A compact array antenna with enhanced-gain end-fire radiators for 28 GHz millimeter-wave user equipment

1 Introduction

Recent studies on millimeter-wave (mmWave) antennas for fifth-generation (5G) wireless communication systems have highlighted the importance of the 28 GHz band due to its wide available bandwidth and suitability for supporting multi-gigabit data rates (Secgin, 2023; Pi and Khan, 2011). This frequency range is regarded as a key enabler for enhanced mobile broadband services and high-capacity wireless connectivity envisioned for future 5G networks (Yang et al., 2025; Spirito and Martens, 2025). However, despite these advantages, mmWave communications suffer from severe propagation challenges, including high free-space path loss and signal blockage, which significantly degrade link reliability in practical deployment scenarios (Pedersen and Fan, 2024; Indumathi et al., 2023). In particular, experimental investigations have demonstrated that human body blockage at 28 GHz can cause substantial signal attenuation in indoor and user-centric communication environments (Dalveren et al., 2019). To compensate for these impairments, antenna systems operating at mmWave frequencies must provide sufficiently high gain while maintaining stable radiation characteristics across the frequency band (Alblaihed et al., 2022). At the user equipment (UE) level, where transmit power is inherently limited, antenna performance becomes a dominant factor in determining overall link quality. As a result, compact, thin, and high-gain antenna elements are essential for space-constrained platforms such as smartphones and portable terminals. Conventional omnidirectional antennas, commonly used at sub-6 GHz frequencies, are generally unsuitable for mmWave applications due to their limited gain and reduced radiation efficiency at higher frequencies (Huang et al., 2025). Consequently, directional antenna elements have gained increasing attention, with end-fire antennas emerging as a particularly attractive solution because they can provide high directivity while preserving a compact planar structure suitable for integration along device edges and printed circuit boards (Abdelgader et al., 2019).

Various antenna element geometries and enhancement techniques were explored in the literature to achieve compact size, high gain, and improved radiation efficiency while maintaining compact structures suitable for modern wireless devices. Slot-loaded and defect-based patch antennas have been widely investigated to enhance radiation performance. For example, a slot-loaded triangular patch antenna reported in (Tiwari R. N. et al., 2024) achieved a gain of 6.5 dBi with an efficiency of 82% at 28 GHz. Similarly, edge-fed circular patch antennas (Ud Din et al., 2023) and Y-shaped patch structures with circular defects (Naqvi et al., 2020) were proposed to improve radiation characteristics; however, their relatively large footprints (up to 25 × 25 mm²) and moderate gain values limit their applicability in highly compact mmWave systems. Tapered-fed elliptical patch designs (Tiwari M. et al., 2024) demonstrated improved gain of 7.5 dBi, albeit at the expense of increased structural complexity. To further reduce antenna size, modified square patch monopoles (Khan et al., 2023) and stub-loaded crescent monopole structures (Ali et al., 2023) were introduced. While these designs offer compact geometries, their radiation efficiency and gain enhancement remain limited, primarily due to insufficient electromagnetic field confinement in the end-fire direction. Complementary split-ring resonator (CSRR)-backed circular patch antennas (Anushkannan et al., 2024) and electromagnetic bandgap (EBG)-loaded patch designs (Elabd et al., 2023) achieved improved efficiency, reaching up to 93% and gain levels of approximately 8 dBi. Several studies have explored defected ground structures (DGS) and metamaterial-inspired loading to enhance antenna performance. A monopole patch with DGS operating at 27 GHz was reported in (Güler and Bayer Keskin, 2023), achieving a gain of 7 dBi, while metamaterial-loaded monopole patch designs (Khan et al., 2022a) showed modest gain improvement but lacked efficiency reporting. Slotted planar patch antennas operating at lower mm-wave frequencies, such as 11 GHz (Khan et al., 2022b), offer limited relevance for 28 GHz 5G systems due to their large physical size and low gain. Rectangular-ring slotted elements (Chettri and Bera, 2022) and slotted E-shaped patch antennas operating at 28 GHz and 37 GHz (Munir et al., 2024) improved impedance characteristics but still suffered from moderate gain values below 6.5 dBi. Furthermore, rectangular patch antennas incorporating multiple strips have been reported at 27.5 GHz (Liang et al., 2022), achieving improved radiation efficiency; nevertheless, the gain level remains limited, and the design complexity increases due to the use of multiple resonant strips. End-fire radiation enhancement has also been investigated using bowtie antennas and dielectric resonator antennas (DRAs). MTM-loaded end-fire bowtie antennas (Liu et al., 2020) achieved gains around 7.2 dBi; however, their large size (30 × 30 mm²) restricts their use in compact devices. Rectangular DRAs with metallic strips (Zhang et al., 2019) provided higher gain (8 dBi) but at the expense of increased volume and fabrication complexity due to the use of dielectric resonator materials. Recent studies have further investigated Yagi and quasi-Yagi antenna configurations for mmWave applications. Gain-enhanced Yagi–Uda designs employing modified parasitic elements and higher-order-mode excitation have been reported in (Bai et al., 2020), demonstrating improved radiation performance through tailored current distribution. Quasi-Yagi antenna configurations have also been explored in (Egels et al., 2025), where performance enhancement is achieved through optimized director arrangements and multi-element structures. In addition, modified Yagi–Uda antennas with improved gain characteristics have been presented in (Korres and Papananos, 2025), where performance improvement is achieved through structural tuning and enhanced director configurations. Further improvements in Yagi-type antennas have been achieved through driven dipole modifications and optimized reflector/director elements, as reported in (Luo et al., 2024), enabling better impedance matching and radiation efficiency. Other compact antenna designs, such as dual-band directive antennas operating at 28/38 GHz (Addepalli et al., 2026), utilize resonant mode control to achieve enhanced gain within limited size, while SIW-based quasi-Yagi configurations (Lak, 2025) employ waveguide-based structures to improve field confinement and radiation performance. Overall, the reviewed antenna element designs reveal a clear trade-off between physical size, achievable gain, and structural complexity. While several approaches provide acceptable radiation performance, most reported antenna elements either occupy relatively large dimensions, exhibit moderate gain levels, or depend on additional loading and resonant structures such as FSS, MTM, or EBG to achieve gain enhancement. For mmWave applications—particularly at 28 GHz—antenna elements must not only be compact but also capable of delivering high gain with well-confined end-fire radiation without relying on auxiliary enhancement structures (Ozturk et al., 2025).

