In this communication, a wideband circularly polarized (CP) Spidron fractal microstrip antenna is proposed based on the concept of embedded structures. The proposed antenna is excited by a tapered microstrip feedline. A wide 3 dB axial ratio (AR) bandwidth of 28.81% (3.09–4.13 GHz) is obtained by merging the CP bands of the Spidron fractal slot and patch antennas. In addition, a measured −10 dB reflection bandwidth of 47.25% (2.57–4.16 GHz) is reported. The measured results are in reasonable concurrence with the simulated results. The measured gain varies between 2.12 dBic and 3.56 dBic within the AR bandwidth.
National Research Foundation of Korea2014R1A5A10114781. Introduction
In modern communication systems, microstrip fractal antennas (MFAs) are useful due to their low profile, light weight, multiband or broadband capabilities, and ease of fabrication. Mostly, MFAs have been utilized as linearly polarized (LP) antennas, with the design objectives of broadband or multiband characteristics and size reductions [1–3].
Circularly polarized (CP) MFAs are usually preferred over the LP type because they offer advantages such as flexible orientation between the transmitter and receiver, the mitigation of multipath interference, and good mobility. In the literature, a single-feed method is employed for the excitation of circular polarization in MFAs, which require less space but possess a narrow 3 dB axial ratio (AR) bandwidth [4, 5]. To cope with the demand for modern wireless communication, a microstrip-fed fractal slot design with a 3 dB AR bandwidth of 22% has been developed [6]. However, only simulated results were presented. Recently, symmetric slot antennas have been proposed and designed in an effort to enhance the AR bandwidth [7, 8].
This study presents a simple and effective technique based on the notion of an embedded structure for the design of a compact wideband CP MFA. The proposed design consists of a Spidron fractal slot with an added rectangular slit and an embedded Spidron fractal patch. A microstrip tapered feedline is deployed for excitation, alleviating the use of a hybrid coupler [9]. Wide reflection and an AR bandwidth are obtained by merging the resonance and CP bands of the Spidron fractal slot and the patch. Details of the proposed antenna design along with the experimental results are discussed in the following sections.
2. Antenna Design
Figure 1 shows the geometry of the proposed Spidron fractal planar antenna, which is fabricated on a square Taconic RF-35 substrate with a dielectric constant (εr) of 3.5, with the width denoted by gw and thickness h. The construction of a Spidron fractal system for a slot antenna was initially proposed in earlier work [10]. A seven-iterated Spidron fractal slot with adjacent side length h1 and angle α is etched from the ground plane on top of the substrate. The adjacent side of the Spidron fractal slot is equidistant from the upper and lower sides and at a distance d from the left side of the substrate. A seven-iterated Spidron fractal patch with length h2 and angle β is placed inside the Spidron fractal slot and is displaced by n1 and n2 from the adjacent side of the Spidron fractal slot along the x-axis and y-axis, respectively. A rectangular slit of length sy and width sw protrudes outward from the Spidron fractal slot. This slit is positioned at a distance of x1 from the vertex and rotated at an angle γ with respect to the hypotenuse of the second right-angled triangle of the Spidron fractal slot. A tapered microstrip feedline of length f2 and tapered-end width y2 is attached to the bottom side of the substrate to excite the proposed antenna. The length and width of the feedline before tapering are f1 and y1, respectively. The feedline is displaced at a distance of fx from the adjacent side of the Spidron fractal slot. The orientation of the surface current in the proposed antenna is such that it provides a left-handed circular polarization (LHCP) sense. The overall dimensions of the proposed antenna are gw×gw×h.
Geometry of the proposed antenna: (a) top view; (b) side view.
In order to validate the AR performance of the proposed antenna, three designs, a Spidron fractal slot (Ant1), a Spidron fractal slot with an embedded patch (Ant2), and a Spidron fractal slot with an added rectangular slit and an embedded patch (Ant3), are investigated. These designs are excited by tapered microstrip feedlines with identical dimensions. Figure 2 shows the geometry of these antennas; the simulated results of the reflection coefficient and AR are depicted in Figure 3. It was found that Ant1 achieves dual-band characteristics with regard to the reflection coefficient, as well as a narrow 3 dB AR bandwidth. The lower band has a −10 dB reflection bandwidth of 11.98% (3.61–4.07 GHz), while the AR bandwidth corresponds to 5.22% (3.92–4.13 GHz). However, the overlapping bandwidth, which satisfies both the reflection coefficient and the AR criteria, corresponds to 3.75% (3.92–4.07 GHz). For Ant2, which is designed with an embedded Spidron fractal patch based on the Ant1 design, a −10 dB reflection bandwidth of 50.53% (2.47–4.14 GHz) was noted, while dual-band characteristics were observed for AR as the level around 3.3 GHz is slightly higher than 3 dB. Consequently, with the addition of a rectangular slit to Ant2, termed Ant3, the AR level is lowered from 3 dB, resulting in a wide 3 dB AR bandwidth of 37.39% (2.74–4.00 GHz). In addition, a −10 dB reflection bandwidth of 49.62% (2.47–4.10 GHz) is achieved, nearly identical to that of Ant2.
Geometries of Ant1, Ant2, and Ant3.
Comparison of Ant1, Ant2, and Ant3 in terms of (a) the reflection coefficient and (b) the axial ratio.
To verify the sense of circular polarization, the simulated magnetic current distributions of a Spidron fractal slot with a time-varying phase at 3.5 GHz in +z-direction are depicted in Figure 4. It should be noted that the vectors (M1,M2,M3,M4) represent the major surface current distributions and that their sum is represented by Mtotal. For t=0, as in Figure 4(a), the resultant vector Mtotal is directed towards lower right corner of the ground plane. When the phase changes to t=T/4, the resultant vector Mtotal rotates in a clockwise direction and becomes orthogonal to Mtotal in Figure 4(a). Therefore, the proposed antenna radiates LHCP waves.
