A Dual Slant-Polarized Cylindrical Array of Tightly Coupled Dipole Antennas

This study proposes a design of a low-profile ultra wide-band cylindrical antenna array with plus/minus 45-degree dual polarization. The proposed compact cylindrical antenna array produces an omnidirectional radiation pattern in the azimuth plane to cover all directions. It consists of $20\times 4$ dual-polarized elements within a diameter of 131 mm and a height of 116 mm. The array elements are tightly coupled slant-polarized wideband dipole antennas, and hence, rotational symmetry of radiation patterns in the horizontal plane is achieved for the two orthogonal polarizations. Furthermore, a metasurface structure has been designed and placed over the interconnected array elements to achieve ultrawideband capabilities. The proposed array provides less than −10 dB reflection coefficient over a frequency band between 1.7 GHz and 5.9 GHz. The cross-polarization discrimination (XPD) is 15 dB at boresight in the azimuth plane. The electromagnetic characteristics of the cylindrical array and its corresponding planar array before bending have been evaluated and compared via simulations, and verified by measurements. The compact size, lightweight, and printable design of the proposed antenna array enable low-cost manufacturing and ease of installation. The proposed array design overcomes many challenges encountered in wide-band MIMO systems by covering the entire sub-6 GHz band while providing wide 360-degree coverage in the azimuth plane, hence, supporting multibeam applications.


I. INTRODUCTION
Beamforming are becoming increasingly important in the fields of communication, sensing, and imaging. In addition, low-profile, broad bandwidth, polarization diversity and wide coverage are also important to maximize performance [1]- [4]. Conformal antenna arrays can meet these requirements in addition to allowing easy integration on a curved surface [5], [6]. Cylindrical array is one of such examples that can provide an omnidirectional coverage in the azimuth while achieving high directivity in the elevation plane. The radiation pattern of the cylindrical array can be further manipulated The associate editor coordinating the review of this manuscript and approving it for publication was Mohammed Bait-Suwailam . by splitting the entire antenna array surface into sectors for multi-beams, using flexible feeding networks and fitting onto a predefined shape [7].
Polarization diversity is one of the key aspects to boost the system performance by providing a diversity between the polarizations [8]. Unlike planar antenna arrays, where the surface of the radiators spread uniformly in every direction, the surface curvature for the elements in the cylindrical array varies with direction. For example, sharp bending in the horizontal plane while no bending in the vertical plane. This leads to nonidentical radiation patterns for dual polarized elements if they are orthogonally polarized in the horizontal and vertical planes. Dual polarization based on ±45 • over the cylindrical surface can solve this problem by providing uniform patterns in both planes since the two ±45 • polarizations are altered in the same way by the curvature. The identical polarized patterns in horizontal and vertical planes of the ±45 • polarized antenna ensure low coupling and thus low correlation in the horizontal plane. Hence, its polarization diversity is more competitive compared to antennas of vertical or horizontal polarization in typical urban-suburban radio environments [9].
Tightly coupled dipole antenna (TCDA) has been widely considered as an approximation implementation of the current sheet model [10]- [12]. It is a low-profile, lightweight, planar design with wideband capability. However, the total thickness of the array is limited by the distance from the ground plane. The minimum thickness for TCDA is reported as slightly shorter than a half wavelength of the highest operational frequency. This is based on the initial concept presented by Munk [13]. A capacitive coupling is introduced between the elements to counter the inductive effect of the ground plane at lower frequencies. A crossed ring structure in conjunction with a metamaterial layer is later found to achieve a broader bandwidth, while maintaining the low-profile structure [14], [15].
The introduction of tip-end capacitance in the dipole array increases the bandwidth by a certain degree, but not sufficiently wide enough, especially when the array is scanned to wide angles from boresight. The conventional solution to this problem is to use wide-angle impedance matching (WAIM) structures made up of a stack of dielectric layers [16]. However, this method potentially introduces an extra loss into the system and it is not cost-effective. Much greater flexibility may be achieved by employing anisotropic slabs with controllable spatial dispersivity. Artificially structured materials (such as metamaterials or metasurfaces) make this approach feasible by allowing the simultaneous control of dielectric and magnetic properties in different directions [17]. A metamaterial layer formed by array of conductive disks was used in this study to ensure that the array can operate over the entire 5G sub-6 GHz band for mobile communication.
