Monopole antennas are widely operated in broadcasting networks. The main benefit of monopole antennas is their wide bandwidth. In addition, they are small in dimension, are small in shape, are cheap, require only a simple fabrication process, and are suitably fitted with mini-sized electronics instruments [1]. Broadband antennas provide a high speed with wide bandwidth, are energy efficient, and have low spectral coverage [2]. Monopole antennas with different shapes and sizes regarding their radiating patches [3] and ground planes are used for wideband applications.

A half-circular patch antenna with a circular-like ground plane energized by a coplanar waveguide feed line that is useful for ultrawide-band application was published in [4]. A modified triangular-like broadband antenna that provides a 0.45–3.060 GHz impedance bandwidth with a 6-dBi peak gain was suggested for TV communication and ultra-high-frequency band application [5]. The antenna provides. An antenna using a modified asymmetric feed line and T slot in the ground plane to increase bandwidth from 0.28 GHz to 2.27 GHz was reported [6]. A monopole antenna containing two tree-like patches, a bent slot, and four split ring resonators that provide a 1.15–2.90 GHz bandwidth was created for a wireless network [7].

An antenna with a leaf-shaped metal patch and a modified bent rectangular ground plane uses two steps coplanar waveguide feed line to provide 3.1–10.6 GHz bandwidth. That antenna was proposed for ground penetrating radar application in one study [8]. A U-like microstrip antenna with two slots on the ground plane was reported for wideband applications [9]. A cross-like antenna that has a symmetrical slant ground at two top corners and two slant slots was suggested for covering a bandwidth of 3.1–11 GHz [10]. A heart-like antenna was made, consisting of a modified ground plane and tapered feed line, covering a bandwidth of 3.04–11 GHz for high-frequency imaging applications [11]. A curve-shaped monopole antenna was proposed for mobile and wideband applications [12]; this antenna gives 1.5–5.5 GHz bandwidth with a maximum 3.2 dBi gain. In [13], a flag-shaped monopole antenna was suggested for covering a large bandwidth (3.1–12 GHz). A broadband monopole antenna consisting of a square patch with a curved edge slot and a rectangular ground plane was described in [14]. A compact elliptical antenna with a Sierpinski fractal structure to broaden the operating bandwidth from 700 MHz to 9 GHz was also proposed [15]. In [16], a circular printed monopole antenna with a rectangular slot was presented for ultra-wideband (UWB) applications. It covers a bandwidth of 3.1 GHz to 11.5 GHz, with a 1.3 dBi gain. A rectangular patch antenna with a rectangular ground plane was found to provide more than 9 GHz of bandwidth [17]. Using a defected ground structure, a rectangular antenna was found to provide 4.6 GHz bandwidth with a 4.3 dBi gain, as reported in [18]. A G-shaped co-planar wave guide feed antenna was used for broadband applications in [19]. UWB was achieved using a folded patch and asymmetric ground plane in [20].

The current paper contains a tree-shaped monopole antenna with a vase-like ground plane. This antenna yields a tested 9.3 GHz (3.8–13.1 GHz) bandwidth, a measured peak gain of 6.5 dBi, and measured peak efficiency of 77.51%. The antenna provides good radiation patterns. These characteristics make the monopole antenna useful for wideband wireless applications.

The proposed antenna is designed and fabricated on an FR4 substrate of thickness = 1.6 mm, relative permittivity = 4.4, and loss tangent = 0.02. A microstrip feed line having 50 Ω resistance is used to energize the patch. This feed line has a length (F_L) of 21 mm and width (F_W) of 3.06 mm. The geometry of the proposed antenna is shown in Fig. 1. The snapshot of the proposed antenna is shown in Fig. 2. The antenna occupies a small volume of 38×20×1.6 mm³ only.

