# Quad-Beam 4 \(\times\) 2 Array Antenna for Millimeter-Wave 5G Applications
## 1 Introduction  
The millimeter-wave (mmWave) spectrum has garnered significant attention due to its ability to offer wide bandwidth and low latency in signal propagation, making it ideal for high-speed data communications and advanced sensing applications. mmWave technology spans a broad range of applications, including both communication and non-communication fields. For instance, mmWave has been employed in radar-based techniques to monitor electrocardiograms (ECGs) [1]. Its application extends across various domains, such as cellular communication, health monitoring [1], vehicular communication [2], and robotic control systems [3]. A key enabler of these applications is the antenna, which plays a central role in ensuring the efficient transmission and reception of mmWave signals. Numerous design methodologies have been explored to realize effective antennas for mmWave frequencies [4, 5, 6, 7, 8, 9]. Among these, planar structures [10] have attracted particular interest due to their compact size, ease of integration, and low-profile configuration. However, planar antennas inherently suffer from lower gain due to their limited aperture size [11; 12]. To address this limitation, array configurations are often employed to enhance gain by increasing the effective aperture area [4]. Multi-beam antennas are especially advantageous in modern wireless communication systems as they facilitate improved beam scanning, enabling wider coverage and allowing more devices or users to connect with a minimal interface. In [7], a complex, leaky wave antenna (LWA) was designed to achieve a quad-beam. Another design in [13] employed a combination of LWA with a substrate-integrated waveguide (SIW) to obtain a quad-beam. However, both designs were intricate and cumbersome due to their large and complex structures. Similarly, in [14], an inclined planar antenna etched with a U-shaped slot and SIW cavity was introduced to achieve circular polarization. The 45\({}^{\circ}\) inclination of the patch and the slot improved polarization, but the SIW feed mechanism contributed to further structural complexity. In [15], multiple patch rings were stacked across different layers, using two orthogonal feeds to achieve dual polarization. To prevent leakage and ensure efficient coupling, an SIW structure was embedded in the feed layer, further completing the design. Simplifying antenna architecture has been a subject of ongoing research. For example, the authors of [16] demonstrates a planar Fracklin array structure with a series feed, reducing complexity by integrating a patch and feed network on the same plane. In this case, the antenna was able to achieve a dual beam at \(\pm 45^{\circ}\). In [17], a straightforward design transformed a non-radiating microstrip line into a radiating element by introducing inverted cone patches at half-wavelength intervals, canceling the antiphase magnetic fields commonly associated with conventional microstrip lines. One of the methods for generating a simultaneous beam is the use of RF circuitry through which the sub-set of an array is activated, thus achieving a dual beam in the azimuthal and the other two in the elevation plane [18]. Similarly, in [19], researchers also demonstrated a multi-beam pattern in the azimuthal plane using a PIN diode and RF circuitry. However, both these structures, due to their RF circuitry, were complex. In another approach, in [20], a curved linear array structure with a non-uniform spacing technique was adopted to form a multi-beam at varied elevation angles. This method is appropriate for smaller array structures achieving decent gain; however, it poses limitations when increasing the antenna aperture for achieving a high-gain performance. Thus, this article focuses on the design of a novel planar array antenna that addresses the above design challenges by strategically placing an array structure to achieve a quad-beam. With quad-beam behavior, the device can simultaneously communicate with two user devices in the azimuthal plane and another two in the elevation plane.