# A Dual Polarized Triple Band Stacked Elliptical Microstrip Patch Antenna for WLAN Applications

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## Abstract

In this paper, a single coaxial feed dual polarized triple band stacked microstrip patch antenna for wireless applications is presented. The proposed stacked antenna has two resonating elements with lower layer consisting of a truncated corner square patch and upper layer with an elliptical patch. Both the layers of proposed antenna are printed on RT Duroid^{®} 5880 substrate (having ϵ_{r }= 2.2). The antenna shows triple band resonance at 4.2, 4.8 and 5.8 GHz with circular polarization behaviour at the first two resonances and linear polarization characteristic in the third resonance respectively. The typical realized gain of proposed antenna is around 7 dB in all three resonating bands.

## Keywords

Circular polarization Elliptical microstrip patch Stacking Triple band## 1 Introduction

Microstrip patch antennas are considered an excellent choice for contemporary wireless communication devices as they are low profile, light weight, easily integrable and small in size thus ideal for portable handheld equipment. Amongst various shapes of microstrip patch antennas like circular, rectangular, triangular, polygon, etc., a very few researchers have focussed their attention on Elliptical Microstrip Patch Antennas (EMPA); though its geometrical shape appears similar to a circular patch. A few researchers who have worked on elliptical patch antennas with very low eccentricity (< 10–20%) assumed and approximated elliptical patch equivalent to a circular patch. However, elliptical microstrip antennas with higher eccentricity are rarely explored. Shen [1] proposed an empirical formula for calculating the resonant frequency of the elliptical patch, keeping a very low range of the eccentricity. Kumprasert [2] proposed simple and relatively accurate technique to calculate the dual resonant frequency *f* _{11} ^{e,o} of the dominant even and odd modes and existence of circular polarization in elliptical microstrip antenna with low eccentricity (i.e. b/a = 0.99–0.96) with best circular polarization at b/a = 0.976; where *a* and *b* correspond to semi-major and semi-minor axis of the elliptical patch, respectively. S. A. Long and M. W. Mc Allister proposed an impedance analysis approach for elliptical patch with very low eccentricity (b/a = 0.98) by considering the resonant circuit model of the elliptical antenna approximating to an average combination of the series equivalent of two different circular antennas, with corresponding radii equal to the semi-major and semi-minor axis of the ellipse; because an elliptical structure with very low eccentricity (b/a = 0.96–0.982) resembles almost a circle. Their equivalent circular patch approximation for two very close resonant frequencies, corresponding to semi-major and semi-minor axis holds good, provided the eccentricity kept is very low [3]. Abboud et al. [4] published a simple and accurate model of EMPA, presenting the varying effect of eccentricity on even and odd resonant modes of EMPA. The recent communication equipment used for transmission of electromagnetic waves requires circularly polarized antennas which can easily be employed in spacecrafts, aircrafts, satellites, missiles, other aerospace equipments where dimension, weight, performance and integration with other devices are the main properties under consideration. By the help of two linearly polarized currents of same amplitude and with 90° phase variation, circular polarization can be achieved. Circularly polarized designs are categorized as dual feed or single feed antennas based upon the number of feed used. However single feed antennas are simple to design compare to dual feed as dual feed involves complex feeding circuitry. Interestingly, one can obtain dual resonance characteristics by exciting an EMPA and also good circular polarization with a single feed point. Circular polarization with a single coaxial feed can be achieved in elliptical microstrip antennas by exciting simultaneous orthogonal modes. This can be done by applying the single coaxial feed at 45° with respect to the semi-major axis of the ellipse, provided that the eccentricity is controlled within 10–20%.

The circularly polarized antennas presented in literature are mainly made up of truncated corners, defected ground, loaded active device, etc. Some design structures are made with separate feeding mechanism for the excitation of circular polarization, however; this type of antenna structures are difficult to fabricate and the antenna occupies more space as well. The present-day portable systems are versatile devices and need several antennas operating at different frequencies for various transmission functions such as Bluetooth, Global Positioning System (GPS), Wireless Local Area Network (WLAN) along with voice-based services and high data rate transmission, which needs multiple antennas. The dual polarized multiband microstrip antennas can be a good option for recent handheld devices as single radiator occupies less chip space and have a large amount of versatility also, however; in the available literature a very few multiband patch antennas are proposed for circular polarization radiation. Yijun et al. [5] proposed stacking of multiple patches with high relative permittivity materials to design triple band antenna, but employed relatively complex dual orthogonal feed. O. P. Falade et al. proposed a design based upon the concept of multiple patches stacked one over other. They stacked three rectangular patches corresponding to each resonance and excited them using a single feed for triple band GNSS and GPS receiver applications [6]. S. Kumar et al. proposed a stacked microstrip antenna with a circular patch truncated on two sides, making it quasi-elliptical patch. The upper parasitic square patch was loaded with a symmetrical slot with two diagonal truncated corners. This stacked antenna shows dual band radiation with circular polarization. However, the gain of this antenna was relatively low (~ 4 dB) [7].

