Wide-Band Wide-Beam Circularly-Polarized Slot-Coupled Antenna for Wide-Angle Beam Scanning Arrays

The design of a wide-band wide-beam circularly-polarized slot-coupled (WWCS) radiating element for wide-angle scanning arrays (WASAs) is addressed. The WWCS radiator exploits a simple geometry composed of a primary (driven) and a secondary (passive) element to generate wide-beam patterns with rotational symmetry and high polarization purity. The synthesis was carried out by means of a customized version of the System-by-Design (SbD) method to derive a WWCS radiator with circular polarization (CP) and wide-band impedance matching. The results of the numerical assessment, along with a tolerance analysis, confirm that the synthesized WWCS radiating element is a competitive solution for the implementation of large WASAs. More specifically, a representative design working at f0=2.45 [GHz] is shown having fractional bandwidth FBW≃15%, half-power beam-width HPBWf0≃180 [deg] in all elevation planes, and high polarization purity with broadside axial ratio ARf0=3.2 [dB] and cross-polar discrimination XPDf0=15 [dB]. Finally, the experimental assessment, carried out on a PCB-manufactured prototype, verifies the wide-band and wide-beam features of the designed WWCS radiator.


Introduction
In recent decades and within the rapid development of modern wireless systems, there has been a continuously growing interest in beam-scanning antennas [1][2][3]. In such a framework, traditional reflectors provide excellent radiation features (e.g., high gain), but they are bulky and heavy. Moreover, mechanical scanning implies a slow reconfigurability of the main beam direction. Phased antenna arrays are excellent alternative since they guarantee an agile/flexible beam scanning [1,4,5]. As a matter of fact, they have been widely employed in satellite communications, radars, and meteorology [1,4]. Moreover, they will be key technology in next-generation mobile communications systems (i.e., 5G/6G and beyond [2,3]).
Microstrip patch antennas are very popular elementary radiators for phased arrays thanks to several advantages, i.e., they are lightweight, have low profiles, and involve simple/low-cost manufacturing [6][7][8]. However, conventional microstrip-based arrays are usually narrowband [9,10] and they generally exhibit limited scanning capabilities [11]. Since these limitations prevent their use in several applications where a large field-of-view (FOV) in a wide-band is required, great efforts have been devoted toward studying innovative Table 1. Comparison in terms of central frequency ( f 0 ), fractional bandwidth (FBW), polarization, elevation HPBW at the central frequency, and overall size (in wavelengths at f 0 , λ 0 ), between the proposed WWCS antenna and wide-beam designs recently appeared in the scientific literature.  [21] Comb-slotloaded 8.25 ÷ 11.5 7.6 ÷ 9.1 LP 83 ÷ 103 0.55 × 0.55 × 0.16 patch [22] Probe-fed u-slotted patch 3 Some interesting approaches implement the wide-beam behavior by adding parasitic elements (e.g., vertical electric walls [22,23], patches [11,19], or rings [20]- Table 1) where additional current components are induced to radiate end-fire patterns that constructively sum with those radiated by the main radiator. Following this guideline, both linearly (LP) [11,[21][22][23] and circularly (CP) polarized [20,25,28] (Table 1) and [11,[21][22][23] and circularly (CP) polarized [20,25,28] (Table 1) wide-beam radiators were synthesized even though the CP ones have several advantages with respect to those with LP. For instance, there is an improved immunity to the multi-path distortion, polarization mismatch losses, and Faraday rotation effects caused by the ionosphere in satellite communications [26,27,31]. Thus, CP wide-beam radiators are a very promising technological asset for many wireless systems including global positioning and navigation systems (GPS and GNSS), radars, satellite communications, radio frequency identification, mobile communications, and wireless local area networks [26][27][28].
