Antennas are considered the fundamental components in wireless communication systems because of the critical role they play in receiving and transmitting electromagnetic waves in free space. In this context, three important factors are integral to the utilization of antennas for practical applications, such as portable devices, health monitoring systems, wearable devices, and so on. The first factor is the operating bandwidth (BW). The required BW of an integrated antenna varies across wireless services. The second factor is flexibility. Flexible antennas, which can be bent and twisted, are preferred over solid planar antennas in modern electronic devices for the purpose of integration with various types of surfaces [1–3]. The third factor is the antenna type, which should be chosen based on the communication environment. Drawing on these factors, this paper focuses on designing an antenna with flexible and controllable BW characteristics for wide-coverage communication systems.

Notably, several flexible antennas suitable for application in industrial, scientific, and medical (ISM) bands have recently been reported [4–8]. In [4], a planar metasurface-based inverted–F antenna was proposed for size miniaturization and gain improvement. A dual-mode antenna comprising fabric material resonating at 2.4 GHz was presented for ISM applications in [5]. Antennas have also been designed for the dual operating bands of 2.3 and 2.6 GHz for ISM and Bluetooth services [6, 7]. In [8], a quad-band flexible antenna with a triband artificial magnetic conductor (AMC) reflector was proposed for WBAN applications. However, all these antennas have limited operating BW and relatively large sizes, which makes them less attractive for use in wideband communication systems and compact devices, respectively. In addition, while using microstrip patches and reflector-backed monopole structures can achieve a unidirectional beam with higher gain radiation, they cannot offer wide coverage.

To address these issues, this study employed a monopole structure to achieve wideband operation with wide coverage while maintaining a compact size. Notably, numerous ultra-wideband (UWB) antennas have recently been reported in the literature [9–17]. Various shapes of monopoles, such as square, rectangle, and elliptical patches, have also been proposed. In this context, the coplanar waveguide (CPW) feeding technique is utilized in most designs featuring single-layer configuration and wideband operation. In [9, 10], antennas with BWs of around 63% were designed for biomedical and 5G applications. Better performance was achieved in [11–14, 16], exhibiting BWs larger than 100%. Such antennas are suitable for use in wearable applications. However, most of the aforementioned designs are large in size—a feature that is not suitable for compact devices. Moreover, the gain and radiation patterns observed across the operating band were not stable. It is also worth noting that while these antenna designs exhibit the ability to provide wideband operation, their BWs could not be controlled.

This paper presents a compact antenna design with adjustable BW characteristics. The proposed antenna uses a monopole structure as its primary radiating element to carry out wide-coverage communications. It is fed by a CPW, making it a single-layer design. To control the operating bandwidth, a single stub was directly connected to the feeding line. Furthermore, two prototypes of the proposed antenna were fabricated and measured under various conditions. As for the results, the design without a stub achieved a wideband of 51.6% in unbending mode, while the design with the stub attained a smaller band of about 19.6%. Moreover, even when bent over a curved surface, both antennas worked efficiently. The proposed antennas also maintained a stable gain and radiation pattern across the operating bandwidth. Finally, it is worth noting that this paper proposes a method to control the operating BW of a monopole antenna using an additional stub. In addition to the normal mode, the proposed method works efficiently even under bending conditions along both the x- and y-axes. Notably, this finding is the main contribution of this paper. In terms of operating BW, the antenna operates within a limited BW, which can be attributed to the target of this study being attaining stable gain and radiation patterns across the operating band rather than a wide operating BW.

