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Miniaturization of microstrip loop antenna for wireless applications based on metamaterial metasurface
Accepted Manuscript Regular paper Miniaturization of microstrip Loop antenna for wireless applications based on metamaterial metasurface Gohar Varamini, Asghar Keshtkar, Mohammad Nasser-Moghadasi PII: DOI: Reference:
S1434-8411(17)31457-7 http://dx.doi.org/10.1016/j.aeue.2017.08.024 AEUE 52020
To appear in:
International Journal of Electronics and Communications
Received Date: Revised Date: Accepted Date:
13 June 2017 11 August 2017 16 August 2017
Please cite this article as: G. Varamini, A. Keshtkar, M. Nasser-Moghadasi, Miniaturization of microstrip Loop antenna for wireless applications based on metamaterial metasurface, International Journal of Electronics and Communications (2017), doi: http://dx.doi.org/10.1016/j.aeue.2017.08.024
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Miniaturization of microstrip Loop antenna for wireless applications based on metamaterial metasurface 1
1
Gohar Varamini, 2,*Asghar Keshtkar, 1Mohammad Nasser-Moghadasi
Dept. of Electrical and Computer Eng., Science and Research Branch, Islamic Azad University, Tehran-Iran. 2
Dept. of Engineering and Technology, Imam Khomeini International University (IKIU), Ghazvin, Iran. E-mail: [email protected] *
Corresponding Author: [email protected]
Abstract: The metamaterial and fractal techniques are two main methods for antenna miniaturization and in this paper, we have modeled an especial shape of the antenna based on loop formation with metamaterial load for this aim. The metamaterial layer is made by multi parallel rings and the result shows that the final antenna size reduced drastically while the frequency shifts from 7 to 4 GHz. The antenna has Omni-directional pattern with the gain of 3.5 dBi, so the size is reduced around 40%for 4.5 GHz and another resonance is made at 2.5 GHz with a return lossless than -6dB with more than 60% frequency shift. The reflection and transmission have been utilized for showing the left hand characteristic based on two port periodic simulations in HFSS full wave software. We show that how the metamaterial load can provide the circular polarization (CP) by controlling the current distribution. We also presented that by making slots we obtained the better Axial Ratio (AR) and miniaturized the antenna with reconfigurable qualification. As a result of fact, we show that by using metasurface we able to miniaturized the antenna and simultaneously achieved the circular polarization. Key word: loop antenna, fractal, metamaterial, circular polarization
I.
Introduction:
Microstrip antenna provides various attractive properties such as high efficiency, lightweight, low cost of manufacturing and low profile in wireless communication. However, these antennas suffer a main disadvantage of narrow impedance bandwidth. Various techniques and configurations have been proposed to achieve bandwidth enhancing characteristic and size reduction [1]. Microstrip antenna mostly consists of a metallic patch on a thin dielectric substrate. For improving the bandwidth, several methods have been used such as thick substrate with low dielectric constant, and also placing parasitic patches in the main patch. Size reduction in patch antenna on the other hand is one of the main features and many studies such as using loaded slots and capacitance [2-3] and using a short circuit piece or pin [4-6] have been investigated to obtain this purpose. The needs of the design of an antenna with multi-band characteristic have become the subject of recent research for broadband applications [7]. Patch antenna with fractal shaped structure is a good candidate for multi band characteristic due to their great properties including appropriate dimension, price, fabrication and weight[8-10]. Fractal antennas with natural shape are broken and irregular fragments, which have self-similarity in their geometrical structures [11]. These structures are relatively small and provide multi resonant and high gain antenna. Some of their common shapes are snowflakes, ferns, coastlines and trees [12]. Various models of ultra wideband (UWB) fractal antenna have been investigated in recent years. A novel compact and broadband coplanar waveguide (CPW) fed flower-shaped microstrip patch antenna is proposed in [13], which provides much wider impedance bandwidth and this structure is capable of providing a good impedance matching with the CPW feed without any modification of feed line and ground plane. Heydari et.al has proposed a new model of antenna with the Jerusalem cross as a fractal slot for CP characteristic [14]. The proposed structure provides wide bandwidth and a compact size. Li et.al have been investigated a novel Koch-like sided fractal bowtie dipole antenna for multi band characteristic and miniaturization [15]. During the last decade, Metamaterial has received considerable attention due to its unusual properties, which do not exist in natural material [16].Metamaterials can provide new techniques in order to miniaturize patch antenna [17]. For this aim, many efforts have been reported for producing highly miniaturizing the patch antenna. One of the main methods is using complementary, split ring resonator place horizontally between the patch and the ground plane. In this design, the structure shows good impedance matching and radiation
