Keywords

1 Introduction

Compact circularly polarized antennas are accelerating their demand in the field of wireless communication systems. In most of the wireless communication applications circularly polarized antennas have been employed to reduce the matching between transmitter and receiver. Generally, the circular polarization will be achieved by feeding two orthogonal signals of 90° out of phase with equal amplitude to the non-radiating and radiating edges of the patch antenna [1,2,3,4]. To have a high circular polarization and better axial ratio, generally dual feeding technique is preferable but it requires a lot of board space and an external polarizer [5, 6]. To avoid these problems with current printed board technology, single feed simple technique is preferred.

To design circular polarization antennas, slot antennas have been receiving much attention from last one decade [7,8,9,10]. These slot antennas have larger bandwidth compared with conventional microstrip antennas due to their bidirectional radiation characteristics. As per the babinet’s principle the slot antennas are nothing but microstrip antenna complementary structures. The perturbation technique can be applied on slot antennas to get circular polarization and these slot antennas can mitigate manufacturing tolerances of perturbation segments [11, 12].

In this chapter, a monopole antenna with defected ground structure is proposed to attain circular polarization. The slots on the ground plane are nothing but combination perturbation of notches and stubs to attain proper acceleration bandwidths. A microstrip line feeding technique is being used in the current structure.

2 Antenna Geometry

Two antenna models are proposed in this work. The antenna model 1 consisting of broadband characteristics and antenna model-2 exhibiting dual band characteristics. A diagonal-fed patch with a pair of notches or stubs along with two opposite edges can provide circularly polarized waves. When the slot edges are modified along with the feeding position by considering orthogonal modes of same magnitude and phase difference of 90°. Two stubs are placed in the design to attain circular polarization and impedance bandwidth. A commercial FR4 substrate with dielectric constant εr = 4.4 and dielectric loss tangent, i.e., \( { \tan }\,\delta = 0.0 2 \) is used in the current design of antennas. The square slot is rotated by 45° and 135° away from the feed line. The total perturbation area \( \Delta {\text{A}} \) is obtained by

$$ \left( {\frac{\Delta A}{A}} \right)Q_{0} \cong \frac{1}{2}, $$
(1)

where, A is area of square patch without perturbations and \( Q_{0} \) is quality factor of patch.

While designing slot antenna with stubs we fixed the stub length to obtain better circular polarization performance and after that the length of the stub is tuned to get impedance matching. The dimensional characteristics and the designed antenna structures are shown in Table 1, Figs. 1 and 2.

Table 1 Antenna dimensions in mm
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Fig. 1
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Broadband antenna model 1

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Fig. 2
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Dual band antenna model 2

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The antenna models are initially designed with CST studio software and the basic simulation work is performed for the identification of performance characteristics. The FDTD simulation approach is considered and the antenna parameters are analyzed in this work.

3 Results and Discussion

The design and simulation of the proposed antenna models are carried through CST and the findings are presented in this section. Figure 3 gives the return loss characteristics of the broadband antenna model 1 and dual band antenna model 2. The broadband antenna model 1 is resonating in the band of 1.9–2.7 GHz and the dual band antenna model 2 is resonating in two bands (1.7–2.3 and 2.5–2.9 GHz). The dual band antenna notching the frequency band of 2.3–2.5 GHz, where Wireless LAN and Bluetooth applications are blocked.

Fig. 3
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Simulated reflection coefficient of antenna models (Broadband and Dual band)

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The simulated voltage standing wave ratios of both the antenna models are presented in Fig. 4. Antenna model 1 is impedance bandwidth of 32% and whereas antenna model 2 is showing 25 and 16% at fundamental and second resonant band. Figure 5 shows the impedance characteristics of the antenna models with respect to resonating frequencies. It is been observed that between 1.7 and 2.1 GHz the impedance is close to the ideal impedance of 50 Ω. The time-domain analysis of the current models with respect to port signals are presented in Figs. 6 and 7.

Fig. 4
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Simulated VSWR of antenna models (Broadband and Dual band)

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Fig. 5
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Simulated impedance characteristics of antenna models (Broadband and Dual band)

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Fig. 6
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Time-domain analysis of antenna model 1

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Fig. 7
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Time-domain analysis of antenna model 2

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The radiation characteristics of the designed antenna models are presented from Figs. 8, 9, 10, 11 and 12. The radiation pattern from Fig. 8 shows that the antenna is providing monopole like radiation at 1.9 GHz and showing gain more than 4.7 dB in the desired direction. Figure 9 provides the similar kind of radiation pattern with gain of 5 dB at 2.1 GHz. At 2.4 GHz the gain is more than 5 dB.

Fig. 8
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Radiation pattern of broadband antenna at 1.9 GHz

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Fig. 9
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Radiation pattern of broadband antenna at 2.1 GHz

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Fig. 10
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Radiation pattern of broadband antenna at 2.4 GHz

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Fig. 11
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Radiation pattern of dual band antenna at 1.9 GHz

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Fig. 12
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Radiation pattern of dual band antenna at 2.1 GHz

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The dual band antenna model 2 radiation characteristics are collected and presented in Fig. 11 for 1.9 GHz. The maximum gain is 4.83 dB at 1.9 GHz and 4.9 dB at 2.1 GHz. The radiation is like monopole like radiation and the antenna models are maintaining desired gain of more than 4.5 dB at all the resonating frequencies and showing very low gain at notch band.

The broadband antenna model 1 is showing circular polarization in the frequency range of 2.2–2.45 GHz. The axial ratio bandwidth for circularly polarized antenna is around 38% (Fig. 13).

Fig. 13
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Axial ratio versus frequency of broadband antenna model 1

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The surface current distribution of the antenna models are presented in Fig. 14. For antenna model 1 the most of the current density is focussed around the defected ground whereas for antenna model 2 the current density is concentrated more at lower half only. The radiating element is contributing towards radiation rather than slotted ground.

Fig. 14
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Surface current density on antenna model 1 and model 2 at 2.1 GHz

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4 Conclusion

Two monopole antenna models are designed in this work. The antenna model 1 is designed to operate in the broadband with circular polarization and the antenna model 2 is designed to operate in the dual band. Antenna model 1 of broadband is showing impedance bandwidth of 32% and gain more than 5 dB in the operating band. Antenna model 2 of dual band is showing impedance bandwidth of 25% at fundamental resonant frequency and 16% at second resonant frequency with gain more than 4.5 dB. The designed antenna models are showing excellent radiation characteristics in the wireless communication applications bands of mobile, wireless LAN and Bluetooth with good impedance matching.