Implications of Metamaterial on Ultra-Wide Band Microstrip Antenna Performance

Abstract: This study is addressing the slotted ring resonator effect on the performance of the ultra-wide band (UWB) microstrip antenna. Two types of metamaterial with double slotted ring resonators (SRR), circular (C-SRR) and square (S-SRR), are studied and implemented on back of the antenna. The design examines the effect of the number of the SRR and its position with respect to the antenna’s ground plane and the rotation of the inner and outer C-SRR rings on different antenna characteristics. The dimensions of the antenna are 45 mm × 31 mm × 1.27 mm. The implementation of the SRR increased the antenna bandwidth to cover the range from 2.2 GHz to 9.8 GHz with rejected bands and frequencies. Antenna simulated characteristics like return loss, maximum gain and radiation pattern are obtained utilizing HFSS. The return loss measurement and the VSWR of the antenna with all SRR configuration studied are in good agreement with simulated results.


Introduction
Antennas are considered significant elements of any wireless communication system. An antenna is defined as a metallic structure used for transmitting and receiving signals or radio waves [1,2]. One of the most popular and utilized antenna types is the microstrip antenna that was first presented in 1950. The microstrip antenna is very prominent due to its small size, low price, simple structure, ease of fabrication and its capability to operate on multiple bands of frequencies; contrarily, it has some significant impediments such as very narrow frequency bandwidth, low efficiency, low power, false feed radiation and poor polarization purity [2,3].
With the high number of wireless devices in information epoch which depends on information transmission with higher data rate, provision security to consumers and business, increase the number of users, the importance of wideband wireless communication technology has increased rapidly [4,5]. UWB is defined as the technology of transmitting data over a broad bandwidth; with wireless transmission of −10 dB bandwidth is more than 25% of a center frequency or −10 dB bandwidth equal or larger than 1.5 GHz [6][7][8]. In 2002, the Federal Communication Commission (FCC) declared the authorization of using the unlicensed frequency band between 3.1 GHz to 10.6 GHz for commercial communication applications. FCC reserved bandwidth of 7.5 GHz for UWB due to its importance [8,9]. Due to the necessity of a low-cost solution for microwave communication systems and the growth in wireless applications, the idea of UWB microstrip antennas was getting noticed by researchers. Antenna researchers and designers have focused on variety of challenges in order to overcome the disadvantages of microstrip antennas. Farhood et al. [10] presented a hexagonal-shaped patch antenna of added two capacitive loaded line resonators (CLLRs) on the ground plane to achieve UWB bandwidth between 1.1 GHz to 10.69 GHz. Their antenna covers the UWB band from 3.1 GHz to 10.6 GHz and frequencies 2.4 GHz and 9.1 GHz for Bluetooth and radar applications. The designed antenna has in UAEU labs will be discussed. It is expected that SRR will increase antenna's bandwidth and provides the possibility to transmit or reject specific frequencies bands based on SRR design parameters.

Antenna Design
A UWB microstrip antenna of circular patch with elliptical slot rings was designed. This antenna consists of a printed circular patch modified with elliptical slot rings excited by a rectangular edge-fed microstrip line. The circular patch design is selected for outer dimensions of antenna design due to its excellent radiation efficiencies, impedance bandwidth and omni-directional far-field beam pattern [26]. In this antenna, Rogers 5880-LZ substrate of a length 45 mm and a width of 31 mm is used with a dielectric constant of ε r = 1.96, substrate thickness h = 1.27 mm and a loss tangent of 0.0009. Moreover, a partial conducting ground plane with a length of 11 mm and a width of 31 mm is implemented; the ground plane length of 11 mm was selected after performing several parametric simulations to produce the best wideband results. Fundamentally, three lunar slots are employed on the patch's circular shape to reduce the antenna reflection loss (S 11 ) and enhance the VSWR. Antenna's detailed geometry is demonstrated in Figure 1, Tables 1 and 2. Crystals 2020, 10, x FOR PEER REVIEW 3 of 23 are fabricated in UAEU labs will be discussed. It is expected that SRR will increase antenna's bandwidth and provides the possibility to transmit or reject specific frequencies bands based on SRR design parameters.

