A Slotted Lotus Shaped Microstrip Antenna based an EBG Structure

The objective of this paper is to study intensively the design of a printed slotted patch based lotus shape structure mounted on a dielectric substrate backed with an electromagnetic band Gap (EBG) layer for wideband applications. The dielectric substrate is made of a Roger RT/duroid®5880 layer. An EBG layer is introduced on the back profile of the substrate to provide a high gain bandwidth product over wide frequency bands. The antenna is fed with a novel coplanar waveguide (CPW) structure of a flared geometry; therefore, the ground plane is mounted on the same substrate surface with the patch structure. A conductive trace is introduced at the substrate back from the bottom connected to the CPW through two shoring plates to remove the effects of the EBG layer on the feed structure. The EBG performance and the antenna design methodology are discussed using analytical analyses and numerical parametric studies, respectively. The numerical simulation is conducted using CST MWS Finally; the optimal antenna design is fabricated and measured for validation to be compared to the simulated results. *Corresponding author: Taha A Elwi, Department of Communication Engineering, Al-Mamon University College, Baghdad, Iraq, Tel: +964 771 961 1221; E-mail: taelwi82@gmail.com Received December 15, 2017; Accepted March 20, 2018; Published March 30, 2018 Citation: Elwi TA (2018) A Slotted Lotus Shaped Microstrip Antenna based an EBG Structure. J Material Sci Eng 7: 439. doi: 10.4172/2169-0022.1000439 Copyright: © 2018 Elwi TA. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.


Introduction
Recently, a high research impact has been applied on introducing EBG structures to enhance the performance of the microstrip antennas [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. EBG structures, as a metamaterial, are periodic-like layers of extraordinary surface wave suppression properties with unique features which cannot be found in nature [1]. Based on their exhibited electromagnetic properties, EBG structures were classified as: near zero refractive index materials [2], soft and hard surfaces [3], highimpedance surfaces [4], and artificial magnetic conductors [5]. It is worth to mention that some of these structures have several relative electromagnetic properties [6]. Due to the unique features of the EBG structures, it can be considered as a special type of metamaterials [4,5,9]; where, their interaction with electromagnetic waves exhibit frequency stop bands, passbands, and band gaps. The concept of EBG structures were originated from the optical domain in 1987 [10] as the terminology of photonic band gap.
Since EBG structures are periodic layers of dielectric or metallic elements arranged as in 1D, 2D, or 3D manners, they provide multiple band gaps based on the unit cell periodicity and the individual unit cell resonance as well [11]. The EBG performance is highly affected by the macroscopic and microscopic resonances of a periodic structure [12,13]. Therefore, the macroscopic resonance, Bragg resonance, is governed by the periodicity; while, the lattice resonance controls the microscopic resonance, the unit cell characteristics, to be called the Mie resonance [8]. Furthermore, the coinciding of the two resonances leads to maximize the structure band gap width. Based on the structure properties and the wave polarization, one of the stop bands dominate over the other bands [4]. In which, at the stop band, the structure reflects back all incident waves, while at other bands acts as a transparent medium.
According to the literature survey, the EBG structures are classified based on their application domains as in the filter designs, gratings, frequency selective surfaces (FSS) [14], photonic crystals [15] and photonic band gaps (PBG) [11]. Moreover, the EBG structures are defined as artificial periodic or sometimes non-periodic objects to prevent and/ or assist the incident wave propagation. Besides that, the EBG structures possess high impedance properties such as artificial magnetic conductors. For example, mushroom-like EBG structures exhibits high surface impedances in both TE and TM modes. In which, illuminating the EBG surface creates an in-phase reflection coefficient. Moreover, soft and hard surfaces operate as EBG structures where are identified as frequency wave number planes [4]. Therefore, such interesting features led to a novel wide range of applications in the antenna engineering researches and industries.

