Super-Wideband Monopole Printed Antenna with Half-Elliptical-Shaped Patch

Abstract

Super-wideband (SWB) antennas have emerged as a promising technology for next-generation wireless communication systems due to their ability to transmit and receive signals across a wide frequency spectrum. A half-elliptical-shaped patch antenna for a super-wideband antenna is proposed in this paper. The proposed antenna was composed of a half-elliptical-shaped patch with a microstrip feedline and a partial ground plane with a triangular inset and a bent edge ground plane. This proposed antenna was designed using Taconic TLY-5 with a dielectric permittivity of 2.2 and a total dimension of 200 × 220 × 1.57 mm³. The proposed antenna demonstrates a bandwidth of 23 GHz (from 0.5 GHz to 23.5 GHz) with a bandwidth ratio of 47:1.

1. Introduction

Modern-day portable wireless communication systems require a high data rate, capacity, and resolution, and thus, the design of an antenna with a wide frequency range is gaining importance in the field of wireless communication technology. With the advancement of wireless technologies, there is a great demand for compact and enhanced bandwidth antenna with great radiation properties and multiband functionality to support various applications for both civilian and military applications [1,2,3]. The compactness of the antenna plays an important role for the integration of RF circuits and handheld devices. The Federal Communications Commission (FCC) designated an unlicensed frequency range of 3.1 to 10.6 GHz for ultra-wideband (UWB) technology in 2002 [4].

A UWB antenna is an antenna with a ratio bandwidth of 3.4:1, which enables short-range and high-speed data transmission between electronic devices. However, the current wireless technology demands a super-wideband (SWB) antenna to cover both short and long-range transmission. An SWB antenna is an antenna with a ratio bandwidth of 10:1 and impedance bandwidth at a 10 dB return loss. Due to its large bandwidth, SWB antennas support high-channel-capacity and high-data-rate applications such as voice and video transmission, making them suitable for various wireless applications along the frequency spectrum. SWB technology can be used in Global Positioning Systems (GPSs), personal communication services (PCSs), industrial, scientific, and medical (ISM) applications, Bluetooth technologies, wireless local arena networks (WLANs), microwave imaging devices, satellite communication systems, military defense systems [5,6], and more recently, massive MIMO applications [7].

An SWB antenna does not have a predefined range of working frequency like a UWB antenna; as long as the antenna bandwidth ratio is 10:1 or more, with a return loss less than −10 dB and a VSWR less than 2 over the range of the frequency, the antenna is considered an SWB antenna.

Several wideband antennas for SWB applications have been demonstrated in numerous studies.

In previous research by Agrawall et al. [8], the authors investigated various patch shapes to identify optimal configurations for antennas with high bandwidth ratios. Their findings suggest that circular and elliptical geometries exhibit the most promising results, achieving bandwidth ratios exceeding 10:1. Moreover, in [9], the authors suggest that halving a circular antenna design may improve the impedance bandwidth. The technique of using a semicircular shape for widening the impedance bandwidth is quite popular; as demonstrated in [10], utilizing a circular-like antenna design can significantly broaden the impedance bandwidth when compared to conventional antenna shapes, and it can achieve a wide bandwidth (1 GHz to 19.4 GHz).

The combination of a semicircular patch antenna with a coplanar waveguide (CPW) feed technique has been shown to be a promising approach for achieving bandwidth ratios exceeding 10:1, a characteristic of super-wideband (SWB) antennas. In [11,12], partial semicircular patch antenna combined with modified CPW ground plane (semi-elliptical shape) has been demonstrated to achieve bandwidth ratios exceeding 10:1, reaching 25 GHz and 60 GHz, respectively. A circular phi-shaped patch also shows the same characteristics as a UWB antenna with a frequency range of 2.75 GHz to 71 GHz [13]. Several studies of CPW with a modified ground plane shape [14,15,16,17] and a microstrip antenna with a modified partial ground plane [18,19,20] have also been investigated. However, most reported designs achieve bandwidths limited to frequencies above 1.3 GHz.

The introduction of a slit or inset at the modified ground plane of microstrip antennas is another promising technique for enhancing the impedance bandwidth [21]. The use of different slit shapes has been studied, with square slits [22,23,24], half-circle slits [25,26], and triangle slits [27] being shown to significantly improve the impedance bandwidth. The latter type of slit shows the lowest resonant frequency of 1.44 GHz.

