## 1 Introduction  
The increasing diversity of internet applications has driven the exponential growth of wireless communication systems, the rapid expansion of machine-to-machine (M2M) communication, and the proliferation of connected devices. The emergence of data-intensive applications, such as augmented reality (AR), virtual reality (VR), and ultra-high-definition (UHD) video streaming, has further accelerated the demand for ultra-high data rates, reaching up to 10 Gbps, as a fundamental requirement for modern communication networks [1]. However, conventional sub-6 GHz frequency bands are becoming increasingly congested, thereby imposing significant limitations on network capacity and hindering further performance improvements [2]. To address this challenge, millimeter-wave (mmWave) technology has emerged as a promising solution, offering access to largely underutilized frequency bands with significantly wider bandwidths, thereby enhancing spectral efficiency and enabling high-speed data transmission [3]. Despite these advantages, mmWave signals suffer from inherent challenges, including high free-space path loss and increased susceptibility to atmospheric absorption, which degrade signal propagation [4]. Consequently, developing highly efficient antennas capable of maximizing the gain while maintaining a compact and cost-effective design is crucial for the widespread adoption of mmWave technology [5]. Various antenna models have been explored for mmWave communication, yet many existing designs fail to meet the stringent requirements of future wireless systems. Although traditional aperture antennas can provide directional radiation characteristics, they are often impractical because of their large size, complex structures, and integration challenges [6]. As a result, microstrip antennas have gained significant attention owing to their lightweight structure, compact form factor, low fabrication cost, and ease of integration with planar circuits, making them strong candidates for high-frequency applications [7]. One critical design parameter of a microstrip antenna is its operating frequency. Regulatory bodies worldwide have designated specific 5G mmWave frequency bands. The United States has allocated the 27.5-28.35 GHz and 37-40 GHz bands, while Japan has adopted the 27.5-28.28 GHz range. Similarly, Europe has designated the 24.25-27.5 GHz spectrum, and China is expected to commercialize the 24.25-27.5 GHz and 37-43.5 GHz bands in next-generation communication networks [8]. Among these bands, the 28- and 38-GHz bands have garnered significant attention owing to their potential for widespread deployment and relatively favorable propagation characteristics, making them a primary focus in antenna research [9; 10]. In mmWave antenna research, significant efforts have been made to enhance the antenna gain to mitigate propagation losses and extend the coverage. Traditional single-element antennas generally struggle to achieve gain values exceeding 5 dBi, which is insufficient for practical applications [11]. As a result, multi-element configurations, such as phased arrays and multiple-input multiple-output (MIMO) antenna systems, have been widely investigated to improve the gain and directivity [12; 13]. Although these array structures provide enhanced performance, they also introduce additional design complexities, increased power consumption, and higher fabrication costs [14; 15]. Hence, striking a balance between high gain and design simplicity remains a key research challenge. Early research efforts in mmWave antenna design typically began with single-element configurations, which were later expanded into array structures to enhance key performance parameters, such as the gain, bandwidth, and beamforming capability [16; 17]. However, achieving a high-gain, dual-band single-element antenna that can outperform array configurations remains an open challenge. This study aims to address this challenge by introducing a novel high-gain, dual-band microstrip antenna designed for operation at 28 GHz and 38 GHz.