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Wireless Personal Communications

, Volume 97, Issue 1, pp 683–693 | Cite as

A Modified Compact Ultra Wideband Antenna with Band Rejection for WLAN Applications

  • Geetanjali Singla
  • Rajesh Khanna
Article
  • 115 Downloads

Abstract

In this paper, a circular ultra wideband antenna is considered which comprises of a planar circular patch element on grounded FR4 substrate of 1.6 mm thickness generating the bandwidth of 6.8 GHz. To enable its extensive use in Wireless Local Area Network (WLAN) applications, it is modified by modulating the microstrip line to generate stopband from 5.9 to 9 GHz rejecting the extended C band and X band. The antenna ground plane is then drilled to form holes implementing EBG structures to obtain desired passband characteristics, from 1.97 to 2.65 GHz and 3.12 to 5.86 GHz suitable for WLAN(2.40–2.484/5.150–5.350/5.725–5.825 GHz) applications. The designed antenna shows good passband and stopband characteristics and return loss S11 and nearly omni-directional field pattern over the frequency band of the passband.

Keywords

Modulated microstripline EBG Filter UWB WLAN 

1 Introduction

With the advancing wireless technology, new approaches and procedures of making antenna circuits with enhanced performance are of great importance to any communication system. Wireless Local Area Network (WLAN), Worldwide Interoperability for Microwave Access (WiMAX) and Ulra WideBand (UWB) standards are most sought after in a variety of wireless application areas [1, 2]. Thereby Electromagnetic Band Gap (EBG) and Defected Ground Structures (DGS) for microwave and millimeter wave applications has gained much concern these years, for example microstrip lines incorporating EBG cells on ground plane have supported the improvement in passband-stopband filter characteristics [3]. These features have been used in bandstop or lowpass filter applications to eliminate the unwanted frequencies and to miniaturize microstrip filter structures [4]. This paper aims at simulation of wideband antenna with interference rejection using filter structures in antenna by incorporating a modulated microstrip line which produces a large stopband. The stripline is modified by inserting square patches in it. Then an EBG based structure is formed by etching holes in the ground plane to improve the passband characteristics. It contribute to the research for EBG based microstrip structures to improve the performance in both the passband and the stopband [5]. A different approach for the implementation of filters with EBGs is investigated.

2 Antenna Design and Parameteric Study

The geometry of the designed antenna is shown in Fig. 1a, b. It comprises of circular ultra wideband antenna with a planar circular patch element constructed on a 40 × 50 mm2 (Ls × Ws) FR4 substrate of 1.6 mm thickness. The reference antenna is taken from [6]. The value of the dielectric constant (εr) of the substrate is 4.4 and loss tangent tan δ is 0.18. It is placed along the x–y plane and centered at (0, 5.3, 0) in the Cartesian coordinate system. A reduced conductor plane is placed on one side of the substrate representing the ground plane. The other side of the substrate consists of a circular radiating patch of radius R = 12 mm which is attached to a 50 Ω microstrip line feed with width, wf = 2.8 mm and length, lf = 17.5 mm. The antenna so designed provides ultra wide bandwidth of 6.8 GHz.
Fig. 1

Geometry of the antenna patch. Geometry of antenna showing a top view. b bottom view

The antenna structure is altered to implement the bandstop property and improve the pass band characteristics. The final results of the antenna structure are the outcomes of different parameteric variations applied to the initial configuration of the design as specified above. The initial UWB antenna is obtained by calculating the length (Ls), width (Ws) of the substrate and ground plane. The radius of the circular patch (R) is calculated using the basic design equations stated in the transmission line theory [7]. Moreover, an additional sweep is applied on the radius of the circular patch as shown in Fig. 2 which finalizes the value of radius of circular radiator to be 12 mm covering 2.2–9 GHz band for UWB applications. Reduced ground plane is being used to optimize the antenna dimensions.
Fig. 2