characteristic by optimizing the geometry of the split rings [18]. Another technique is proposed a novel zerothorder resonant (ZOR) antenna on via-less CPW based on a composite right/left-handed CPW transmission lines[19-20]. This structure utilizes the ZOR condition to achieve a compact size, and its reactive parameters determine the resonant frequency. The bandwidth of 6.8% and efficiency of 62% are experimentally obtained. Its bandwidth is enhanced compared with other ZOR antennas [21]. The loop antenna with special shape based on fractal techniques have developed in this paper and we have utilized a metamaterial load on the other side of the substrate. We show that by the implementation of this load we have miniaturized the antenna size more than 50%. We show that this metamaterial loaded antenna is useful for reconfigurable applications by making some slots, if they replaced by PIN diode or other kind of switch. Furthermore, these slots can be used for current controlling on the surface current, which creates the circular polarization. The load permittivity and permeability is obtained by Eq. (1) and (4). The antenna is fabricated for measurement, and for the feeding of the loop, we have used a balun. The experiments are confirmed by simulations. II.
Design Theory and Background:
In this article, we applied Minkowski fractal arrangement for the first layer of the proposed structure. Since fractal structures increased the effective size of the antenna, the Minkowski fractal method has been noticed in microstrip antenna. In [13] with the ratio of
Minkowski order 1 to 3 can be designed and the resonant
frequency is reduced [22]. We utilized Split Ring Resonator (SRR) metamaterial in the second layer of the prototyped antenna. As mentioned in previous sections, metamaterial are a kind of material which are not found in nature and can make artificially. Metamaterials exhibit special properties such as negative or zero permittivity ( )/permeability (
).Such material have complex permittivity
and complex permeability
which can be obtained using expressions for reflection and transmission shown in (1) and (2) [23]. (1)
(2) Where
and Z is impedance.
The impedance (Z) and refractive index (n) are calculated by Eq. (1) and (2). (3) (4) Recently, reconfigurable antenna has been considered as one of the best candidates that can improve antenna performance. In this section, we present a reconfigurable modified antenna in order to enhance multi band characteristics. Lee et.al proposed a new reconfigurable CP microstrip antenna using a dual embedded dual slotted ring perturbation [24]. In this study, the dual slotted ring perturbation can create circularly polarized waves by changing the effective size of the dual slotted ring. A novel compact, ultra wideband microstrip monopole antenna with the reconfigurable polarization capability investigated by Aboufoul et.al, in which the structure can be switched from linear polarization to right-hand circular polarization (RHCP) or left-hand circular polarization (LHCP) [25]. III.
Antenna Design:
Fig. 1(a) and (b) show the schematic configuration of the basic antenna and as shown here, the combination of a fractal loop and SRR loads with the fractal arrangement is presented in order to miniaturize the microstrip antenna. Fig. 1(a) illustrates a loop fractal as a top layer and Fig. 1(b) shows metamaterial fractal load as a bottom layer of the proposed antenna, respectively. The proposed antenna is composed of two metallic layers which are separated by a dielectric layer. The antenna is printed on a FR-4 low cost substrate with relative permittivity of 4.4, thickness of 1.6 mm and loss tangent of 0.02. A 50 Ω microstrip transmission lines is used as a feeding source and is located in the middle of the structure. We utilize a balun in order to feed the antenna with the maximum matching. The total size of the designed structure is 40mm×40 mm. All dimensions as shown in Fig.1 in the top layer are ,
,
,
,
,
and
and in the bottom layer are
,
,
and
.The top layer (fractal
loop) is created based Minkowski fractal formation.
(a) (b) Fig.1 the geometry of the antenna (a) the top view (b) the metamaterial load at the bottom layer IV.
