Abstract
A comparative analysis of compact multiband bio-inspired Asymmetric microstrip fed antennas (BioAs-MPAs) is presented in this paper for the first time. The proposed antennas are based on semi-Carica papaya-leaf shaped, semi-Monstera deliciosa-leaf shaped, semi-Vitis vinifera shaped, Defected Ground Structure (DGS) and L-slit techniques. The antennas are built on a 33 × 15 mm2 Rogers duroid 5880 substrate. The modelling equations for resonant frequencies of the proposed arbitrarily shaped radiating patch is based on modified circular patch modelling equations. The semi-Carica papaya-leaf antenna operates at 2.4 GHz and 4.4 GHz, Monstera deliciosa-leaf antenna operates at 2.6 GHz, 4.4 GHz and 5.5 GHz, while Vine-leaf antenna operates at 2.5 GHz and 5.4 GHz. The proposed BioAs-MPAs antennas radiation patterns at E-plane are Bi-directional in all the operating frequencies with suitable X-Pol purity and have Omnidirectional radiation patterns at H-Plane in all the operating frequencies. As a result of the analysis, it was found that each of the bio-inspired structures has its unique merit over the others. Owing to the small size, stable radiation pattern, acceptable gain and high radiation efficiency, the proposed BioAs-MPAs antennas are suitable for ISM band, Bluetooth, Wi-Fi, WiMAX, LTE, UMTS, Sub6 GHz 5 G band, ZigBee and RF-Altimeter used in unmanned aerial vehicle and Aviation industry.
1. Introduction
Antennas for wireless sensor network must be compact and narrowband because of its space and data purity constraint. More so, in airborne applications such as aviation, the antenna should be flat on the body of the airplane; hence, the use of patch antennas. RF altimeter operating at a frequency band of 4.2–4.4 GHz is used in aviation industry to measure the altitude of the airborne and this, of a necessity, should not be interfered by other wireless protocol to ensure the accuracy of the measured altitude, hence the use of narrowband antenna operating at a center frequency of 4.3 GHz. Nonetheless, the majority of the works in the literature concentrated on multiband antennas for WLAN, WiMAX and ISM band applications [1–5]. Slot etching on the ground plane or on the radiating patch has been used in the literature to achieve multi-frequency resonances [1, 6–8]. Metamaterial has also been used for multiband antennas realization as reported by authors in [3, 9, 10].
Furthermore, the authors in [11, 12] have used electromagnetic Band Gap (EBG) and meandering to realize compact multiband antennas. Parasitic loading has also been used by authors in [13–15] for multiband antenna realization. One of the advantages of patch antennas is the availability of different feeding techniques such as coaxial, microstrip, coplanar waveguide (CPW). Coplanar waveguide can be divided into symmetrical and Asymmetrical coplanar waveguide. Due to the benefits presented by CPW such as small radiation leakage, less dispersion, the independence of its characteristic impedance on the thickness of the substrate, uniplanar and ease of integration with other microwave devices, it has been popularly employed as the feeding techniques of patch antennas [16]. For symmetrical CPW, the strip is situated at the center of the two ground planes [5, 17]. On the other hand, Asymmetric CPW usually referred to as asymmetric coplanar strip (ACS) has a strip shifted from the center of the ground plane and usually has its ground plane on one side of the strip [2, 18].
In this work, an Asymmetric microstrip feedline is used to achieve compactness. The contributions of this work are: (1) The proposition of a novel feeding technique called Asymmetric microstrip feeding technique to achieve compactness; (2) proposition of Semi-bioinspired antenna structures for multiband wireless application for antenna size miniaturization; (3) proposition of the resonant frequency prediction formula for arbitrarily shaped patch antenna using modified circular radiating patch; and (4) proposition of the use of L-slit for Impedance matching enhancement and frequency tuning. As far as we know, no one has presented a compact narrow-multiband antenna operating at 2.4 GHz and 4.4 GHz in the literature, which is another contribution of this work. These two bands are important in the aviation industry as WLAN is used for onboard passengers’ Internet browsing and C-band (4.4 GHz) is used for altitude measurement. Therefore, incorporating the two bands in a single antenna element is necessary to reduce the aircraft overall weight and minimize cost. The proposed patch shapes are based on (1) the Papaya-leaf shape first reported in [19] where the authors used a full papaya-leaf shape as the radiating patch for a wideband application; (2) Monstera deliciosa-leaf; and (3) Vine-leaf.