Antenna Design
A UWB microstrip antenna of circular patch with elliptical slot rings was designed. This antenna consists of a printed circular patch modified with elliptical slot rings excited by a rectangular edgefed microstrip line. The circular patch design is selected for outer dimensions of antenna design due to its excellent radiation efficiencies, impedance bandwidth and omni-directional far-field beam pattern [26]. In this antenna, Rogers 5880-LZ substrate of a length 45 mm and a width of 31 mm is used with a dielectric constant of ε r = 1.96, substrate thickness h = 1.27 mm and a loss tangent of 0.0009. Moreover, a partial conducting ground plane with a length of 11 mm and a width of 31 mm is implemented; the ground plane length of 11 mm was selected after performing several parametric simulations to produce the best wideband results. Fundamentally, three lunar slots are employed on the patch's circular shape to reduce the antenna reflection loss ( S 11 ) and enhance the VSWR. Antenna's detailed geometry is demonstrated in Figure 1, Tables 1 and 2.  The parametric simulation was conducted on a ground length ranges between 8 mm-13 mm to achieve optimal design performance. Figure 2 shows the return loss with the ground plane variations, which in return affects the resonant frequency. As a consequence of the return loss value and UWB requirements, 11-mm length of the ground plane is noticeable to be the most appropriate value with  The parametric simulation was conducted on a ground length ranges between 8 mm-13 mm to achieve optimal design performance. Figure 2 shows the return loss with the ground plane variations, which in return affects the resonant frequency. As a consequence of the return loss value and UWB requirements, 11-mm length of the ground plane is noticeable to be the most appropriate value with Crystals 2020, 10, 677 4 of 24 a frequency range between 3.5 GHz and 9 GHz; the best return loss obtained is −40 dB at 4.6 GHz, as illustrated in Figure 3. The VSWR of this design is less than two for the impedance bandwidth between 3.5 GHz and 9 GHz. The maximum gain obtained is almost 5 dB in the UWB band, as shown in Figure 4. in Figure 4.
Moreover, the radiation pattern is another essential antenna parameter; The proposed antenna's radiation pattern at different frequencies of 4 GHz, 6 GHz, 8 GHz and 10 GHz in both E-and Hplanes are presented in Figure 5 at Phi = 0° and Phi = 90°. Obviously, the radiation pattern is almost omni-directional at Phi = 0° plane. The proposed antenna with 11 mm partial ground was fabricated and displayed in Figure 6. The return loss measurement shows that the antenna has a frequency bandwidth between 3.5 GHz and 8.4 GHz and a return loss up to −38 dB at 4.2 GHz, Figure 7. The simulated and measured results are in good agreement with a compression in the measured results between 5 GHz to 6 GHz. The small variation between the measured and simulated results can be attributed to the use of SMA-N adapter during the VNA calibration process. Overall, both results have acceptable performance over the design range.    Moreover, the radiation pattern is another essential antenna parameter; The proposed antenna's radiation pattern at different frequencies of 4 GHz, 6 GHz, 8 GHz and 10 GHz in both E-and Hplanes are presented in Figure 5 at Phi = 0° and Phi = 90°. Obviously, the radiation pattern is almost omni-directional at Phi = 0° plane. The proposed antenna with 11 mm partial ground was fabricated and displayed in Figure 6. The return loss measurement shows that the antenna has a frequency bandwidth between 3.5 GHz and 8.4 GHz and a return loss up to −38 dB at 4.2 GHz, Figure 7. The simulated and measured results are in good agreement with a compression in the measured results between 5 GHz to 6 GHz. The small variation between the measured and simulated results can be attributed to the use of SMA-N adapter during the VNA calibration process. Overall, both results have acceptable performance over the design range.    Moreover, the radiation pattern is another essential antenna parameter; The proposed antenna's radiation pattern at different frequencies of 4 GHz, 6 GHz, 8 GHz and 10 GHz in both E-and Hplanes are presented in Figure 5 at Phi = 0° and Phi = 90°. Obviously, the radiation pattern is almost omni-directional at Phi = 0° plane. The proposed antenna with 11 mm partial ground was fabricated and displayed in Figure 6. The return loss measurement shows that the antenna has a frequency bandwidth between 3.5 GHz and 8.4 GHz and a return loss up to −38 dB at 4.2 GHz, Figure 7. The simulated and measured results are in good agreement with a compression in the measured results between 5 GHz to 6 GHz. The small variation between the measured and simulated results can be attributed to the use of SMA-N adapter during the VNA calibration process. Overall, both results have acceptable performance over the design range.    Moreover, the radiation pattern is another essential antenna parameter; The proposed antenna's radiation pattern at different frequencies of 4 GHz, 6 GHz, 8 GHz and 10 GHz in both E-and Hplanes are presented in Figure 5 at Phi = 0 • and Phi = 90 • . Obviously, the radiation pattern is almost omni-directional at Phi = 0 • plane. The proposed antenna with 11 mm partial ground was fabricated and displayed in Figure 6. The return loss measurement shows that the antenna has a frequency bandwidth between 3.5 GHz and 8.4 GHz and a return loss up to −38 dB at 4.2 GHz, Figure 7. The simulated and measured results are in good agreement with a compression in the measured results between 5 GHz to 6 GHz. The small variation between the measured and simulated results can be attributed to the use of SMA-N adapter during the VNA calibration process. Overall, both results have acceptable performance over the design range.