Polarization purity of the TCDA or the interconnected cross ring antenna is satisfactory as this structure is essentially originated from a dipole structure. In a planar array structure, this feature is preserved and the radiations patterns of both polarizations are identical for orthogonally polarized two elements. In a cylindrical array configuration, the dual polarized elements are bent differently if they are polarized in the horizontal or vertical planes. Hence the radiation patterns of two polarizations vary significantly in azimuth plane, leading to an asymmetry between the two polarizations. However, in the case of employing ±45 • polarized elements, the radiation patterns of both polarizations are identical even after the array plane becomes deformed on a cylindrical surface. As a result, this study adopted ±45 • dual polarized elements to establish the array and provide uniform azimuth coverage for the two orthogonal polarizations.
Dual polarization of ±45 • is also expected to combat multipath and other scattering propagation effects present in modern mobile communication systems, by offering symmetrical propagation characteristics [18]. Cross-polarization discrimination (XPD) is widely used to evaluate the polarization performance. As a rule of thumb, the desired XPD is at least 20 dB at boresight and 10 dB in the range of main beams. However, this requirement becomes increasingly more stringent as more ports are introduced in antenna arrays. As indicated in [19], there are not sufficient studies on ±45 • polarized antennas, and parasitic elements have been placed around the dual polarized element to enhance the XPD performance. Studies on slant polarized arrays on a curved surface were even less covered. For an antenna array of dual polarization, it is equally important to achieve symmetrical radiation patterns for dual polarization while maintaining a reasonable XPD performance across the main lobe in azimuth plane.
An omnidirectional radiation pattern is required to cover the entire surrounding of the antenna array for wireless communication networks. The cylindrical array structure produces this omnidirectional coverage in the azimuth plane while each antenna pair provides uniform dual polarization of ±45 • , i.e., slant polarization, with a reasonable XPD. There are very few publications about slanted omnidirectional antennas of ultrawide bandwidth. In [20], a ±45 • polarized omnidirectional antenna operational between 1.9 GHz and 2.2 GHz is introduced, with the XPD at 15 dB. A method to enhance cross polarization performance of a ±45 • slantpolarized antenna is reported in [19] where XPD is more than 20 dB at boresight and 10 dB within ±45 • off the boresight direction.
Massive multiple-input-multiple-output (MIMO) is one of the key technologies to achieve huge data rates in 5G and beyond communication networks since it exploits spatial diversity to communicate with multiple users within the same time-frequency resources [21], [22]. However, the antenna array design and its features have an impact on the channel correlation among users, and hence, effecting the capacity of massive MIMO communication channels [23]. These design features include radiation pattern, mutual coupling, array gain, bandwidth and total efficiency, that are highly critical to the overall performance of massive MIMO communication systems. The proposed antenna array is a potential solution for wideband massive MIMO base stations as it provides a nearly ommnidirectional radiation pattern and a dual polarization with a satisfactory discrimination between the two polarizations, resulting in a low channel correlation between them. Moreover, it is shown that cylindrical arrays perform satisfactorily in massive MIMO base stations while occupying a relatively smaller space [24].
In this paper, the properties of the cylindrical array formed by tightly coupled dipole antennas of slanted ±45 • polarization were investigated. The cylindrical antenna array curvature was transformed from a slanted square grid based VOLUME 10, 2022 planar array. The planar array antenna was manufactured first and rolled into a cylindrical shape by connecting the two ends, filled up with expandable polyethylene foam between the center ground plane tube and the curved surface of the antenna array. Both the cylindrical array antenna and its corresponding planar array antenna have the same ±45 • polarization and consist of the same number of antenna elements.
To access the array elements for measurement and characterization, a bended coplanar waveguide (CPW) design has been used for feeding. The CPW strip was made flexible by using thin microwave substrate. The TCDAs have been fed with a single-ended feed structure [25], [26] or through a balanced feedline [27], [28]. Folded or perforated Marchand balun have been recently used to feed the TCDAs in [29], [30]. These designs add a complex structure between array surface and ground plane, hence, require complicated fabrication process. The CPW feeding method was adopted for its relative simplicity. In order to avoid crossover of the two feed lines for dual polarization at the array surface, the elements for ±45 • polarization were designed in offset position vertically, and manufactured on the opposite side of the same board.