2. Antenna Design and Analysis
The Bio-inspired asymmetric microstrip fed antennas (BioAs-MPAs) proposed in this work are based on a semi-papaya leaf, Semi-Monstera deliciosa-leaf, and Semi-Vine leaf structures having an equal area of 119 mm2 with an L-slit fed using an asymmetric microstrip feedline as shown in Figures 1–3 respectively. A typical Carica papaya-leaf, Monstera deliciosa-leaf, Vine-leaf is as shown in Figures 4(a)–4(c) respectively. There are three stages involved in achieving the final BioAs-MPA structure as shown in Figures 1(a)–1(c), 2(a)–2(c), and 3(a)–3(c) for Carica papaya based, Monstera deliciosa based, and Vine-based patches respectively. In Figure X(a) (X = 1, 2, and 3), the half (semi) shape of the full bio-inspired structures with the full-width ground is first used; then, Defected Ground-Width is performed as shown in Figure X(b) and an L-slit is introduced to the semi-papaya leaf shape in Figure X(b) as shown in Figure X(c).
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The substrate used is Rogers duroid 5880 having a of 2.2, a loss tangent of 0.0009 and a thickness of 1.57 mm. The parameters as depicted in Figure X(c) (X = 1, 2, and 3) is presented in Table 1. The antenna is fed with a 50 Ω feedline and equations (1) and (2) are the modified circular patch predicting modelling equations for the proposed BioAs-MPA antennas which is adapted from [20]. The points coordinate of the proposed structures are presented in Table 2 for repeatability purposes. The coordinates are derived through the graphing of the proposed structures.where; is the effective area of the patch, is the area of the radiating patch, is the effective relative permittivity, is the zeros of the derivative of the Bessel function , is the free space velocity and is the thickness of the substrate.
Using Table 2, the value of the area of the radiating patch can be determined by equation (4);where, “a” represents the starting point which is 1, “N” represents the end point. It can be observed that all the radiating patch has an Area of 119.00 mm. , , and .
3. Result and Discussion
3.1. Reflection Coefficient (S11)
The S11 of the three stages of the BioAs-MPA antennas are presented in Figures 5–7 respectively. It can be observed that the Slitless semi-papaya structure with full-width ground (i.e., ) has two resonance frequencies at 2.4 GHz as the mode and 5 GHz as the mode with a S11 of −12.74 dB and −6.32 dB respectively as shown in Figure 5. This shows that, the antenna structure in stage one is barely suitable for WLAN application. In order to enhance the reflection coefficient at the dominant mode, the ground plane width is defected as shown in Figure 1(b). As can be seen in Figure 6, the reflection coefficient becomes −30.86 dB and −5.5 dB at 2.5 GHz and 5.7 GHz respectively. But, since our target is to achieve multiband antenna, in stage II, a L-slit was introduced to create another resonance at 4.4 GHz as the mode for airborne applications with a reflection coefficient of −26.54 dB and further enhances the reflection coefficient at 2.4 GHz and 5.7 GHz to −45.40 dB and −6.41 dB respectively as shown in Figure 7.
On the other hand, it can be observed that the slitless semi-Vine-leaf structure with full-width ground (i.e., ) has two resonance frequencies at 2.5 GHz as the mode and 5.1 GHz as the mode with a S11 of −16.07 dB and −8.01 dB respectively as shown in Figure 5. In order to enhance the reflection coefficient at dominant mode, the ground plane width was defected just like in the case of papaya leaf as shown in Figure 2(b). As can be seen in Figure 6, the reflection coefficient becomes −41.14 dB and −6.08 dB at 2.6 GHz and 5.6 GHz respectively. Just like the case of papaya leaf, a L-slit was also introduced but it does not result to another resonance, instead, the resonance frequencies are lowered to 2.5 GHz and 5.4 GHz with a reflection coefficient of −32.71 dB and −10.02 dB respectively as shown in Figure 7.
In the same light, it can be observed that the slitless semi-Deliciosa structure with full-width ground (i.e., ) has two resonance frequencies at 2.5 GHz as the mode and 5.1 GHz as the mode with a S11 of −15.15 dB and −10.63 dB respectively as shown in Figure 5. In order to enhance the reflection coefficient at 2.5 GHz, the ground plane width was defected as shown in Figure 3(b). Consequently, as can be seen in Figure 6, the reflection coefficient becomes −21.39 dB, −4.97 dB, −10.35 dB and −4.73 dB at 2.6 GHz, 4.9 GHz, 5.7 GHz and 7.0 GHz respectively. Seeing that this can only operate at 2.6 GHz conveniently, a L-slit is introduced to enhances the impedance matching. The introduction of L-slit results in resonance at 2.5 GHz as the mode, 4.4 GHz as the mode, 5.5 GHz as the mode with a reflection coefficient of −20.26 dB, −33.05 dB, and −19.27 dB respectively as shown in Figure 7.