Simulation Results
The prototyped antenna has been simulated with HFSS as full wave software and at the first step, we have assumed 6 slots which is replaced by switches in the loop radiator in five different cases. Fig. 2 shows fifth steps of the process of proposed antenna designing. In each case, we consider slots with different placement. Here, these slots can be equivalent with a PIN diode as on/off switches (Fig.2 (b)). Firstly, we analyzed these five cases in the absence of metamaterial loads. Fig.3 (a) illustrates S11 results of our structure for various cases. As shown in Fig. 3(a), multi band characteristics cannot be seen and only a resonance appeared at 7 GHz. Then, we added metamaterial loads in our structure and simulated five steps of the structure and Fig. 3(b) shows S11 results in the presence of metamaterial loads. As shown in Fig. 3(b), the antenna exhibit dual band and multi band characteristics for various cases of the structure. In the first case at Fig. 3(b), when the switch is off the frequency of resonance occurs at 6 GHz and two other resonances are appearing at lower frequencies, however the S11 value is over the -10 dB and these resonances are not suitable. In the other case, by implementing two slots we are able to make dual band characteristics at 4.2 and 6 GHz and in addition, by changing the slot placement, we are able to create a reconfigurable resonance between 4.2 to 4.6 GHz.
(a)
(b) Fig.2 (a) various case of the prototype antenna (b) switches placement
(a)
(b)
Fig.3 magnitude of S11 Results (a) without metamaterial loads (b) with metamaterial loads We have modified the structure for two ports analysis by using HFSS simulation software. Then, we extracted the magnitude and the phase of S11 by Matlab software to obtain the real part of the permittivity ( ) and
permeability ( ) for the first case with metamaterial load. As shown in Fig. 4, the permittivity and permeability in 4.5 and 6 GHz is negative, which causes resonances at these frequencies. The negative permeability at 2.5 and 4.5 GHz has made two resonances for the first case with metamaterial load at Fig.3 (b). In conclusion , we can see that the negative characteristic of the metasurface is affected directly on the antenna resonances by altering the wave impedance in substrate as reported by Yousefi et.al based on 3D model [26] and in addition the 3D μ-negative (MNG ) and double negative (DNG) are noticed for Subwavelength operation of the antenna[27] which is noticed in this work with 2D formation.
(a)
(b)
Fig.4 the permittivity and permeability of the metamaterial for the basic model (a) by the effect of the antenna (b) without the effect of the antenna
V.
Antenna pattern and current distribution studies
Fig. 5 illustrates the antenna current distribution in the presence and absence of the metamaterial load, and as shown here the antenna current maximum value is increased drastically by applying the metamaterial load from 86 A/m to 379 A/m. In this structure, in the absence of the metamaterial load, the antenna current distributes around the whole loop while after adding the metamaterial load, the current is concentrated at two corners. Here we can see that the current can be controlled by metamaterial loads. In Fig.6, we have studied this effect for
case 2 and by comparing this antenna with and without metamaterial, we have realized that the current vector also can be shaped which have to be noticed for achieving the circular polarization.
(a)
(b)
(c)(d)
Fig. 5 current antenna Case 1(a) at 4 GHz without metamaterial load (b) at 7 GHz without metamaterial load(c) at 4.5 GHz with metamaterial load(d) at 6 GHz with metamaterial load
(a)
(b)
(c)
(d)
Fig. 6 current antenna Case 2(a) current density for case 2 without metamaterial at 7 GHz(b) current distribution vector for case 2 without metamaterial at 7 GHz(a) current density for case 2 with metamaterial at 4 GHz(d) current distribution vector for case 2 with metamaterial at 4 GHz
The simulated radiation pattern for case 1 and 2 are presented in fig. 7 and 8, respectively and we have checked the pattern with and without metamaterial loads. Fig.7 (a) shows the radiation pattern for 7 GHz without metamaterial loads, and at this frequency, the antenna has a bidirectional shape with the gain of 5.8 dBi. After adding the metamaterial the radiation at lower frequencies is activated and the antenna gives an omnidirectional pattern having a tube-liked shape with the gain value of 3.1 dBi (Fig.7 (b)). At the second frequency, the bidirectional pattern remains for the antenna (Fig.7 (c)). Fig.8 illustrates the antenna gain for the second case in the absence and presence of metamaterial load as well. The antenna gain without loads for LHCP and RHCP are exhibited at 7 GHz in Fig. 8 (a) and (b).Fig. 8 (c) show antenna gain with metamaterial load at 4 GHz and Fig. 8
(d) and (e) present the LHCP and RHCP gain at 4 GHz, respectively. When the metamaterial load added to the first case, the antenna shows fewer tendencies to circular polarization, but in the second case the tendency toward circular polarization increases drastically. Metamaterial loads change the first resonance pattern, i.e. the Omni-directional pattern became dumbbell-shaped or bidirectional. By comparing the patterns, we can say that the structure is oriented toward circular polarization.