The rest of the article is organized as follows. In Section II, the full cylindrical array model was established, analyzed and compared with its correspondent planar array, experimental verification is presented in Section III, discussion is in Section IV, and Section V draws the conclusion.

II. CYLINDRICAL ARRAY DESIGN
The overall structure and fundamental layers of the proposed cylindrical array are depicted in Fig. 1. This cylindrical array design is achieved by wrapping up a planar array around a cylinder. The planar array and the associated unit cell model is illustrated in Fig. 2, which consists of three conductive layers. The active antenna layer is composed of dual polarized antenna elements in the middle, which is backed up by a ground plane within a proper distance, and a metasurface layer is placed over the active antenna layer. The substrate for the metasurface is ultrathin polyimide (PI) film with a thickness of 25 µm and the substrate for the antenna elements is a thin Polytetrafluoroethylene (PTFE) board with a thickness of 0.254 mm. The elements polarized in +45 • are placed on one (top) side of the board and the elements of −45 • polarization are placed on the opposite side (bottom) of the same substrate. The polarization directions are orthogonal to each other. The unit cell of the dual polarized elements in the array is illustrated in Fig. 2(b). The basic building block of the array can be considered as dual polarized wideband dipoles (crossed disks) with an enhanced coupling between them and a metasurface layer is formed by conductive disks following the same pattern as the elements in the array. For the planar array operating between 1.7 and 5.9 GHz, the optimal element spacing is 26 mm (L + F = 26 mm), the distance between the array surface and the ground plane is 16 mm, and the separation between the planar array and the metasurface is 7 mm. The optimum capacitance of the capacitor between two adjacent elements in the proposed array design is 0.34 pF. The parameters for the unit cell design and the physical dimension of the cylindrical array with the target frequency band between 1.7 and 5.9 GHz are given in Table 1 and  Table 2.

A. INPUT IMPEDANCE ANALYSIS
For an infinite current sheet in free space with infinitesimal ''unit cell'' radiators, infinite operational bandwidth and frequency-independent behaviour were demonstrated [31], where the scanning radiation impedance within the whole visible space (u 2 where t stands for the x-and y-polarized currents in (u, v, w) = sin θ cos φ, sin θ sin φ, cos θ domain, η 0 is the wave impedance in free space. No reactance component exists in (1). Hence, this expresses infinite bandwidth in theory. In conformal arrays, a practical backing plane is necessary. However, placing an electric conductor next to the current sheet limits its bandwidth. In fact, a conductor (or the ground plane) merely produces an inductance effect to the input terminals of the array. Bandwidth of the array can be expanded by introducing capacitors between the adjacent elements in the current sheet. This is due to the fact that the end tip capacitor can cancel out the short-circuit effect of the ground plane at the low frequency band. At the high frequency end, the dipole element in the array shows inductance characteristic, and the extra inductive effect introduced by the ground plane constrains the bandwidth of the array. Fig. 3 illustrates the input impedance of the four individual elements in the same array column, and other elements in the array exhibit similar input impedance characteristics as the elements in other columns are under similar boundary condition. The locations of elements ''1,'' ''2,'' ''3'' and ''4'' were shown in Fig. 1. The real part of the input impedance of element ''1'' is 28 on average, which is smaller than the other three elements over the frequency range from 1.5 GHz to 6 GHz. However, the imaginary parts of the impedance for the four elements are closer to each other with a standard deviation of 5.6 for the elements ''2,'' ''3,'' and ''4.'' The reactance values are negative below 4.5 GHz, which demonstrates the capacitive nature of dipole elements at lower frequencies. In order to yield broader frequency bandwidth, mutual coupling between elements in the array is crucial.