Metamaterials Design
The new idea of structured materials known as metamaterials was studied to benefit from its functionality and behavior. This material can produce new types of materials with negative electric permittivity, negative magnetic permeability and a negative refractive index [27]. Metamaterials are classified according to the properties of electric permittivity ε and magnetic permeability µ into four groups [28]: (1) double-positive (DPS) materials are materials with positive permittivity ε > 0 and positive permeability µ > 0. Most of the materials in nature such as dielectrics belong to DPS; (2) epsilon negative (ENG) materials are materials that have negative permittivity ε < 0 and positive permeability µ > 0 like plasmas. For microwave applications, thin metal wires are used to produce ENG material; (3) Mu-negative (MNG) materials are materials with positive permittivity ε > 0 and negative permeability µ < 0. Gyrotropic material is an example of a material that has negative permeability, in addition, split-ring resonator (SRR) is the most widely used MNG material; (4) double negative (DNG) materials are materials that are not available in nature so it should be produced artificially. In this class both permittivity and permeability are negative ε < 0 and µ < 0. SRR material with MNG properties was used extensively in antenna and microwave applications. In 1999, John Pendry proposed the double slotted ring resonator structure, where both rings are split [29]. SRRs have many geometric shapes, but in this study, the analysis will be concentrated on circular and square SRR.
A unit cell of double SRR structure consists of two concentric metallic rings with slots (gaps) at opposite sides; the two rings have a small separation between them and are made of nonmagnetic metal such as copper [30,31]. Exciting the SRR by a time-varying electromagnetic wave, with a magnetic field (H-field) is perpendicular to the plane of the SRR, this induces a current in the two rings. The two gaps on the opposite side prevent current flow around the rings but allow an induced current to flow on the ring surface and pass from one ring to the other as a displacement current through the capacitive split between the two rings. The high value of distributed capacitance on the gap between the two rings, which store the same amount of charges, but an opposite sign at both sides of the gap, prompts strong coupling between the two rings and lower the resonant frequency. The total capacitance of an SRR structure is the series distributed capacitance on the two rings and the two gaps capacitance [31][32][33][34][35][36]. Moreover, SRR inductance is specified according to the length of the rings and it is becoming larger when ring length increased; due to SRR capacitance and inductance, it behaves as a resonant LC circuit [34,35,37]. Figure 8 shows how the current flow on circular and square SRR.