Simulated magnetic current distributions of a Spidron fractal slot with time period T at 3.5 GHz: (a) t=0; (b) t=T/4.
3. Parametric Analysis
A parametric analysis is conducted in order to investigate the effects of varying the angle α of the Spidron fractal slot and the distance n1 (between the Spidron fractal patch and the slot along the x-axis) on the reflection coefficient and the AR. These simulations are performed using the ANSYS High-Frequency Structure Simulator (HFSS) software. During this analysis, only one design parameter is allowed to change at a time.
The size of the Spidron fractal slot changes with the value of angle α; the effect of this variation on the reflection coefficient and AR is shown in Figure 5. In the figure, it is evident that an increase in angle α provides better impedance matching as the capacitance of the slot counterbalances the inductance of the tapered feedline. The 3 dB AR bandwidth is also very sensitive to the value of angle α and continues to widen as α is increased. For α=30° and α=31.5°, dual-band CP characteristics are observed. The upper band, which lies around 4.5 GHz, does not satisfy the −10 dB reflection bandwidth criterion. When α=33°, the CP bands of the Spidron fractal slot and patch lie close to each other; therefore, a wide 3 dB AR bandwidth is realized. A further increase in the value of angle α makes the proposed design unrealizable, as the size of the Spidron fractal slot does not fit within the given dimensions of the ground plane.
Effect of variation of angle α of the Spidron fractal slot on (a) the reflection coefficient and (b) the axial ratio.
The effect of variation of distance n1 on the reflection coefficient and AR is depicted in Figure 6. It was observed that the −10 dB reflection bandwidth of the proposed design is determined by the variation of n1 and that an increase in n1 decreases the reflection bandwidth as the resonant bands of both structures move closer to one another. With this variation, the feeding position of the Spidron fractal patch is altered, which in turn modifies the surface current densities of the Spidron fractal slot and patch; hence the AR is subjected to variation. It was also noted that the upper CP band remains nearly identical, whereas the lower CP band is very sensitive to variations in n1. While, for n1=3mm, the reflection bandwidth is the widest, the AR bandwidth is not as wide as compared to its width when n1=5mm. Therefore, on the basis of the AR performance, the chosen value is n1=5mm.
Effect of variation in distance n1 on (a) the reflection coefficient and (b) the axial ratio.
Based on the parameter analysis, the finally determined parameters of the proposed antenna are shown in Table 1.
Geometric parameters of the proposed antenna.
Parameter
Value
h1
35 mm
α
33°
h2
20 mm
β
30°
n1
5 mm
n2
0.5 mm
h
1.52 mm
d
2.5 mm
gw
40 mm
sy
6 mm
sw
0.5 mm
γ
96°
x1
3.83 mm
f2
18 mm
y2
11.1 mm
fx
20 mm
f1
7 mm
y1
3 mm
4. Measurements and Discussion
After obtaining the optimum values of the design parameters from Table 1, a prototype of the proposed antenna was fabricated, as shown in Figure 7. The reflection coefficient of the fabricated antenna was measured using an Agilent 8510C network analyzer. Figure 8 depicts the results of a comparison of the measured and simulated reflection coefficients of the proposed design. The antenna attains measured and simulated −10 dB reflection bandwidths of 47.25% (2.57–4.16 GHz) and 49.62% (2.47–4.10 GHz), respectively. The discrepancy between the simulation and the measurement at the second resonant frequency band (3.8 GHz) can be attributed to the slight misalignment between the Spidron fractal slot on the top layer and the feedline on the bottom layer during the fabrication process. Figure 9 shows the measured and simulated ARs as well as the LHCP gains of the proposed antenna in the boresight direction (θ=0°). The measured 3 dB AR bandwidth corresponds to 28.81% (3.09–4.13 GHz), while the simulated result is 37.39% (2.74–4.00 GHz). It is of practical importance to note that the total AR bandwidth lies within the reflection bandwidth; therefore, the overall AR bandwidth can be utilized for CP applications. These figures demonstrate that the measured results are in reasonably good agreement with the simulated results, apart from certain discrepancies which can be attributed to fabrication tolerance. The measured LHCP gain of proposed antenna varies from 2.12 dBic to 3.56 dBic within the AR bandwidth.
Photograph of the fabricated antenna: (a) top view; (b) bottom view.
Measured and simulated reflection coefficients of the proposed antenna.
Measured and simulated axial ratios and LHCP gains of the proposed antenna.
The measured and simulated radiation patterns of the proposed antenna on the xz (ϕ=0°) and yz (ϕ=90°) planes, for both the LHCP and right-handed circular polarization (RHCP) gains, at 3.6 GHz are shown in Figure 10. On both planes, the measured LHCP is stronger than the RHCP by more than 20 dB.
Measured and simulated radiation patterns of the proposed antenna at 3.6 GHz.
5. Conclusion
Based on the concept of an embedded design, a compact wideband CP Spidron fractal microstrip antenna was designed, fabricated, and tested. The experimental results have demonstrated that a wide 3 dB AR bandwidth of 28.81% is obtained in conjunction with a −10 dB reflection bandwidth of 47.25% when using the Spidron fractal slot and patch. In the boresight direction, the measured LHCP exceeds the RHCP by more than 20 dB. Therefore, the proposed design has the potential to be used in various CP applications owing to its structural simplicity.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (2014R1A5A1011478).
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