In a tightly coupled array, mutual coupling can be constructively utilized to achieve a stable active impedance over a broad frequency bandwidth, and a sinusoidal current distribution has to be maintained at each frequency. Since the size of each element is approximately half-wavelength (λ/2) of the highest frequency of the band, its size is too small to radiate at the low frequency end. Therefore, in order to operate at lower frequencies, mutual coupling is the key to achieve the desired current distribution. However, mutual coupling between the elements in a cylindrical array is different compared to a planar array as a result of bending on the elements. The mutual coupling between the center element ''3'' and its surrounding elements in both the cylindrical array configuration and the corresponding planar array is shown in Fig. 4. It shows that the effect of mutual coupling in the cylindrical array is less distributive than the one in the planar array configuration. The average mutual coupling between the center element and the surrounding elements was −12 dB in the cylindrical array whereas −8 dB mutual coupling at 3 GHz between adjacent elements was observed in the planar array.
Furthermore, this distinction can be reflected by studying the active input impedance characteristics. A 3 × 3 subarray was investigated for the both array configu-  (2) where n denotes the middle element of 3 × 3 subarray, N is equal to 9 representing the total number of the elements in the subarray. Then, the active input impedance of the central where Z 0 is the characteristic input impedance of the array element, which is 120 for the crossed disc dipole design in this study. The active input impedance of the element of interest in the subarray within the cylindrical array and planar array is given in Fig. 5. The active reactance part of the impedance for the element in the cylindrical array were closer to zero at the low frequency than the element in the planar array, which further enhances the bandwidth towards the low frequency end. More specifically, the reflection coefficient is less than −15 dB for the center element in the cylindrical array at 1.7 GHz and above, it is less than −15 dB from 2.5 GHz and above in the case of planar array. In addition to the constructive mutual coupling effect in the tightly coupled array, the metasurface placed above the array was another important factor contributing to increasing the bandwidth. This is investigated in the following section.

B. METAMATERIAL WAIM
The use of innovative metamaterial and metasurface as coatings in antenna arrays can increase the bandwidth, enhance wide-scan capability or manipulate polarization status. For the tightly coupled dipole array configuration, the designed metasurface is based on conductive disks that are laid out on a thin sheet to achieve a wider bandwidth covering the entire sub-6GHz band, and it is placed above the planar array. The metasurface, the unit cell, and its equivalent circuit model are shown in Fig. 6. The disks were scattered following the same pattern as the array elements and finally form a metasurface. It is working as an impedance transformer between freespace and the feeding terminals of the antenna elements. Consequently, a broader frequency bandwidth is achieved. The effectiveness of the metasurface is clearly shown by observing the improvement on the reflection coefficient of the array element as shown in Fig. 7. Both the elements in the infinite planar array and the elements in the cylindrical array demonstrated a significant improvement (reflection coefficient changed from −10 dB to −15 dB on average) on impedance matching with the added metasurface. Fig. 6 (c) gives the Equivalent Circuit Model (ECM) for the metasurface, where βd denotes the distance from the metasurface to the plane of the array. When the radius of disk is 7 mm, the separation between the disks for one polarization is 20 mm, and the size of unit cell is 40 mm, the values of components for the ECM of the metasurface layer are summarized in Table 3.

C. RADIATION PATTERN CHARACTERISTICS
The co-polar radiation patterns of the cylindrical array are given in Fig. 8. The gain stability of the cylindrical array in the horizontal plane is high, and the variance of the gains over horizontal angles for the frequencies of 1.7 GHz, 2.5 GHz, 3.4 GHz, 4 GHz and 5.9 GHz are within 2 dB range, as expected. There are more fluctuations at the high frequency VOLUME 10, 2022  end, nevertheless they are still well within 2 dB. The XPD is greater than 15 dB for the cylindrical array radiation in the azimuth. It is worth mentioning that the cross polarization performance of the array is somewhat different from that for a single element excitation. The XPD performance of an element (element 3) in the array is shown in Fig. 9. The XPD is the lowest at 2.5 GHz in the frequency band of interest, where it is slightly less than 10 dB. It is noted that the metasurface has a minor effect on the polarization status, especially in the main direction, where θ = 0 • . However, in the case of array, all elements become active, therefore XPD is greater than 15 dB for each frequency monitored over the entire Sub-6 GHz band. Fig. 10 shows the gain and XPD of the entire cylindrical array when all elements for one polarization were excited. The gain varies between 3 and 8 dB, and the XPD is in the range of 15 dB to 24 dB through the sub-6 GHz band.