Metamaterials Design
The new idea of structured materials known as metamaterials was studied to benefit from its functionality and behavior. This material can produce new types of materials with negative electric permittivity, negative magnetic permeability and a negative refractive index [27]. Metamaterials are classified according to the properties of electric permittivity and magnetic permeability into four groups [28]: (1) double-positive (DPS) materials are materials with positive permittivity > 0 and positive permeability > 0. Most of the materials in nature such as dielectrics belong to DPS; (2) epsilon negative (ENG) materials are materials that have negative permittivity < 0 and positive permeability > 0 like plasmas. For microwave applications, thin metal wires are used to produce ENG material; (3) Mu-negative (MNG) materials are materials with positive permittivity > 0 and negative permeability < 0 . Gyrotropic material is an example of a material that has negative permeability, in addition, split-ring resonator (SRR) is the most widely used MNG material; (4) double negative (DNG) materials are materials that are not available in nature so it should be produced artificially. In this class both permittivity and permeability are negative < 0 and < 0. SRR material with MNG properties was used extensively in antenna and microwave applications. In 1999, John Pendry proposed the double slotted ring resonator structure, where both rings are split [29]. SRRs have many geometric shapes, but in this study, the analysis will be concentrated on circular and square SRR.
A unit cell of double SRR structure consists of two concentric metallic rings with slots (gaps) at opposite sides; the two rings have a small separation between them and are made of nonmagnetic metal such as copper [30,31]. Exciting the SRR by a time-varying electromagnetic wave, with a magnetic field (H-field) is perpendicular to the plane of the SRR, this induces a current in the two rings. The two gaps on the opposite side prevent current flow around the rings but allow an induced current to flow on the ring surface and pass from one ring to the other as a displacement current through the capacitive split between the two rings. The high value of distributed capacitance on the gap between the two rings, which store the same amount of charges, but an opposite sign at both sides of the gap, prompts strong coupling between the two rings and lower the resonant frequency. The total capacitance of an SRR structure is the series distributed capacitance on the two rings and the two gaps capacitance [31][32][33][34][35][36]. Moreover, SRR inductance is specified according to the length of the rings and it is becoming larger when ring length increased; due to SRR capacitance and inductance, it behaves as a resonant LC circuit [34,35,37]. Figure 8 shows how the current flow on circular and square SRR.
SRR, circular and square, behaves as an LC resonator that can be excited by an electromagnetic field [38]; the equivalent LC circuit model is displayed in Figure 9; L s is the resonator self-inductance and the two SRR rings have the capacitance in halves, C 0 2 ⁄ [39]. SRR structure's total capacitance is the series capacitance of its two rings [35].  SRR, circular and square, behaves as an LC resonator that can be excited by an electromagnetic field [38]; the equivalent LC circuit model is displayed in Figure 9; L s is the resonator self-inductance and the two SRR rings have the capacitance in halves, C 0 /2 [39]. SRR structure's total capacitance is the series capacitance of its two rings [35]. From the equivalent circuit of the SRR, it can be noticed that the structure acts as an LC circuit with the resonant frequency specified by the total inductance and capacitance of the SRR structure [40]. In the next analysis, the designing equations and HFSS analysis for circular and square SRR are presented. Figure 10 shows the schematic view of circular SRR and its resonance frequency is expressed as:

Circular SRR (C-SRR) and Square SRR (S-SRR) Design
where C 0 is the distributed capacitance, C g is the gap capacitance and L is the self-inductance of the rings.
The resonant frequency of square SRR, demonstrated in Figure 11, is given by [41,42]: L T is the total inductance and C eq is the total equivalent capacitance. For detailed C-SRR and S-SRR design equations, refer to references [29,34,35].  From the equivalent circuit of the SRR, it can be noticed that the structure acts as an LC circuit with the resonant frequency specified by the total inductance and capacitance of the SRR structure [40]. In the next analysis, the designing equations and HFSS analysis for circular and square SRR are presented.
3.1. Circular SRR (C-SRR) and Square SRR (S-SRR) Design Figure 10 shows the schematic view of circular SRR and its resonance frequency is expressed as: where C 0 is the distributed capacitance, C g is the gap capacitance and L is the self-inductance of the rings. From the equivalent circuit of the SRR, it can be noticed that the structure acts as an LC circuit with the resonant frequency specified by the total inductance and capacitance of the SRR structure [40]. In the next analysis, the designing equations and HFSS analysis for circular and square SRR are presented. Figure 10 shows the schematic view of circular SRR and its resonance frequency is expressed as:

Circular SRR (C-SRR) and Square SRR (S-SRR) Design
where C 0 is the distributed capacitance, C g is the gap capacitance and L is the self-inductance of the rings.
The resonant frequency of square SRR, demonstrated in Figure 11, is given by [41,42]: L T is the total inductance and C eq is the total equivalent capacitance. For detailed C-SRR and S-SRR design equations, refer to references [29,34,35].  The resonant frequency of square SRR, demonstrated in Figure 11, is given by [41,42]: Crystals 2020, 10, x FOR PEER REVIEW 8 of 23 Figure 11. Schematic view of a square split-ring resonator.