3.2. Antenna Gain, Radiation Efficiency and Radiation Pattern
The gain and radiation efficiency of the BioAs-MPA antennas are presented in Figures 8–10 for Carica papaya-based antenna, Monstera deliciosa-based antenna, and Vine-leaf based antenna respectively. For Carica papaya patch antenna, the gain is 2.17 dB and 1.73 dB at 2.4 GHz and 4.4 GHz respectively as shown in Figure 8. In the case of Monstera deliciosa-leaf based antenna, the gain is 2.25 dB, 1.46 dB and 4.67 dB at 2.6 GHz, 4.4 GHz and 5.5 GHz respectively as shown in Figure 9. More so, it can be observed in Figure 10 that for the Vine-leaf patch antenna, the gain is 2.23 dB and 3.48 dB at 2.5 GHz and 5.4 GHz respectively.
In terms of the radiation efficiency, the efficiency of the papaya-leaf patch is 97.4% and 93.0% at 2.4 GHz and 4.4 GHz respectively as shown in Figure 8. More so, Figure 9 revealed that, the efficiency of Deliciosa-leaf patch antenna is 97.5%, 86.6%, and 98.3% at 2.6 GHz, 4.4 GHz and 5.5 GHz respectively. The efficiency of the Vine-leaf patch antenna at 2.5 GHz and 5.4 GHz is 97.2% and 99.9% respectively as shown in Figure 10. It can be observed that the gain of the proposed antennas is generally low at 4.4 GHz. This is because the resonance at 4.4 GHz is created through the L-slit and agree with the theoretical analysis of effect of slot in the radiating patch to generate new resonance.
The 2D radiation pattern of the BioAs-MPA antennas in the E-plane and H-plane is presented in Figures 11 and 12 respectively. It is observed that all the proposed antennas have bi-directional radiation patterns in E-plane at all the operating frequencies as shown in Figure 11. The BioAs-MPA antennas proposed have considerable polarization purity at E-plane. The cross polarization of Vine-based antenna is high at 5.4 GHz on the E-plane and this could be traced to the poor impedance matching at this frequency as seen in Figure 7. The H-plane radiation patterns of the proposed antennas are omnidirectional in all the operating frequencies with considerable polarization purity as shown in Figure 12. Hence, the proposed antennas are suitable for wireless applications as listed in Table 3.
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3.3. The Distribution of the Surface Current Density
The distribution of current density of the BioAs-MPA antennas are presented in Figures 13–15 for Carica papaya based antenna, Vine-leaf based antenna, and Monstera deliciosa-leaf based antenna respectively. It can be observed that for Papaya-leaf based antenna, at 2.4 GHz (dominant mode), the surface current density concentrated on the feed branch than the semi-papaya as shown in Figure 13(a). This shows that the feed-branch contributed greatly to the resonance at the papaya dominant mode. But, at 4.4 GHz ( mode), the distribution of the current density is concentrated on the top-edge of the L-slit as shown in Figure 13(b). This shows that, 4.4 GHz resonance is achieved through the introduction of L-slit.
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The current distribution for the Vine-leaf based antenna is presented in Figure 14. It can be observed that both feed branch and the L-slit contribute to the resonance at 2.5 GHz and 5.4 GHz but comparatively small in the case of 5.4 GHz. This explains while the S11 at 5.4 GHz is higher than at 2.5 GHz. More so, for Deliciosa, the surface current at 2.5 GHz is evenly distributed at both the feed-branch and the center of the patch as shown in Figure 15(a). The surface current is concentrated around the L-slit in the case of the mode, which shows that L-slit contributed to the resonance at 4.4 GHz as shown in Figures 15(b) and 15(c) revealed that, the mode is as a result of the contribution of both the feed-branch and the patch.
These analyses show that L-slit helps in improving the impedance matching and create a new resonance at 4.4 GHz in Carica papaya-leaf based antenna and Monstera deliciosa-leaf antenna while it only improves the impedance matching of the Vine-leaf based.
3.4. Comparative Parametric Analysis
The parametric analysis of the Bio-ACS antenna is presented to study the effect of variations on the S11 of each of the proposed Bio-ACS antenna.
