A 28 GHz Broadband Helical Inspired End-Fire Antenna and Its MIMO Conﬁguration for 5G Pattern Diversity Applications

: In this paper, an end-ﬁre antenna for 28 GHz broadband communications is proposed with its multiple-input-multiple-output (MIMO) conﬁguration for pattern diversity applications in 5G communication systems and the Internet of Things (IoT). The antenna comprises a simple geometrical structure inspired by a conventional planar helical antenna without utilizing any vias. The presented antenna is printed on both sides of a very thin high-frequency substrate (Rogers RO4003, ε r = 3.38) with a thickness of 0.203 mm. Moreover, its MIMO conﬁguration is characterized by reasonable gain, high isolation, good envelope correlation coefﬁcient, broad bandwidth, and high diversity gain. To verify the performance of the proposed antenna, it was fabricated and veriﬁed by experimental measurements. Notably, the antenna offers a wide − 10 dB measured impedance ranging from 26.25 GHz to 30.14 GHz, covering the frequency band allocated for 5G communication systems with a measured peak gain of 5.83 dB. Furthermore, a performance comparison with the state-of-the-art mm-wave end-ﬁre antennas in terms of operational bandwidth, electrical size, and various MIMO performance parameters shows the worth of the proposed work.


Introduction
The fifth-generation (5G) of mobile communications is expected to revolutionize the manner in which communications take place globally. This is based on a user-centric approach which will guarantee improved mobile broadband, and aims for a peak data rate of 20 Gbps [1]. Moreover, 5G technology is expected to upgrade the Internet of Things (IoT) capabilities of the current mobile network, which will be a transformation for future communication technologies [2]. Consequently, to achieve these highly promising features of 5G technology, antennas operating at the designated bands capable of supporting 5G functionalities are required [3]. In addition, 5G antennas must support other features, such as accurate beam coverage, and have the ability to meet the requirements of ever-changing scenarios [4]. In addition to reducing antenna losses to provide coverage with considerable depth and width, precise control of the antenna pattern to direct the beam towards the intended direction, and high spectral efficiency to allow the sharing of resources in the time and frequency domain between several users, are highly desired [5].

Antenna Geometry
The geometrical configuration of the proposed end-fire antenna is shown in Figure 1. The antenna has a compact size, with a length (A X ) and width (A Y ), which correspond to 1.36λ C × 0.9λ C , where λ C is the free-space wavelength at the central frequency of 28 GHz. The presented antenna is imprinted on both sides of Rogers RO4003 substrate with a thickness (H) of only 0.203 mm, dielectric constant (ε r ) of 3.38, and dissipation factor (tan δ) of 0.0027 [29]. The antenna was fed using a grounded coplanar waveguide (GCPW) technique. The radiator consists of a planar helical antenna with an inspired V-shaped structure, where both arms are expanded by an angle of θ. The backside of the substrate consists of a partial ground plane with a length L 3 . A hole with a radius of 1 mm was etched from the substrate to insert the screw of the end-launch connector, which is used to feed the proposed antenna, as depicted in Figure 1d.

Antenna Geometry and Design Methodology
This section presents the details for the proposed design geometry, and the design evolution and impact of design parameters.

Antenna Geometry
The geometrical configuration of the proposed end-fire antenna is shown in Figure  1. The antenna has a compact size, with a length (AX) and width (AY), which correspond to 1.36λC × 0.9λC, where λC is the free-space wavelength at the central frequency of 28 GHz. The presented antenna is imprinted on both sides of Rogers RO4003 substrate with a thickness (H) of only 0.203 mm, dielectric constant of 3.38, and dissipation factor (tan δ) of 0.0027 [29]. The antenna was fed using a grounded coplanar waveguide (GCPW) technique. The radiator consists of a planar helical antenna with an inspired V-shaped structure, where both arms are expanded by an angle of θ. The backside of the substrate consists of a partial ground plane with a length L3. A hole with a radius of 1 mm was etched from the substrate to insert the screw of the end-launch connector, which is used to feed the proposed antenna, as depicted in Figure 1d.

Antenna Designing and Radiation Mechanism
The design methodology and various steps involved in the design of the proposed antenna are depicted in Figure 2. The antenna design comprises of the following three steps.

Prototype-1
Prototype-2 Proposed design Step-1: Design of a T-shaped monopole antenna operating at 28 GHz.
Step-2: Modifying the conventional T-shaped antenna to improve impedance bandwidth and end-fire radiation.

Antenna Designing and Radiation Mechanism
The design methodology and various steps involved in the design of the proposed antenna are depicted in Figure 2. The antenna design comprises of the following three steps.