To examine the proposed concept, a commercially available finite element method-based High-Frequency Structure Simulator (HFSS) was used. The proposed ultrathin and flexible antenna with a compact size is based on a quadrilateral patch monopole, as shown in Fig. 1. Antenna-1 comprises a quadrilateral monopole as its primary radiating element. It is printed onto an ultra-thin Roger 5880 substrate with a thickness of 0.127 mm, a dielectric constant of 2.2, and a loss tangent of 0.0004 [18]. A feeding technique (CPW) is applied to excite the monopole. However, the overall dimensions of this antenna are quite large. To address this issue, the literature has reported various techniques for antenna miniaturization, including defected ground structures, meandered-line radiators, and fractal geometries. Among these methods, fractal geometries have shown promise in achieving 30%–70% miniaturization [19]. Therefore, this method is employed in this paper. The fractal monopole is presented in Fig. 1 as Antenna-2, which is also the second iteration of Antenna-1. Notably, for each iteration, additional patches that are two times smaller in size than those used in the previous iteration are added. Furthermore, for better comparison, Antenna-1 and Antenna-2 are optimized so that their operating bands are optimized so that their lowest operating frequencies are similar. The optimized dimensions for these antennas are listed in Table 1.

The simulated performance of the quadrilateral monopole (Antenna-1) and the miniaturized fractal monopole (Antenna-2) are outlined in Fig. 2. Fig. 2(a) shows that although the lowest resonances for both antennas are similar, the volume of Antenna-2, which is 32 mm × 38 mm × 0.127 mm, is about 30% less than that of Antenna-1, which is 40 mm × 44 mm × 0.127 mm. This finding can be attributed to the use of the fractal structure in Antenna-2, which contributes to increasing the electrical length of the antenna. In terms of the realized gain, Fig. 2(b) presents the gain attained in the broadside direction to better demonstrate the stable radiation pattern of the antennas. In this context, it should be noted that the peak gain is not relied on, since it represents the maximum gain over all user-specified directions in the far-field infinite sphere. As shown in Fig. 2(b), both antennas maintain a stable gain across the operating BW. Interestingly, the broadside gain values decrease significantly beyond the impedance BW. Notably, although the peak gain, which refers to the maximum gain in an arbitrary direction, is considered for most monopole antennas, it does not reflect stability in their operation. Moreover, the gain and radiation patterns are not consistent across all frequencies within the operating BW. In this study, the stable gain and radiation patterns of antennas are considered more significant than a wide operating BW.

In Section II, the compact monopole antenna with wideband operation is considered Antenna-2. In this section, a technique for controlling its BW is proposed. Fig. 3 displays the geometry of the compact monopole antenna with controllable BW, which is considered Antenna-3 in this study. To achieve this, a single stub arranged parallel to the ground plane was directly connected to the feeding line. The BW of Antenna-3 could be adjusted by tuning the stub's length. The dimensions (unit: mm) of Antenna-3 are as follows: L = 38, W = 32, w₁ = 22.6, w₂ = 11.3, w₃ = 5.65, w_f = 2.8, s = 0.4, w_g = 3.0, g = 0.2, and w_o = 1.0. The stub's length l_o is chosen to range from 0 to 16.0 mm.

The simulated results of the reflection coefficient |S₁₁| and the broadside gain with regard to variations in the stub's length, l_o, are shown in Fig. 4. The data indicate that l_o has a strong effect on both impedance BW and broadside gain. In Fig. 4(a), the −10 dB impedance BW is smaller when using a longer stub. The impedance BW gradually decreases from 58.1% (2.2–4.0 GHz) for the antenna without a stub (Antenna-2) to 20.4% (2.2–2.7 GHz) for the antenna with the 16-mm-long stub (Antenna-3). In terms of broadside gain, the results in Fig. 4(b) demonstrate significantly degraded gain values in the frequency band outside the impedance BW.

For better understanding, the current distributions of Antenna-2 and Antenna-3 at 3.4 GHz are depicted in Fig. 5. For Antenna-2, the current flows symmetrically along the edges of the radiating patch. In contrast, the phenomenon observed for Antenna-3 is completely different. In this design, instead of flowing along the radiating element, the current is highly concentrated around the stub, resulting in a significant degradation in matching performance at the given frequency. Therefore, it is concluded that the function of the additional stub is to alter the current flowing through the monopole, thus controlling the operating BW.