(a)
(b)
(c)
Fig.7 antenna gain (a) antenna case 1 without metamaterial at 7 GHz (b)antenna case 1 with metamaterial at 4 GHz (c)antenna case 1 with metamaterial at 6 GHz
(a)
(b)
(e)
(c)
(d)
(f)
Fig.8. antenna gain (a) antenna case 2 without metamaterial at 7 GHz for LHCP (b) antenna case 2 without metamaterial at 7 GHz for RHCP(c) antenna case 2 with metamaterial at 4 GHz (d) antenna case 2 with
metamaterial at 4 GHz for LHCP (e) antenna case 2 with metamaterial at 4 GHz for RHCP(6) antenna case 2 with metamaterial at 4 GHz The antenna axial ratio (AR) is investigated for all structures in the presence of the metamaterial load and as shown here (Fig.9) the metamaterial helped us to improve the AR and the proposed antenna (case 5) shows a good agreement between the axial ratio and the resonance bandwidth at 4 GHz.
Fig.9 the AR simulation for the various models of the antenna with metamaterial loads VI.
Experimental and measurement results
Fig.10 shows the fabricated antenna. It is shown in Fig.10a that we have used a balun with the length of 10mmfor feeding the antenna. the FR-4 substrate is selected for the substrate and for the prototyped antenna we have chosen case 5 since that was modified for creating circular polarization.
(a)(b) Fig.10 the fabricated antenna for measurement (a) top view and the feeding structure (b) the metamaterial load at bottom layer
The antenna return loss is presented in Fig. 11 for the proposed antenna, comparing the simulation and experimental results. As it is shown in Fig. 11, the experimental return loss shows good agreement with simulation. However, the Balun junction with the patch has made a great loss in this antenna and we have some frequency shift, which may also because of the impurity of the substrate. The antenna has triple band resonance at 2.7, 4.5 and 5.9 GHz. It is noteworthy that by antenna resizing and optimization, frequencies of operation can be modified for the other desired band as well.
Fig.11 The return loss of the antenna for simulation and experimental
The antenna pattern is obtained in the chamber room for the 4.5 GHz, and for this frequency we present the coand cross-polarization for phi=0º and 90º (Fig.12 (a) and (b)).For showing the circular polarization with circular antenna, we have a test on RHCP and LHCP and the results show the circular polarization at Fig.12 (c) and (d). The antenna gain and efficiency are obtained experimentally in table.1 in details. As shown in Fig.12 (a) and (b), the balun has small effect on antenna pattern in side of the balun however, the bidirectional pattern remains for the antenna.
(a)
(b)(c)
(d)
Fig.12 the measurement of antenna radiation pattern (a) the co- and cross-polarization for phi=0º (b) the co- and cross-polarization for phi=90º (c) the LHCP pattern (d) the RHCP pattern. For (a) and (b) the solid lines show co- polarization and dash lines for cross-polarization.
Table.1 the antenna resonance comparing frequency
gain
efficiency
BW
polarization
2.5 GHz
3.5 dBi
72 %
300 MHz
linear
4.5 GHz
4.5 dBi
78%
360 MHz
circular
5.9 GHz
4.2 dBi
70%
240 MHz
linear
For measurement of the axial ratio we have checked the E-field in both axes and the result is checked for all resonances for a few points around 4.5 GHz with the step of 20 MHz as shown in Fig.13. The antenna has circular polarization radiation at 4.5 GHz and for 2.7 and 5.9 GHz the antenna has linear polarization.
Fig.13 the antenna AR comparing for simulation and experimental
VII.
Conclusion
The first aim of this paper is reducing the frequency miniaturization and obtained the frequency is reduced from 7 to 4.5 GHz for the loop antenna so around 40% miniaturized and for 2.7 GHz we have the return loss of -6 dB which is sufficient for indoor application with more than 64% miniaturization. In comparison with conventional patch antenna, we have around 30% of miniaturization for physical area. The second aim of this paper is making a reconfigurable characteristic which is realized by controlling the effective length. For the third aim, circular polarization is noticed for the proposed antenna, metamaterial loads is implemented as a metasurface for shaping the current distribution which results in circular polarization. Acknowledgment The Authors would like to thank Navid P. Gandji (Michigan Technological University) for his helpful discussions and co-operations.
