1 Introduction

Compact printed monopole antennas with multiband operation is gaining attention as a result of the tremendous development of compact devices for wireless applications [1, 2]. Antenna is the main component of wireless communication devices for adopting standards like WLAN, WiMAX etc. Now a days market demand for compact wireless portable device is increasing rapidly. Hence, in regard to this demand, design of miniaturized multiband antenna has attracted many researchers [3,4,5,6]. Multiband antenna works for more than one standard is vital component for portable devices. Modified monopole antennas is considered as proficient among other antennas structures due to its simplest structures [7, 8].

The literature details various dual and multiband antennas [9,10,11,12,13,14,15,16,17,18,19,20]. In [9], a pair of L-shaped strips are combined to obtain triple band operation. However, large antenna dimension (i.e., \(33 \times 28\) mm2) is a major drawback. Two self similar ring radiator was designed in [10] for achieving triple band operation with a dimension of \(38 \times 25\) mm2. This antenna size is also large for compact devices. Li et al. [11] proposed a rectangular ring monopole antenna exhibiting triple band operation is designed using L-strips and inverted T-shaped stubs. In [12], L-Shaped strips on a \(30 \times 42\) mm2 substrate are used to obtain tri-band operation. Similarly, CPW fed slotted antenna with relatively larger dimension has been proposed in [13, 14]. Li et al. [15] proposed a triple band antenna that satisfies WLAN and WiMAX standards by employing fork shaped strips within a rectangular ring. The multiband operation is exhibited in [16] using fractal antenna using Koch iteration, however antenna size is considerably bigger i.e., \(33.5 \times 23.5\) mm2. Recently, in [17], using split ring resonator (SRR) a single narrowband at 1.1 GHz and ultra wide band (UWB) operation ranging from 2.3 to 6.1 GHz have been reported with an large antenna size of about \(34 \times 44\) mm2. A \(\lambda /4\) rectangular stub loaded with metamaterial for 2.45, 3.5 and 5.8 GHz frequency bands has been designed in [18]. In [19], a fractal multiband antenna with Sierpinsiki triangle geometry with L-Shaped strips was reported. With a defected ground structure and compact slot, UWB antenna was presented in [20]. However, it exhibits less gain at operating frequencies. However, smaller antenna operational bandwidth and large size continues to remain the major loopholes in the aforementioned antenna designs.

In this research article, a compact tri-band antenna of \(33 \times 17\) mm2 dimensions with four T-shaped strips stacked one above another inside a rectangular ring monopole is proposed. The antenna operating bands are 2.47–2.77 GHz, 3.3–3.7 GHz and 5.10–6.62 GHz. The effect of adding strips on frequency and impedance matching are detailed. The parametric analysis are performed to justify the selection of antenna dimension.

The organization of the paper is briefed as follows: Section II details the methodology of the proposed design. The influence of various antenna parameters on performance has been explained in the third section. Section IV highlights the comparison between simulation and measured results. Section V concludes the paper.

2 Antenna Design

The development stages of the proposed antenna is depict in the Fig. 1. The first stage i.e., Antenna I consists of a rectangular ring (\(w_{1} \times h_{1}\)), strip line (\(w_{8} \times h_{6}\)), incremented strip line (\(w_{7} \times h_{5}\)) and ground palne (\(w \times h_{7}\)). From [21], it is obvious that the tapered feed line gives better performance in terms of S11. It is observed from the Fig. 2 that, a dual band operation at 2.5 and 5 GHz is produced by the Antenna I. Then four T-shaped strips are stacked one over another inside the rectangular ring to produce Antenna II. This results in triple band operation as can be seen from the Fig. 2. Further to enhance the impedence bandwidth, a rectangular slot has been etched in the ground. Finally, the proposed antenna is evolved with triple oprating bands with ranges 2.47–2.77 GHz, 3.3–3.7 GHz and 5.10–6.62 GHz. This range corresponds to WLAN and WiMAX applications. The Fig. 3 depicts a detailed dimension layout of Antenna III. Figure 4 shows the fabricated antenna. The final dimensions of the proposed antenna are presented in Table 1.