Finally, a performance comparison of the three analyzed antennas is summarized in Table 2, showing that Antenna-1 attained the largest operating BW of 81.6%. However, its gain variation of 7.8 dBi is very large. Meanwhile, gain variations at different operating states of Antenna-3 are significantly lower than that of Antenna-1—about 1.5 dBi compared to the latter's 7.8 dBi.

For validation, two antenna prototypes, a narrowband (Antenna-3) and a wideband (Antenna-2) design, are fabricated using a standard chemical etching process. Photographs of the fabricated antennas are shown in Fig. 6. The reflection coefficient is measured using an E5063A vector network analyzer and an antenna measurement chamber is utilized to test the far-field parameters with regard to the gain and radiation pattern.

Fig. 7 illustrates the theoretical and measured reflection coefficient results for the proposed antennas. In general, it is evident that the simulation and measurement results are well matched. For the antenna without a stub, wideband performance is achieved with a −10 dB impedance BW of 51.6%, ranging from 2.3 to 3.9 GHz. Meanwhile, the estimation for the antenna with the stub is smaller (about 19.6%, ranging from 2.3 to 2.8 GHz).

Fig. 8 shows the simulated and measured realized gain values of the proposed antennas. For the wideband antenna, the gain is more than 2.1 dBi across the operating BW of 2.3–3.9 GHz, and a maximum gain of 2.7 dBi is attained. For the narrowband antenna, the gain in the operating band (2.3–2.8 GHz) is greater than 1.8 dBi, with the maximum gain being 2.3 dBi.

Gain radiation patterns at selected frequencies of 2.5 GHz for Antenna-3 and 3.4 GHz for Antenna-2 are plotted in Fig. 9. Patterns are plotted for the two principal planes: x-z and y-z. Overall, the radiation patterns at the frequencies appear to be almost similar—omnidirectional in the y-z plane and monopole-like bidirectional in the x-z plane. In addition, the radiation patterns are symmetric around the broadside direction.

Since the proposed antennas are modeled on an ultra-thin substrate with a thickness of 0.127 mm, they are expected to work effectively when mounted on curved surfaces. Therefore, to conduct measurements in the conformal condition, the antennas are wrapped across a cylinder with a radius of 10 mm in both the x- and y-axes, as shown in Fig. 10.

The measured |S₁₁| results of the fabricated antennas in conformal and non-conformal conditions are presented in Fig. 11. It is evident that when the antennas are bent along the x- and y-axes, the measured reflection coefficients are stable, and the operating BWs remain unchanged for both designs.

The advantages of the proposed antenna can be interpreted from Table 3, which summarizes the performance of other flexible antennas with a monopole structure. The data in Table 3 indicate that, despite its compact size, the proposed antenna is able to achieve a comparable operating BW. Moreover, the gain variation between the operating BW is small at 0.6 dB. Stable radiation is another advantage of the proposed design compared to the others.

Although a larger BW was achieved in [12, 14, 16], these designs feature some critical drawbacks, such as significantly large dimensions, high gain variations, and unstable radiation patterns across the operating BW.

This paper presents the design for a flexible antenna featuring compact size and controllable BW characteristics. A monopole structure with an omnidirectional radiation pattern was employed to achieve wide-coverage communications. To control the operating BW, an additional stub was directly connected to the feeding line. The −10 dB impedance matching BW was adjusted by tuning the stub's length. Furthermore, to validate the proposed concept, two antenna prototypes—one with wideband operation and the other with narrowband operation—were fabricated and tested. In the non-conformal condition, the design without the stub exhibited a measured BW of 51.6%, while the other design comprising the stub attained a smaller BW of about 19.6%. Under the conformal condition, the measured data demonstrated that the proposed designs work effectively in the event of bending in both the x- and y-axes. Therefore, the proposed antenna can be used for wearable applications. The antenna is also suitable for UWB systems, in which it can contribute to changing the operating BW based on the BW of the signal.