Keywords

1 Introduction

Printed antennas are widely used in wireless communication systems owing to the essential characteristics such as lightweight, low cost, easy fabrication, and low profile. CPW-fed antennas can be easily integrated with the surface-mount devices as compared to microstrip-fed antennas owing to the presence of radiator and ground in a solitary plane. Further, the wireless device that works at multiple frequencies has led to the design of diverse categories of antennas. Therefore, the CPW-fed printed antenna is a worthy solution for the reason that diverse resonance modes can be easily achieved. In the open literature, the various CPW-fed/microstrip-fed/ACS-fed printed/planar antenna structures have been investigated [1,2,3,4,5,6,7,8,9] to achieve the diverse design objectives. Numerous low profile printed multiband antennas have been realized by employing multiple radiating elements in the form of an inverted L-shaped and T-shaped strips/stubs [1,2,3], F-shaped slot [4], split-ring resonator (SRR) [5], stub-loaded rectangular patch [6, 7], vertex-fed pentagonal ring slot [8], and a meandered strips [9]. Unfortunately, the reported multiband antennas have been important shortcomings such as massive size and unusable bandwidths. In comparison, this work has small size (14.5 × 30 × 1.6 mm3) and simple structure to realize the desired bands for wireless applications.

In this paper, a compact asymmetrical CPW-fed antenna loaded with two L-shaped branched stubs and a horizontal stub is proposed to achieve triple-band characteristics for 2.4/5.2/5.5 GHz WLAN and 3.5/5.5 GHz WiMAX applications. The reflection coefficient behavior of each configuration is compared to demonstrate the influence of distinct stubs on the antenna behavior, and also, the current distribution with radiation patterns at three resonant frequencies is discussed. Antenna design, configuration, and analysis of the proposed multistubs loaded printed antenna are described in Sect. 2. The conclusion is made in Sect. 3.

2 Design Procedure, Antenna Configuration, and Analysis

This section is describing the design procedure, antenna configuration, and analysis of the proposed antenna. The initial antenna design starts from an asymmetric CPW-fed printed antenna [i.e., Ant. 1]. The structure of the first stage of the proposed antenna is shown in Fig. 1 [see Ant. 1], is formed from an asymmetrical 50 Ω CPW-fed rectangular antenna, and is simulated on CST MWS simulation software. From the |S11| response of Ant. 1, shown in Fig. 2, it is noticeable that the antenna exhibits resonance at about 4.2 GHz. In the second stage, by integrating an asymmetrical long L-shaped stub in Ant. 1, Ant. 2 is formed and it creates the additional resonance at about 2.6 GHz. To shift the resonances in lower side to become usable bands of Ant. 2, an asymmetrical short L-shaped stub is integrated into Ant. 2 with a specific gap in the upper side to form Ant. 3. Further, by embedding a horizontal stub in Ant. 3 with a specific gap in the lower side, Ant. 4 [proposed antenna] is formed to create surplus resonance band at about 5 GHz in Ant. 3 without affecting the lower resonances.

Fig. 1
figure 1

Step-by-step evolution of the different antenna structures

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Fig. 2
figure 2

|S11| comparison of the evolution stages of the antenna structures

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These antennas are designed on a FR-4 dielectric substrate with εr = 4.3, thickness of 1.6 mm, and tanδ = 0.025. These are fed by an asymmetrical CPW feed with width Wf = 2.5 mm and identical gaps of 0.3 mm. |S11| comparison of the evolution stages [see Fig. 1] is shown in Fig. 2. The comparison of design stages and their respective impedance bandwidths (IBWs) and fractional bandwidths (FBWs) is summarized in Table 1. The geometry of the proposed antenna with labeled parameters is illustrated in Fig. 3. The optimized dimensions of the proposed multiband antenna are as follows: L = 30 mm, W = 14.5 mm, W1 = 8.5 mm, W2 = 2.9 mm, W3 = 8 mm, Wf = 2.5 mm, Wg = 9.2 mm, Ws = 1 mm, L1 = 18.2 mm, L2 = 6.8 mm, Lf = 13.5 mm, Lg= 6.5 mm, d = 5.3 mm, d1 = 8.5 mm, and d2 = 3 mm.

Table 1 Comparison of impedance and fractional bandwidth variations for evolution stages of the antenna structures
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Fig. 3
figure 3

Geometry of the proposed asymmetrical CPW-fed multistubs loaded  printed antenna

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To elucidate the mechanism of resonances, the surface current vectors at three resonances at 2.44, 3.66, and 5.25 GHz are illustrated in Fig. 4a–c, respectively. It can be perceived from Fig. 4a, the current mainly on the asymmetrical long L-shaped stub (i.e., Lg, d, W1, and L1 pointed out in Fig. 3) in the forward direction which designates that the first resonance mode is excited at about 2.44 GHz.