Circular and Square SRR Design Analysis
In order to analyze the behavior of SRR unit cell, all structures are designed using the same geometry in [36,41] and printed on Rogers 5880-LZ substrate with a dielectric constant of ε r = 1.96, with dielectric loss tangent of tanδ = 0.0009 and substrate thickness of 1.27 mm. Parameters values of Figure 11. Schematic view of a square split-ring resonator. L T is the total inductance and C eq is the total equivalent capacitance. For detailed C-SRR and S-SRR design equations, refer to references [29,34,35].

Circular and Square SRR Design Analysis
In order to analyze the behavior of SRR unit cell, all structures are designed using the same geometry in [36,41] and printed on Rogers 5880-LZ substrate with a dielectric constant of ε r = 1.96, with dielectric loss tangent of tan δ = 0.0009 and substrate thickness of 1.27 mm. Parameters values of circular and square SRRs are shown in Table 3. The resonance behavior of the circular SRR, in Figure 12, has resonant frequencies at 3.6246 GHz, 13.5906 GHz and 19.7908 GHz, as displayed in Figure 13. It is noticeable that SRR has a resonance at different frequencies where it has a good transmission. A jumping phenomenon occurs at the resonant frequency where there is a phase change in (S 11 ), which confirms some material characteristics such as negative permeability. Figure 14 shows the behavior of the magnetic permeability of the proposed circular SRR. It is clearly shown that the permeability takes negative values at the transmission parameter (S 21 ). The surface current density for the circular SRR at the first resonant frequency 3.6246 GHz is shown in Figure 15; it is observed that most the current is concentrated at the inner side of each ring opposite to its gap and the same figure displays the direction of the surface current that flows in the same direction, clockwise, for the two rings.                The next unit cell to be analyzed is the S-SRR, Figure 16, which has two resonant frequencies at 4.975 GHz and 17.6226 GHz, as displayed in Figure 17. At the transmission parameter S 21 , the permeability of the square SRR is having a negative value, Figure 18. Metamaterial characteristic is shown clearly in the permeability behavior of changing from positive to negative values at specific frequency. The current concentration is at the inner side of each ring opposite to the gaps at resonant frequency 4.975 GHz, and the flow of the current for the two rings is in the same direction counter-clockwise, Figure 19.