The tree-like radiating patch (Fig. 1(a)) begins with a circular disk with a radius of 9 mm. The upper portion of the circular disk is etched so as to shape it as a semicircular patch. Two identical circular disks of a radius of 3 mm are removed from the top corner portion of the semicircular patch. Two identical circular patches of a radius of 4 mm are placed at two coordinate positions P₁ and P₂ of the patch, as shown in Fig. 1(a). The reference coordinate position is indicated as P₀ (0, 0, 1.6), and the coordinate positions of the two circular patches are P₁ (23, −5, 1.6) and P₂ (24, 4, 1.6), as depicted in Fig. 1(a). Next, a patch (circular) of a radius 3.6 mm is added in the middle position to complete the radiating patch. A vase-shaped ground plane is shown in Fig. 1(b), which consists of a rectangular ground plane of length = 16 mm and width = 20 mm. An elliptical ground is added to the rectangular ground plane. The elliptical ground portion has dimensions of L₂ = 15.38 mm and W₂ = 7 mm; it is placed at the coordinate position of P₃ (15.5, 5, 0) as shown in Fig. 1(b). The top and bottom views of the fabricated antenna are shown in Fig. 2.

The four-stage detailed design procedure of the proposed antenna is illustrated in Fig. 3. The proposed antenna design procedure starts with a circular patch antenna. A rectangular slit of length L₁ and width W₁ is made on the initial circular patch, resulting in Antenna A. Furthermore, two circular slits of radius R₂ are made on Antenna A, generating Antenna B. Next, a circular patch of radius R₃ is inserted on the top edge of the semicircular patch, along with two circular patches of radius R₄ at positions P₁ and P₂, resulting in Antenna C. Finally, an elliptical patch with a major axis of 3 mm and ratio of 2.5 is added on the top edge of the partial ground plane of Antenna C, generating the proposed antenna design (Antenna D).

A comparison of the S₁₁ versus frequency responses of all the antenna designs discussed in the four-stage antenna design procedure is given in Fig. 4. Fig. 4 shows that the proposed antenna (Antenna D) offers the widest impedance bandwidth compared with the other intermediate antenna designs.

A detailed parametric study on the key design parameters has been carried out to understand the effect of each parameter on antenna performance.

The parametric study is conducted on the radius of the initial circular patch antenna (R₁), where R₁ is varies from 7 mm to 10 mm with an increment of 1 mm. The S₁₁ and gain versus frequency responses corresponding to the variation of R₁ are given in Fig. 5. From the S₁₁ versus frequency plot, the impedance bandwidth of the antenna increases with an increase of R1. However, from the gain versus frequency plot, the antenna gain decreases as R₁ increases. Considering both the impedance bandwidth and gain, R₁ = 9 mm is considered the optimal value to implement the proposed antenna.

Next, the width of the rectangular slit (W₁) made on the initial circular patch varies from 3 mm to 8 mm, with an increment of 1 mm. The S₁₁ and gain versus frequency response corresponding to the variation of W₁ is shown in Fig. 6. From the S₁₁ versus frequency plot, the impedance bandwidth of the antenna increases with an increase of W₁. However, from the gain versus frequency plot, the antenna gain decreases as W₁ increases. Considering both impedance bandwidths and gain, W₁ = 6 mm is considered the optimal value to implement the proposed antenna.

Next, the radius of the two circular slits (R₂) of Antenna B varies from 1 mm to 5 mm, with an increment of 1 mm. The S₁₁ and gain versus frequency responses corresponding to the variation of R₂ are shown in Fig. 7. From the S₁₁ versus frequency plot, the impedance bandwidth of the antenna increases with an increase in R₂. However, from the gain versus frequency plot, the antenna gain decreases as R₂ increases. Considering both the impedance bandwidth and gain, R₂ = 3 mm is considered the optimal value to implement the proposed antenna.

Next, the parametric variation of the radius (R₃) of the circular patch inserted on the top edge of the semicircular patch and radius (R₄) of the two circular patches at positions P₁ and P₂ of Antenna C is studied simultaneously. R₃ varies from 1.6 mm to 3.6 mm, and R₄ varies from 2 mm to 4 mm, both with an increment of 1 mm, respectively. The S₁₁ and gain versus frequency responses corresponding to the variation of R₃ and R₄ are shown in Fig. 8. From the S₁₁ and gain versus frequency plots, both the impedance bandwidth and gain of the antenna increase with an increase in R₃ and R₄. Considering both the impedance bandwidth and gain, R₃ = 3.6 mm and R₄ = 4 mm are considered the optimal values to implement the proposed antenna.