Fig. 4
figure 4

Surface current distributions of the proposed multistubs loaded antenna at three resonance frequencies: a 2.44 GHz, b 3.66 GHz, and c 5.25 GHz

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Correspondingly, it can be observed from Fig. 4b that the current mainly on the asymmetrical short L-shaped stub (i.e., Lg, d1, W2, and L2 pointed out in Fig. 3) in the forward direction which can lead to excite the second resonance mode at about 3.66 GHz. The third resonance mode is excited at about 5.25 GHz due to current flow in forward direction on horizontal stub (i.e., Lg, d2, and W3 pointed out in Fig. 3) as shown in Fig. 4c. So, it may be concluded that the respective resonances are mainly reliant on the total length of respective multistubs resonators.

Further, the design procedure for the excitation of resonances, the resonant frequency \(f_{{ri}}\), and effective dielectric constant \(\varepsilon_{\text{eff}}\) can be calculated by Eqs. (1) and (2) as:

$$f_{ri} = \frac{c}{{2L_{ti} \sqrt {\varepsilon_{\text{eff}} } }};\quad i = 1,2,3$$
(1)
$$\varepsilon_{\text{eff}} \approx \frac{{\varepsilon_{r} + 1}}{2}$$
(2)

where c = speed of light in vacuum (3 × 108 m/s), \(\varepsilon_{r}\) = dielectric constant of the substrate, and \(L_{ti}\) = total length of the ith stub.

The total length of first (i = 1), second (i = 2), and third (i = 3) resonances can be calculated by Eqs. (3), (4), and (5) as:

$$L_{t1} = L_{g} + d + W_{1} + L_{1} = 38.5\;{\text{mm }}$$
(3)
$$L_{t2} = L_{g} + d_{1} + W_{2} + L_{2} = 24.7\;{\text{mm}}$$
(4)
$$L_{t3} = L_{g} + d_{2} + W_{3} = 17.5\;{\text{mm}}$$
(5)

For first resonance, the simulated \(f_{r1}\) is 2.44 GHz while calculated by (1) is 2.39 GHz. Correspondingly, for second and third resonances, the simulated \(f_{r2}\) and \(f_{r3}\) are 3.66 and 5.25 GHz while calculated by (1) are 3.73 and 5.26 GHz, respectively, which is much close. Hence, the calculated resonances using design procedure are well agreeing full-wave simulation procedure.

Further, the effect of variation in length L2 of asymmetrical short L-shaped stub and length W3 of horizontal stub is studied while other parameters are kept constant as illustrated in Fig. 5a, b, respectively. When the length L2 varied from 5.8 mm to 7.8 mm, it mainly affects second resonance and shifted toward lower frequency side from 3.78 GHz to 3.54 GHz, respectively, while first and third resonances are almost unaffected. Similarly, when length W3 varied from 7 mm to 9 mm, it mainly affects third resonance and shifted toward lower frequency side from 5.79 GHz to 4.81 GHz, respectively, while first and second resonances are almost unaffected. From Fig. 5, it has been realized that L2 = 6.8 mm and W3 = 8 mm are taken for the desired band of operation.

Fig. 5
figure 5

Surface current distributions of the proposed multistubs loaded antenna at three resonance frequencies: a 2.44 GHz, b 3.66 GHz, and c 5.25 GHz

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The co- and cross-polarized components of the proposed antenna at three resonance frequencies 2.44, 3.66, and 5.25 GHz in xz- and yz-plane are shown in Fig. 6. Omnidirectional co-polarized patterns in xz-plane and bidirectional co-polarized patterns in yz-plane are observed. The gain at 2.44/3.66/5.25 GHz is 1.97/2.29/3.41 dBi, respectively, as displayed in Fig. 7.

Fig. 6
figure 6

Radiation patterns of the proposed multistubs loaded antenna at three resonant frequencies at a 2.44 GHz, b 3.66 GHz, and c 5.25 GHz

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Fig. 7
figure 7

3D patterns of the proposed multistubs loaded antenna at three resonant frequencies at a 2.44 GHz, b 3.66 GHz, and c 5.25 GHz

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3 Conclusion

An asymmetrical CPW-fed compact printed multiband antenna with branched multistubs resonators is proposed. By incorporating two L-shaped branched stubs and a horizontal stub at a suitable position on asymmetric CPW-fed printed antenna, the triple resonances at 2.44/3.66/5.25 GHz are achieved. It provides bandwidths of 140 MHz (2.36–2.50 GHz), 630 MHz (3.40–4.03 GHz), and 800 MHz (5.06–5.86 GHz). The design procedure and analysis of the proposed multistubs resonator are presented. Owing to the small size, multiband functionality, very simple structure with good radiation patterns, the proposed antenna is suitable for WLAN and WiMAX applications.