Motivated by these limitations, this work proposes a modified dipole–Yagi antenna element specifically tailored for mmWave operation at 28 GHz. The Yagi antenna, known for its inherent end-fire radiation and directional characteristics, represents a promising candidate for mmWave applications (Farahat et al., 2025; Bi et al., 2025). However, conventional Yagi–Uda configurations are typically optimized for lower-frequency operation and employ director elements arranged only along a single plane in front of the driven element. When directly scaled to mmWave frequencies, such configurations often suffer from limited gain enhancement, inefficient field confinement, and reduced design flexibility within compact form factors. In conventional planar Yagi and end-fire, director elements are typically arranged along a single central axis in front of the driven dipole. While such configurations are effective, they often suffer from limited gain enhancement due to weak electromagnetic coupling and inefficient field confinement, particularly under compact design constraints. In contrast, the proposed design introduces a distinctly different director arrangement, in which paired director elements are symmetrically placed on both the top and bottom layers and intentionally shifted away from the conventional central front region of the dipole. This non-centralized dual-layer configuration enhances the interaction between the driven element and the directors, resulting in stronger induced currents that are well aligned in phase with the driven dipole. Consequently, electromagnetic energy is more effectively guided and confined in the end-fire direction, leading to a significant gain enhancement compared to conventional single-layer Yagi designs of similar dimensions. Comprehensive parametric investigations and full-wave electromagnetic analyses are carried out using CST software (Systèmes and Microwave Studio, 2024) to elucidate the operating mechanism of the proposed antenna, demonstrating a significant gain enhancement compared to conventional single-layer Yagi antennas of comparable dimensions. In this work, the time-domain solver was employed due to its efficiency for broadband analysis at mmWave frequencies. Open boundary conditions were applied to emulate free-space radiation. A hexahedral mesh was used with adaptive mesh refinement to ensure convergence and accuracy of the results. The frequency sweep was performed around 28 GHz to capture the antenna performance within the operating bandwidth. Building upon the proposed gain-enhancement concept, a four-element end-fire array antenna operating at 28 GHz is subsequently designed and realized. The array configuration further increases the overall gain while preserving a compact geometry suitable for practical device integration. A prototype of the proposed array antenna is fabricated, and experimental measurements are conducted to validate the design. The measured reflection coefficients, radiation patterns, and realized gain exhibit good agreement with simulated results, thereby confirming the effectiveness of the proposed approach. Moreover, to assess the applicability of the proposed antenna for smartphone platforms, the multiple-input multiple-output (MIMO) performance of the four-element array is thoroughly evaluated. Key MIMO performance metrics are analyzed at 28 GHz, and the obtained results demonstrate satisfactory MIMO characteristics, indicating that the proposed antenna design can support high-capacity mmWave communication systems while meeting the stringent integration constraints of modern mobile devices.