Antenna Geometry and Design Methodology
This section presents the details for the proposed design geometry, and the design evolution and impact of design parameters.

Antenna Geometry
The geometrical configuration of the proposed end-fire antenna is shown in Figure  1. The antenna has a compact size, with a length (AX) and width (AY), which correspond to 1.36λC × 0.9λC, where λC is the free-space wavelength at the central frequency of 28 GHz. The presented antenna is imprinted on both sides of Rogers RO4003 substrate with a thickness (H) of only 0.203 mm, dielectric constant ( ) of 3.38, and dissipation factor (tan δ) of 0.0027 [29]. The antenna was fed using a grounded coplanar waveguide (GCPW) technique. The radiator consists of a planar helical antenna with an inspired V-shaped structure, where both arms are expanded by an angle of θ. The backside of the substrate consists of a partial ground plane with a length L3. A hole with a radius of 1 mm was etched from the substrate to insert the screw of the end-launch connector, which is used to feed the proposed antenna, as depicted in Figure 1d.

Antenna Designing and Radiation Mechanism
The design methodology and various steps involved in the design of the proposed antenna are depicted in Figure 2. The antenna design comprises of the following three steps.

Prototype-1
Prototype-2 Proposed design Step-1: Design of a T-shaped monopole antenna operating at 28 GHz.
Step-2: Modifying the conventional T-shaped antenna to improve impedance bandwidth and end-fire radiation. Step-1: Design of a T-shaped monopole antenna operating at 28 GHz.
Step-2: Modifying the conventional T-shaped antenna to improve impedance bandwidth and end-fire radiation. Step-3: Design of helical inspired broadband antenna to cover 28 GHz band spectrum with an end-fire radiation pattern.
First, a T-shaped GCPW fed monopole antenna was designed to resonate at the desired frequency (f c ) of 28 GHz. The length (P 1 ) of the monopole antenna was determined by using the following equation [30]: where c is the free-space speed of light which is approximately equal to 3 × 10 8 m s −1 , and ε eff is the effective dielectric constant of the substrate, which can be estimated by using the following relation [30]: where ε r is the dielectric constant of the substrate, d is the width of the monopole, and h is the substrate thickness. The bandwidth of the T-shaped antenna was 650 MHz, ranging from 27.78 GHz to 28.43 GHz, as shown in Figure 3. The antenna exhibits a bi-directional radiation pattern due to equal distribution of current on both sides of the head of the T-shaped antenna, as depicted in Figure 4, with a maximum gain value of 3.96 dB, as illustrated in Figure 5. The bandwidth of the T-shaped antenna was not enough to cover the globally allocated band spectrum of 28 GHz with a range of 26.5-29.5 GHz. Moreover, the reported gain and radiation patterns were not suitable for 5G mm-wave communication systems.

