In recent years, the landscape of wireless communication has witnessed a significant transformation driven by the emergence of private LTE (long-term evolution) and 5G networks. These networks have rapidly evolved from being a niche segment to becoming the foundation of connectivity solutions across various industries. The liberalization of the radio frequency spectrum is a key driver accelerating the adoption of private LTE and 5G networks. Regulatory authorities around the world have provided access to shared and locally licensed spectrum. For instance, the three-tiered citizens broadband radio service (CBRS) spectrum-sharing system in the United States, the allocation of 3.7–3.8 GHz and 28 GHz licenses for 5G campus networks in Germany, the issuance of 3.7 GHz and 26 GHz permits in Sweden, regulation governing local networks in the 3.8–4.2 GHz range of Norway, the local 5G network licenses in the 4.6–4.9 GHz and 28 GHz bands of Japan, the provision of the 4.8–4.9 GHz spectrum for private 5G networks in Taiwan [1]. Notably, the Korean government allocated 4.7 GHz and 28 GHz bands for e-UM 5G use, demonstrating its commitment to expanding the 5G ecosystem [2].

To support both 5G around 3.5 GHz and private LTE bands, an antenna must exhibit a fractional bandwidth of approximately 34.5% covering 3.4–4.82 GHz. Achieving such a wide bandwidth within the limited volume of user equipment (UE) poses a major design challenge. Conventional microstrip-fed and coplanar-fed antennas suffer from narrow bandwidth and strong sensitivity to substrate thickness. While, stripline-fed slot-coupled structures have recently attracted attention for their ability to provide strong electromagnetic coupling through multilayer configurations, offering improved impedance bandwidth. In [3], the authors presented a dual-band stripline antenna using RT/Duroid 5880 substrate. The achieved bandwidth was relatively narrow in the low band (10%) and high band (25%) and the large size of it needs further miniaturization. In [4], the introduction of vias around the slot enhanced the radiation efficiency of slot-fed patch antennas. In [5], similarly, a lumped-element model of the stripline coupling mechanism was reported in. Li et al. [6] proposed a 50 mm × 10 mm × 10 mm folded slots antenna for multiband operations. In [7], a 41.32 mm × 41.32 mm × 11.5 mm stripline-fed aperture-coupled antenna element was introduced. Including an extra slot in the bottom ground plane enables the bandwidth of this antenna to be 400 MHz for a return loss better than 15 dB with the center frequency 2.4 GHz. However, despite their improved performance, the antennas in previous studies still suffer from bulky geometries that demand additional miniaturization.

In this paper, a novel wideband antenna realized on an ultra-thin film laminate with a total thickness of only 0.146 mm is proposed. The antenna employs a stripline-fed dual-slot configuration etched on the top and bottom ground layers to enhance electromagnetic coupling and achieve broadband impedance matching. The proposed structure attains wideband operation across both private LTE and 5G frequency ranges while maintaining compact dimensions of 28 mm × 6 mm, making it highly suitable for edge integration in modern UE platforms. Our experimental results satisfactorily align with the intended design objectives.

Fig. 1 displays the structure of our proposed antenna consisting of two main sections, GCPW and stripline connected through a feeding via. This feeding via with a radius of 0.3 mm was designed to meet both transition performance and fabrication requirements (not shown here). The stripline is terminated by a shorting via. The stack-up is depicted in Fig. 2(c) embracing top and bottom copper layers applied onto dual polyimide (PI) substrates. This combination of copper/PI layers are adhered together through an adhesive center layer. By making use of the PI layer as the foundational substrate, the antenna design possesses the advantageous properties associated with PI film including superior electrical performance, heightened thermal stability, and robustness against environmental factors [8].

The dielectric constant (ɛ_r ) and loss tangent (tanδ) of the PI are 3.3 and 0.005, while those of the adhesive substance are 4 and 0.015, respectively. GCPW serves as the feeding line and a via fence is used to suppress higher order modes and improve its electromagnetic performance while meeting the fabrication requirements [9]. In the stripline section, a strip of a width (w_strip) 0.2 mm is enclosed by adhesive and PI layers (Table 1).