Implementation of Circular-SRR
The UWB circular microstrip antenna with elliptical rings was combined with C-SRR to improve the antenna return loss and increase the bandwidth. The experiment started with implementing the same C-SRR with dimensions detailed in Table 3, on the back of the antenna (ground side of the substrate). Beginning with one C-SRR at the center (origin (0, 0)), then the number of C-SRRs are increased gradually; by this addition of C-SRRs, arrays of 3 × 2 and 3 × 3 are created. Figure 20 shows the implementations of different numbers of C-SRR, and the results are presented in Table 4 and Figure 21.
substrate). Beginning with one C-SRR at the center (origin (0, 0)), then the number of C-SRRs are increased gradually; by this addition of C-SRRs, arrays of 3 × 2 and 3 × 3 are created. Figure 20 shows the implementations of different numbers of C-SRR, and the results are presented in Table 4 and Figure 21.
The return loss ( S 11 ) of the circular antenna affected by increasing C-SRRs number was noticeable for 3 × 3 array while the lower numbers of C-SRRs have the no effect or small change enhancement occurs by implementing 3 × 3 C-SRR on the back of the antenna with a one-millimeter separation between each SRR unit, Figure 22. The effect of C-SRR on antennas return loss is shown in Figure 23. The introduction of the SRR resulted in a shift of the UWB start at 2.8 GHz and improved return loss over the entire band. The antenna's maximum gain is presented in Figure 24; it is almost 5 dB. The antenna's radiation pattern with implemented C-SRR is examined for different frequencies of 4, 6, 8 and 10 GHz with φ = 0° and 90°; it is noticeable that its radiation pattern has maintained omnidirectional behavior, Figure 25.
The proposed circular microstrip antenna with an array of 3 × 3 C-SRR is fabricated in the lab and presented in Figure 26. The frequency bandwidth of the antenna is between 2.9 GHz to 8.8 GHz, Figure 27, with VSWR less than 2 for antenna's impedance bandwidth. The simulated and measured results are in good agreement.     The return loss (S 11 ) of the circular antenna affected by increasing C-SRRs number was noticeable for 3 × 3 array while the lower numbers of C-SRRs have the no effect or small change enhancement occurs by implementing 3 × 3 C-SRR on the back of the antenna with a one-millimeter separation between each SRR unit, Figure 22. The effect of C-SRR on antennas return loss is shown in Figure 23. The introduction of the SRR resulted in a shift of the UWB start at 2.8 GHz and improved return loss over the entire band. The antenna's maximum gain is presented in Figure 24; it is almost 5 dB. The antenna's radiation pattern with implemented C-SRR is examined for different frequencies of 4, 6, 8 and 10 GHz with ϕ = 0 • and 90 • ; it is noticeable that its radiation pattern has maintained omnidirectional behavior, Figure 25.             The proposed circular microstrip antenna with an array of 3 × 3 C-SRR is fabricated in the lab and presented in Figure 26. The frequency bandwidth of the antenna is between 2.9 GHz to 8.8 GHz, Figure 27, with VSWR less than 2 for antenna's impedance bandwidth. The simulated and measured results are in good agreement.

C-SRR and Ring Rotation
Studying the arrangements of the SRR double-ring orientation with respect to each other will provide an insight on the effect of the ring orientation on antenna's performance. Thus, Inner ring rotation of the SRR effect is investigated. Inner ring rotation of 10°, 3° and 17° is applied along either the row or the column configuration of the array. Figure 28 presents the antenna with SRR in case 1, where the rotation angles applied to the rows of the array. Case 2 displays the SRR arrangement along the column of the array. Figure 28 shows the return loss results with inner ring rotation, which results in a rejection or notched band from 3.91 GHz to 4.25 GHz for rotation along the row. On the other hand. Implementing ring rotation along the column resulted in a rejection bandwidth from 4.82 GHz to 5.6 GHz, as shown in Figure 29. Antenna's gain in case 1, case 2, is around 5 dB as displayed in Figure 30.
The next step for applying rotated inner rings will utilize positive (clockwise) and negative (anticlockwise) angles rotation as displayed in Figure 31. Three cases are studied: Case 1: inner ring rotations for 47°, 33° and 10° are applied to the array's column on the back of the antenna. Such arrangement resulted in a rejection at three frequencies 4.8 GHz, 5.4 GHz and 7 GHz, Figure 32. In Case 2 negative angle rotation of −47° is implemented on the upper row, and the other two rows have positive angle rotation of 33° and 10°, respectively. This design has no rejection frequencies in its bandwidth, Figure 32. In Case 3 all SRRs are implemented with negative angles rotation of −47°, −33° and −10°. Figure 32 shows two rejection frequencies at 4.8 GHz and 7.2 GHz. The three designs have an improved frequency bandwidth between 2.2 GHz to 9.8 GHz and a maximum gain around 5.3 dB, Figure 33.
The final C-SRR implementation is to study inner and outer ring rotation with positive/negative angles. Three cases are investigated with angle rotation of 7°, 13° and 30° as illustrated in Figure 34. The three studied cases are as follows: Case 1: angles 7°, 13° and 30° are applied the inner rings. In the scenario the antenna has two rejection frequencies at 4.8 GHz and 5.4 GHz, Figure 35. In Case 2 the same rotation angles 7°, 13° and 30° are used for outer ring. In this case the antenna did not exhibit any rejection bands, Figure 35. For Case 3: negative angles −7°, −13° and −30° are implemented on the outer rings, this implementation resulted in rejection bandwidth between 4.69 GHz to 5.44 GHz and a second rejection frequency at 7 GHz as depicted in Figure 35. The three design cases have an improved bandwidth from 2.2 GHz to 9.8 GHz. The antenna gain for the 3 cases is between 5.4 dB and 5.55 dB, as shown in Figure 36.