Sweep results for the radius of main radiating circular patch

The microstripline is then modulated by inserting square patches of length la and width wa which is varied along x and y axis to obtain the requisite values la = 2.5 mm and wa = 3 mm as shown in Figs. 3 and 4 respectively. It affects the impedance bandwidth of the antenna by creating a stopband at 5.9 GHz, thereby rejecting the extended C band and X band.
Fig. 3

Sweep results for length of square patches on microstripline

Fig. 4

Sweep results for width of square patches on microstripline

The passband characteristics of the antenna are refined by scratching a column of four circular EBG structures in the reduced ground plane of the antenna beneath the modulated microstrip line. The radius of the circles is optimized to be 2 mm (as shown in Fig. 5) which thus obtains two bands from 1.97 to 2.65 GHz and 3.12 to 5.86 GHz for WLAN systems.
Fig. 5

Sweep results for radius of circular EBG structures scratched in the ground plane

3 Results and Discussions

The modified antenna formed by using modulated microstrip line by inserting square patches follows the principle of Bragg Reflection [8]. The length and width of square patches is symbolized by la and wa respectively. The width of the microstrip line is wf such that:
$${\text{w}}_{\text{a}} > {\text{w}}_{\text{f}}$$
(1)
This condition creates an EBG structure which increases the coupling between the microstrip line and the ground plane. This planar EBG edifice exhibits a bandgap when the Bragg reflection condition is fulfilled [7]. The period of the structure, d is specified by the distance amid the centers of the two adjacent patches. The period d, as defined by the Bragg reflection condition, is specified by:
$$\upbeta \cdot {\text{d}} =\uppi$$
(2)
where β is the guided wavenumber in the substrate material and \(\uplambda_{\text{g}}\) is the guided wavelength given by:
$${\uplambda}_{\text{g}} = \frac{\text{c}}{{{\text{f}}_{\text{o}} \sqrt {{\upepsilon}_{\text{eff}}}}}$$
(3)
where c is the speed of light in free space, \({\text{f}}_{\text{o}}\) is the center frequency of the stopband and \({\upepsilon}_{\text{eff}}\) is the effective permittivity of the substrate [7] given by:
$${\upepsilon}_{\text{eff}} = \frac{{{\upepsilon}_{\text{r}} + 1}}{2} + \frac{{{\upepsilon}_{\text{r}} - 1}}{{2\sqrt {1 + \frac{{12{\text{h}}}}{\text{w}}}}}$$
(4)
Using these equations, the period d equals half the guided wavelength given as:
$${\text{d}} = \frac{{\uplambda_{\text{g}} }}{2}$$
(5)
Using the relation described above the period d of the patches obtained is 8.97 mm and the length and width of the inserted patches in the microstrip line are both fixed to 4.49 mm × 4.49 mm respectively. This structure creates a stop band above 5.9 GHz.

A column of n identical circles is scratched in the reduced ground plane, beneath the microstrip line with n = 4. It reduces the coupling between the transmission line and the ground plane. The radius of the circles is r = 2 mm. The ratio r/d1 represents the filling factor which is indicative of the relative size of the EBG cell to the period of the configuration. To avoid overlapping between any two adjacent circles, it varies from 0 to 0.5 [9]. The optimal value of r/d1 is found to be 0.25 where a good compromise between the stopband and passband performance can be obtained.

The designed antenna is simulated using Computer Simulation Technology (CST) Microwave Studio (High Frequency FDTD Structure Simulator). It assimilates simulation of design, its visualization, modeling of the structure and its automation. The return loss of the antenna is obtained using S11 parameter. The surface current density/power flow and radiation pattern of the designed antenna are plotted. The designed antenna using the same geometry without the EBG structures is also included for comparison with the modulated microstrip line with EBG structures forming wideband reject antenna. The optimized antenna is then photolithographically fabricated and tested for its performance using Vector Network Analyser (VNA). Photographs of the fabricated antenna are shown in Fig. 6. The comparative return losses of the simulation with CST and the measurement on VNA are shown in Fig. 7. The simulated results and measured data have good agreement except for minor deviation which may arise due to fabrication process. According to the measured Voltage Standing Wave Ratio (VSWR), the proposed antenna with the EBG structures covers the bandwidth from 1.93 to 5.90 GHz with two pass bands from 1.97 to 2.65 GHz and 3.12 to 5.86 GHz with a band rejection performance 5.86 to 9.0 GHz.
Fig. 6