2 Investigation into the antenna element

This section investigates the key characteristics and radiation performance of the proposed single-element antenna. To clearly demonstrate the effectiveness of the proposed gain-enhancement approach, several antenna configurations are studied and compared. These include a basic printed dipole antenna, a dipole antenna with a single director placed on the top layer and another director positioned on the back layer, as well as the proposed single-element antenna design with modified director placement, as illustrated in Figures 1a–c respectively. All antenna configurations are implemented on a Rogers RT/duroid 5880 substrate with a thickness of 0.5 mm, a permittivity of 2.2, and a loss tangent of 0.0019, which is well suited for mmWave frequency operation. The simulated reflection coefficient (S₁₁) and realized gain of the investigated antennas are presented in Figures 2a,b, respectively. As shown in Figure 2a, all antenna configurations exhibit similar impedance matching characteristics, resonating at 28 GHz with an impedance bandwidth exceeding 1 GHz. This indicates that the introduction and modification of the director elements have a negligible impact on the impedance matching performance of the antenna (Cao et al., 2018). In contrast, the realized gain results shown in Figure 2b reveal a significant improvement in radiation performance. The basic dipole antenna achieves a peak gain of approximately 5 dBi. By introducing the first set of director elements, the antenna gain increases to more than 7 dBi.

Furthermore, when the director’s configuration is modified as shown in Figure 1c, the realized gain is substantially enhanced, exceeding 9 dBi. This notable gain improvement is achieved without increasing the antenna footprint or profile, highlighting the effectiveness of the proposed director arrangement. To the best of the authors’ knowledge, this gain-enhancement mechanism—based on the unconventional placement of paired directors—has not been widely reported in existing literature and represents a unique feature of the proposed design. Consequently, the proposed single-element antenna is well suited for compact, high-gain planar Yagi antennas intended for mmWave applications. Moreover, the three-dimensional radiation patterns of the investigated antennas at 28 GHz are shown in Figure 3. As expected, the proposed configuration exhibits a more directional end-fire radiation pattern with a higher realized gain at 28 GHz compared to the other configurations. Further investigations were carried out on several dipole–Yagi configurations in which the director elements are positioned directly in front of the driven dipole. The examined cases include two directors arranged in a vertical column on the right side of the top layer, two directors arranged in a vertical column on the left side of the bottom layer, two directors placed at the center, and the proposed director configuration, as illustrated in Figure 4. These four configurations were analyzed in terms of their reflection coefficient (|S₁₁|) and realized gain across the operating bandwidth, with the results shown in Figure 5. The simulation results indicate that all configurations exhibit nearly identical |S₁₁| responses, resonating at 28 GHz with comparable impedance bandwidths. However, a clear distinction is observed in the gain performance. While all conventional configurations achieve a realized gain of approximately 7 dBi, the proposed configuration provides a significantly higher gain, exceeding 9 dBi at 28 GHz. Moreover, the three-dimensional radiation patterns at 28 GHz, presented in Figure 6, further confirm the enhanced directivity and end-fire radiation of the proposed design. These results demonstrate that simply positioning director elements directly in front of the driven dipole is insufficient for substantial gain enhancement (Liu et al., 2013; Choudhary et al., 2024). To further support these observations, a parametric analysis was conducted to investigate the effect of key design parameters on antenna performance. In particular, the spacing between the driven dipole and the director elements, as well as the relative positioning of the directors on the top and bottom layers, were systematically adjusted to achieve optimal impedance matching and maximum gain at 28 GHz. The results indicate that the antenna performance is highly sensitive to director placement, with the proposed dual-layer configuration providing stronger end-fire radiation compared to conventional centrally aligned designs. In addition, the substrate thickness and dielectric properties were selected to ensure stable resonance behavior and consistent radiation performance.

