In the rapidly developing landscape of wireless communication systems, the microstrip patch antenna has garnered considerable attention due to its compact size, lightweight nature, affordability, and low-profile characteristics [1–5]. However, conventional patch antenna arrays exhibit two pertinent limitations—low aperture efficiency and narrow impedance matching bandwidth. Therefore, drawing on the concept of a tightly coupled array (TCA), a new approach emerged, which involved constructing a radiator using a group of elements at a subwavelength scale. This innovative approach led to the development of the tightly coupled patch array (TCPA) [6] or the tightly coupled dipole array (TCDA) [7], which show high potential for wide bandwidth and high aperture efficiency. These antennas not only show remarkable wideband capabilities but also have the ability to scan beams effectively. However, one of the critical disadvantages of the concept of a TCA is the requirement for too many antenna elements (that is, high-cost implementation) to conduct normal operations, such as impedance matching. Therefore, in this work, a compact subarray, equipped with a compact power divider to construct a subarray composed of 3 × 3 elements, is proposed to reduce the excitation ports. In addition, since a dense array is one of the characteristics of TCA, it can help achieve high aperture efficiency. Therefore, the proposed subarray antenna can simultaneously address both of the limitations mentioned earlier.

As shown in Fig. 1(a), the proposed subarray is composed of 3 × 3 TCDA unit elements. These elements are illustrated in Fig. 1(b). The prepreg (PPG) layer is 970LF (LD), with ε_r = 3.2 and tanδ = 0.004, and the core layer is 972LF (LD), with ε_r = 3.4 and tanδ = 0.004. In this work, the thickness of the core and other layers is set to 40 μm and 80 μm, respectively, while both the top and bottom layers are 30 μm thick. The unit element is a bow-tie shaped TCDA, with its left arm capacitively fed by coupling pads (Fig. 1(c)) and the other one grounded. In addition, it is covered by a frequency selective surface (FSS) at the top, as shown in Fig. 1(c), for wideband operation [7]. The values of the structural dimensions depicted in Fig. 1 are W = 5, H = 0.742, E = 0.1, S = 0.8, P = 0.22, F_g = 0.03, F_w = 0.235, w = 1, C_c = 0.03, C_d = 0.44, C_g = 0.06, d_c = 0.427, d_g = 0.03, d_l = 0.15, and d_w = 0.7 mm. Fig. 2 presents the reflection coefficient of the unit cell, which is the unit element within the infinitely periodic boundary in the xy-plane. The impedance matching bandwidth (IBW), considering 50 Ω port impedance under −10 dB, is 26.5–39.3 GHz (38.9%). However, since the unit cell size is about 0.11λ_c at the center frequency, as mentioned in the introduction, it is too small for low-cost implementation.

Therefore, in this work, a subarray of size 0.58λ_c is proposed conserving wideband characteristics. Notably, a small or large number of unit elements of the subarray can result in narrowband operation or a beam scanning limit due to grating lobes. Furthermore, side extended arms of length S and square rings along the edges are used for impedance matching of the finite 3 × 3 array. The 3 × 3 unit elements are combined using a power divider with quarter-wave transformers of the same length, as shown in Fig. 1(c). Notably, some transformers are ring-shaped to save space. All elements are designed with 50 Ω port impedance matching, and each element is transformed to 100 Ω using 70.7 Ω transformers. Effectively, the parallel impedance from the three stems is 33.3 Ω. This is then extended by a proper length and transformed to 150 Ω with 70.7 Ω again. Ultimately, the three transformed stems at the final terminal are parallelly combined to reach 50 Ω. Fig. 3 depicts the reflection coefficient and broadside array gain. The IBW at 50 Ω port impedance under −10 dB is 28.6–41.6 GHz (37.0%), which is well preserved compared to that of the unit cell. Furthermore, the broadside array gain within the IBW is 4–6.3 dBi.

Fig. 1(d) illustrates the 4 × 4 array of subarrays composed of Fig. 1(a). Fig. 4 presents the reflection coefficient and array gain attained by scanning the beam along the xz- and yz-planes. With regard to the xz-plane (E-plane), the IBW is found to be 28.5–41.8 GHz (37.8%) when all elements are equally excited. This value remains well preserved up to 45°. In contrast, the reflection coefficient for the yz-plane (H-plane) degrades slightly to reach −8 dB, while the broadside array gain within the IBW is 14.6–18.6 dBi. Furthermore, it is observed that on scanning the beam from 0° to 45°, it degrades by 1 dB to reach 3 dB on both planes. In this context, it is worth noting that the broadside array gain almost reaches the gain limit, where it is calculated using 4πW² × 16/λ². In other words, a very high aperture efficiency of around 77.2% is achieved. Fig. 5 illustrates the array gain patterns of the scanned beams at three different frequencies. They were scanned using high directive beam patterns up to 45°. At 38 GHz, it is seen that a side lobe grows when scanning at 45°.

Table 1 presents a comparison of the performance of various wideband patch array antennas. In [2], researchers employed subarray techniques and series patch configuration, achieving wideband fractional bandwidth (FBW) (21.1%) and a very thin profile. However, the aperture efficiency attained was poor since the series patch structure used was not an efficient radiator with regard to its physical size. In contrast, [3] and [4] achieved aperture efficiencies of 43.2% and 46.6%, respectively, although their FBWs were below 7%. Meanwhile, [5] attained improved FBW and aperture efficiency compared to [2], but the thickness was about three times thicker than it. Furthermore, the large unit cell size in [5] could not steer the beam because of grating lobes. For [6], the FBW was found to be overwhelmingly wider than those noted in the other studies due to the application of the tightly coupled concept. However, it represents a high-cost solution and its unit cell size was too small (0.26λ_c × 0.26λ_c). In addition, it employed two separate plates, resulting in a high profile (0.14λ_c). Therefore, considering all the performance factors, the array antenna proposed in this study offers the advantages of wideband FBW characteristics (37.8%), not too small or too large unit cell size (0.58λ_c × 0.58λ_c), beam scanning capability, low-profile configuration (0.08λ_c), and high aperture efficiency (77.2%) at a low cost.

This letter demonstrates that the proposed low-profile tightly coupled dipole subarray offers a fitting solution to address the two limitations of conventional array antennas—narrow band and low aperture efficiency. Therefore, this antenna design is poised to excel in various wireless communication scenarios in which lowprofile, wideband, and beam-scanning capabilities are crucial.