C-SRR and Ring Rotation
Studying the arrangements of the SRR double-ring orientation with respect to each other will provide an insight on the effect of the ring orientation on antenna's performance. Thus, Inner ring rotation of the SRR effect is investigated. Inner ring rotation of 10 • , 3 • and 17 • is applied along either the row or the column configuration of the array. Figure 28 presents the antenna with SRR in case 1, where the rotation angles applied to the rows of the array. Case 2 displays the SRR arrangement along the column of the array. Figure 28 shows the return loss results with inner ring rotation, which results in a rejection or notched band from 3.91 GHz to 4.25 GHz for rotation along the row. On the other hand. Implementing ring rotation along the column resulted in a rejection bandwidth from 4.82 GHz to 5.6 GHz, as shown in Figure 29. Antenna's gain in case 1, case 2, is around 5 dB as displayed in Figure 30.      The next step for applying rotated inner rings will utilize positive (clockwise) and negative (anti-clockwise) angles rotation as displayed in Figure 31. Three cases are studied: Case 1: inner ring rotations for 47 • , 33 • and 10 • are applied to the array's column on the back of the antenna. Such arrangement resulted in a rejection at three frequencies 4.8 GHz, 5.4 GHz and 7 GHz, Figure 32. In Case 2 negative angle rotation of −47 • is implemented on the upper row, and the other two rows have positive angle rotation of 33 • and 10 • , respectively. This design has no rejection frequencies in its bandwidth, Figure 32. In Case 3 all SRRs are implemented with negative angles rotation of −47 • , −33 • and −10 • . Figure 32 shows two rejection frequencies at 4.8 GHz and 7.2 GHz. The three designs have an improved frequency bandwidth between 2.2 GHz to 9.8 GHz and a maximum gain around 5.3 dB, Figure 33.             The final C-SRR implementation is to study inner and outer ring rotation with positive/negative angles. Three cases are investigated with angle rotation of 7 • , 13 • and 30 • as illustrated in Figure 34. The three studied cases are as follows: Case 1: angles 7 • , 13 • and 30 • are applied the inner rings. In the scenario the antenna has two rejection frequencies at 4.8 GHz and 5.4 GHz, Figure 35. In Case 2 the same rotation angles 7 • , 13 • and 30 • are used for outer ring. In this case the antenna did not exhibit any rejection bands, Figure 35. For Case 3: negative angles −7 • , −13 • and −30 • are implemented on the outer rings, this implementation resulted in rejection bandwidth between 4.69 GHz to 5.44 GHz and a second rejection frequency at 7 GHz as depicted in Figure 35. The three design cases have an improved bandwidth from 2.2 GHz to 9.8 GHz. The antenna gain for the 3 cases is between 5.4 dB and 5.55 dB, as shown in Figure 36.          The circular antenna, with rotated inner ring angles of 47 • , 33 • and 10 • is fabricated in the lab and displayed in Figure 37. The measurement shows that the antenna has frequency bandwidth between 3.4 GHz and 9.4 GHz as shown in Figure 38; with VSWR is less than 2 in its bandwidth range. The simulated bandwidth is wider than the measured one, but both results are in good agreement.

Implementation of Square-SRR
The S-SRR has been utilized in many antenna and microwave applications; such that, to design multi-band and single frequency notch filters and reduce the UWB antenna interference [41]. In this section, we study the effect of the array size on the antenna performance. The analysis starts with 2 × 1 array and increases up to 2 × 5 array size as shown in Figure 39a-e. The return loss results and geometric configuration for all arrays 2 × 1-2 × 5 are presented in Figure 40 and Table 5. Maximum gain for the circular antenna with the S-SRR implementation of the studied arrays is around 5 dB; the gain is 4.897 dB for 2 × 1 and increased up to 5.053 dB for 2 × 5, Figure 41. The simulated radiation pattern of the antenna with the 2 × 4 array of the double S-SRR, Figure 42, shows that the antenna