To further validate this conclusion, additional analysis was conducted using configurations with four identical director elements. In the conventional configuration, two rows of directors are placed on the top layer and two rows on the bottom layer, all centrally located in front of the driven dipole, as shown in Figure 7. This configuration is compared with the proposed director arrangement, as illustrated in Figure 8. Both configurations exhibit nearly identical S₁₁ responses with comparable impedance bandwidths; however, a significant difference is again observed in the realized gain. The proposed configuration achieves a substantially higher gain than the conventional centrally placed director arrangement. These findings further confirm that the proposed dual-layer, non-centralized director configuration is more effective in enhancing end-fire radiation and antenna gain than merely increasing the number of director elements (Naqvi and Lim, 2018). The surface current distribution of the proposed antenna at the 28 GHz resonant frequency is shown in Figure 9. Strong current concentration is observed along the driven dipole arms, confirming fundamental resonance operation. In addition, significant induced currents are excited on the paired director elements located on the top and bottom layers. The induced currents on the directors are well aligned and largely in phase with those on the driven dipole, forming an effective guiding structure that reinforces electromagnetic energy propagation in the end-fire direction. This enhanced coupling confines surface currents toward the forward region and suppresses backward radiation, resulting in improved directivity and higher realized gain. These observations explain the gain enhancement achieved by the proposed dual-layer director configuration at 28 GHz (Parchin, 2024). This gain enhancement can be physically explained by the interaction between the driven dipole and the asymmetrically distributed director elements. In conventional planar Yagi antennas, directors are typically placed along a single central axis, resulting in relatively uniform current distribution with limited control over field confinement in the end-fire direction. In contrast, the proposed configuration employs symmetrically paired directors on the top and bottom layers, positioned away from the central axis. This non-centralized arrangement modifies the current paths, thereby enhancing the electromagnetic coupling between the driven element and the parasitic directors. As observed from the current distribution in Figure 9, strong induced currents are generated on the director elements, which are largely in phase with the driven dipole. The vertical separation between the top and bottom directors introduces additional coupling paths, enabling improved control of the radiated fields, while the lateral displacement suppresses undesired transverse radiation and directs electromagnetic energy more efficiently along the end-fire direction. Consequently, the combined effect of in-phase current reinforcement, multi-path coupling, and improved field confinement results in enhanced directivity and higher realized gain compared to conventional single-layer, centrally aligned Yagi configurations. This design approach provides a systematic method for gain enhancement without increasing the antenna footprint or relying on additional array elements.

3 Antenna array performance investigation

This section investigates the phased array performance of the proposed antenna. The phased array arrangement and performance of the suggested antenna using a four-element linear configuration are shown in Figure 10 and investigated in the following. Four identical antenna elements are arranged in a linear array with uniform spacing. The simulated S-parameters and gain characteristics of the array are presented in Figures 11a,b, respectively. As shown in Figure 11a, all array elements exhibit good impedance matching, with S_nn resonating at 28 GHz and providing a satisfactory impedance bandwidth comparable to that of the single-element antenna. The mutual coupling between adjacent elements is also well controlled, remaining below −20 dB at 28 GHz, which indicates good isolation within the array. The gain results in Figure 11b show that the realized gain of each antenna element is approximately 8.5 dBi, which represents a slight reduction compared to the isolated single-element case due to mutual coupling, as expected. However, when operated as a phased array, the main beam of the four-element array achieves a peak gain exceeding 13 dBi at 28 GHz. Furthermore, the three-dimensional radiation patterns and beam-steering performance of the phased array are illustrated in Figure 12. The results demonstrate effective end-fire beam steering with high gain across the steering range, confirming the suitability of the proposed antenna for phased-array applications (Amar et al., 2023a; Parchin and Abd-Alhameed, 2018).