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S1434-8411(17)31457-7 http://dx.doi.org/10.1016/j.aeue.2017.08.024 AEUE 52020
To appear in:
International Journal of Electronics and Communications
Received Date: Revised Date: Accepted Date:
13 June 2017 11 August 2017 16 August 2017
Please cite this article as: G. Varamini, A. Keshtkar, M. Nasser-Moghadasi, Miniaturization of microstrip Loop antenna for wireless applications based on metamaterial metasurface, International Journal of Electronics and Communications (2017), doi: http://dx.doi.org/10.1016/j.aeue.2017.08.024
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Miniaturization of microstrip Loop antenna for wireless applications based on metamaterial metasurface 1
1
Gohar Varamini, 2,*Asghar Keshtkar, 1Mohammad Nasser-Moghadasi
Dept. of Electrical and Computer Eng., Science and Research Branch, Islamic Azad University, Tehran-Iran. 2
Dept. of Engineering and Technology, Imam Khomeini International University (IKIU), Ghazvin, Iran. E-mail: [email protected] *
Corresponding Author: [email protected]
Abstract: The metamaterial and fractal techniques are two main methods for antenna miniaturization and in this paper, we have modeled an especial shape of the antenna based on loop formation with metamaterial load for this aim. The metamaterial layer is made by multi parallel rings and the result shows that the final antenna size reduced drastically while the frequency shifts from 7 to 4 GHz. The antenna has Omni-directional pattern with the gain of 3.5 dBi, so the size is reduced around 40%for 4.5 GHz and another resonance is made at 2.5 GHz with a return lossless than -6dB with more than 60% frequency shift. The reflection and transmission have been utilized for showing the left hand characteristic based on two port periodic simulations in HFSS full wave software. We show that how the metamaterial load can provide the circular polarization (CP) by controlling the current distribution. We also presented that by making slots we obtained the better Axial Ratio (AR) and miniaturized the antenna with reconfigurable qualification. As a result of fact, we show that by using metasurface we able to miniaturized the antenna and simultaneously achieved the circular polarization. Key word: loop antenna, fractal, metamaterial, circular polarization
I.
Introduction:
Microstrip antenna provides various attractive properties such as high efficiency, lightweight, low cost of manufacturing and low profile in wireless communication. However, these antennas suffer a main disadvantage of narrow impedance bandwidth. Various techniques and configurations have been proposed to achieve bandwidth enhancing characteristic and size reduction [1]. Microstrip antenna mostly consists of a metallic patch on a thin dielectric substrate. For improving the bandwidth, several methods have been used such as thick substrate with low dielectric constant, and also placing parasitic patches in the main patch. Size reduction in patch antenna on the other hand is one of the main features and many studies such as using loaded slots and capacitance [2-3] and using a short circuit piece or pin [4-6] have been investigated to obtain this purpose. The needs of the design of an antenna with multi-band characteristic have become the subject of recent research for broadband applications [7]. Patch antenna with fractal shaped structure is a good candidate for multi band characteristic due to their great properties including appropriate dimension, price, fabrication and weight[8-10]. Fractal antennas with natural shape are broken and irregular fragments, which have self-similarity in their geometrical structures [11]. These structures are relatively small and provide multi resonant and high gain antenna. Some of their common shapes are snowflakes, ferns, coastlines and trees [12]. Various models of ultra wideband (UWB) fractal antenna have been investigated in recent years. A novel compact and broadband coplanar waveguide (CPW) fed flower-shaped microstrip patch antenna is proposed in [13], which provides much wider impedance bandwidth and this structure is capable of providing a good impedance matching with the CPW feed without any modification of feed line and ground plane. Heydari et.al has proposed a new model of antenna with the Jerusalem cross as a fractal slot for CP characteristic [14]. The proposed structure provides wide bandwidth and a compact size. Li et.al have been investigated a novel Koch-like sided fractal bowtie dipole antenna for multi band characteristic and miniaturization [15]. During the last decade, Metamaterial has received considerable attention due to its unusual properties, which do not exist in natural material [16].Metamaterials can provide new techniques in order to miniaturize patch antenna [17]. For this aim, many efforts have been reported for producing highly miniaturizing the patch antenna. One of the main methods is using complementary, split ring resonator place horizontally between the patch and the ground plane. In this design, the structure shows good impedance matching and radiation
characteristic by optimizing the geometry of the split rings [18]. Another technique is proposed a novel zerothorder resonant (ZOR) antenna on via-less CPW based on a composite right/left-handed CPW transmission lines[19-20]. This structure utilizes the ZOR condition to achieve a compact size, and its reactive parameters determine the resonant frequency. The bandwidth of 6.8% and efficiency of 62% are experimentally obtained. Its bandwidth is enhanced compared with other ZOR antennas [21]. The loop antenna with special shape based on fractal techniques have developed in this paper and we have utilized a metamaterial load on the other side of the substrate. We show that by the implementation of this load we have miniaturized the antenna size more than 50%. We show that this metamaterial loaded antenna is useful for reconfigurable applications by making some slots, if they replaced by PIN diode or other kind of switch. Furthermore, these slots can be used for current controlling on the surface current, which creates the circular polarization. The load permittivity and permeability is obtained by Eq. (1) and (4). The antenna is fabricated for measurement, and for the feeding of the loop, we have used a balun. The experiments are confirmed by simulations. II.