= 4
(1) where c is the free-space speed of light which is approximately equal to 3 × 10 8 m s −1 , and ɛeff is the effective dielectric constant of the substrate, which can be estimated by using the following relation [30]: where ɛr is the dielectric constant of the substrate, d is the width of the monopole, and h is the substrate thickness. The bandwidth of the T-shaped antenna was 650 MHz, ranging from 27.78 GHz to 28.43 GHz, as shown in Figure 3. The antenna exhibits a bi-directional radiation pattern due to equal distribution of current on both sides of the head of the Tshaped antenna, as depicted in Figure 4, with a maximum gain value of 3.96 dB, as illustrated in Figure 5. The bandwidth of the T-shaped antenna was not enough to cover the globally allocated band spectrum of 28 GHz with a range of 26.5-29.5 GHz. Moreover, the reported gain and radiation patterns were not suitable for 5G mm-wave communication systems. Thus, to further improve the bandwidth of the antenna and radiation characteristics, the symmetric T-shaped antenna designed in the first step was modified as an asymmetric T-shaped antenna, as depicted in Figure 2. The asymmetric structure changed the current distribution on the surface and resulted in increased current flowing through the large arm compared to the short arm of the T-shaped head, as depicted in Figure 4. This redistribution of current and unbalanced geometrical configuration resulted in more electromagnetic waves bending in a specific direction and, therefore, an end-fire radiation pattern, as depicted in Figure 5. Moreover, it was also observed that the impedance bandwidth of the modified T-shaped antenna increased from 650 MHz to 4300 MHz, with a range of 26.9-31.1 GHz, and the gain improved from 3.96 GHz to 5.11 dB, as illustrated in Figure 3 and Figure 5, respectively.  Thus, to further improve the bandwidth of the antenna and radiation characteristics, the symmetric T-shaped antenna designed in the first step was modified as an asymmetric T-shaped antenna, as depicted in Figure 2. The asymmetric structure changed the current distribution on the surface and resulted in increased current flowing through the large arm compared to the short arm of the T-shaped head, as depicted in Figure 4. This redistribution of current and unbalanced geometrical configuration resulted in more electromagnetic waves bending in a specific direction and, therefore, an end-fire radiation pattern, as depicted in Figure 5. Moreover, it was also observed that the impedance bandwidth of the modified T-shaped antenna increased from 650 MHz to 4300 MHz, with a range  In the final step, a single turn helix-inspired antenna was formed, as depicted in Fig  ure 2. Helix antennas are well known for their end-fire radiation pattern if they satisfy th conditions for the axial mode of operation and their broadband operation. However, th conventional helix antenna requires a larger volume, therefore, planar helix antennas ar becoming popular [31]. The equivalent models of an uncoiled single turn helix and a con ventional helix are depicted in Figure 6. The planar configuration of the helix antenna use the following parameters, as discussed in [32]: the diameter of the helix (D), the circum ference of the helix (C = πD), the spacing between helix turns (x), the total length of th single turn (LT = √ + ), and the pitch angle (θ = sin −1 ). In most planar helix anten nas, the top and bottom arms are connected by means of a via to form a single turn, how ever, the insertion of a via results in additional unwanted inductance which may affec antenna performance. Thus, to overcome the aforementioned challenge in the presente work, a via free single turn helix antenna was formed by placing two arms on the top sid of the substrate. A helix antenna only radiates in the end-fire direction if it satisfies th following relation for operation in the axial mode: < C < [32]. Therefore, the selecte spacing between the helix arms is x = and the relation between the circumference an the free space wavelength at the central frequency of 28 GHz was chosen to be = 1, t satisfy the relation for the end-fire radiation pattern. The optimized pitch angle was foun to be 30°, which was selected after a detailed numerical analysis. The resultant antenn and its corresponding S-parameter results are depicted in Figures 2 and 3. The antenn exhibits broad impedance bandwidth of 3.8 GHz, ranging from 26 GHz to 29.8 GHz fo |S11| < −10 dB, covering the globally allocated band spectrum of 28 GHz. The numericall Prototype-I Prototype-II Proposed design In the final step, a single turn helix-inspired antenna was formed, as depicted in Figure 2. Helix antennas are well known for their end-fire radiation pattern if they satisfy the conditions for the axial mode of operation and their broadband operation. However, the conventional helix antenna requires a larger volume, therefore, planar helix antennas are becoming popular [31]. The equivalent models of an uncoiled single turn helix and a conventional helix are depicted in Figure 6. The planar configuration of the helix antenna uses the following parameters, as discussed in [32]: the diameter of the helix (D), the circumference of the helix (C = πD), the spacing between helix turns (x), the total length of the single turn (LT = √ + ), and the pitch angle (θ = sin −1 ). In most planar helix antennas, the top and bottom arms are connected by means of a via to form a single turn, however, the insertion of a via results in additional unwanted inductance which may affect antenna performance. Thus, to overcome the aforementioned challenge in the presented work, a via free single turn helix antenna was formed by placing two arms on the top side of the substrate. A helix antenna only radiates in the end-fire direction if it satisfies the following relation for operation in the axial mode: < C < [32]. Therefore, the selected spacing between the helix arms is x = and the relation between the circumference and the free space wavelength at the central frequency of 28 GHz was chosen to be = 1, to satisfy the relation for the end-fire radiation pattern. The optimized pitch angle was found to be 30°, which was selected after a detailed numerical analysis. The resultant antenna and its corresponding S-parameter results are depicted in Figures 2 and 3. The antenna exhibits broad impedance bandwidth of 3.8 GHz, ranging from 26 GHz to 29.8 GHz for |S11| < −10 dB, covering the globally allocated band spectrum of 28 GHz. The numerically In the final step, a single turn helix-inspired antenna was formed, as depicted in Figure 2. Helix antennas are well known for their end-fire radiation pattern if they satisfy the conditions for the axial mode of operation and their broadband operation. However, the conventional helix antenna requires a larger volume, therefore, planar helix antennas are becoming popular [31]. The equivalent models of an uncoiled single turn helix and a conventional helix are depicted in Figure 6. The planar configuration of the helix antenna uses the following parameters, as discussed in [32]: the diameter of the helix (D), the circumference of the helix (C = πD), the spacing between helix turns (x), the total length of the single turn (L T = , and the pitch angle (θ = sin −1 x L T ). In most planar helix antennas, the top and bottom arms are connected by means of a via to form a single turn, however, the insertion of a via results in additional unwanted inductance which may affect antenna performance. Thus, to overcome the aforementioned challenge in the presented work, a via free single turn helix antenna was formed by placing two arms on the top side of the substrate. A helix antenna only radiates in the end-fire direction if it satisfies the following relation for operation in the axial mode: 3 4 < C < 4 3 [32]. Therefore, the selected spacing between the helix arms is x = λ 4 and the relation between the circumference and the free space wavelength at the central frequency of 28 GHz was chosen to be C λ = 1, to satisfy the relation for the end-fire radiation pattern. The optimized pitch angle was found to be 30 • , which was selected after a detailed numerical analysis. The resultant antenna and its corresponding S-parameter results are depicted in Figures 2 and 3. The antenna exhibits broad impedance bandwidth of 3.8 GHz, ranging from 26 GHz to Electronics 2021, 10, 405 6 of 15 29.8 GHz for |S11| < −10 dB, covering the globally allocated band spectrum of 28 GHz. The numerically calculated gain of the helix-inspired antenna was reported to be 5.92 dB with an end-fire radiation pattern, as depicted in Figure 5. It is important to note that in all three steps the dimension of the ground plane remains unchanged. The optimized parameters of the proposed antenna are as follows: A Y = 15 mm; A X = 10 mm; L 1 = 5 mm; L 2 = 3 mm; L 3 = 7 mm; W 1 = 3 mm; W 2 = 6 mm; P 1 = 5 mm; P 2 = 4 mm; d = 0.7 mm; f = 0.45 mm; θ = 30 • .