Furthermore, for practical implementation and potential experimental validation, as well as to enable a fair comparison between passive and active linear array configurations, a single-fed linear antenna array incorporating a feeding network is also investigated. The configuration and detailed design of the proposed passive linear array are illustrated in Figure 13, while the corresponding design parameters are summarized in Table 1. This investigation provides insight into the performance trade-offs between active phased-array operation and passive array excitation using a corporate feeding network. The linear array antenna is designed on a Rogers RT/duroid 5880 substrate with a thickness of 0.5 mm. The overall physical dimensions of the array are W × L. A corporate feeding network optimized for 28 GHz operation is employed to excite the antenna elements. The feeding network is designed to provide equal-amplitude and equal-phase excitation to all elements while maintaining proper impedance matching. To achieve uniform power distribution, the input signal is successively divided using impedance-transforming transmission-line sections, ensuring that each antenna element is fed with the same power level. Quarter-wavelength impedance transformers are utilized to match the input impedance of each branch to the system impedance, thereby minimizing reflection and insertion loss. This equal power and phase distribution is essential for generating a coherent radiation pattern with a well-defined main beam and suppressed sidelobes (Khaleel et al., 2022). As a result, the proposed feeding network enables efficient excitation of all array elements and ensures stable radiation performance of the passive linear array at 28 GHz. The simulated reflection coefficient (S₁₁) of the linear array antenna with the integrated feeding network is shown in Figure 14. As observed, the array exhibits good impedance matching at 28 GHz, with a reflection coefficient well below −10 dB and a satisfactory impedance bandwidth, confirming proper operation of the feeding network at the target frequency. The surface current distribution of the array antenna at 28 GHz is shown in Figure 15. Strong and nearly uniform current excitation is observed along all driven dipole elements, confirming effective equal-amplitude and equal-phase power distribution provided by the feeding network. In addition, significant induced currents are clearly observed on the director elements, indicating active electromagnetic coupling between the driven elements and the directors. These induced currents on the directors are properly aligned with those on the driven dipoles, forming an effective guiding structure that reinforces radiation in the end-fire direction. The combined current excitation on both the driven elements and the directors results in constructive interference along the array axis while suppressing backward and lateral radiation. The three-dimensional radiation characteristics of the array at 28 GHz are presented in Figure 16, including side and top views. The results demonstrate a well-defined end-fire radiation pattern with good directivity and suppressed sidelobe levels. A peak realized gain of approximately 12.8 dBi is achieved at 28 GHz, confirming the effectiveness of the proposed passive linear array design for high-gain mmWave applications (Kobal et al., 2021; Parchin et al., 2019).

TABLE 1. Design values of the antenna parameters.

In the following, the performance of the active phased array (Figure 10) and the passive linear array with a feeding network (Figure 13) is compared in terms of reflection coefficient and realized gain, as shown in Figures 17a,b, respectively. Both configurations exhibit similar impedance matching and radiation characteristics at 28 GHz. A slight gain reduction is observed in the passive array, mainly due to feeding network losses. Overall, the results confirm that the radiation performance is primarily governed by the antenna element.

4 Prototyping and measurements of the array antenna design

In this section, the fabricated prototype of the proposed array antenna is presented, followed by a detailed discussion of the experimental measurements and their comparison with simulation results. A photograph of the fabricated prototype is shown in Figure 18. The antenna is realized on a single-layer Rogers substrate and consists of a four-element linear array integrated with a corporate feeding network. The prototype was experimentally characterized to evaluate its impedance matching and radiation performance. Figure 19 illustrates the measured and simulated reflection coefficient (S₁₁). As observed, the measured results show good agreement with simulations, successfully covering the targeted 28 GHz band, with only minor discrepancies. These discrepancies are expected at mmWave frequencies and can be attributed to fabrication tolerances, variations in substrate properties, and connector and soldering effects. In addition, measurement setup uncertainties and alignment errors may also contribute to the observed deviations. Furthermore, the two-dimensional radiation patterns of the antenna, including both E-plane and H-plane cuts, are presented in Figure 20 at 28 GHz, comparing measured and simulated results (Jensen and Wallace, 2004). The radiation patterns confirm that the prototype exhibits stable end-fire radiation characteristics, achieving high gain with suppressed sidelobe levels. A close correspondence between measured and simulated patterns is observed, indicating consistent beam shape and comparable gain levels.