Design Theory and Background:
In this article, we applied Minkowski fractal arrangement for the first layer of the proposed structure. Since fractal structures increased the effective size of the antenna, the Minkowski fractal method has been noticed in microstrip antenna. In [13] with the ratio of
Minkowski order 1 to 3 can be designed and the resonant
frequency is reduced [22]. We utilized Split Ring Resonator (SRR) metamaterial in the second layer of the prototyped antenna. As mentioned in previous sections, metamaterial are a kind of material which are not found in nature and can make artificially. Metamaterials exhibit special properties such as negative or zero permittivity ( )/permeability (
).Such material have complex permittivity
and complex permeability
which can be obtained using expressions for reflection and transmission shown in (1) and (2) [23]. (1)
(2) Where
and Z is impedance.
The impedance (Z) and refractive index (n) are calculated by Eq. (1) and (2). (3) (4) Recently, reconfigurable antenna has been considered as one of the best candidates that can improve antenna performance. In this section, we present a reconfigurable modified antenna in order to enhance multi band characteristics. Lee et.al proposed a new reconfigurable CP microstrip antenna using a dual embedded dual slotted ring perturbation [24]. In this study, the dual slotted ring perturbation can create circularly polarized waves by changing the effective size of the dual slotted ring. A novel compact, ultra wideband microstrip monopole antenna with the reconfigurable polarization capability investigated by Aboufoul et.al, in which the structure can be switched from linear polarization to right-hand circular polarization (RHCP) or left-hand circular polarization (LHCP) [25]. III.
Antenna Design:
Fig. 1(a) and (b) show the schematic configuration of the basic antenna and as shown here, the combination of a fractal loop and SRR loads with the fractal arrangement is presented in order to miniaturize the microstrip antenna. Fig. 1(a) illustrates a loop fractal as a top layer and Fig. 1(b) shows metamaterial fractal load as a bottom layer of the proposed antenna, respectively. The proposed antenna is composed of two metallic layers which are separated by a dielectric layer. The antenna is printed on a FR-4 low cost substrate with relative permittivity of 4.4, thickness of 1.6 mm and loss tangent of 0.02. A 50 Ω microstrip transmission lines is used as a feeding source and is located in the middle of the structure. We utilize a balun in order to feed the antenna with the maximum matching. The total size of the designed structure is 40mm×40 mm. All dimensions as shown in Fig.1 in the top layer are ,
,
,
,
,
and
and in the bottom layer are
,
,
and
.The top layer (fractal
loop) is created based Minkowski fractal formation.
(a) (b) Fig.1 the geometry of the antenna (a) the top view (b) the metamaterial load at the bottom layer IV.