Antenna Design and Discussions
The most fundamental challenges to be overcome in the proposed antenna design are the antenna size reduction in flat profile, frequency allocation must be subjected according to the FCC recommendations, high gain enough for medium and long communication distances to be over 2 dB, and |S 11 | should be less than -10 dB over a wide range of frequencies.
In this section, the performance of the proposed EBG structure in both analytical and numerical aspects is introduced firstly. Whereas, the electromagnetic properties of the proposed EBG structure can be investigated through evaluating the propagation constant, reflection phase and dispersion characteristics of a single unit cell structure. However, in order to realize the enhancements in the introduced microstrip antenna, the author postpone introducing the design methodology for overall structure after EBG structures analysis.
Next, the microstrip patch geometry is derived from a triangular shape. The operation mechanism of each design parameter is optimized in order to determine its initial value. The aime of the proposed design methodolgy is to maximize the antenna bandwidt in which |S 11 |<-10. The antenna substrate is considered a Roger RT/duroid ® 5880 layer of 0.5 mm thickness (h) and ε r =2.2 with tanδ=0.0009. The overall antenna dimensions are 32 mm × 28 mm in length and width, respectively. In the proposed design, a 50 Ω feed line is connected to the patch and printed on the substrate. Therefore, the antenna design methodology is presented by the flow chart that is seen in Figure 1.

EBG construction and performance
The proposed EBG structures are printed on the back panel of the substrate for two purposes: Suppresses the surface waves on the substrate and reduces the interference of the surface current on the ground plane edges [16]. The EBG layer is constructed of 6 × 7 array of square metal pads, to create 5 × 6 slots, mounted on a rectangular area of 24 mm × 28 mm (Figure 2a). The dimensions of the proposed unit cell, (Figure 2b), are 4 mm × 4 mm. The EBG structure is based on conductive square pads (p) of 3.6 mm × 3.6 mm size spaced with a gap (g) 0.4 mm mounted on the substrate.
Such array creates a capacitive gap (Cgap) between neighboring cells which can be calculated using the following formula [16].
The proposed EBG structures exhibit a capacitive behavior, so works as high pass filter that allows passing the high frequencies and prevent the lower bands. On the other hand, each unit cell can be considered unbalanced circuit because the inductive absence where the influence of the capacitive reactance on the EBG width is higher than its length for the surface current.
One of the main factors which control the proposed EBG performance is the number of slots (m × n) in their array. For this analysis, the number of slots along the length (m) is considered 5, and the number of slots along the width (n) is considered 6. However, it is worth to mention that the EBG slots repetition on the width influence has been selected depending on the size of proposed EBG structure and the maximum size of the antenna and the fabrication limitation. So, the capacitive reactance between the unit cells on the width has higher suppression for surface currents.
For an additional validation, the dispersion diagram based on CST MWS and reflection phase based on Agilent ADS are realized. The Eigen mode simulation of the CST MWS formulations is invoked to localize the band-gap, the natural resonances and the dispersion properties that are exhibited by the proposed EBG unit cell. The dispersion characteristics of the TE and TM modes can be performed at the First Brillion Zone (FBZ) [17] in the crystal lattice (Γ, χ, M) vertex. This is by considering a triangle of two equidistant sides (Γ to X and X to M), while, the other longer side is (M to Γ). Each side can be described by a dispersion characteristics graph that can be merged in one continues dispersion diagram [18][19][20].
From the resulted dispersion diagram (Figure 3), the horizontal axis represents phase differences along FBZ boundaries. However, it is seen in the frequency range of interest that the structure supports a fundamental TM mode which electric filed is mostly longitudinal  creation of several concatenated TL and layouts embedded as sub networks within a single design. Finally, the analytical expressions based on the derived analysis in previous study [18] are compared to the results of CST and ADS simulations.
It is found that the unit cell shows reflection phase behavior between the 8 GHz and 12 GHz in Figure 5a. Nevertheless, the electromagnetic properties including ε r and µ r for the proposed unit cell are evaluated and presented in Figure 5b with and without substrate. It is found that the introduction of the substrate layer has a significant effect on the retrieved ε r and µ r due to the change of the guided wave length [10].