Utilizing a tapered microstrip feedline in conjunction with a small gap between the partial ground plane and the antenna presents another intriguing approach for improving the bandwidth in super-wideband (SWB) antennas. Studies such as [28,29,30,31,32] have demonstrated the effectiveness of this technique in increasing the maximum resonant frequency up to 70 GHz. Combining this with a semicircular shape, as described in [33,34,35,36], can reduce the minimum resonant frequency to as low as 1.3 GHz and 0.3 GHz, respectively, while maintaining bandwidth ratios exceeding 10:1. These techniques have also been studied in the Thz region [37], achieving a similar bandwidth enhancement to that in the mmWave region.

The development of fractal antenna designs also contributes to the advancement of super-wideband (SWB) antennas. As shown in [38,39], octagonal fractal geometries can achieve a wide resonant frequency range, spanning from 10 GHz to 50 GHz. Furthermore, incorporating a Sierpinski fractal within the octagonal shape and introducing an inset at the partial ground plane, as presented in [40], can further reduce the minimum frequency of the SWB antenna. The benefits of employing elliptical partial ground planes extend to fractal antenna designs, as demonstrated in [41,42,43]. These studies report that hexagonal- and star-shaped fractal antennas achieve bandwidth ratios exceeding 11:1 when incorporating elliptical partial ground planes. While the aforementioned discussion on fractal patterns highlights their potential application in SWB antenna design, the complexity of designing such elements presents a significant challenge.

This paper presents a novel super-wideband (SWB) antenna design utilizing a half-elliptical radiator and a modified partial ground plane with a triangular inset. The proposed antenna exhibits a bandwidth ratio exceeding that of previously cited references and without design complexity. The proposed antenna achieves a bandwidth ratio greater than 10:1 across the observed frequency range of 0.5–23.5 GHz.

The proposed antenna’s low operating frequency of 0.5 GHz, coupled with a bandwidth ratio exceeding 10:1, makes it suitable for a wide range of applications, including UHF/VHF applications, mobile communication systems, and radar systems.

2. Antenna Design

The geometrical parameters of the patch antenna are computed using rectangular patch formulas and then tuned to two elliptical-shaped patches to achieve the desired lowest working frequency. One of the methods of achieving a low resonant frequency is by increasing the capacitance between the patch and the ground plane of the microstrip antenna. Increasing the capacitance between the patch and ground plane can be achieved by making the patch wider, adding parasitic elements such as slots and shorting pins, or adding an additional patch on the ground plane side, or it can also be achieved by modifying the patch shape to adopt meandering or serpentine structures. Rectangular patch formulas are determined using Equations (1)–(4) [44].

$$W={\displaystyle \frac{c}{2f\sqrt{{\epsilon }_r}}}$$

(1)

where W is dimension of patch width.

$${\epsilon }_{eff}={\displaystyle \frac{({\epsilon }_r+1)}{2}}+{\displaystyle \frac{({\epsilon }_r-1)}{2}}{\left[1+10{\displaystyle \frac{h}{W}}\right]}^{-{\displaystyle \frac{1}{2}}}$$

(2)

$${\displaystyle \frac{\Delta L}{H}}=0.412{\displaystyle \frac{({\epsilon }_{eff}+0.3)(\frac{W}{h}+0.262)}{({\epsilon }_{eff}-0.258)(\frac{W}{h}+0.813)}}$$

(3)

$$L={\displaystyle \frac{c}{2f\sqrt{{\epsilon }_{eff}}}}-2\Delta L$$

(4)

where L is dimension of patch length.

The proposed super-wideband monopole antenna is shown in Figure 1. The antenna is printed on a Taconic TLY-5 dielectric substrate with a permittivity (ε_r) of 2.2 and a height (h) of 1.57 mm. The physical dimensions of the substrate are Ws (200 mm) × Ls (220 mm). These large dimensions are proposed to accommodate the low working frequency of 0.5 GHz.

Based on the rectangular patch dimensions from Equations (1) and (4), the proposed antenna is tuned to achieved an SWB performance by reshaping it into a semicircular shape with a partial ground plane (Figure 1a). Then, in Step 2 (Figure 1b), the antenna ground plane edge is modified into a bent shape. The next step is to adjust the R1 and R2 of the semicircular shape (Figure 1c). The final step is to add a triangular slit at the ground plane to achieve an SWB performance (Figure 1d). The performance of the antenna in each step will be explained in Section 3.

The dimensions of the proposed antenna are listed in

Table 1. Dimensions of proposed SWB antenna in mm.