Accordingly, this paper proposes a novel wide-band wide-beam CP slot-coupled (WWCS) antenna based on the combination of a primary (driven) and a secondary (passive) element to generate large-HPBW patterns with rotational symmetry and high polarization purity. More specifically, a 3D microstrip layout is obtained by placing a dielectric layer hosting a metallic ring at a proper distance from a circular patch. By properly exciting a CP current within such a parasitic element, a torus-shaped pattern with maximum gain on the azimuth plane is radiated, thus triggering an increased end-fire gain which, combining to the broadside radiation of the underlying patch, results in a wide beam along every elevation plane.
Unlike the narrowband design in [20] (having a fractional bandwidth of FBW = 1.2%- Table 1), the proposed radiating element is characterized by (i) a wide-band impedance matching (i.e., FBW = 15%-  Table 1) as well as (iii) a simpler feeding mechanism for CP (i.e., slot-coupling versus probe feeding) [7]. Moreover, unlike the single-element design in [20], the possibility to exploit such an element in a WASA is addressed, as well.
Therefore, the main novelties of this work consist of (a) the design of a new wide-beam CP radiator exploiting an aperture coupling feeding mechanism to significantly widen the impedance bandwidth and overcome spurious radiation, narrowband operation, and more complex manufacturing of probe-fed layouts in the literature [20], (b) the formulation of the arising synthesis problem, unlike the parametric trial-and-error approach used in [20], as a global optimization enabling a more effective control of the CP in the complete radiating semi-sphere and a proper impedance matches within the user-defined wide bands, (c) its efficient solution by means of a customized system-by-design (SbD) methodology, and (d) the wide-band assessment of the suitability of the WWCS for implementing large planar WASAs, differently from [20] where only the single radiator is considered.
The manuscript is organized as follows. Section 2 describes the layout of the WWCS radiator. The SbD-based synthesis strategy, which is used for the synthesis of this radiating element, is detailed in Section 3. A representative example, which is concerned with a LHCP design, is illustrated in Section 4 to numerically assess, via full-wave (FW) simulations along with a tolerance analysis, the effectiveness of the proposed radiator when implementing wide-band WASAs. Finally, the experimental assessment of the designed WWCS radiator, carried out on a PCB-manufactured prototype, is shown in Section 5. Eventually, some conclusions and final remarks are presented in (Section 6). Figure 1 shows a geometric sketch of the layout of the proposed WWCS radiator. The antenna lies on the (x, y) plane and it comprises L = 3 square dielectric layers l , l = 1, . . . , L of side L a . The thickness, the relative permittivity, and the loss tangent of the l-th (l = 1, . . . , L) layer are denoted with H l , ε rl , and tan δ l , respectively. The two stacked bottom layers (i.e., 1 and 2 ) are relative to the primary antenna element, which consists of a circular microstrip patch of radius R p , printed on the layer 2 [ Figures 1 and 2b]. Such a patch is fed with an aperture-coupling mechanism. Towards this end, a cross-shaped slot is etched in the ground plane that separates the layers 1 and 2 [ Figures 1 and 2a], which is in turn excited with a microstrip feeding line of width W f and characteristic impedance Z 0 . This latter is printed on the bottom face of 1 [Figures 1and 2a]. To maximize the EM coupling, the microstrip line, the slot, and the patch are aligned with respect to the (x, y) plane ( Figure 1). Moreover, the feeding line is terminated into an open-circuited stub whose length L f [ Figure 2a] is properly tuned so that the standing-wave current, induced within the microstrip, is maximum at the slot barycenter [6]. It is worth pointing out that, even though a multiple-layer etching manufacturing process is required, the adopted aperture feeding enables some advantages with respect to a probe/pin-based choice [6]. For instance, (a) is a wide-band impedance matching, (b) is a an easier construction, since it avoids the vertical pin that would require additional drilling and soldering processes, and (c) is the higher polarization purity and pattern symmetry since the vertical pin would behave as an additional monopole degrading the overall axial ratio (AR) and cross-polar discrimination (XPD). Moreover, the use of independent substrates for both the circular patch (i.e., 2 ) and the feeding line (i.e., 1 ) gives the designer more flexibility in selecting the optimum dielectric support for each antenna "building block" with respect to a solution with coplanar edge feeding (either direct or inset-based) [6].