5 MIMO evaluation of the suggested array antenna

This section investigates the multiple-input multiple-output (MIMO) performance of the proposed array antenna, targeting its potential application in next-generation wireless communication systems, particularly handheld and portable devices such as smartphones and tablets operating at mmWave frequencies. To realize the MIMO configuration, four identical array antenna elements are symmetrically positioned at four different edges of the system circuit board, as illustrated in Figure 21. This placement strategy is well suited for compact handheld platforms, as it enables pattern diversity, spatial diversity, and polarization robustness, while maintaining a low mutual interaction between antenna elements (Chen et al., 2018). The S-parameter characteristics of the MIMO system are presented in Figure 22. The results demonstrate that all antenna elements exhibit consistent impedance matching across the targeted frequency band, confirming identical operating behavior. Moreover, the measured mutual coupling between antenna elements remains very low, indicating excellent isolation performance without the need for additional decoupling structures. This high isolation ensures minimal correlation between ports, which is essential for achieving enhanced MIMO channel capacity and link reliability (Hao et al., 2021).

To further assess the radiation behavior, three-dimensional radiation patterns of the individual antenna elements at 28 GHz are illustrated in Figure 23, both with and without linear scaling, to provide clear visualization of the radiation characteristics. The radiation patterns confirm that each antenna element maintains stable end-fire radiation, high realized gain, and similar beam shapes, regardless of its position on the board. The close similarity among the radiation patterns verifies the uniformity and repeatability of the MIMO antenna elements. Overall, the proposed MIMO antenna configuration demonstrates excellent bandwidth performance, low mutual coupling, high gain, and stable end-fire radiation characteristics, making it a strong candidate for future mmWave handheld wireless devices (Amar et al., 2023b). To further emphasize its practical applicability, the compact layout, consistent radiation performance, and effective MIMO operation confirm the potential of the proposed design for 5G and beyond wireless communication systems. From a practical perspective, the compact size and planar structure of the proposed antenna make it well suited for integration into modern mmWave user equipment, such as smartphones and tablets. The use of a compact substrate and simple geometry facilitates easy fabrication and integration with existing RF front-end modules. In addition, the end-fire radiation characteristic is particularly advantageous for edge-mounted configurations in handheld devices, where directional radiation can help mitigate human body blockage and improve link reliability. These features demonstrate its effectiveness for real-world 28 GHz mmWave communication systems. It should also be noted that, at mmWave frequencies, antenna performance can be influenced by user interaction, such as hand blockage and nearby objects. However, the end-fire radiation characteristic and the possibility of edge-mounted placement can help mitigate these effects by directing radiation away from the user’s hand. As a result, the proposed antenna configuration is expected to maintain stable performance in practical usage scenarios.