Parameters	Dimension	Parameters	Dimension
Ws	200	R2	20
Ls	220	R3	15
Wf	4.8	W1	3
Lf	50	L1	35
D	177	Lg	49
R1	140

.

3. Results and Discussion

3.1. S-Parameters

The proposed SWB monopole antenna is analyzed in terms of the reflection coefficient (S₁₁), as shown in Figure 2 and Figure 3.

Figure 2 demonstrates the reflection coefficient (S₁₁) of the proposed SWB antenna for all steps. In Step 1, a patch with values of R1 = 100 mm and R2 = 60 mm and with a rectangular defected ground plane of Lg = 49 mm was used; the circle-like patch caused many ripples throughout the span of the frequency, which caused the S₁₁ value to be higher than −10 dB. Step 2 involved bending the edge of the defected ground plane with a radius R3 = 15 mm; this bending technique reduced the rippling throughout the span of the frequency, and both Step 1 and Step 2 had the same lowest −10 dB frequency shift at 0.56 GHz. Step 3 involved changing the patch from a circle-like shape to an elliptical-shaped patch and increasing the R1 value (R1 = 140 mm) and reducing the R2 value (R2 = 20 mm); this elliptical configuration achieved the same S₁₁ ripple as in Step 2, but the lowest −10 dB frequency shift from 0.56 GHz to 0.54 GHz. Steps 1, 2, and 3 all have the same issue, which is that in the frequency range from 1 to 1.5 GHz, there are frequencies that have an S₁₁ value higher than −10 dB, which caused the antenna to have a low bandwidth and bandwidth ratio. This issue can be solved by introducing a triangle inset on the defected ground plane (Step 4), which shifts the lowest −10 dB frequency back to 0.5 GHz, and achieving a super-wideband characteristic from 0.5 GHz to over 40 GHz with a bandwidth of over 39.5 GHz and a bandwidth ratio of over 80.

Figure 3 demonstrates the reflection coefficient of the proposed SWB antenna with a variation in the ground triangular inset length (L1). At L1 = 5 mm, this exhibits a working frequency of 0.536–1.03 GHz and 1.18–40 GHz where there are frequencies above −10 dB in the range of 1.03–1.18 GHz, which causes a small bandwidth ratio value. At L1 = 15 mm, this enhances the S₁₁ performance at the range of 1.03–1.18 GHz, which now means the antenna has a working frequency from 0.53 to 40 GHz. At L1 = 15 mm, this shifts the lower frequency from 0.53 GHz to 0.525 GHz. This shift in the lower working frequencies is caused by the increment of L1, wherein at L1 = 35 mm, the lowest working frequency of the proposed antenna is 0.5 GHz.

Table 2. Electrical characteristic summary of designed antenna in various L1 values.

No	Design	Operating Frequency, GHz	Bandwidth	Bandwidth Ratio
1	L1 = 5 mm	0.536–1.03	0.494	1.92
		1.18–40	38.82	33.89
2	L1 = 10 mm	0.53–40	39.47	75.47
3	L1 = 15 mm	0.525–40	39.475	76.2
4	L1 = 20 mm	0.52–40	39.48	76.92
5	L1 = 25 mm	0.514–40	39.486	77.82
6	L1 = 35 mm	0.5–40	39.5	80

summarizes the electrical characteristics for the various L1 values including the bandwidth and bandwidth ratio, which are computed using Equations (5) and (6).

$$Bandwidth=f_H-f_L$$

(5)

$$Bandwidth Ratio={\displaystyle \frac{f_H}{f_L}}$$

(6)

where $f_H$ is the highest working frequency, which is 40 GHz, and $f_L$ is the lowest working frequency of the proposed antenna.

3.2. Gain and Radiation Characteristics

The proposed SWB antenna is simulated using CST Microwave Studio and produces peak gains of 11.2 dBi. At 0.5 GHz, the proposed antenna exhibits an omnidirectional radiation pattern due to the excitation of the dominant mode. However, as the frequency increases, higher-order modes begin to excite, and the presence of a partial ground plane disrupts the ideal radiation pattern from the ground plane towards the patch, resulting in the emergence of increased minor lobes (Figure 4).

Figure 5 presents the normalized radiation pattern of the antenna in a polar plot. As the frequency increases, the minor lobes become more distinct, and the antenna exhibits a more directional radiation pattern with the main beam oriented towards the top semicircle. Figure 6 depicts the simulated gain and the radiation efficiency of the proposed antenna across a frequency range of 0.5 GHz to 40 GHz. The antenna exhibits two peak gains: 8 dBi at 10 GHz and 10 dBi at 38 GHz, while the radiation efficiency is more than 80% at all frequency range.