WWCS Antenna Layout
As for the shape of the slot, a 45-degrees rotated cross, with unequal arms of width W 1 (W 2 ) and length L 1 (L 2 ) [ Figures 1 and 2a], was adopted to realize the desired circular polarization (CP). As a matter of fact, the introduced asymmetry allows one to excite, by injecting a current into the feeding line and exploiting the aperture coupling mechanism, two orthogonal current components having a phase difference of 90 [deg] onto the patch. As a result of the combination of such excited modes, a CP current is yielded, which in turns radiates a CP field. More specifically, left-hand (LHCP) or right-hand (RHCP) CPs are obtained by simply letting L 1 < L 2 or L 1 > L 2 , respectively [7]. Otherwise, the polarization switching (LHCP ⇔ RHCP) could be yielded by simply mirroring the cross aperture with respect to the y-axis. Thanks to such a modeling, it is possible to enforce a CP by means of a simple design and manufacturing process, since there is no need for two separate orthogonal microstrip lines. Moreover, a simple circular patch can be used by avoiding more complex solutions such as, for instance, a primary element with elliptic-shape (that would imply the tuning of the two semi-axes) or electrically-small perturbations of the external border of the patch (e.g., stubs or notches) to yield an AR close to one [7].
The top layer (i.e., 3 ) hosts the secondary element of the antenna, which is implemented as a metallic ring of inner radius R r and width W r [ Figures 1 and 2c]. Such a parasitic element is "activated" by an air coupling mechanism by placing the layer 3 at a proper distance D above the patch ( Figure 1). Overall, the total height of the WWCS antenna turns out to be ( Figure 1) The secondary passive element shares a geometric rotational symmetry with the primary active one to obtain a high polarization purity and an azimuth-invariant radiation pattern, which is a highly desirable feature for WASAs [11]. Indeed, by properly exciting a CP current within the parasitic ring [20], a torus-shaped pattern with maximum gain on the azimuth plane [i.e., θ = 90 [deg]- Figure 1a)] is radiated. The metallic ring shape is selected to assure that the arising parasitic radiation mode triggers an increased end-fire gain. As a consequence, the combination of the field radiated by the primary element [having maximum gain at broadside, i.e., θ = 0 [deg]- Figure 1a] and the secondary radiator generates a wide beam with half-power beamwidth close to HPBW(ϕ) = 180 [deg] along every elevation plane ϕ ∈ [0, 360] [deg] [ Figure 1a].

Design Methodology
In order to address in a computationally-effective way the synthesis problem at hand, a customized implementation of the system-by-design (SbD) paradigm [32] is exploited and briefly summarized in the following. More specifically, the "Problem Formulation" SbD functional block [32] is customized to (i) define a proper set of geometric descriptors of the WWCS layout and (ii) formulate a suitable multi-objective cost function accounting for several user-defined requirements on both impedance matching and radiation features. Concerning (i), once the characteristics of the substrates (i.e., material/thicknesses) of the layers l , l = 1, . . . , L, and the width of the microstrip feeding line W f are determined as detailed in [8] (p. 148, Equation 3.197) to yield the desired characteristic impedance Z 0 (e.g., Z 0 = 50 [Ω]), the set Ω = {Ω k ; k = 1, . . . , K} of geometric descriptors (Figures 1 and 2) is being auxiliary parameters (0 < α < 1; 0 < β < 1) that avoid the generation of physicallyunfeasible geometries for the secondary element by enforcing the constraints R r < L a 2 and W r < L a 2 − R r , respectively [ Figure 2c]. The synthesis problem at hand can then be stated as follows: WWCS Antenna Design Problem-Determine the optimal setup of the degreesof-freedoms (DoFs), Ω (opt) , such that the corresponding WWCS radiator (i) exhibits a suitable impedance matching within the user-defined wide frequency range f min ≤ f ≤ f max , (ii) radiates an azimuth-invariant wide-beam pattern suitable for WASAs, and (iii) implements a LHCP/RHCP with high polarization purity within the half-space region Figure 1a].
As for (ii), because of the conflicting requirements on the bandwidth and the radiation features as well as the non-linear dependence of these latter on Ω, the original synthesis problem is recast into a global optimization one, where Φ(Ω) being the cost function, which quantifies the mismatch with the synthesis targets, given by where Γ = {S 11 , HPBW, AR, XPD}, and w γ is a real weight associated with the γ-th cost function term Φ γ (Ω). More in detail, the impedance bandwidth term of the cost function Φ (γ = S 11 ) is defined as follows is the reflection coefficient at the antenna input port, Z in f q Ω being the input impedance.
Moreover, S th 11 is the desired threshold and ] is the q-th (q = 1, . . . , Q) frequency sample, Q being the number of spectral components analyzed with full-wave (FW) simulations. Finally, H{ . } is the Heaviside's function, equal to H{ζ} = 1 if ζ > 0 and H{ζ} = 0, otherwise. As for the wide-beam features, the HPBW cost term (γ = HPBW) is given by where HPBW th is the user-defined requirement, while Figure 1a], M being the number of elevation planes considered for the numerical evaluation of the HPBW.
The last two cost function terms in (3) (i.e., γ = AR and γ = XPD) are related to the CP and they are defined as follows and In the previous expressions, Figure 1a], AR th is the maximum AR given by [6] AR where the subscripts "C" and "X " denote the co-polar and the cross-polar field components, respectively (i.e., C ← LHCP and X ← RHCP if a LHCP antenna is designed, and viceversa for RHCP operation), equal to being the far-field electric field, (·) is the dot product, and ( . ) * stands for complex conjugate. Moreover, ρ C and ρ X are the polarization unit vectors for the two CPs Finally, XPD th is the minimum XPD being where is the gain related to the C/X -th field component, respectively, η 0 is the free-space impedance, while P acc f q Ω is the accepted power at the antenna terminals for a given incident power P inc The overall SbD-driven design work-flow consists of the following procedural steps:

1.
Input phase. Define the bounds of the target's operating band, f min and f max , the required CP (i.e., LHCP or RHCP), and the threshold value for each key performance indicator, Γ th = S th 11 , HPBW th , AR th , XPD th . Perform the following operations (a) Set L a = λ 0 2 , λ 0 being the free-space impedance at the central frequency Select from an off-the-shelf data-sheet the material/thickness of the l-th (l = 1, . . . , L) layer l ; (c) Compute the width of the feeding line W f to yield the desired characteristic impedance Z 0 (p. 148, Equation 3.197 [8]); (d) Derive an analytic guess, R p , for the radius of the primary element of the radiator as detailed in [33] (p. 846, Equation 14.69), then set its optimization range Ω (min) 1 and Ω (max) 1 as a percentage of R p , being Ω 1 = R p ; (e) Define the optimization bounds of the remaining

3.
Design Initialization (i = 0)-Define an initial swarm of P particles, P 0 = Ω (p) SbD Design Loop (i = 1, . . . , I) -Iteratively update the swarm positions and velocities by applying the PSO-OK/C updating rules [32], and leveraging on both the cost function predictions and the associated "reliability estimations" outputted by the SM. As for the latter, the training set at the i-th (i = 1, . . . , I) iteration, T i , of size S i = (S 0 + i), comprises progressively-added training samples according to the SbD "reinforcement learning" strategy [32] aimed at refining the prediction accuracy within the attraction basin of Ω (opt) ; 5.
Output Phase-Output the final setup of the DoFs, Ω (opt) , whose corresponding layout best fits all user-defined requirements.

Numerical Assessment
This section is aimed at illustrating the performance of the proposed WWCS antenna model. Towards this end, the synthesis of a LHCP-polarized radiator working in the [20]) was addressed. The Rogers RO4350B substrate was chosen for the L = 3 layers (ε rl = 3.66, tan δ l = 0.004, l = 1, . . . , L) with thicknesses set to According to [8], the width of the microstrip feeding line turns out to be W f = 6.65 [mm] for Z 0 = 50 [Ω], while the analytic guess of the patch radius is set to R p = 17.46 [mm] [33]. The PSO-OK/C parameters were chosen by following the literature guidelines to yield a time saving of ∆t sav = 86% with respect to a standard optimization based on a bare integration of the global optimizer and the FW simulator to compute the cost function values in correspondence with each trial antenna layout [32].
More specifically, the swarm size, the number of iterations, the social/cognitive acceleration coefficients, the inertial weight, and the initial training size were set to P = 9, I = 200, C 1 = C 2 = 2, ω = 0.4, and S 0 = 45, respectively. Moreover, the numerical evaluation of (3)  The geometric descriptors of the SbD-optimized layout are reported in Table 2, while the corresponding layout, modeled in the Ansys HFSS FW simulator [38] and having an overall height of T = 71.98 [mm] (T = 0.59 [λ 0 ] - Table 1), is shown in Figure 3. Going to the analysis of the antenna performance, Figure 4 shows the simulated reflection coefficient at the antenna input port versus the frequency. As it can be observed, such a radiating element fully complies with the requirement since S In more detail, it turns out that S (dB) 11 f |Ω (opt) ≤ S th 11 for an even wider frequency interval ( f ∈ [2.29, 2.66] [GHz]) by assessing the wide-band behavior of the proposed design with an overall fractional bandwidth of FBW| WWCS = 15% [39], while, for instance, the state-of-the-art solution in [20] is limited to FBW| [Pan 2014] = 1.2% ( Figure 4 and Table 1).
(a) (b)     As for the radiation features, Figure 5 shows  Figure 5c], it is reasonable to indicate the proposed antenna such as a wide-beam one suitable for implementing WASAs. It is worth noticing that such a feature has been yielded thanks to the constructive combination of the fields radiated by the primary and secondary sources. To better illustrate the EM phenomena and interactions, Figure 6 shows the 2D plot of the magnitude of the electric field, |E(x, z)|, on a vertical surface parallel to the (x, z)-plane and crossing the barycenter of the antenna. As it can be observed, the air coupling between the bottom (primary) and the top (secondary) element of the radiator at hand guarantees a proper excitation of the parasitic element by enabling the generation of a wide beam in the far-field region. The wide-beam behavior of the synthesized WWCS antenna in a wide frequency is "detailed" in Figure 7a    The optimized WWCS layout exhibits the desired LHCP operation and is pointed out by both the co-polar, G LHCP ( f 0 , θ, ϕ), and the cross-polar, G RHCP ( f 0 , θ, ϕ), gain patterns in Figure 5, where it can be clearly observed that G( f 0 , θ, ϕ) ≈ G LHCP ( f 0 , θ, ϕ) and G LHCP ( f 0 , θ, ϕ) G RHCP ( f 0 , θ, ϕ) for 0 ≤ θ ≤ 90 [deg] with broadside AR and XPD equal to AR( f 0 , θ = 0, ϕ = 0) = 3.2 [dB] and XPD( f 0 , θ = 0, ϕ = 0) = 15 [dB], respectively. Moreover, such a good polarization purity is kept almost unaltered in the complete radiating upper semi-sphere with the exception of the elevation angles close to the antenna end-fire as illustrated by the 2D maps of AR( f 0 , θ, ϕ) [ Figure 8a] and XPD( f 0 , θ, ϕ) [ Figure 8c] as well as by the corresponding thresholded pictures aimed at highlighting the fulfilment of the design requirements [ Figure 8b,d]. It is worth remarking that the slight degradation of both AR and XPD appears only in the most challenging region (i.e., θ 90 [deg]) and is possibly due to the spurious radiation by the slot along the directions of its major arms (i.e., ϕ = 135 [deg] and ϕ = 315 [deg]- Figure 8 and Figure 3).  In order to assess the excitation of a LHCP, the plot of the magnitude of the instantaneous surface current density, J sur f (x, y; t) , is reported in Figure 9 at four consecutive instants ]. One can observe that the fundamental mode TM 11 is properly excited on the circular patch [33] and there is a clock-wise rotation of the corresponding surface current distribution (Figure 9). The vector plot of the electric field distribution at a quota of z = 10λ 0 , E(x, y; t), shown in Figure 10 for the same instants, further verifies the desired CP of the radiated wave, which evolves in time according to a LHCP.
For comparison purposes, Figure 11 plots the broadside gain G( f , θ = 0, ϕ = 0) [ Figure 11a] and the AR AR( f , θ = 0, ϕ = 0) [ Figure 11b] within the band of interest of the proposed WWCS model and of the design in [20]. It turns out that the synthesized radiator exhibits a good AR performance, especially within the band f ∈ [2.4, 2.6] [GHz] where AR| WWCS ≤ 6 [dB] [40], that results in an AR bandwidth (ARBW) equal to ARBW| 6 dB WWCS = 8%, while ARBW| 6 dB  Finally, the suitability of the WWCS radiator as elementary building block of circularlypolarized wide-band WASAs was assessed. Towards this end, the radiation features of a large planar uniform phased array, comprising N = (50 × 50) WWCS identical elementary radiators, were studied. To account for the mutual coupling in this large aperture, a periodic model was simulated in HFSS.   Figure 12c]. The SL on both elevation planes is always smaller than 8.5 [dB] at the central frequency [ Figure 12a]. Moreover, it is worth noticing that there is a good stability of the sidelobe level (SLL) when scanning the beam on both planes [i.e., −13.6 ≤ SLL ≤ −8.2 [dB]- Figure 13a].      In addtion to the numerical assessment, a tolerance analysis has been carried out to give the interested reader some insights into the reliability and robustness on the fabrication tolerances of the proposed antenna layout both stand-alone and within an array arrangement. First, the height of the parasitic element, D, has been supposed to deviate of ±5% and ±10% from the nominal value D (opt) (Tab. II) because of some manufacturing tolerances. Figure 15 summarizes the results of the tolerance analysis versus the frequency for the input reflection coefficient [ Figure 15a], the broadside AR [ Figure 15b], and the HPBW along the ϕ = 0 [deg] [ Figure 15c] and the ϕ = 90 [deg] [ Figure 15d] planes. As it can be inferred, the proposed antenna layout turns out to be quite robust. More precisely, the wide-band [ Figure 15a] and the wide-beam [ Figure 15c,d] characteristics of the WWCS radiator are confirmed regardless of the non-negligible fabrication tolerances on D, the fractional bandwidth being equal to FBW = 12.1% in the worst case [i.e., D = D (opt) − 10%D (opt) - Figure 15a]. As a consequence, the scan loss value of the array, SL( f ), within the working frequency range, f min ≤ f ≤ f max , is quite stable in both elevation planes (Figure 16), as well.

Experimental Assessment
The experimental validation of the performance of the designed WWCS has been carried out next ( Figure 19). In order to exploit available off-the-shelf RO4350B PCB boards, two (layers 2 and 3 - Figure 1) and three (layer 1 - Figure 1) substrates of thickness h = 1.52 [mm] have been stacked to realize the different layers of the antenna. The overall structure has been assembled using four nylon M4 threaded rods and sixteen nylon bolts, stacking together the PCBs and placing the parasitic ring at distance D = D (opt) from the driven patch ( Figure 19). An RS 759-5252 SMA connector has been used to feed the antenna prototype. Figure Figure 20. Experimental Assessment-Comparison between the simulated and measured reflection coefficients at the antenna input port.
As for the radiation features of the fabricated antenna, the far field patterns have been measured inside an anechoic chamber having dimensions 9 × 6 × 6 [m 3 ]. The AUT has been placed on a remotely controlled rotating frame and the electric field has been measured by means of a circularly polarized probe connected to a signal analyzer, both placed on a dielectric mast at a distance of 3 [m] from the AUT. In order to avoid field perturbations due to cablings, the AUT has been connected with a short coaxial cable to a small transmitter able to generate a constant amplitude and frequency signal at f = f 0 = 2.45 [GHz]. The transmitter has been placed just behind the layer 1 of the AUT. Similarly, the presence of a long coaxial cable connected to the field probe has been avoided thanks to the use of the PMM 9060 EMI Receiver/Signal Analyzer (30 [MHz]-6 [GHz]) that can be remotely controlled by means of a fiber optic link. A good matching between the simulated and measured gain pattern has been obtained. As a matter of fact, both pattern cuts along the ϕ = 0 [deg] [ Figure 20a] and the ϕ = 90 [deg] [ Figure 21b] elevation planes closely match the outcomes of the numerical assessment. Moreover, it turns out that the measured HPBW verifies the wide-beam behavior of the radiator on both planes, being HPBW( f 0 , ϕ = 0)| meas = 151 [deg] [ Figure 21a] and HPBW( f 0 , ϕ = 90)| meas = 172 [deg] [ Figure 21b], respectively. Finally, the measured gain, AR, and XPD are equal to G( f 0 , θ = 0, ϕ = 0)| meas = 2.8 [dB], AR( f 0 , θ = 0, ϕ = 0)| meas = 3.3 [dB], and XPD( f 0 , θ = 0, ϕ = 0)| meas = 14.8 [dB], respectively, thus verifying a good matching with the simulated values.

Conclusions
The design of a novel wide-band wide-beam circularly-polarized elementary radiator has been proposed for WASAs. Such a WWCS structure leverages on a cross-shaped aperture-coupling feeding mechanism to achieve wide-band LHCP/RHCP operation using a simple circular patch and a single microstrip line. Moreover, it takes advantage of the air coupling between the primary and secondary EM sources to realize rotationalsymmetric patterns with large elevation HPBWs and high polarization purity in the complete upper semi-sphere. The computationally-efficient synthesis of the layout of the WWCS antenna, which supports the desired CP operation, has been carried out with a customized implementation of the SbD paradigm. Accordingly, the main advancements with respect to the state-of-the-art [20] include (i) the exploitation of an aperture feeding mechanism instead of a probe feeding to significantly widen the impedance bandwidth, mitigate spurious radiation, and enable an easier manufacturing, (ii) the formulation of the design problem as a global optimization one rather than a parametric trial-and-error approach, enabling to better control the AR and the XPD in the complete radiating semisphere, (iii) the study over a wide-band of the radiation features of the resulting planar array, as well as (iv) the fabrication tolerance analysis on both single element and array performance.
The numerical results, concerned with the representative design of a WWCS radiator working at the central frequency of f 0 = 2.45 [GHz], have demonstrated that the proposed radiating structure provides 1.
wide-band fractional impedance bandwidth (FBW 15%), which is 12.5 times larger than that in state-of-the-art solutions based on similar EM mechanisms [20]; 2.
As for the arising WASA, the numerical assessment has pointed out the potential of the proposed layout of the elementary radiator for the realization of wide-band circularlypolarized WASAs. Finally, the reliability and robustness on the fabrication tolerances of the proposed antenna layout have been verified for both the stand-alone and the array arrangement. Furthermore, the experimental assessment of a PCB-manufactured prototype has verified the FW-simulated outcomes, confirming both the wide-band and the wide-beam features of the designed WWCS radiator (Figures 20 and 21).
It should be pointed out that the proposed design concept and methodology are general since they can be applied to synthesize wide-band wide-beam CP radiators working in different operative bands. Indeed, the designer is given the freedom to choose the materials of the different layers as well as the desired target performance (i.e., bandwidth, HPBW, AR, and XPD) for the specific applicative scenario at hand.
Future works, beyond the scope of the current manuscript, will be aimed at assessing the possibility to exploit the stripline technology to feed the antenna and at investigating the resulting advantages and drawbacks.