Antenna design methodology
The antenna design methodology, with the parameters depicted in this section, has been modeled step by step. The antenna exhibits a wideband response with an impedance bandwidth of |S 11 |<-10 dB. Therefore, the antenna design derivation steps are presented as: The first design modeling (Antenna 1): Figure 6 shows the geometry of the microstrip antenna (Antenna 1) based on a triangular geometry. The antenna design is started from an equilaterial triangular patch backed with a full ground plane of dimensions are depicted in Table 1. The S 11 spectrum is evaluated by CST MWS as seen in Figure  7. It is found that such antenna design provides narrow bandwidths around multiple frequency bands.
The S 11 spectra of the Antenna1, shown in Figure 8, are carried out of different patch Widths (Wt). It is found that Wt variation has an effect on the antenna bandwidth as well as the center frequency. While, Figure 9 shows the S 11 spectra of the Antenna 1 after varying the length of the feed line (Lt). It is observed that the Lt variation has insignificant effects on the center frequency.
The S 11 changes with different feed line widths (Wc) are shown in Figure 10. It is shown the matching can be improved by increasing Wc.
to the direction of the propagation, followed by TE mode which is predominantly transverse to the direction of propagation. Consequently, any propagation can be considered prohibited in TM mode under 8 GHZ while in TE mode under 12 GHz.
The propagation constant (γ) for the proposed unit cell can be expressed with the absence of the inductance component as: where α is the attenuation loss constant and β is the phase constant. Such γ involves only the conductor and the dielectric losses in R and G, respectively, are dispersive values in dB and given in previous study [21] as: where µ is permeability of the conductor, σ is conductivity, t conductor thickness, and λ g is the guide wavelength.
ADS simulation is carried out for characterizing the reflection characteristics in terms of phase in order to validate the effects of adding the EBG pads on the antenna performance. From Figure 4, the simulation is evaluated based on the transmission line circuit via      The second design modeling (Antenna 2): In this section, the ground plane with antenna 1 is changed to a partial conductive geometry as seen in Figure 11 to be called Antenna 2. By optimizing the ground plane dimensions as shown in Table 2, a significant enhancement is     achieved in the bandwidth; however, with poor matching. The partial ground plane removal reduces the back-lobe radiation of the proposed antenna by suppressing the surface wave diffraction from the edges of the ground plane [12]. Figure 12 shows the S 11 of the Antenna 2.
A sweep of the parameter (Lg) has been taken, where Figure 13 shows samples of the S 11 spectra of Lg sweep. It is clearly shown that the variation of Lg has a little shift in the center frequency.

The second design modeling (Antenna 3):
The Antenna 2 is developed to the model in Antenna 3 as seen in Figure 14 by using a coplanar transmission line with flared ground planes from to connect the SMA port to the patch continously. Such novel feeding desing is adopted to maximize the matching between the input impedance of the patch and the SMA over a wide range of frequencies. Nevertheless, the flared ground planes act as matching circuits as well as reflectors in which the electromagnetic radiation may towarded to the end-fire direction. The geometrical dimensions of the antenna and the relative S 11 spectra are shown in Table 3 and Figure 15, respectively.
This section represents the effects of changing the reflector length (Lf) on the bandwidth. From Figure 16, it can be seen that increasing Lf restricts the bandwidth.
The best separation distance between the reflectors and the transmission line can be localized to maximize the matching. The effects of separation distance on the S 11 spectra are monitored by changing it as 0.05 mm, 0.1 mm, and 0.15 mm as seen in Figure 17. It is found that the coupling gap exhibits a significant effect on the antenna bandwidth.

The second design modeling (Antenna 4):
A further modification is considered in this design as presented in Antenna 4 by including triangular slots on the patch to create new resonance modes by increasing surface current paths on the patch. Then, the surface electrical area of the patch within the same physical area is increased. Figure 18 and Table 4 show the antenna configuration and design parameters, respectively. In addition, the internal areas of the coplanar transmission line reflectors are etched to improve the matching impedance of the antenna as shown in Figure 19. Furthermore, removing the internal area of the coplanar transmission line reflectors insures the avoidance of any possibility of the radiation coupling effects            due to the current motion on the conductor surface [13] that may led to side radiation lobs.
A sweep of the parameter (Wfr) has been taken in to account. Figure 20 shows samples of the return loss responses of the sweep of the fractal width Wfr. It is obvious that the variation of the width has insignificant effects on the center frequency position only. It is worth to mention that the surface current of the antenna with solid reflector is mainly distributed along the edges of the reflector as seen Figure 22. Therefore, both reflectors are etched to improve the directivity and reduced the side lobes without extending the antenna dimensions.

Design the reference antenna with shorting plates (Antenna 5):
In this section, the Antenna 4 are modified to Antenna 5 by adding shorting plates to the coplanar waveguide to work as matching circuits as shown in Figure 23. The antenna dimensions and the S 11 spectrum are presented in Table 5 and Figure 24, respectively.
The return loss responses of the modeled antenna, shown in Figure  25, have been carried out for different sizes of the shorting plate width (Ws) as it is shown in Table 6. It is obvious that the variation of the width has insignificant effects on the center frequency position.

