A Dual Polarized Triple Band Stacked Elliptical Microstrip Patch Antenna for WLAN Applications
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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 ϵr = 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.
KeywordsCircular polarization Elliptical microstrip patch Stacking Triple band
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  proposed an empirical formula for calculating the resonant frequency of the elliptical patch, keeping a very low range of the eccentricity. Kumprasert  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 . Abboud et al.  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.  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 . 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) .
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
Design specifications of the proposed stacked antenna
Rogers RT Duroid 5880®
Substrate height, h1
Substrate height, h2
Ground, X s
Semi-major axis of the elliptical patch, a
Semi-minor axis of the elliptical patch, b
Side of the square patch, S
Radius of truncation, R
3 Theoretical Considerations
4 Results and Discussion
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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