A penta‐band microstrip slot antenna for S‐, C‐ and X‐band applications

Introduction

As multiple new wireless technologies have been added to modern wireless communication systems, including as many of them into a single wireless device would be ideal. Multi-band antennas can play a crucial role in meeting this demand for wireless services. Recent years have seen much interest in printed slot antennas to achieve multi-band operation due to their compact size, low profile, lightweight, and low cost.

Many multi-band antennas using different techniques have been proposed in the literature. These techniques for multiband antennas include a triple-band antenna by three separate meander-line types inverted-L radiators [1], a dual-band B-shaped patch antenna [2] and a penta-band patch antenna [3] with the partial ground, metamaterial integrated antennas for multi-band operation by employing a triangular split ring resonator (TSRR) [4] and a complementary split-ring resonator (CSRR) [5], using circular and triangular fractal structures [6], a quad-band magneto-electric (ME) dipole antenna designed by four Γ-shaped electric plates for achieving four operating bands [7], and a CPW-fed multi-band antenna [8]. Besides, several multiband antennas have been developed by the utilization of non-conventional radiating and substrate materials. In [9], a quad-band wearable antenna, designed with conductive fabric as a radiating patch and flexible denim as the substrate, has been presented. A rectenna operating at four frequencies, on the other hand, has been designed on a polyimide flexible substrate on which both the antenna and rectifier circuits are fabricated [10]. Additionally, two triple-band antennas realized on glass [11] and synthetic paper (Teslin) [12] substrates have been demonstrated.

Slot antennas are another popular design for multiband applications. Several slotted antenna structures have been reported in [13–18]. In [13], a technique to achieve dual-band dual-polarization operation by using a triangular-ring slot antenna with a substrate-integrated waveguide (SIW) structure has been reported. A triple-band microstrip slot antenna using a trapezoidal slot and a strip on the ground plane has been presented in [14] for worldwide interoperability for microwave access (WiMAX) and wireless local-area network (WLAN) applications. On the other hand, a slotted antenna by integrating a kite-shaped slot on the radiating patch [15], two slotted antennas with and without defected ground structure (DGS) [16, 17], and a reconfigurable fractal slot antenna [18] have been reported for penta-band operation. However, these antennas usually have comparatively complicated structures.

This letter reports a compact penta-band slot antenna for wireless applications of S-, C- and X-bands. The proposed antenna has a feed line on the upper plane and a slotted ground on the lower plane of the substrate. Five operating frequencies are achieved by integrating trapezoidal and triangular-shaped slots and two strips on the ground plane. The antenna shows a bi-directional radiation pattern with adequate gain at each band. The operating bands make the antenna suitable for Bluetooth, WLAN, WiMAX, Sub-6 GHz 5G applications, radiometry, intelligent transport systems (ITS), and radar applications.

Antenna design

Figure 1 shows the configuration of the proposed penta-band microstrip slot antenna designed on a dielectric substrate. The antenna consists of a microstrip feed line and a slotted ground plane. The microstrip feed line having a characteristic impedance value of 50 Ω is used to excite the antenna centrally. Simple slots and strips are introduced on the ground plane to realize different frequency bands with proper impedance matching.

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Figure 2 illustrates the design steps with reflection coefficient characteristics. A rectangular slot (30 mm × 20 mm) is etched on the ground plane having a dimension of 35 mm × 30 mm and found to operate just like a conventional slot antenna at 5.12 GHz, as shown in Step-1.

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In Step-2, a trapezoidal-shaped slot on the ground plane, placed just beneath the microstrip feed line, is added to the rectangular slot. This slot creates a higher frequency band at 11.06 GHz with a slight shift of the initial frequency to a lower band of 4.38 GHz.

In Step-3, a pair of triangular-shaped slots is embedded beside the trapezoidal-shaped slot. This addition creates a lower frequency band at 2.54 GHz with a better than 10-dB reflection coefficient along with 4.38 and 11.06 GHz.

In Step-4, a new operating frequency at 5.78 GHz is obtained with a better than 10-dB reflection coefficient by adding another pair of triangular-shaped slots on both sides of the previous triangular-shaped slots. In this step, four operating frequencies of 2.39, 4.28, 5.78, and 10.79 GHz are achieved.

In Step-5, embedding a pair of horizontal strips on the side walls of the rectangular slot while decreasing the side walls width of the slot located on the upper portion of the added horizontal strips helps to get better impedance matching at the 1st and 3rd operating bands. A slight frequency shifting is observed at both bands.

Finally, in Step-6, another frequency band at 4.60 GHz can be established between the 2nd band (3.46 GHz) and 3rd band (5.28 GHz) by adding another pair of horizontal strips above the previously added strips in Step-5. So, the proposed antenna operates at 2.22, 3.46, 4.60, 5.28, and 10.86 GHz.