Fabricated antenna. a Front view. b Bottom view

Fig. 7

Comparison of simulated and measured results on Vector Network Analyser (VNA)

Figure 8a, b, c shows the excitation of surface currents and power flow at the resonant frequency of 2.43, 3.89 GHz and at frequency of 5.14 GHz respectively. Since the circular EBG structures are scratched on ground plane beneath the square patches on the microstripline, therefore it achieves high magnetic coupling between the patch and the circles. This magnetic coupling inhibits signal propagation at a certain frequency which enhances the band-stop characteristic. As can be seen in Fig. 8a, the current flow dominates on the circular EBG rings with their track opposite to the patch current flow. Due to this current distribution, the antenna does not respond at this frequency, whereas, in Fig. 8b feeble surface current distribution on the EBG circles is observed at the pass band.
Fig. 8

Surface current density of the antenna and power flow on the antenna at a 2.43 GHz, b 3.89 GHz, c 5.14 GHz

The simulated radiation patterns of the projected antenna at frequencies 2.43, 3.89 GHz and at frequency of 5.14 GHz are illustrated in Fig. 9. The antenna is tested in an anaehoic chamber and measurements are done and analyzed on Vector Network Analyzer. The radiation patterns exhibit that the antenna essentially radiates over a wide frequency band, almost omnidirectional for the three frequencies.
Fig. 9

Radiation pattern plots of the antenna. ai Azimuth radiation pattern plots of the antenna at 2.43 GHz, aii elevation radiation pattern plot of the antenna at 2.43 GHz, bi azimuth radiation pattern plots of the antenna at 3.89 GHz, bii elevation radiation pattern plot of the antenna at 3.89 GHz, ci azimuth radiation pattern plots of the antenna at 15.14 GHz, cii elevation radiation pattern plot of the antenna at 5.14 GHz

Results after Parametric analysis are illustrated in Figs. 10, 11 and 12. The fabricated antenna attains two pass bands from 1.97 to 2.65 GHz and 3.12 to 5.86 GHz with a band rejection performance from 5.86 to 9.0 GHz. The bandwidth of stopband is controllable by altering the width of the patches on the modulated microstripline. The circular periodic EBG structures are bored in the reduced ground plane of the antenna which creates passband with desired characteristics in WLAN bands with VSWR less than one.
Fig. 10

a Basic UWB microstrip antenna and its b return loss (S11 parameter)

Fig. 11

a UWB antenna with modulated microstripline and corresponding b return loss (S11 parameter)

Fig. 12

a UWB antenna with modulated microstripline and circular EBG structures in the reduced ground plane and corresponding b return loss (S11 parameter)

4 Conclusion

In this paper, a UWB circular antenna using Modulated microstripline and EBG structures is proposed and fabricated to be used in WLAN applications. The antenna creates a stopband from 5.9 to 9 GHz. It also provides permissible return loss in the frequency range from 1.92 to 5.9 GHz. The rejected band is obtained by integrating a Modulated microstripline that resonates at the required rejection frequency band with the circular EBG structures drilled in the ground plane of antenna. The measured data of the antenna matches well with the corresponding simulated results. Moreover it provides VSWR less than 1 for its working frequencies and almost omnidirectional patterns, over an entire UWB frequency band apart from the rejected band.

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Copyright information

© Springer Science+Business Media New York 2017

Authors and Affiliations

  1. 1.Department of Electronics and Communication EngineeringThapar UniversityPatialaIndia

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