This paper proposes a novel triple band stacked antenna for modern wireless local area network applications. The novelty of the design is that by applying only two layers in stack formation three distinct resonant bands are obtained, exhibiting a combination of circular and linear polarizations with relatively high gain (~ 7 dB for each resonance). The problem of orientation mismatch between transmitting base station antenna and receiving mobile station antenna get solved by using polarization diverse antennas. A theoretical analysis is also proposed for the proposed triple band antenna structure using equivalent circuit approach and modal expansion cavity model. The simulation of the proposed design is done using FEM based available software, Ansys HFSS.

## 2 Antenna Design

*c*), having a semi-major axis (

*a*) and semi-minor axis (

*b*) is excited by a single 50 Ω coaxial feeding probe, passing through a small via-hole in the lower layer of the substrate. The driven patch is rotated, so that the coaxial feed is aligned at 45° to the semi-major axis, in order to generate orthogonal modes required for circular polarization.

*S*) diagonally truncated by two quarter circles of radius (

*R*) excited by electromagnetic coupling as it encloses the resonant cavity formed by the elliptical patch on the top and ground plane on the bottom side. The via-hole concept in the lower square patch also provides a capacitive effect to compensate the inductive effect introduced by vertical component of the inner conductor of coaxial feed. Figures 2 and 3 shows the top view and fabricated prototype of proposed antenna configuration respectively. The antenna specifications are listed in Table 1.

Design specifications of the proposed stacked antenna

Antenna parameters | Value |
---|---|

Substrate | Rogers RT Duroid 5880 |

Substrate height, | 0.76 mm |

Substrate height, | 0.76 mm |

Ground, | 30 mm |

Semi-major axis of the elliptical patch, | 12 mm |

Semi-minor axis of the elliptical patch, | 9.6 mm |

Eccentricity, | 0.6 |

Side of the square patch, | 22 mm |

Radius of truncation, | 5.1 mm |

## 3 Theoretical Considerations

*h*

_{1},

*h*

_{2}and

*r*are thickness of lower substrate, thickness of upper substrate and radius of circular patch respectively. The Eq. 9, is derived from the relation of equivalence between a square radiating patch (with resonating side = S) and a circular radiating patch (with radius = r) resonating at identical frequency, i.e. r = S/2 [9]. Equal circumference is considered as the basis of equivalence, like equal surface areas in [10], to account for equal static fringing fields.

*w*

_{ e }and \( \varepsilon_{r}^{\prime} \) are calculated by iteration method given in Ref. [11]. The effective radius of resonant cavity magnetic wall gets enlarged by the dielectric superstrate placed above the resonant patch, which consequently changes the effective radius size of the patch specified by [4]

*c*is the free space velocity of light and \( \alpha \) is the first zero of the derivative of the Bessel function of first order. The equivalent circuit of the square radiating patch is a parallel resonant circuit having equivalent inductance (

*L*

_{1}) capacitance (

*C*

_{1}), and resistance

*R*

_{1}. The input impedance of lower part of the antenna is calculated as [12]

*S*is the side (length) of square patch and \( \omega_{1} = 2\pi f_{1} \).

*R*

_{2}), capacitance (

*C*

_{2}) and inductance (

*L*

_{2}) are given as

*Z*

_{ o }is the coaxial feed characteristic impedance and

## 4 Results and Discussion

_{11}and frequency of proposed stacked antenna. The theoretical, simulated and measured results are in a good match for the three radiating bands. An independent elliptical patch with high eccentricity exhibits dual band characteristics, due to simultaneously generated orthogonal modes and the third resonant band is obtained by exciting parasitically coupled truncated square patch placed at the lower side. The dimensions of stacked antenna structure are optimized for achieving the triple band resonance. The first resonance band occurs around 4.2 GHz, the second resonance band near 4.8 GHz and the third resonance band near 5.8 GHz thus achieving a triple band operation. The resonant bands of the truncated square radiator vary inversely with small changes in radius

*R*of truncated corner. This occurs due to the increase in the path of surface currents circulating in the radiating patch, shifting resonant frequency towards lower side. However, the optimized dimension of

*R*has been considered with maximum circularly polarized radiation.

## 5 Conclusion

A single feed triple band stacked microstrip antenna has been designed and analyzed in this paper. The main advantage of proposed dual polarized antenna structure is its small size and simple geometry without any loading of active devices, slots in the patch or ground for exciting circularly polarized radiations. The measured results are in decent match with simulated and theoretical results. A small deviation in measured and simulated results is seen, which may arise due to customary photolithography fabrication process, alignment of lower/upper part of stacked antenna and SMA connector positioning and soldering. Thus, the designed antenna may be a preferable choice for various applications including wireless local area network.

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