Simulation Results
The prototyped antenna has been simulated with HFSS as full wave software and at the first step, we have assumed 6 slots which is replaced by switches in the loop radiator in five different cases. Fig. 2 shows fifth steps of the process of proposed antenna designing. In each case, we consider slots with different placement. Here, these slots can be equivalent with a PIN diode as on/off switches (Fig.2 (b)). Firstly, we analyzed these five cases in the absence of metamaterial loads. Fig.3 (a) illustrates S11 results of our structure for various cases. As shown in Fig. 3(a), multi band characteristics cannot be seen and only a resonance appeared at 7 GHz. Then, we added metamaterial loads in our structure and simulated five steps of the structure and Fig. 3(b) shows S11 results in the presence of metamaterial loads. As shown in Fig. 3(b), the antenna exhibit dual band and multi band characteristics for various cases of the structure. In the first case at Fig. 3(b), when the switch is off the frequency of resonance occurs at 6 GHz and two other resonances are appearing at lower frequencies, however the S11 value is over the -10 dB and these resonances are not suitable. In the other case, by implementing two slots we are able to make dual band characteristics at 4.2 and 6 GHz and in addition, by changing the slot placement, we are able to create a reconfigurable resonance between 4.2 to 4.6 GHz.
(a)
(b) Fig.2 (a) various case of the prototype antenna (b) switches placement
(a)
(b)
Fig.3 magnitude of S11 Results (a) without metamaterial loads (b) with metamaterial loads We have modified the structure for two ports analysis by using HFSS simulation software. Then, we extracted the magnitude and the phase of S11 by Matlab software to obtain the real part of the permittivity ( ) and
permeability ( ) for the first case with metamaterial load. As shown in Fig. 4, the permittivity and permeability in 4.5 and 6 GHz is negative, which causes resonances at these frequencies. The negative permeability at 2.5 and 4.5 GHz has made two resonances for the first case with metamaterial load at Fig.3 (b). In conclusion , we can see that the negative characteristic of the metasurface is affected directly on the antenna resonances by altering the wave impedance in substrate as reported by Yousefi et.al based on 3D model [26] and in addition the 3D μ-negative (MNG ) and double negative (DNG) are noticed for Subwavelength operation of the antenna[27] which is noticed in this work with 2D formation.
(a)
(b)
Fig.4 the permittivity and permeability of the metamaterial for the basic model (a) by the effect of the antenna (b) without the effect of the antenna
V.
Antenna pattern and current distribution studies
Fig. 5 illustrates the antenna current distribution in the presence and absence of the metamaterial load, and as shown here the antenna current maximum value is increased drastically by applying the metamaterial load from 86 A/m to 379 A/m. In this structure, in the absence of the metamaterial load, the antenna current distributes around the whole loop while after adding the metamaterial load, the current is concentrated at two corners. Here we can see that the current can be controlled by metamaterial loads. In Fig.6, we have studied this effect for
case 2 and by comparing this antenna with and without metamaterial, we have realized that the current vector also can be shaped which have to be noticed for achieving the circular polarization.
(a)
(b)
(c)(d)
Fig. 5 current antenna Case 1(a) at 4 GHz without metamaterial load (b) at 7 GHz without metamaterial load(c) at 4.5 GHz with metamaterial load(d) at 6 GHz with metamaterial load
(a)
(b)
(c)
(d)
Fig. 6 current antenna Case 2(a) current density for case 2 without metamaterial at 7 GHz(b) current distribution vector for case 2 without metamaterial at 7 GHz(a) current density for case 2 with metamaterial at 4 GHz(d) current distribution vector for case 2 with metamaterial at 4 GHz
The simulated radiation pattern for case 1 and 2 are presented in fig. 7 and 8, respectively and we have checked the pattern with and without metamaterial loads. Fig.7 (a) shows the radiation pattern for 7 GHz without metamaterial loads, and at this frequency, the antenna has a bidirectional shape with the gain of 5.8 dBi. After adding the metamaterial the radiation at lower frequencies is activated and the antenna gives an omnidirectional pattern having a tube-liked shape with the gain value of 3.1 dBi (Fig.7 (b)). At the second frequency, the bidirectional pattern remains for the antenna (Fig.7 (c)). Fig.8 illustrates the antenna gain for the second case in the absence and presence of metamaterial load as well. The antenna gain without loads for LHCP and RHCP are exhibited at 7 GHz in Fig. 8 (a) and (b).Fig. 8 (c) show antenna gain with metamaterial load at 4 GHz and Fig. 8
(d) and (e) present the LHCP and RHCP gain at 4 GHz, respectively. When the metamaterial load added to the first case, the antenna shows fewer tendencies to circular polarization, but in the second case the tendency toward circular polarization increases drastically. Metamaterial loads change the first resonance pattern, i.e. the Omni-directional pattern became dumbbell-shaped or bidirectional. By comparing the patterns, we can say that the structure is oriented toward circular polarization.