Performance analysis and discussion

The photograph of the fabricated antenna is shown in Figure 3. The prototype has a dimension of 0.29λ₀ × 0.25λ₀ × 0.007λ₀ (λ₀ @ 2.48 GHz) and is etched on a polytetrafluoroethylene (PTFE) substrate with a dielectric permittivity of 2.15.

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Figure 4 illustrates the current distribution of the proposed penta-band antenna. It is observed that as the frequency increases, the current becomes more concentrated in smaller areas. At 2.23 GHz, the edge of the entire slot region contributes to accumulating the current. Conversely, currents at 3.46 and 4.60 GHz are distributed around both the pair and only the top pair of horizontal strips, respectively. During the following two higher operating frequencies, lower edges surrounding the triangular-shaped slots for 5.28 GHz and the trapezoidal-shaped slot for 10.86 GHz are responsible for concentrating more currents.

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Figure 5 depicts the reflection coefficient of the penta-band antenna. The performance of the prototype antenna is measured using a vector network analyzer (HP8510C). The measured 10-dB impedance bandwidths (BW) are 5.65% (2.41–2.55 GHz), 8.14% (3.31–3.58 GHz), 5.47% (4.52–4.76 GHz), 19.47% (5.10–6.20 GHz), and 20.10% (10.14–12.40 GHz). A slight mismatch due to the fabrication discrepancy and imperfect simulation environment is observed between measured and simulated results.

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Figure 6 shows the simulated gain pattern of the antenna in the xz-plane. It is seen that all five bands show a bi-directional linear polarization pattern with excellent cross-polarization (X-pol) performance. Moreover, adequate gains of 3.21 dBi at 2.22 GHz, 3.27 dBi at 8.14 GHz, 3.10 dBi at 4.64 GHz, 4.19 dBi at 5.65 GHz, and 5.16 dBi at 11.27 GHz are achieved.

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Table 1 compares the proposed antenna and the previously reported multi-band antennas. It is found that the proposed antenna has a compact size with five operating bands. Moreover, other performances, like the bandwidth and gain, are comparatively similar or better than other antennas.

Table 1 Comparison of the proposed antenna with other multi-band antennas.

Ref.	Antenna size (λ0 × λ0)	No. of band	Centre frequency (GHz)	10-dB Imp. BW (%)	Gain (dBi)
4	0.35 × 0.58	4	3.5, 4.1, 5.6, 9.7	—-	1.2, 0.6, 0.7, 2.09
6	0.73 × 0.51	3	2.5, 3.8, 5.3	42.5, 30, 11.3	2.98, 2.58, 3.34
14	0.29 × 0.25	3	2.5, 3.5, 5.5	22.2, 12.3, 23.2	3.86, 3.52, 4.32
15	0.37 × 0.37	5	3.5, 5.9, 6.7, 8.5, 9.8	7, 21, 8.12, 3.8, 36	1.2, 1.6, 2.1, 2.5, 2.7
17	0.18 × 0.15	5	1.57, 2.90, 3.82, 4.50, 5	5.1, 2.76, 2.09, 1.78, 1.6	2.65, 3.84, 1.07, 1.93, 3.92
This ant.	0.29 × 0.25	5	2.48, 3.45, 4.64, 5.65, 11.27	5.65, 8.14, 5.47, 19.47, 20.1	3.21, 3.27, 3.10, 4.19, 5.16

Conclusion

A compact penta-band microstrip slot antenna has been proposed and fabricated for S-, C- and X-band wireless applications. As penta-band is achieved through etching simple slots along with two pairs of strips on the ground plane, the structure is simple to fabricate. The antenna covers the frequency bands of 2.41–2.55, 3.31–3.58, 4.52–4.76, 5.10–6.20 and 10.14–12.40 GHz with an impedance BW of 5.65%, 8.14%, 5.47%, 19.47%, and 20.10%, respectively. Additionally, the antenna gives a gain in the range of 3.21–5.16 dBi. Therefore, the proposed antenna can be used for Bluetooth, WLAN, WiMAX, Sub-6 GHz 5G applications, radiometry, ITS, and radar applications.

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Author contributions

Muhammad Asad Rahman—Conceptualization; Formal analysis; Investigation; Software; Supervision; Writing—original draft; Writing—review and editing. MD Shahidul Islam—Conceptualization; Formal analysis; Investigation; Software. MD Shakil Hossain—Conceptualization; Formal analysis; Methodology; Software. Maodudul Hasan—Formal analysis; Validation; Writing—review and editing. Eisuke Nishiyama—Formal analysis; Validation; Writing—review and editing. Ichihiko Toyoda—Formal analysis; Validation; Writing—review and editing.

Funding information

The authors received no specific funding for this work.

Conflict of interest statement

The authors declare no conflicts of interest.

Data availability statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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