Fig. 1
figure 1

Development stages of the proposed antenna

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Fig. 2
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S11 of various antenna in development stages

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Fig. 3
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Geometry of the proposed antenna

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Fig. 4
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a Front view and b Back view of the fabricated antenna

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Table 1 Measurements of the proposed antenna
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3 Parametric Analysis

The influence of various geometric parameters on the frequency parameter characteristics are detailed in parametric analysis. Initially, the effect of various stubs is analyzed. The Fig. 5 illustrates the different structures of the proposed design. The corresponding S11 performance is given in the Fig. 6. It is clear from the figure, the final shape marked w2 gives better performance in comparison with other structures. Additionally, it can be easily observed that on adding each stub the performance of the antenna gradually increases. After finalizing the structure, the effect of dimensions of the stubs are studied through parametric analysis.

Fig. 5
figure 5

Different structures of the proposed antenna

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Fig. 6
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S11 for different structures of the proposed antenna

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First, we analyze the effect of variation in length of the ground slot on the S11 performance as described in Fig. 7. It can be inferred that w9 = 7.2 mm produces only dual band operation, when w9 is increased to 8.2 mm, the antenna operates at three bands. Hence, in the antenna design the ground slot width is chosen as 8.2 mm.

Fig. 7
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Variation of S11 with respect to the variation in w9

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Next, to understand the effect of gap between the strips and ring monopole, the length of h4 is differed from 1.4 to 3.4 mm. The various S11 performance corresponding to different h4 values are plotted in the Fig. 8.

Fig. 8
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Variation of S11 with respect to the variation in h4

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It is obvious from Fig. 8 that the length of h4 affects the second operating band at 3.5 GHz. This parameter gives the flexibility of tuning the antenna and set at any frequency between 3 and 4 GHz. Here, we fixed the length as 2.4 mm in order to get 3.5 GHz as the operating frequency.

The Fig. 9 shows the variation of S11 for the various values of w5. To get the operating band at 3.5 GHz 3.1 mm has been chosen for the fabricated antenna. It is observed Fig. 9 that, the other values of w5 shifts the operating band towards 3 GHz.

Fig. 9
figure 9

Variation of S11 with respect to the variation in w5

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4 Measurement Results

Agilent N5230A vector network analyzer (VNA) is used to perform the proposed antenna design measurements. The Fig. 10 shows that there is minimal difference between measured and simulated results. Additionally, it can be inferred from the Fig. 10, that three bands of antenna operation are 2.47–2.77 GHz, 3.3–3.7 GHz and 5.10–6.62 GHz. These operating regions finds application in WLAN and WiMAX standards.

Fig. 10
figure 10

Comparative analysis between the simulated and measured result

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The surface current densities are given in Fig. 11. It is observed from figure that at 2.5 GHz, the current concentrates at the peripheral ring. The operating band at 3.5 GHz is due to the current at microstrip line. At 5.5 GHz, the current concentrates on ground slot as well as rectangular ring, thus validating the process of antenna design.

Fig. 11
figure 11

Surface current density at a 2.5 GHz, b 3.5 GHz and c 5.5 GHz

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Figure 12 shows the measured and simulated radiation pattern. The far field radiation pattern exhibits bidirectional pattern at center frequency, i.e., 2.5 GHz, 3.5 GHz and 5.5 GHz in E-Plane.

Fig. 12
figure 12

Simulated and measured radiation patterns of the proposed antenna at a 2.5 GHz, b 3.5 GHz and c 5.5 GHz

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Omni directional pattern is observed in the H-Plane. The antenna gains and radiation efficiency is simulated over the operating ranges. It can be seen from the Fig. 13 that the gain varies from 2 to 3.8 dB and the efficiency varies from 60 to 80% over the operating bands. Comparative analysis of the proposed antenna with the existing similar antenna designs is highlighted in Table 2, thus implying the superiority of the proposed design in terms of size, gain and bandwidth over its counterpart.

Fig. 13
figure 13

Simulated gain and efficiency of the proposed antenna

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Table 2 Comparison of proposed antenna with the existing designs
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5 Conclusion

A compact triple band antenna has been reported in this research article. The proposed design is compact in nature with dimension of \(33 \times 17 \times 1.6\) mm3. The printed monopole antenna exhibits triple band operation which supports important wireless communication standards like WLAN and WiMAX. The designed antenna achieves the omni-directional radiation pattern. It also achieves high gain and bandwidth. With these features and compact size, the proposed antenna could easily be adopted for real-time compact wireless communication devices.