Parametric Analysis
Changing the diameter of the helix eventually changes the circumference of the which in turn changes the wavelength of the resonating antenna, because the prev selected relation for the end-fire radiation pattern was = 1. The change in wave (λ) shifts the frequency, and the radiation pattern is also affected, due to the relati C < . Therefore, the diameter of the helix, which is equal to the length of the mon was not investigated. Other key parameters of the helix-inspired antenna were pit gle, the total length of single helix turns, and spacing between helix turns. By cha the pitch angle, other parameters attain new values because they depend upon each therefore, the parametric analysis of the pitch angle is presented in this section.
By reducing the pitch angle (θ) from 30° to 15°, x corresponds to 1.1 mm and responds to 11 mm. In this case, the antenna shows poor performance in terms of loss, and the resonating frequency shifts to the higher side, whereas the targeted 2 band was notched from the resonating region, as depicted in Figure 7. It can also served that, in this case, the gain of the antenna decreases from 5.92 dB to 3.89 dB a back lobes become more prominent, as illustrated in Figure 8. In contrast, when the of θ was increased from 30° to 45°, then x and LT correspond to 2.43 mm and 11.2 respectively. In this case, the resonating frequency of the antenna was slightly shif ward the lower side, with the disadvantage of reduced bandwidth and poor impe matching, as depicted in Figure 7. Moreover, it was also observed that the gain antenna also decreases from 5.92 dB to 5.05 dB, and the radiation pattern of the an is also distorted from purely end-fire to dual-beam, as depicted in Figure 8. Thus, be concluded from the aforementioned discussion that the presented antenna sho best performance with optimized values of θ = 30°, x = 1.67 mm, LT = 11.13 mm.

Parametric Analysis
Changing the diameter of the helix eventually changes the circumference of the helix, which in turn changes the wavelength of the resonating antenna, because the previously selected relation for the end-fire radiation pattern was C λ = 1. The change in wavelength (λ) shifts the frequency, and the radiation pattern is also affected, due to the relation 3 4 < C < 4 3 . Therefore, the diameter of the helix, which is equal to the length of the monopole, was not investigated. Other key parameters of the helix-inspired antenna were pitch angle, the total length of single helix turns, and spacing between helix turns. By changing the pitch angle, other parameters attain new values because they depend upon each other, therefore, the parametric analysis of the pitch angle is presented in this section.
By reducing the pitch angle (θ) from 30 • to 15 • , x corresponds to 1.1 mm and L T corresponds to 11 mm. In this case, the antenna shows poor performance in terms of return loss, and the resonating frequency shifts to the higher side, whereas the targeted 28 GHz band was notched from the resonating region, as depicted in Figure 7. It can also be observed that, in this case, the gain of the antenna decreases from 5.92 dB to 3.89 dB and the back lobes become more prominent, as illustrated in Figure 8. In contrast, when the value of θ was increased from 30 • to 45 • , then x and L T correspond to 2.43 mm and 11.27 mm, respectively. In this case, the resonating frequency of the antenna was slightly shifted toward the lower side, with the disadvantage of reduced bandwidth and poor impedance matching, as depicted in Figure 7. Moreover, it was also observed that the gain of the antenna also decreases from 5.92 dB to 5.05 dB, and the radiation pattern of the antenna is also distorted from purely end-fire to dual-beam, as depicted in Figure 8. Thus, it can be concluded from the aforementioned discussion that the presented antenna shows the best performance with optimized values of θ = 30 • , x = 1.67 mm, L T = 11.13 mm.