Without the slot, the stripline is directly shorted to the ground plane, preventing any radiation from the structure. In a conventional stripline-fed antenna, radiation typically occurs directionally through a single slot. However, to achieve omnidirectional radiation, a dual open-slot configuration is introduced, which effectively behaves as a folded monopole structure. In this configuration, the two parallel slots support coupled current paths that increase the effective electrical length while maintaining a compact physical size. This folded-monopole-like behavior enhances impedance matching to broader bandwidth and enables the radiator length to be reduced while maintaining resonance at 3 GHz. And then, the slot position and slot width are optimized to ensure proper impedance matching across the desired frequency range. Fig. 2(a) and Fig. 2(b) show the detailed geometric dimensions of the proposed antenna from the top and bottom views, respectively, while Figs. 3 and 4 illustrate the S₁₁ responses under variations of key design parameters. The most critical factors influencing performance are the slot dimensions and their positions, as these determine the effective electrical length of the antenna and thus its operating frequency, as observed in Fig. 3. Specifically, the width of the slot governs the coupling strength between the radiating element and the ground plane. As shown in Fig. 3(a), increasing the slot width reduces the coupling, resulting in a shift of the second resonance toward a higher frequency and a corresponding increase in reflection. The top slot position d_topslot, top slot width w_topslot significantly affect the high-frequency behavior of the antenna. As illustrated in the Smith charts, these parameters primarily impact the reactance in the high-frequency band, while their influence on the low-frequency band remains minimal. Additionally, the relative size of the top and bottom slots, expressed as the ratio w_botslot and w_topslot, along with the bottom slot position, plays a key role in determining the quality of impedance matching and reflection characteristics.

Fig. 5 shows the fabricated antenna prototype in the top and bottom view with an end-launch connector. For reflection coefficient measurement, the antenna is connected to the vector network analyzer (VNA) Anritsu MS46122B offering a frequency measurement range from 1 MHz to 43.5 GHz. The simulated and measured results of S₁₁ depicted in Fig. 6(a) exhibit an agreement on a wide bandwidth that meets the research objectives. The measured impedance bandwidth was recorded from 2.98 GHz to 5.14 GHz, and the fractional bandwidth is approximately 53.2% for S₁₁ < −10 dB, the reference level. The fabricated antenna was experimented in an anechoic chamber to evaluate its radiation efficiency and radiation pattern. The comparison of simulated and measured radiation efficiency versus frequencies is shown in Fig. 6(b). The radiation efficiency exceeds 50% over the entire frequency range and the peak is recorded at 3.5 GHz with a value of 84%, making it well-suited for wireless devices operating within mobile communication systems. The disparities observed between simulation and measurement results arise from several factors. Since proposed antenna uses a newly developed thin-film dielectric material, which may introduce slight dielectric inhomogeneity in initial prototypes due to early-stage fabrication processes. Fabrication tolerances and copper trace effects can also contribute to these differences. To address this, process uniformity is improved through supplier collaboration, dielectric mapping, and thickness verification, while simulations are updated to reflect real substrate properties. In the practical design, the impact of dielectric variation will also be analyzed through simulation to confirm that it remains within acceptable specified.

Fig. 7 presents the evaluation of the simulation and measurement results in terms of the normalized radiation pattern at 3 GHz, 4 GHz, and 5 GHz. This comparison focuses on the normalized pattern in the XY plane (or H-plane) with θ = 90° and the XZ plane (or E-plane) with φ = 0°. The proposed antenna exhibits an omnidirectional radiation pattern in the E-plane, with the measured results closely aligning with the simulation. A figure-donut shape is observed in the H-plane, and slight differences may arise from previously mentioned factors. In practical implementation, particularly when the antenna is integrated along the edge of UE, additional effects may occur due to asymmetric coupling with the chassis and nearby components. Such coupling can cause minor impedance detuning, efficiency reduction, and pattern distortion, which can be mitigated through optimized ground clearance, small matching adjustments, and careful placement away from metallic or high-dielectric structures.

Table 2 provides a performance comparison of our proposed antenna and prior works in thin antenna with λ₀ referring to the free-space wavelength of the lowest operational frequency [3, 10–16]. Compared to previously relevant works, our design exhibits significantly reduced physical dimensions, a broadened bandwidth, improved gain and radiation efficiency. These advantages position our antenna as a promising choice for a wide range of modern wireless communication applications

This study introduces a broad band strip feed line-based antenna design and demonstrates its practical feasibility. By integrating it with GCPW by via, the feeding method of the strip line has been simplified, enhancing its usability. Double asymmetric slots in the top and bottom ground layers and a shorting via connecting these layers to the strip result in resonance within the conventional strip line structure. The proposed antenna was fabricated and measured, and the experimented results closely align with the simulated data. Featuring a compact dimension of 28 mm × 6 mm with a thickness of 0.149 mm, this antenna provides a broad impedance bandwidth ranging from 2.98 to 5.14 GHz. Furthermore, it offers high gain, radiation efficiency, and an omnidirectional radiation pattern. Given these attributes, this antenna holds promise as a potential candidate for integration into mobile devices designed for private LTE and 5G communication systems.