D. THE FEED NETWORK FOR ARRAY ELEMENTS
This study focuses on characterizing the slant polarized cylindrical array. Therefore, flexible coplanar waveguide (CPW) feed network was designed to access the elements externally for measurements. The fundamental structure to form the tightly coupled dipole array is balanced, and the elements are spreading over the ground plane with the distance of a quarter of wavelength at the highest frequency of the operational band. In this study, for the planar array, the optimal distance between the antenna elements and the ground plane is 16 mm, and it is 21 mm for the cylindrical array. The feed lines to extend the feed points from the surface of the tightly coupled dipoles to the points at the ground plane can introduce common mode resonance in the structure.
To reduce this common mode resonance, flexible coplanar waveguides (CPW) were designed to feed the broadband dipole elements. The illustration of the design and the prototype for measurements are shown in Fig. 11 and Fig. 12,    where it can be seen that each element is fed through a CPW line terminated with an SMA connector. A parity strip is needed to eliminate the common mode propagation in the feeding structure, and this was investigated in greater details in [32]. If the parity strip is absent, and when the total length of the CPW is approximately half the wavelength, a resonance will occur at the corresponding frequency that is within the operational frequency band. The total length of the arch formed by the CPW and the parity strip is expected to be less than a wavelength of the highest frequency. It has VOLUME 10, 2022 been verified in [32] that the effect from the bending of the CPW board on the wave propagation through the CPW was negligible. The CPW lines made from flexible materials offers a simple feeding approach, potentially leading to the realization of a fully printed phased array, where planar array and feed are fully printed and connected through vias.

III. MEASUREMENT AND VERIFICATION
The cylindrical array of ±45 • slant polarization was fabricated and measured. The fabricated prototype model is shown in Fig. 13(a). The active layer and the metasurface are made from two types of substrates. The substrate for the active layer is 0.254 mm in thickness and the relative permittivity is 2.55 (Rogers AD255C, tan δ = 0.0014). The dual-polarized elements of the array are chemically etched on the opposite sides of the same board. The thickness of the substrate for the metasurface is PI (Polyimide) of 25 µm in thickness and the relative permittivity is 4.1 (PMTK182518JBB). The disks for the metasurface are chemically etched on one side of the thin PI sheet. The CPWs were manufactured by using PTFE based F 4 BM-2-A255 (ε r = 2.55, tan δ = 0.0015) with the thickness of 0.3 mm. The separation between the metasurface and the active layer is 7 mm, which is the same for both the planar array and the cylindrical array. The optimal spacing between the active layer and the ground plane is 16 mm for the planar array and 21 mm for the cylindrical array, which is corresponding to a quarter wavelength at 4.7 GHz and 3.6 GHz, respectively. For the planar array before wrapping up into a cylindrical array, the element spacing is 26 mm, which is half a wavelength at 5.8 GHz. Considering the desired highest operational frequency of 5.9 GHz for the proposed array, 26 mm is 0.51 times of the wavelength, which is slightly more than a half wavelength. The cylindrical array can be extended vertically for higher gains, the current form of 116 mm in height, that is approximately one wavelength at the highest frequency. This height is chosen to accommodate the minimum number of elements so that the input impedance of the centre element in the cylindrical array becomes stable (i.e., the active reflection coefficient was less than −10 dB over the entire frequency band). With this size, there are four rows of elements in each column, and 20 columns together in the array. This array configuration was adopted for several factors and will be discussed in more details in the next section. The reflection coefficient for the element 3 is shown in Fig. 13(b) where the results from measurement and simulation were compared when all the other elements in the cylindrical array were terminated with matched loads. It indicates that the embedded element in the cylindrical array can operate effectively from 2.75 GHz and above without exciting the neighboring elements. The isolation between ports of dual polarization is more than 20 dB. Moreover, the radiation patterns for the center element have been measured. Radiation pattern was measured for element ''3'' as shown in Fig. 1, Fig. 14 compares the measured and calculated co-polar and cross-polar patterns in the horizontal plane for element ''3.'' Simulation results were compared with measured results at the higher frequencies 3.4 GHz, 4 GHz, and 5.9 GHz where the reflection coefficient for the observed element was less than −10 dB without requiring excitation on neighboring elements. The XPD is over 15 dB in the horizontal plane for all three frequencies observed at boresight direction. At the high end of the frequency, i.e., 5.9 GHz, the cross polarization rises at wide angles. Nevertheless, it is approximately 10 dB lower than the copolar component within 60 • off the boresight direction.