Single Element
To verify the numerical findings, a prototype of the proposed antenna was fabricated and tested as shown in the inset of Figure 9. The standard chemical etching method was adopted to fabricate the antenna, and an end-launch connector by South West Corp. was used to excite the antenna. It is important to mention that the end-launch connector has been used to demonstrate the concept due its better performance and ease of use in 28 GHz mm-wave designs [33][34][35].
3.1.1. Return Loss Figure 9 presents the comparison between the simulated and measured return loss of the proposed antenna. It can be seen that the antenna exhibits an impedance bandwidth of 3.8 GHz, ranging from 26 GHz to 29.8 GHz, and the measured impedance bandwidth was observed to be 3.89 GHz, ranging from 26.25 GHz to 30.14 GHz for |S11| < −10 dB

Single Element
To verify the numerical findings, a prototype of the proposed antenna was fabricated and tested as shown in the inset of Figure 9. The standard chemical etching method was adopted to fabricate the antenna, and an end-launch connector by South West Corp. was used to excite the antenna. It is important to mention that the end-launch connector has been used to demonstrate the concept due its better performance and ease of use in 28 GHz mm-wave designs [33][34][35]. Figure 9 presents the comparison between the simulated and measured return loss of the proposed antenna. It can be seen that the antenna exhibits an impedance bandwidth of 3.8 GHz, ranging from 26 GHz to 29.8 GHz, and the measured impedance bandwidth was observed to be 3.89 GHz, ranging from 26.25 GHz to 30.14 GHz for |S11| < −10 dB. The small discrepancy between numerically calculated and tested results occurred due to imperfections in fabrication and the measurement setup.

Single Element
To verify the numerical findings, a prototype of the proposed antenna was fabricated and tested as shown in the inset of Figure 9. The standard chemical etching method was adopted to fabricate the antenna, and an end-launch connector by South West Corp. was used to excite the antenna. It is important to mention that the end-launch connector has been used to demonstrate the concept due its better performance and ease of use in 28 GHz mm-wave designs [33][34][35].  Figure 10 presents the far-field results of the proposed antenna. Measurements wer carried out using an anechoic chamber. It can be seen that antenna exhibits an end-fir radiation pattern in the principal E-plane ( = 0°), where the main lobe is pointed towar θ = −105°. A similar pattern is also observed in the H-plane ( = 90°), where the main lob is pointed toward θ = −135°. A good agreement between simulated and measured radia tion patterns was observed for both E-and H-planes. The antenna possesses a simulate and measured peak gain of < 5.83 dB in the operational bandwidth, and the numericall calculated and measured radiation efficiency of the antenna was also reported <85% in th achieved band, as depicted in Figure 10b.  Figure 9 presents the comparison between the simulated and measured return loss of the proposed antenna. It can be seen that the antenna exhibits an impedance bandwidth of 3.8 GHz, ranging from 26 GHz to 29.8 GHz, and the measured impedance bandwidth was observed to be 3.89 GHz, ranging from 26.25 GHz to 30.14 GHz for |S11| < −10 dB. The small discrepancy between numerically calculated and tested results occurred due to imperfections in fabrication and the measurement setup. Figure 10 presents the far-field results of the proposed antenna. Measurements were carried out using an anechoic chamber. It can be seen that antenna exhibits an end-fire radiation pattern in the principal E-plane (φ = 0 • ), where the main lobe is pointed toward θ = −105 • . A similar pattern is also observed in the H-plane (φ = 90 • ), where the main lobe is pointed toward θ = −135 • . A good agreement between simulated and measured radiation patterns was observed for both E-and H-planes. The antenna possesses a simulated and measured peak gain of < 5.83 dB in the operational bandwidth, and the numerically calculated and measured radiation efficiency of the antenna was also reported <85% in the achieved band, as depicted in Figure 10b.