6 Comparison

Table 2 provides a quantitative comparison between the proposed antenna element and recently reported mmWave antenna designs (Tiwari R. N. et al., 2024; Ud Din et al., 2023; Naqvi et al., 2020; Tiwari M. et al., 2024; Khan et al., 2023; Ali et al., 2023; Anushkannan et al., 2024; Elabd et al., 2023; Güler and Bayer Keskin, 2023; Khan et al., 2022a; Khan et al., 2022b; Chettri and Bera, 2022; Munir et al., 2024; Liang et al., 2022; Liu et al., 2020; Zhang et al., 2019; Bai et al., 2020; Egels et al., 2025; Korres and Papananos, 2025; Luo et al., 2024; Addepalli et al., 2026; Lak, 2025) in terms of operating frequency, physical size, gain, radiation efficiency, and employed enhancement techniques. It should be noted that the number of reported compact end-fire Yagi-type antenna elements operating at mmWave frequencies remains relatively limited. Therefore, Table 2 includes a broader range of compact antenna designs to provide a general performance perspective. This broader coverage offers a more comprehensive evaluation of antenna performance across different design approaches. In addition, several recent Yagi and end-fire antenna configurations (Bai et al., 2020; Egels et al., 2025; Korres and Papananos, 2025; Luo et al., 2024; Addepalli et al., 2026; Lak, 2025) have been included to enable a more direct and meaningful comparison with the proposed design. Furthermore, some of the cited works correspond to MIMO or array-based antenna designs. In such cases, the comparison is based on the performance of individual antenna elements (single-port characteristics) to ensure consistency. It can be observed that most reported compact antennas achieve gains in the range of 6–7 dBi. In comparison, the proposed antenna provides a peak gain of 9 dBi, corresponding to an improvement of approximately 15%–40% compared to typical designs. In addition, the proposed antenna maintains a compact size of 7 × 9.5 mm², which is comparable to or smaller than many reported antennas while achieving higher gain. It is evident that most existing antenna elements achieve moderate gain levels typically below 8 dBi or require relatively large physical dimensions and additional loading structures to enhance radiation performance. In contrast, the proposed antenna element achieves a significantly higher gain while maintaining a compact form factor. The proposed antenna element does not employ any external loading, backing, or periodic structures. Instead, gain enhancement is achieved purely through an optimized antenna geometry and an innovative director arrangement, resulting in a simpler and more integration-friendly design. Furthermore, many compact patch- and monopole-based antennas reported in the literature suffer from insufficient electromagnetic field confinement in the end-fire direction, which limits their achievable gain. By contrast, the proposed modified dipole–Yagi configuration effectively confines the electromagnetic fields and strengthens end-fire radiation through the use of symmetrically placed paired directors on both the top and bottom layers, positioned away from the conventional central front region of the driven element. This configuration enables more efficient utilization of surface currents and significantly enhances forward radiation. As summarized in Table 2, the proposed antenna element occupies a compact area of only 7 × 9.5 mm², while delivering a peak gain of 9 dBi and achieving a high simulated radiation efficiency of approximately 97% and a total efficiency of around 80% at 28 GHz, under ideal simulation conditions. This performance offers a favorable trade-off compared to existing antenna element designs. The reported efficiencies correspond to simulated results, where the radiation efficiency accounts for conductor and dielectric losses, while the total efficiency also includes impedance mismatch effects. The high radiation efficiency is attributed to the use of a low-loss Rogers 5880 substrate, whereas the reduction in total efficiency is mainly due to mismatch losses at the operating frequency. This superior trade-off between size, gain, and efficiency demonstrates that the proposed antenna element is well suited for mmWave user equipment, where compactness and high directional gain are critical to overcoming severe propagation losses. Overall, the proposed antenna offers a simple, compact, and high-performance solution for 28 GHz mmWave applications, effectively addressing the limitations of previously reported designs and making it a good candidate for next-generation user equipment.

TABLE 2. Comparison table of the suggested high gain antenna element.

Bold values indicate the best performance among the compared antenna designs.

7 Conclusion

This paper presented a gain-enhancement technique for a compact dipole–Yagi antenna operating at 28 GHz. By rearranging and modifying the director elements into paired structures symmetrically placed on the top and bottom layers and positioned away from the conventional central front region of the driven dipole, a significant enhancement in end-fire radiation was achieved without increasing the antenna footprint or profile. Parametric studies and electromagnetic analyses confirmed that the proposed director configuration enables stronger field confinement and more effective electromagnetic coupling, resulting in a substantially higher realized gain compared to conventional Yagi designs. The proposed concept was first validated at the single-element level, where a peak gain exceeding 9 dBi was achieved while maintaining good impedance matching and a wide operating bandwidth around 28 GHz. The design was then extended to a four-element linear array, which was evaluated under both active phased-array and passive single-fed configurations. The phased array demonstrated a peak gain exceeding 13 dBi with effective end-fire beam steering, while the passive array achieved comparable performance with only a minor gain reduction due to feeding network losses. In addition, the MIMO performance of a four-element configuration suitable for handheld devices was investigated. These results confirm the suitability of the proposed antenna for mmWave portable device applications. Overall, the proposed antenna offers a compact and high-gain solution that supports single-element, array, phased-array, and MIMO operation at 28 GHz. In addition, the proposed antenna concept can be extended to other mmWave frequency bands through appropriate scaling of the geometrical dimensions. It can also be adapted for multi-element and MIMO configurations, enabling its application in advanced wireless communication systems. This flexibility further highlights the potential of the proposed antenna for future 5G and beyond applications.

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Footnotes

Edited by: Francesca Benassi, University of Bologna, Italy

Reviewed by: Srividhya R., Rajalakshmi Engineering College, India

Guo Weisen, Xidian University, China