Implementation of Square-SRR
The S-SRR has been utilized in many antenna and microwave applications; such that, to design multi-band and single frequency notch filters and reduce the UWB antenna interference [41]. In this section, we study the effect of the array size on the antenna performance. The analysis starts with 2 × 1 array and increases up to 2 × 5 array size as shown in Figure 39a-e. The return loss results and geometric configuration for all arrays 2 × 1-2 × 5 are presented in Figure 40 and Table 5. Maximum gain for the circular antenna with the S-SRR implementation of the studied arrays is around 5 dB; the gain is 4.897 dB for 2 × 1 and increased up to 5.053 dB for 2 × 5, Figure 41. The simulated radiation pattern of the antenna with the 2 × 4 array of the double S-SRR, Figure 42, shows that the antenna exhibits an omnidirectional radiation pattern and low gain at the rejection frequency 5.6 GHz.

Implementation of Square-SRR
The S-SRR has been utilized in many antenna and microwave applications; such that, to design multi-band and single frequency notch filters and reduce the UWB antenna interference [41]. In this section, we study the effect of the array size on the antenna performance. The analysis starts with 2 × 1 array and increases up to 2 × 5 array size as shown in Figure 39a-e. The return loss results and geometric configuration for all arrays 2 × 1-2 × 5 are presented in Figure 40 and Table 5. Maximum gain for the circular antenna with the S-SRR implementation of the studied arrays is around 5 dB; the gain is 4.897 dB for 2 × 1 and increased up to 5.053 dB for 2 × 5, Figure 41. The simulated radiation pattern of the antenna with the 2 × 4 array of the double S-SRR, Figure 42, shows that the antenna exhibits an omnidirectional radiation pattern and low gain at the rejection frequency 5.6 GHz.

Conclusions
UWB circular microstrip antenna with elliptical rings was designed covering bandwidth between 3.5 GHz to 9 GHz. In order to enhance the antenna performance, multiple techniques used based on implementing metamaterials SRR. Arrays with different structure and position with respect to the UWB antenna were investigated. The study covered two types of double-ring SRR, these are C-SRR and S-SRR. The C-SRR array effect is noticed on antenna's behavior, such as increasing the frequency bandwidth to be 2.2 GHz to 9.8 GHz with rejection bands and frequencies. Additionally, C-SRR ring rotation was investigated; rotation resulted in a larger bandwidth, exhibited frequency band rejection, increased antenna gain and could obtain multiple rejection frequencies. A similar performance was observed with the implementation of the S-SRR array structure. Other designs of SRR can be analyzed and implemented; different designs have different resonant frequencies, which will affect the antenna's performance. This study will open the door for future investigation of tunable array configuration to suite desired metamaterial applications. Furthermore, antenna's width and length, patch shape, the radius of a circular patch, patch slots and ground size are parameters that affect the whole performance of the antenna. In this case, the antenna can be selective and operates at other frequency bands rather than UWB.

Conclusions
UWB circular microstrip antenna with elliptical rings was designed covering bandwidth between 3.5 GHz to 9 GHz. In order to enhance the antenna performance, multiple techniques used based on implementing metamaterials SRR. Arrays with different structure and position with respect to the UWB antenna were investigated. The study covered two types of double-ring SRR, these are C-SRR and S-SRR. The C-SRR array effect is noticed on antenna's behavior, such as increasing the frequency bandwidth to be 2.2 GHz to 9.8 GHz with rejection bands and frequencies. Additionally, C-SRR ring rotation was investigated; rotation resulted in a larger bandwidth, exhibited frequency band rejection, increased antenna gain and could obtain multiple rejection frequencies. A similar performance was observed with the implementation of the S-SRR array structure. Other designs of SRR can be analyzed and implemented; different designs have different resonant frequencies, which will affect the antenna's performance. This study will open the door for future investigation of tunable array configuration to suite desired metamaterial applications. Furthermore, antenna's width and length, patch shape, the radius of a circular patch, patch slots and ground size are parameters that affect the whole performance of the antenna. In this case, the antenna can be selective and operates at other frequency bands rather than UWB.