MIMO Antenna
MIMO communication systems have gained considerable attention during the past decade due to the advantage of enhanced data throughput under the effects of signal interference, multiple paths, and signal fading. Consequently, to satisfy the basic requirements of the MIMO communication system, i.e., a MIMO antenna, the proposed antenna was further transformed into a MIMO antenna by placing two identical elements orthogonal to each other. The fabricated prototype of the MIMO antenna was used to test the various MIMO performance parameters, including scattering parameters (i.e., both reflection and transmission coefficients), envelope correlation coefficient (ECC), channel capacity loss (CCL), pattern diversity, diversity gain (DG), and mean effective gain (MEG). radiation pattern in the principal E-plane ( = 0°), where the main lobe is pointed toward θ = −105°. A similar pattern is also observed in the H-plane ( = 90°), where the main lobe is pointed toward θ = −135°. A good agreement between simulated and measured radiation patterns was observed for both E-and H-planes. The antenna possesses a simulated and measured peak gain of < 5.83 dB in the operational bandwidth, and the numerically calculated and measured radiation efficiency of the antenna was also reported <85% in the achieved band, as depicted in Figure 10b.

MIMO Antenna
MIMO communication systems have gained considerable attention during the past decade due to the advantage of enhanced data throughput under the effects of signal interference, multiple paths, and signal fading. Consequently, to satisfy the basic requirements of the MIMO communication system, i.e., a MIMO antenna, the proposed antenna was further transformed into a MIMO antenna by placing two identical elements orthogonal to each other. The fabricated prototype of the MIMO antenna was used to test the various MIMO performance parameters, including scattering parameters (i.e., both reflection and transmission coefficients), envelope correlation coefficient (ECC), channel capacity loss (CCL), pattern diversity, diversity gain (DG), and mean effective gain (MEG).

Scattering Parameters
A comparison between the simulated and measured value of reflection (|S11|) and transmission coefficients (|S12|) is presented in Figure 11. It can be observed that the simulated and measured reflection bandwidths are from 26-29.8 GHz and 26.25-30.14 GHz, respectively. The simulated value of |S12| was observed to be <−30 dB over the complete operational region, and the measured value of |S12| was observed to be less than −31 dB.

Scattering Parameters
A comparison between the simulated and measured value of reflection (|S 11 |) and transmission coefficients (|S 12 |) is presented in Figure 11. It can be observed that the simulated and measured reflection bandwidths are from 26-29.8 GHz and 26.25-30.14 GHz, respectively. The simulated value of |S 12 | was observed to be <−30 dB over the complete operational region, and the measured value of |S 12 | was observed to be less than −31 dB. Moreover, the minimum value of −57 dB was observed at the frequency of 27.61 GHz. The small discrepancy between numerically calculated and tested results occurred due to imperfections in fabrication and the measurement setup. Frequency(GHz) S12 Measured S12 Simulated S11 Measured S11 Simulated Figure 11. Simulated and measured scattering parameters of the multiple-input-multiple-output (MIMO) antenna.

Envelope Correlation Coefficient (ECC)
The envelope correlation coefficient (ECC) defines how one antenna is independent in its performance with respect to another antenna. Ideally, the value of ECC should be 0, however, for real cases, ECC < 0.5 is acceptable. ECC can be calculated using the following relation, provided in [36]: For the presented antenna, the simulated value of ECC is <0.013, whereas the measured value was observed to be <0.005, as depicted in Figure 12a. The value of the proposed antenna is due to low mutual coupling and thus one element has less impact on the performance of the other element of the MIMO antenna.

Channel Capacity Loss (CCL)
Channel capacity loss (CCL) refers to the losses which may occur in the system due to correlation effects. The unit of CCL is bits/s/Hz and 0.5 is the maximum acceptable

Envelope Correlation Coefficient (ECC)
The envelope correlation coefficient (ECC) defines how one antenna is independent in its performance with respect to another antenna. Ideally, the value of ECC should be 0, however, for real cases, ECC < 0.5 is acceptable. ECC can be calculated using the following relation, provided in [36]: For the presented antenna, the simulated value of ECC is <0.013, whereas the measured value was observed to be <0.005, as depicted in Figure 12a. The value of the proposed antenna is due to low mutual coupling and thus one element has less impact on the performance of the other element of the MIMO antenna. Frequency(GHz) S12 Measured S12 Simulated S11 Measured S11 Simulated Figure 11. Simulated and measured scattering parameters of the multiple-input-multiple-output (MIMO) antenna.