Furthermore, the parametric variation of the ground plane length (G_L) is also studied. G_L varies from 14 mm to 17 mm, with an increment of 1 mm. The S₁₁ and gain versus frequency responses corresponding to the variation of G_L are shown in Fig. 9. From the S₁₁ and gain versus frequency plots, both the impedance bandwidth and gain of the antenna increase with an increase in G_L. Considering both the impedance bandwidth and gain, G_L = 16 mm is considered the optimal value to implement the proposed antenna.

Finally, a parametric study of the different dimensions of the elliptical patch inserted on the top edge of the ground plane is also conducted. The major axis and ratio of the elliptical patch vary from 2 mm to 3 mm and 1.5 to 2.5, respectively, with an increment of 0.5 mm. The S₁₁ and gain versus frequency responses corresponding to the variation of different dimensions of the elliptical patch are shown in Fig. 10. From the S₁₁ and gain versus frequency plots, both the impedance bandwidth and gain of the antenna increase with an increase in the different dimensions of the elliptical patch. Considering both the impedance bandwidth and gain, major axis of 3 mm and ratio of 2.5 are considered the optimal values of the elliptical patch for implementing the proposed antenna.

In this section, the simulated and tested results (Fig. 11) of the proposed antenna are discussed. The measured S₁₁ parameter is examined using the R&S ZNB20 vector network analyzer (Rohde & Schwarz, Munich, Germany). The 9.4 GHz (3.6–13 GHz) bandwidth is obtained by simulating the proposed antenna using the HFSS software simulation tool, which provides a simulated resonant frequency of 6.5 GHz and percentage bandwidth of 144.62%.

The results show a 9.3-GHz bandwidth for the proposed antenna. The bandwidth ranges from 3.8 GHz to 13.1 GHz, providing a percentage bandwidth of 136.76%. The resonant frequency occurs at 6.8 GHz. Using three intermediate stages, the proposed antenna provides 9.4 GHz broadband ranges from 3.6 GHz to 13 GHz.

The gain versus frequency plot is shown in Fig. 12. A peak gain of 6.5 dBi (measured) is achieved at 10 GHz. The radiation efficiency of the proposed antenna is measured at three distinct frequencies of 6.8 GHz, 7.3 GHz, and 8 GHz. Here, 73.46%, 77.51%, and 68.11% of measured radiation efficiencies are obtained at 6.8 GHz, 7.3 GHz, and 8 GHz respectively.

The normalized radiation patterns (Φ = 0° plane and Φ = 90° plane) of the proposed antenna appear in Fig. 13(a)–(c), giving monopole-type radiation patterns in both the Φ = 0° plane and Φ = 90° plane.

In the case of the initial circular patch antenna, only the 0.25 GHz (3.75–4.0 GHz) bandwidth has been obtained. Using three intermediate stages, the proposed antenna provides a 9.4 GHz (simulated) bandwidth ranging from 3.6 GHz to 13 GHz. Much more bandwidth has been obtained from the proposed design.

The performance of the proposed antenna is compared with other antennas in the literature based on the electrical dimension, bandwidth, peak gain, efficiency, and fractional bandwidth of the antenna. The results are listed in Table 1. Table 1 shows that regarding the bandwidth and gain, the designed antenna provides the best performance. Here, λ₀ is the free-space wavelength at the lower edge measured frequency of 3.8 GHz. In the proposed design, the main objective has been to design a broadband antenna with high gain, which makes the proposed design highly suitable for broadband applications.

A small monopole antenna is suggested. It provides broadband with good gains. The measurements have been performed on the fabricated antenna. The experimental results are in parity with the simulated results. The bandwidth, gain and radiation patterns of the fabricated antenna are found to be in parity with the simulated results.