IV. DISCUSSION
The tightly coupled wideband circular dipole array with ±45 • polarization was successfully realized on a cylindrical surface. The array produces omnidirectional radiation pattern in azimuth plane with rotational symmetry characteristics. The proposed antenna array covers the sub-6 GHz (between 1.7 GHz and 5.9 GHz) frequency band for mobile communications. The cross polarization performance was satisfactory with XPD lower than −15 dB at boresight over the entire frequency range. In addition to these features, it was demonstrated that the mutual coupling among elements in the cylindrical array was more concentrated than in the case of planar array. Hence elements in a small subarray of 3 × 3 can achieve a reasonable impedance matching and a combination of many subarrays can then be established from the cylindrical array to steer multiple independent beams. This is one of the reasons to have 80 ports per polarization in the design to study the properties of the cylindrical array. There are several factors that have been considered to determine the size of the array: (1) solid impedance matching for elements in the array requires at least 3 elements in one column; (2) to achieve a reasonable gain in the elevation plane: 4 elements in one column was a good compromise to have the right value with a minimum number of elements; (3) space is needed at the axis region of the cylinder for element accessing. Hence, the diameter for the ground plane cylinder was an important factor to consider the number of columns. Having many ports, i.e., two ports for each dual polarized element, enables the proposed cylindrical array to be implemented in massive MIMO communications or radar imaging applications, where each antenna must be connected to a separate RF chain for maximum performance. Finally, the CPW feed lines made from thin material for feeding elements on curved surface can be further explored to realize entirely printed low cost phased arrays. The proposed cylindrical array design was compared with other reported antenna and array designs of linear slant-polarization in Table 4. The ultra-wide frequency bandwidth of the new design is distinct to other existing designs. Another important feature is that many ports offered by the proposed cylindrical array design can be employed to form multiple beams simultaneously to perform precise beamforming. This shall be attractive to certain applications where flexibility is needed in the frontend subsystem, e.g., massive MIMO and phased array radar applications. It is worth mentioning that the previous studies on slant-polarized arrays have focused on applications for the past wireless communication systems where directional radiation was dominating and multi-beam capability was yet to be implemented. The large number of ports with ultra-wideband capability offered by the proposed design will potentially meet the requirement of the next generation wireless networks, where sensing and communication are expected to be integrated within the same system.

V. CONCLUSION
A slant-polarized omnidirectional cylindrical array antenna has been designed and developed by utilizing tightly coupled dipole antenna elements of ±45 • polarization. 80 dualpolarized elements were arranged in a cylindrical form consisting of 4 circular rows and 20 vertical columns. The main characteristics of the cylindrical array were investigated and compared to the corresponding planar array. A bandwidth of 110% (1.7-5.9 GHz) was obtained with the criteria of 10 dB return loss. It produces an omnidirectional radiation pattern in the azimuth plane with a gain variation of 2 dB at any frequency over its entire bandwidth. The cross-polarization at boresight was below −15 dB over the entire sub-6 GHz frequency band. The rotational symmetry of the radiation patterns in the azimuth plane was achieved due to geometrical symmetry between the ±45 • polarized elements with respect to the horizontal plane. The proposed ultra-wideband array antenna design and the demonstrated performance offer a great flexibility to be used for a wide range of applications. It can be excited simultaneously for omnidirectional coverage or grouped into sub-arrays for multi-beam steering applications. In addition, size and number of elements can be easily adjusted to suite specific applications. Furthermore, the low profile and ease of manufacturing offer an additional advantage. Therefore, the proposed design is a promising solution for future wireless, radar and communication systems.