Envelope Correlation Coefficient (ECC)
The envelope correlation coefficient (ECC) defines how one antenna is independent in its performance with respect to another antenna. Ideally, the value of ECC should be 0, however, for real cases, ECC < 0.5 is acceptable. ECC can be calculated using the following relation, provided in [36]: For the presented antenna, the simulated value of ECC is <0.013, whereas the measured value was observed to be <0.005, as depicted in Figure 12a. The value of the proposed antenna is due to low mutual coupling and thus one element has less impact on the performance of the other element of the MIMO antenna.

Channel Capacity Loss (CCL)
Channel capacity loss (CCL) refers to the losses which may occur in the system due to correlation effects. The unit of CCL is bits/s/Hz and 0.5 is the maximum acceptable

Channel Capacity Loss (CCL)
Channel capacity loss (CCL) refers to the losses which may occur in the system due to correlation effects. The unit of CCL is bits/s/Hz and 0.5 is the maximum acceptable value for a system. CCL of any MIMO antenna can be calculated using the following relation [36]: For the presented antenna, the value of i = j = 1, 2. Therefore, the simulated value of the MIMO antenna is <0.2 bits/s/Hz, whereas the measured value was observed to be <0.12 bits/s/Hz, as demonstrated in Figure 12b. Both simulated and measured values are less than the acceptable range of CCL, thus the presented MIMO antenna has a simple effect on the diversity performance of the system.

Pattern Diversity
Because the antenna radiates in the end-fire direction, the MIMO antenna elements were placed orthogonal to each other to achieve pattern diversity. Figure 13 illustrates the pattern diversity performance of the MIMO antenna when either port-1 or port-2 were excited. When port-1 is excited, the antenna shows a peak value of the radiation pattern located at θ = −105 • in the E-plane, and the maximum beam was observed in the H-plane pointed toward θ = −115 • . When port-2 is excited, the antenna exhibits an end-fire radiation pattern pointed toward θ = 115 • in the principal E-plane, whereas for the principal H-plane, the antenna maximum beam was directed toward θ = −110 • . Thus, the pattern of the proposed antenna can be switched from the +x-axis to the −y-axis depending upon the user requirements.
For the presented antenna, the value of i = j = 1, 2. Therefore, the simulated value of the MIMO antenna is <0.2 bits/s/Hz, whereas the measured value was observed to be <0.12 bits/s/Hz, as demonstrated in Figure 12b. Both simulated and measured values are less than the acceptable range of CCL, thus the presented MIMO antenna has a simple effect on the diversity performance of the system.

Pattern Diversity
Because the antenna radiates in the end-fire direction, the MIMO antenna elements were placed orthogonal to each other to achieve pattern diversity. Figure 13 illustrates the pattern diversity performance of the MIMO antenna when either port-1 or port-2 were excited. When port-1 is excited, the antenna shows a peak value of the radiation pattern located at θ = −105° in the E-plane, and the maximum beam was observed in the H-plane pointed toward θ = −115°. When port-2 is excited, the antenna exhibits an end-fire radiation pattern pointed toward θ = 115° in the principal E-plane, whereas for the principal H-plane, the antenna maximum beam was directed toward θ = −110°. Thus, the pattern of the proposed antenna can be switched from the +x-axis to the −y-axis depending upon the user requirements.

Diversity Gain (DG)
Diversity gain (DG) is one of the key parameters for any MIMO antenna system and is defined as the loss occurring in transmission power when the diversity scheme is performed. The DG can be calculated by using the ECC of the MIMO antenna by following relation [37]: DG = 10 1 − ECC (5) The ideal case, in which ECC is 0, results in DG = 10 dB. Consequently, for the real case, the ECC should be very small so that DG must be approximately equal to 10 dB. For the presented case, the simulated results show that the antenna exhibits diversity gain of >9.998 dB, whereas the measured results depict DG >9.999 dB, as depicted in Figure 14a. Thus, the low value of ECC eventually results in a high value of DG, which makes the proposed antenna a strong candidate for diversity applications.

Diversity Gain (DG)
Diversity gain (DG) is one of the key parameters for any MIMO antenna system and is defined as the loss occurring in transmission power when the diversity scheme is performed. The DG can be calculated by using the ECC of the MIMO antenna by following relation [37]: DG = 10 1 − ECC 2 The ideal case, in which ECC is 0, results in DG = 10 dB. Consequently, for the real case, the ECC should be very small so that DG must be approximately equal to 10 dB. For the presented case, the simulated results show that the antenna exhibits diversity gain of >9.998 dB, whereas the measured results depict DG >9.999 dB, as depicted in Figure 14a. Thus, the low value of ECC eventually results in a high value of DG, which makes the proposed antenna a strong candidate for diversity applications. 12

Mean Effective Gain (MEG)
Another key parameter for diversity applications is mean effective gain (MEG) of the MIMO antenna, which is defined as the mean of received power by a system inside a fading environment. The value of MEG can be calculated using the following relation, given in [37]: For the proposed MIMO antenna l = 2, i = j = 1, 2, where μir represents the efficiency of the element of the MIMO antenna under observation. Practically, the acceptable value of MEG should be less than −3 dB. It can be observed that, for both simulation and measurement results, the MEG value is within the acceptable range, as depicted in Figure 14b. Thus, the good agreement between simulated and measured results of various performance parameters of the MIMO antenna validates the potential of the proposed work for MIMO and diversity applications.

Comparison with State-of-the-Art Works
The presented antenna is compared in Table 1 with the state-of-the-art mm-wave end-fire antennas to show the worth of this design. It can be noted that the presented antenna exhibits a compact size compared to [23][24][25][26], although the antenna presented in [22] has a compact size, but has limited bandwidth and low gain. Conversely, the gain values reported by [23][24][25][26] were higher than those of the current study, but they exhibit bigger dimensions, a via-based design [24][25][26], and less bandwidth [24,26]. Moreover, to provide a closer examination of MIMO performance, a comparison of the proposed MIMO antenna with other 28 GHz MIMO antennas is presented in Table 2. The proposed MIMO antenna exhibits very low mutual coupling, and low values of ECC and CCL, and a high value of DG was also observed. Therefore, it can be deduced from the aforementioned discussion that the presented end-fire antenna outperforms the related work due to its compact dimension, wide operational bandwidth, and moderate gain value, in addition to its via-free design, good MIMO antenna performance parameters, and pattern diversity for MIMO applications.

Mean Effective Gain (MEG)
Another key parameter for diversity applications is mean effective gain (MEG) of the MIMO antenna, which is defined as the mean of received power by a system inside a fading environment. The value of MEG can be calculated using the following relation, given in [37]: For the proposed MIMO antenna l = 2, i = j = 1, 2, where µ ir represents the efficiency of the element of the MIMO antenna under observation. Practically, the acceptable value of MEG should be less than −3 dB. It can be observed that, for both simulation and measurement results, the MEG value is within the acceptable range, as depicted in Figure 14b. Thus, the good agreement between simulated and measured results of various performance parameters of the MIMO antenna validates the potential of the proposed work for MIMO and diversity applications.

Comparison with State-of-the-Art Works
The presented antenna is compared in Table 1 with the state-of-the-art mm-wave end-fire antennas to show the worth of this design. It can be noted that the presented antenna exhibits a compact size compared to [23][24][25][26], although the antenna presented in [22] has a compact size, but has limited bandwidth and low gain. Conversely, the gain values reported by [23][24][25][26] were higher than those of the current study, but they exhibit bigger dimensions, a via-based design [24][25][26], and less bandwidth [24,26]. Moreover, to provide a closer examination of MIMO performance, a comparison of the proposed MIMO antenna with other 28 GHz MIMO antennas is presented in Table 2. The proposed MIMO antenna exhibits very low mutual coupling, and low values of ECC and CCL, and a high value of DG was also observed. Therefore, it can be deduced from the aforementioned discussion that the presented end-fire antenna outperforms the related work due to its compact dimension, wide operational bandwidth, and moderate gain value, in addition to its via-free design, good MIMO antenna performance parameters, and pattern diversity for MIMO applications.

Conclusions
In this paper, a planar helix-inspired wideband antenna for 28 GHz 5G applications was presented. The antenna geometry was extracted from a grounded-CPW fed T-shaped antenna by transforming it into an asymmetric antenna. Then, the concept of the planar helix antenna was utilized to improve the performance of the antenna. The via-free antenna offers a compact size of 1.36λ C × 0.9λ C while exhibiting wideband measured impedance bandwidth of 3.89 GHz (26.25-30.14 GHz), with an end-fire radiation pattern having a maximum measured gain of 5.83 dB and maximum radiation efficiency of 88%. A single element was also utilized to design a MIMO antenna system, in which both elements were placed orthogonal to each other to achieve pattern diversity. The MIMO antenna showed parameter measurements of ECC < 0.005, CCL < 0.12 bits/s/Hz, and DG > 9.99 over the entire resonating bandwidth. The performance of the presented antenna was compared with state-of-the-art end-fire antennas and it could be observed that antenna outperforms related works due to its better performance.