Design the reference antenna with EBG structures (Antenna 6):
Finally, the EBG-defects are introduced, see Figure 26, to the partial ground plane of Antenna 5 to improve the antenna matching. Excellent enhancements are found in the antenna directivity, gain and beamwidth due to suppressing the surface waves. These EBG structures on the ground plane are defected as square pads to enhance the tangential component of the electric field and the broadness the antenna radiation toward the end fire. For the proposed antenna design, Antenna 6, all      the related parameters are described in Table 6. The S 11 spectrum is presented in Figure 27.
The effects changing the EBG pad dimensions on the S 11 spectrum is depicted in Figure 28. As can be observed, the best bandwidth enhancement is found when P=3.6 mm and g=0.4 mm.

Effects of the substrate permittivity on the proposed antenna S 11 :
The consideration of the effect of substrate permittivity on S 11 spectrum is conducted. From Figure 29, it can be seen the Roger RT 5880 with ε r =2.2 and tanδ=0.00134 provides a wider bandwidth than other substrates within the same thickness. Therefore, under this condition, the Roger RT 5880 is used for the proposed antenna.

Radiation patterns of the proposed antenna:
The objective of this section is to study the antenna radiation properties using CST MWS as shown in Figure 30. The radiation patterns of the antenna are illustrated in the E-plane (XZ-plane at θ=0) and H-planes (YZ-plane at ф=90). It is clearly seen that the radiation at 4 GHz is half wave dipole       Gap between two EBG neighboring cells G 0.4    like whereas the antenna produces directional broadside pattern at 6 GHz. It can be noticed that the radiation pattern of the antenna design is toward the end-fire direction at 8 GHz to 10 GHz. This is accomplished by forcing the radiation to be directed along the end-fire by applying the flared reflectors. Nevertheless, the introduction of the EBG structure is to reduce the radiation leakage from the ground plane and providing more focusing toward the tangential radiation. However, the antenna shows broad side radiation at 14 GHz and the radiation goes back to be end fire at 17 GHz.

Antenna Fabrication and Measurements
A wideband antenna is generally fabricated by photo-lithographic chemical etching method. Photo-lithographic method produces highly accurate etched patterns [9]. The fabrication accuracy is very critical whereas any errors in the fabrication could shift the resonant frequency [10]. Photo-lithographic method is a chemical etching process which removes the unwanted metal regions of the metallic layer [8]. The fabricated prototype is depicted in Figure 31.
The performance in terms of S 11 of the implemented prototype is tested and measured then compared against the numerical simulations as seen in Figure 32a. The HFSS simulation is invoked as a further validation for the obtained results from CST MWS before the antenna fabrication. A good agreement between the simulated and measured S 11 spectra is achieved after invoking an Agilent N5230A Vector Network Analyzer (VNA). The measured S 11 of the fabricated antenna shows a slight frequency shift with insignificant the S 11 magnitude change. This may occur due to the unavoidable manufacturing tolerances. The far-field radiation patterns of the principal planes (E and H) are measured in a fully equipped anechoic chamber. The proposed antenna is placed on a positioner and aligned to a horn antenna with adjustable polarization. In Figure 32b and 32c, the far-field radiation patterns of the fabricated prototype in the E-and H-planes are provided at 4 GHz with a gain of 5 dBi [22][23][24][25][26][27][28][29][30][31][32][33][34][35][36].

Conclusion
In this paper, an intensive systematic study based on a parametric numerical simulation supported by both analytical derivation and measurements to design a lotus shaped microstrip antenna for wideband applications. The proposed antenna is constructed from a microstrip patch of lotus geometry defected with slots. The patch structure is fed with a novel flared CPW; where, the ground plane on the same substrate surface of the patch. The antenna ground plane is based on a partial defected EBG pads. The proposed antenna performance is tested numerically using CSTMWS and HFSS, then, compared against measurements for validation. It is found that the defected ground plane has a significant effect on the antenna bandwidth with excellent gain enhancements at different frequencies. This observation is due to the ability of the EBG defects on suppressing the surface waves on the substrate. On top of that, the flared ground plane edges of the CPW structure provides a wide range of matching impedance that matches the antenna bandwidth over a wide range of frequencies. Therefore, the antenna bandwidth increased from 7.8 GHz up to 15 GHz and a matched frequency mode, |S 11 |<-10 dB, at 4 with 5 dBi boresight gain.