(a)
(b)
(c)
Fig.7 antenna gain (a) antenna case 1 without metamaterial at 7 GHz (b)antenna case 1 with metamaterial at 4 GHz (c)antenna case 1 with metamaterial at 6 GHz
(a)
(b)
(e)
(c)
(d)
(f)
Fig.8. antenna gain (a) antenna case 2 without metamaterial at 7 GHz for LHCP (b) antenna case 2 without metamaterial at 7 GHz for RHCP(c) antenna case 2 with metamaterial at 4 GHz (d) antenna case 2 with
metamaterial at 4 GHz for LHCP (e) antenna case 2 with metamaterial at 4 GHz for RHCP(6) antenna case 2 with metamaterial at 4 GHz The antenna axial ratio (AR) is investigated for all structures in the presence of the metamaterial load and as shown here (Fig.9) the metamaterial helped us to improve the AR and the proposed antenna (case 5) shows a good agreement between the axial ratio and the resonance bandwidth at 4 GHz.
Fig.9 the AR simulation for the various models of the antenna with metamaterial loads VI.
Experimental and measurement results
Fig.10 shows the fabricated antenna. It is shown in Fig.10a that we have used a balun with the length of 10mmfor feeding the antenna. the FR-4 substrate is selected for the substrate and for the prototyped antenna we have chosen case 5 since that was modified for creating circular polarization.
(a)(b) Fig.10 the fabricated antenna for measurement (a) top view and the feeding structure (b) the metamaterial load at bottom layer
The antenna return loss is presented in Fig. 11 for the proposed antenna, comparing the simulation and experimental results. As it is shown in Fig. 11, the experimental return loss shows good agreement with simulation. However, the Balun junction with the patch has made a great loss in this antenna and we have some frequency shift, which may also because of the impurity of the substrate. The antenna has triple band resonance at 2.7, 4.5 and 5.9 GHz. It is noteworthy that by antenna resizing and optimization, frequencies of operation can be modified for the other desired band as well.
Fig.11 The return loss of the antenna for simulation and experimental
The antenna pattern is obtained in the chamber room for the 4.5 GHz, and for this frequency we present the coand cross-polarization for phi=0º and 90º (Fig.12 (a) and (b)).For showing the circular polarization with circular antenna, we have a test on RHCP and LHCP and the results show the circular polarization at Fig.12 (c) and (d). The antenna gain and efficiency are obtained experimentally in table.1 in details. As shown in Fig.12 (a) and (b), the balun has small effect on antenna pattern in side of the balun however, the bidirectional pattern remains for the antenna.
(a)
(b)(c)
(d)
Fig.12 the measurement of antenna radiation pattern (a) the co- and cross-polarization for phi=0º (b) the co- and cross-polarization for phi=90º (c) the LHCP pattern (d) the RHCP pattern. For (a) and (b) the solid lines show co- polarization and dash lines for cross-polarization.
Table.1 the antenna resonance comparing frequency
gain
efficiency
BW
polarization
2.5 GHz
3.5 dBi
72 %
300 MHz
linear
4.5 GHz
4.5 dBi
78%
360 MHz
circular
5.9 GHz
4.2 dBi
70%
240 MHz
linear
For measurement of the axial ratio we have checked the E-field in both axes and the result is checked for all resonances for a few points around 4.5 GHz with the step of 20 MHz as shown in Fig.13. The antenna has circular polarization radiation at 4.5 GHz and for 2.7 and 5.9 GHz the antenna has linear polarization.
Fig.13 the antenna AR comparing for simulation and experimental
VII.
Conclusion
The first aim of this paper is reducing the frequency miniaturization and obtained the frequency is reduced from 7 to 4.5 GHz for the loop antenna so around 40% miniaturized and for 2.7 GHz we have the return loss of -6 dB which is sufficient for indoor application with more than 64% miniaturization. In comparison with conventional patch antenna, we have around 30% of miniaturization for physical area. The second aim of this paper is making a reconfigurable characteristic which is realized by controlling the effective length. For the third aim, circular polarization is noticed for the proposed antenna, metamaterial loads is implemented as a metasurface for shaping the current distribution which results in circular polarization. Acknowledgment The Authors would like to thank Navid P. Gandji (Michigan Technological University) for his helpful discussions and co-operations.
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