A Compact, Wideband, Dual Polarized CPW-Fed Asymmetric Slot Antenna for Wireless Systems

— Wideband, dual sense circularly polarized (CP), asymmetric circular slot fed by coplanar waveguide is proposed and designed. Dual polarizations are obtained by using a horizontal stub to the feed line and embedding the L-shaped strip onto the slotted ground plane. The simulated result of impedance bandwidth shows wideband performance is produced by four resonant modes. Some key parameters are studied to analyze the characteristics of the proposed antenna. The overall volume of the proposed antenna is 18 mm × 18 mm × 1.6 mm. The measured value of 10 dB impedance bandwidth is 112%at frequencies from 4.2 GHz to 14.9 GHz with voltage standing wave ratio (VSWR) ≤ 2. The 3-dB axial ratio bandwidths are 39.3% for the lower band which senses right-hand circular polarization (RHCP) and 2.8% for the upper band which senses left-hand circular polarization (LHCP) relative to the center frequency of 6.1GHz and 14.1 GHz, respectively. The proposed structure gives a good radiation pattern with moderate gain. the multipath shortcomings. For CP radiation, a patch should support two orthogonal modes of the same amplitude however, 90 o phase difference. To achieve these modes a single patch with appropriate excitations or a properly structured array of patches with proper phasing is required. Using a printed slot antenna with coplanar waveguide (CPW) feed is one of the methods to prevent the increasing size of the antenna and improve the operating bandwidth. The main features of CPW feed are wider bandwidth, a single metallic layer, easily from the proposed antenna. The small deviation between experimental results comes out from the environmental effect by measurement and manufacturing tolerances. The experimental results show that the -10 dB impedance bandwidths obtained a wideband of 10.7 GHz and dual sense 3-dB ARBW of 2.4 GHz and 0.4 GHz for lower and upper bands, respectively. Therefore, the proposed antenna is an attractive choice for broadband and satellite applications.

integrated with active elements, and a small mutual coupling between two adjacent lines of the coplanar structure.
In recent times, some techniques are utilized to produce circularly polarized antennas. Reference [2] proposed and investigated the stage 1 and stage 2 of a Koch fractal boundary CP microstrip antenna.
In [3] and [4], a dual-band, dual sense CP antenna is obtained by four unequal linear slots incorporated to the annular slot and by two spiral-shaped slotted ground planes, respectively. A tripleband circularly polarized operation is achieved by a single layer, a single-feed antenna with asymmetrical fractal boundary is presented in [5]. A circular patch slotted with eight curves and a coaxial probe with the integrated disk also generates a dual sense polarization operation that is proposed in [6]. Another structure is proposed in [7] for dual sense polarization operation that consists of a bent feeding configuration with a slotted ground structure. To obtain dual-band circular polarization in [8], a rectangular and L-shaped adjusting stub is used additionally. The proposed design in [9] consists of the stacking of two truncated patches having an inclined slot and horizontal slits to produce quad-band circular polarization operation. Dual-band circularly polarized spiral slotloaded antenna is proposed in [10]. To achieve circular polarization incorporation of three slit arms of L shaped onto the hexagonal slot is presented in [11]. Various other slot combinations are reported to obtain broadband and multiband circular polarization with wide impedance bandwidth [12]- [20].
In view of the above kinds of literature, a novel structure of a CPW fed slot antenna consist of a horizontal stub and L-shaped strip to achieve RHCP and LHCP with the wideband operation is  Table I.  The asymmetric structure of the antenna design generates two different modes of the radiated fields.
Therefore, by setting the perturbation segment to the edge of the circular slot, the generated mode is separated into two orthogonal modes Ex and Ey. The radiated fields excited by these two modes are in general perpendicular to each other and orthogonally polarized in the direction of maximum radiation.
This enables the antenna to operate as a CP radiator. This proposed antenna has a number of advantages such as small size, wide impedance bandwidth and can excite dual sense CP radiation without using an external polarizer as compared to other dual sense CP radiators.

III. EVOLUTION OF THE PROPOSED ANTENNA
To obtain large impedance bandwidth and dual sense CP operation, two main structures have been  Impedance bandwidth results for antenna I and antenna II give only shifted lower and upper resonance frequency. Second and third resonance frequency does not appear below -10 dB for the S11 value. In this case, results for axial ratio are not appreciable. Antenna III gives a good and wideband S11 with four resonance frequencies but the axial ratio is not below 3-dB. Finally, the antenna with an L-shaped strip lying onto the top right side of the circularly slotted ground plane for generating two orthogonal resonant modes is designed. