# Wide-Band Wide-Beam Circularly-Polarized Slot-Coupled Antenna for Wide-Angle Beam Scanning Arrays
## 1 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 innovativesolutions for implementing wide-band wide-angle scanning arrays (_WASAs_) [12; 13]. In such a framework, wide angle impedance matching (_WAIM_) layers [14; 15; 16] have been proposed as effective tools to compensate for the mutual coupling among the array elements when steering the beam towards the end-fire for mitigating the decrease of the gain [i.e., the scan loss (_SL_)] due to the mismatch between the antenna and the free-space. Otherwise, pattern-reconfigurable elementary radiators [17; 18] have been used to yield large _FOV_s when arranged into arrays thanks to the electronic tilt of the element pattern in the scan direction. However, complex architectures with extra circuits and components are necessary with a higher complexities of the overall system, which may cause a pattern degradation [11; 19]. _WASAs_ can also be obtained starting from wide-beam radiating elements [10; 11; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31] according to the different microstrip-based implementations summarized in Table 1. It is worth pointing out that paramount challenges must be properly addressed to design microstrip radiators able to afford radiation patterns with a very large half-power beamwidth (_HPBW_) on the elevation planes and over the wide frequency band (such as, for instance, the design in [11], yielding \(HPBW\in[156\div 360]\) [deg] over a fractional bandwidth of \(FBW=40\%\)--Table 1). Indeed, many state-of-the-art methods for the synthesis of wide-beam antennas broaden the pattern beamwidth either in one plane (i.e., E-plane or H-plane such as, for instance, the design based on \"electric walls\" [22], which widens the pattern only in the plane containing such parasitic structures--Table 1) or in a narrowband (e.g., \(FBW=1.2\%\) in [20]--Table 1). Moreover, some of the available designs have rather complex layouts involving quite difficult manufacturing processes, potentially more prone to fabrication tolerances (such as, for instance, the magnetic dipole in [24] and the magnetoelectric dipole with meta-columns loading in [25]--Table 1).