposed low-SAR penta-band microstrip patch antenna with human model.Figure 15
S11 comparison of the proposed antenna with and without human model.Figure 16
SAR analysis at various resonating frequencies. (a) 3.01 GHz. (b) 3.83 GHz. (c) 4.14 GHz. (d) 5.46 GHz. (e) 6.04 GHz.
(a)(b)(c)(d)(e)Table 3
Centre frequency vs. SAR.
Centre frequency (GHz)3.013.834.145.466.04SAR (W/kg)1.060.4120.421.041.61In Table4, the proposed antenna is compared with literature antennas presented for mobile applications. The recent methodologies adopted to reduce SAR value in mobile phone antenna were taken for comparing proposed antenna. From the comparison, the AMC structure reduces the SAR value with better gain, but the number of operating bands is very limited; additional AMC structure is developed along with main structure. By optimizing electric and magnetic fields, SAR can be reduced, but there is no evidence of gain improvement using those structures. Even though EBG structures reduce SAR at greater level without adding additional structures with more resonant frequencies, gain is lower than that of other methodologies. The SRR available in the literature reduces SAR either by adding additional structures or by compromising gain values. But the proposed compact rectangular patch antenna loaded with split-ring resonator reduces SAR at a greater level in all five different resonant frequencies with improved gain and minimum return loss.Table 4
Proposed antenna vs. literature antennas.
Ref no. and yearApplied design methodsDimension (mm2)Requirement and name of additional structureNo. of operating bandsMaximum bandwidthMinimum return loss (dB)Maximum gain (dBi)Minimum SAR valueReference [13], 2021AMC structure40.5 × 40.5Yes and AMC structure1510 MHz−407.474.8 (1 gram)Reference [14], 2020AMC structure30.54 × 30.54Yes and AMC structure1472 MHz−168.690.353 (10 gram)Reference [15], 2021OptimizingE field154 × 74Yes and metal rim4980 MHz−27Not specified1.19 (10 gram)Reference [16], 2021OptimizingH field151 × 71Yes and metallic bezels190 MHz−24Not specified0.183 (10 gram)Reference [17], 2022EBG structure50.3 × 40No31 GHzPoor2.50.27 (1 gram)Reference [18], 2019EBG structure91 × 91No1244 MHz−242.950.498 (1 gram)Reference [19], 2021EBG structure36.67 × 40Yes and dual feeding4750 MHz−404.550.62 (10 gram)Reference [40], 2021SRR structure20 × 13Yes and SRR structure31.9 GHz−3431.64 (1 gram)Reference [41], 2021SRR structure14 × 28No4550 MHz−24Not specified0.21 (1 gram)Reference [42], 2021SRR structure90 × 42Yes and SRR structure260 MHz−38Not specified0.50 (10 gram)Proposed antennaSRR structure30 × 26No additional structure5400 MHz−356.20.41 (10 gram)


## 6. Conclusions

A simple compact rectangular patch antenna with dual-ring SRR is presented for the modern wireless personal communication. The antenna is designed and fabricated on an FR4 substrate. The proposed structure operates on five different bands with resonance at 3.0 GHz, 3.8 GHz, 4.1 GHz, 5.4 GHz, and 6.0 GHz. In all the operating bands, the impedance bandwidth is well maintained with maximum gain of 6.2 dBi. The introduction of the SRR is responsible for the multiband operation, which is evident from the parameter analysis and surface current distribution presented in this article. The critical antenna parameters are analyzed, and its optimum values are identified using parametric analysis. The antenna is further analyzed for the SAR level, and it is found that the antenna exhibits very low values of SAR at all the five resonating frequencies. The simulated SAR value at all the resonating bands is less than 2 W/kg. The depicted simulated results ofS11 and radiation pattern are on par with the measured results. The antenna is further validated with the surface current, gain, E Plane, and H plane pattern results. Simple structure with stable pattern, reasonable gain, good impedance bandwidth, and low SAR makes the proposed antenna more suitable for the 5G mobile application.



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*Source: 3298866-2022-12-30.xml*