3.3. Measurement Results

The proposed antenna (Figure 7) was connected by a 2.92 mm connector and measured using a Rohde & Schwarz ZNB40 vector network analyzer (VNA) (Munich, Germany) operating at frequencies up to 40 GHz. Figure 8 illustrates a comparison between the simulated and measured results.

The simulated and measured results exhibit a discrepancy. The S₁₁ parameter (re-flection coefficient) starts to increase at 10 GHz, resulting in a limited −10 dB bandwidth only up to 22.5 GHz. To address the discrepancy between the simulated and measured results, the 2.92 mm connector (Figure 9) was accounted for in the simulation model to analyze its impact on the S₁₁ parameter compared to a waveguide port.

Adding the 2.92 mm connector model to the simulation clearly shows a change in the results when compared to the simulation without it (Figure 10). The simulated −10 dB bandwidth with the connector only reaches 23.5 GHz, while by using waveguide port, the −10 dB bandwidth can reach up to 40 GHz. Figure 11 shows a comparison of the measurements with the simulation model that includes the 2.92 mm connector. The results agree very well up to 23.5 GHz, with a bandwidth ratio of 47. Beyond this point, the simulation shows a significant increase in the reflection coefficient, followed by another decrease at around 38 GHz.

A significant difference exists in the simulation results obtained with and without the connector model, despite both simulations yielding the same characteristic impedance (Zo) of 50 Ω. To investigate the source of the differences, the surface current is analyzed across a range of frequencies.

Anomalous surface current distributions are observed at frequencies above 23 GHz. At this frequency, in simulations without connector model, the direction of the surface current appears to propagate back towards the port rather than radiating outwards towards the antenna (Figure 11), as is the case for frequencies between 0.5 GHz and 23 GHz. Figure 11a–d shows the surface current at 29 GHz, when the phase are 90°, 135°, 180°, and 235°, respectively.

However, when the connector model is included, the simulations show a bidirectional surface current flow in the transmission line. While some energy propagates from the antenna towards the port, additional energy also propagates from the connector towards the antenna. This phenomenon highlights the distinct energy flow difference between the two models (Figure 12). As illustrated in Figure 1, the surface current direction appears to be opposite to each other. The differences in the S₁₁ results between the simulations with and without a connector model at frequencies exceeding 23 GHz are likely attributed to the significantly altered energy flow patterns observed in this regime.

4. Comparison of Proposed SWB Antenna with Reported Works

Table 3. Comparison of the reported SWB antennas.