3.4.1. Effect of the Width of Ground Plane on S11
(1) Carica papaya based Antenna. It can be observed from Figure 16 that the three resonances are maintained, but the reflection coefficient when is at 5 mm is worst at 2.4 GHz and 4.4 GHz while there is a shift on the higher resonance from 5 GHz to 7 GHz with a reflection coefficient of −23.42 dB. It can be seen also from Figure 16 that the reflection coefficient is better at the lowest (2.4 GHz) and middle resonance (4.4 GHz) when the is 10 mm but starts increasing when is around 15 mm. This implies that, the ground width of an offset microstrip feedline can be used for impedance matching enhancement and frequency tuning.
(2) Monstera Deliciosa based Antenna. It can be observed from Figure 17 that the reflection coefficient when is at 5 mm is worst at 2.4 GHz and 4.4 GHz just like the case of Papaya-leaf based antenna while there is a shift on the higher resonance from 5 GHz to 7 GHz with a reflection coefficient of −24.77 dB. Furthermore, it can be seen from Figure 17 that the reflection coefficient is better at the lowest (2.4 GHz) and middle resonance (4.4 GHz) when the is 10 mm and 15 mm. Nonetheless, there is a shift in the resonant frequency from 5.2 GHz to 6.1 GHz when changes from 15 mm to 10 mm. This shows that the ground plane is an effective means of optimizing the impedance matching as well as frequency tuning of an offset microstrip feedline.
(3) Vine-leaf based Antenna. The S11 ≤−10 dB at equals 5 mm occurred 7.0 GHz as can be seen in Figure 18. It can also be observed that the S11 of equals 10 mm is better compared to when it is 15 mm. It can be observed that though the ground width defection enhances the impedance matching yet the extent of its use is bounded.
3.4.2. Effect of the Length of Ground Plane on S11
(1) Carica papaya based Antenna. It can be observed from Figure 19, the lowest S11 (−45.4 dB) at 2.4 GHz occurred with equals 4 mm and a −27.15 dB at 4.4 GHz. It can be observed that the lowest S11 (−30 dB) at 4.4 GHz occurred at equals 5 mm but at 2.4 GHz, its S11 is −20.18 dB. It is can be seen also that, though the S11 keeps improving as the increases until 4 mm after which it started increasing at both resonant frequencies (2.4 GHz and 4.4 GHz). It can be observed that there is no shift in the resonant frequencies as changes. Hence, ground length only affects the impedance matching of the antenna without tuning the frequency of the proposed Carica papaya based antenna.
(2) Monstera deliciosa based Antenna. In the case of Monstera deliciosa, has a considerable effect in terms of shifting on the resonant frequencies apart from the fundamental frequency (2.6 GHz) as shown in Figure 20. It can be observed that variation in only affect the S11 at 2.6 GHz with the lowest S11 being −32.35 dB when is 4 mm. It can also be observed that equals to 4 mm has the best S11 (−39.34 dB) at the second resonant frequency (4.5 GHz) but the third resonant frequency has S11 >−10 dB at 6.0 GHz as shown in Figure 20. On the other hand, = 6 mm has a S11 ≤−10 dB at 2.6 GHz, 4.4 GHz, and 5.0 GHz with a S11 of −14.86 dB, −28.55 dB and −21.79 dB respectively as shown in Figure 20. At = 3 mm, the antenna resonates at 2.6 GHz, 4.4 GHz and 5.5 GHz with a −20.12 dB, −33.05 dB and −19.2 dB reflection coefficient respectively as shown in Figure 20. It can also be seen from Figure 20 that as increases from 4 mm, the impedance matching keeps decreasing across all the resonant frequencies.
(3) Vine-leaf based Antenna. S11 changes with the variation of , as seen in Figure 21 increases with the increase in . It can be observed that only = 3 mm has S11 ≤−10 dB at 2.5 GHz and 5.3 GHz. It can be observed from Figure 21 that, the maximum value of that can result in S11 ≤−10 dB at 2.5 GHz is 6 mm. This could be traced to the sawtooth effect of the Vine-leaf edge. Though, the S11 at 5.3 GHz is −10.22 dB, yet it can be engaged in 5.2 GHz WLAN in mobile device application where a VSWR of 2.5 is acceptable.
3.4.3. Effect of on S11
(1) Carica Papaya based Antenna. Figure 22 shows the effect of on the reflection coefficient. It can be observed that to achieve the mode resonance (4.4 GHz) must be ≥5 mm otherwise, there would not be middle resonance as can be seen in the case of 3 mm and 4 mm in Figure 22. It can also be observed that can be used to tune the frequency as shown from 5 mm, 6 mm and 6.5 mm but with a trade-off in S11 as presented in the case of = 5 mm. It can also be observed that, does not shift the mode resonance frequency (2.4 GHz).