Ref.	Dimension (λ0)	Substrate Material	Operating Frequency (GHz)	Bandwidth Ratio	Result Type
[8]	0.98 × 0.98	Metal Disc	1.21–13	1:10.7	Measurement
[9]	0.38 × 0.38	Metal Disc	3–12	1:4	Measurement
[10]	3.8 × 3.8	FR4 (h = 0.8 mm)	1–19.4	1:19.4	Measurement
[11]	0.67 × 0.5	FR4 (h = 1.6 mm)	4.9–25	1: 5.1	Measurement
[12]	4.9 × 3.9	FR4 (h = 0.8 mm)	1.99–60	1:30	Measurement
[13]	3.18 × 3.07	FR4 (h = 1.6 mm)	2.75–71	1:25.8	Measurement
[14]	1.11 × 0.98	RT5880 (h = 1.6 mm)	0.95–13.8	1:14.52	Measurement
[15]	2.16 × 1.44	AgHT−8 (h = 3 mm)	3.15–32	1:10.5	Measurement
[16]	2.09 × 1.96	FR4 (h = 1.6 mm)	1.3–40	1:31	Measurement
[17]	1.67 × 3.49	FR4 (h = 1.6 mm)	4.1–60	1:14.6	Measurement
[18]	1.79 × 1.36	FR4 (h = 1 mm)	2.9–40	1:13	Measurement
[19]	3.65 × 4	FR4 (h = 0.787 mm)	1.25–50	1:40	Measurement
[20]	1.45 × 1.35	FR4 (h = 1.6 mm)	2–32	1:16	Measurement
[21]	0.9 × 0.9	RT5880 (h = 1.57 mm)	2.75–30	1:10.9	Simulation
[22]	1.41 × 0.97	FR4 (h = 1.6 mm)	2.5–29	1:11.6	Measurement
[23]	1.03 × 0.69	FR4 (h = 1.2 mm)	2.3–23	1:10	Measurement
[24]	0.97 × 1.46	FR4 (h = 1.6 mm)	2.6–27	1:10.3	Measurement
[25]	1.46 × 1.4	FR4 (h = 1.6 mm)	1.7–20	1:11.8	Simulation
[26]	1.01 × 1.24	FR4 (h = 1.6 mm)	1.44–18.8	1:13	Measurement
[27]	3.47 × 3.08	RT5880 (h = 1.57 mm)	1.25–47.5	1:38	Measurement
[28]	1.09 × 1.09	FR4 (h = 1.6 mm)	3.6–36.5	1:10	Simulation
[29]	2.35 × 2.35	FR4 (h = 1.6 mm)	3–50	1:16	Measurement
[30]	2.22 × 2.22	RT5880 (h = 0.787 mm)	3.4–70	1:20.5	Measurement
[31]	1.44 × 0.9	FR4 (h = 0.8 mm)	2.9–30	1:10.3	Measurement
[32]	2.66 × 3.32	RT5880 (h = 1.57 mm)	3.6–43.5	1:11.8	Measurement
[33]	1.62 × 1.3	RT5870 (h = 1.57 mm)	1.3–20	1:15.3	Measurement
[34]	2.62 × 2.62	RT5880 (h = 1.57 mm)	0.3–20	1:66.6	Measurement
[35]	3.1 × 3.1	RT5880 (h = 0.254 mm)	5.1–100	1:19.6	Simulation
[36]	0.84 × 1.16	FR4 (h = 1.6 mm)	1.05–32.7	1:31	Measurement
[37]	1.73 × 2.12	FR4 (h = 0.001 mm)	2200–60,000	1:27.2	Simulation
[38]	4 × 4	RogersTMM (h = 1.524 mm)	10–50	1:50	Simulation
[39]	7.78 × 8.89	RT5880 (h = 0.787 mm)	3.71–337	1:90	Simulation
[40]	1.06 × 1.78	RT5880 (h = 1.57 mm)	3–35	1:11.6	Measurement
[41]	1.55 × 1.5	FR4 (h = 1.6 mm)	4.6–52	1:11.3	Measurement
[42]	1.69 × 1.57	FR4 (h = 1.6 mm)	3.4–37.2	1:11	Measurement
[43]	1.2 × 1.2	RT5880 (h = 1.6 mm)	1.9–20	1:10.5	Measurement
This Work	7.6 × 7.6	TLY-5 (h = 1.57 mm)	0.5–23.5	1:47	Measurement

shows a comparison of the performance of the proposed design with recently published SWB antenna works. As is well known, achieving a high bandwidth ratio requires minimizing the lowest operating frequency and maximizing the highest operating frequency. In this regard, the proposed antenna exhibits a combined performance of the high bandwidth ratio (1:47) and the lowest minimum resonant frequency (0.5 GHz) amongst the compared designs.

In references [12,16,19] and [27], the bandwidth ratio exceeds 1:30, but the lowest frequency is limited to 1.25 GHz. Some other works also demonstrate the capability to extend the maximum frequency up to 71 GHz, as seen in reference [13]. Additionally, references [17,29] and [30] show maximum frequencies exceeding 40 GHz. However, while extending the maximum frequency, the lowest frequency also increases and never drops below 2 GHz.

References [10,14,33] and [36] demonstrate the lowest frequencies of 1 GHz, 0.95 GHz, 1.3 GHz, and 1.05 GHz, respectively. In none of them did the frequency go lower than 0.95 GHz, although there were bandwidth ratios exceeding 1:10. Reference [34] demonstrated a lower minimum resonant frequency (0.3 GHz) when compared to our proposed antenna, but the operational bandwidth was limited, only reaching a maximum frequency of 20 GHz.

5. Conclusions

A super-wideband monopole antenna with a half-elliptical-shaped patch was designed for various applications. The proposed antenna was developed using a Taconic TLY-5 substrate with a microstrip feedline and a half-elliptical-shaped patch for which the dimensions of the elliptical shape were derived from a rectangular microstrip patch formula with an optimization to achieve the desired lower frequency. The proposed antenna exhibits a bandwidth of 23 GHz (from 0.5 GHz to 23.5 GHz) with a bandwidth ratio of 47:1.

When designing super-wideband (SWB) antennas for the millimeter-wave (mm-wave) regime, it is crucial to carefully consider the connector model within the simulation environment. Accurately modeling the connector can ensure the simulation results closely match the measurements.