(2) Monstera deliciosa based Antenna. Figure 23 shows the effect of of Monstera deliciosa on S11. It can be observed that to achieve the middle RESONANCE (4.4 GHz) must be greater than 3 mm otherwise, there would not be mode resonance as can be seen in the case of 3 mm in Figure 23. It can also be observed that can be used to tune the middle resonance frequency as shown in 4 mm, 5 mm, 6 mm, 7 mm and 8 mm but with a trade-off in S11 as shown in the case of = 5 mm and 4 mm. does not affect 2.4 GHz ( mode) resonance frequency which proves that the slot fundamental effect is for the generation and tuning of the mode resonance frequency as well as enhancement of the impedance matching. More so, only = 6 mm results in S11 ≤−10 dB reflection coefficient at 5.5 GHz. Hence, taking as the optimized for Monstera deliciosa-leaf antenna.
(3) Vine-leaf based Antenna. Figure 24 shows that has considerable effect on the mode resonance and the mode resonance frequencies. It can also be observed that to have mode resonance, must greater than 7 mm.
3.4.4. Effect of on S11
Due to the trend observed on the effect of on S11 in Section 3.4.3, only Carica papaya-based antenna is used to examine the effect of on S11. As seen in Figure 25, contributes to the mode resonance just like with an insignificant shifting effect on mode frequency.
It can be observed that as decreases, the mode resonance fades. A 0.5 mm variation in has a tremendous effect on the mode resonance frequency as can be seen in Figure 25.
Summarily, the S11 of Carica papaya-leaf and Vine-leaf based antenna are highly sensitive to the variation in while Deliciosa is less sensitive to at the fundamental ( mode) and mode resonance frequencies. In the case of , the S11 of Carica papaya-leaf and Vine-leaf based antennas is sensitive to variation of but it does not cause considerable shift in the resonance frequencies. Contrarily, the and resonant frequencies of Monstera deliciosa-leaf based antenna changed with a change in . For , only resonance frequency has considerable changes with variation of in the proposed BioAs-MPAs antennas and the same effect is noted in the case of .
4. Comparative Analysis
4.1. Comparative Analysis of the Proposed BioAs-MPA Antennas
Table 3 presents the comparative analysis of the proposed antennas. It can be observed that Carica papaya, and Vine-leaf patch antennas are Dual-band while Monstera deliciosa-leaf antenna is a triband antenna. It can also be noted that papaya-leaf antenna has the lower S11 at both operating frequencies. It can also be observed that the least radiation efficiency of the proposed antennas is 86.6% at resonance of Monstera deliciosa-leaf antenna. The proposed antennas are narrowband multiband antennas with acceptable gain, radiation pattern and efficiency, suitable for mobile wireless applications. The small variation in the resonance frequency is due to variation in the shape of the radiating patch despite their equal patch area .
4.2. Comparative Analysis of the Proposed Antennas with Recent Works in the Literature
Table 4 shows the comparative study of the proposed antennas with the recently reported works in the literature. is the guided wavelength in the substrate at the lowest resonant frequency. It can be observed that only the work reported in [5] combined 2.49 GHz and 4.2 GHz but with a wide impedance bandwidth of 1 GHz which can result into inference with other devices in the environment if used for a sensitive application such as radio altimeter in airborne systems. Though, the results presented in this work are based on simulation using HFSS built on a Finite Element Method (FEM), it is can be seen that the proposed BioAs-MPAs antennas have a compact dimension, good radiation efficiency and gain when compared with several of the works reported in the literature.
5. Conclusion
Compact-size multiband asymmetric-fed bio-inspired antennas called BioAs-MPAs are presented. Three semi-bioinspired antenna structures are presented to achieve compactness and multiband applications for the first time in this work. A novel feeding technique called Asymmetric microstrip feedline has been proposed in this work to achieve compactness. Also, a new modelling equation for predicting resonant frequency of arbitrarily shaped antennas by modification of circular patch modelling equation has been proposed and validated through simulation. The proposed BioAs-MPAs antennas have succinct characteristics such as compactness, good return loss, acceptable gain, stable radiation pattern and efficiency. Hence, the proposed antennas are suitable for wireless sensor network, Bluetooth, RF-Altimeter, WLAN, WiMAX, LTE, Wi-Fi, ISM band and airborne applications, such as airplane and unmanned aerial vehicle (UAV) owing to its compactness and flush-ability into the body of the airplane.
Data Availability
The data supporting the findings of this study are all presented within the article.
Conflicts of Interest
There is no conflicts of interest among the authors.
Acknowledgments
This work was sponsored and supported by the African Union through the Pan African University Institute of Basic Sciences, Technology, and Innovation.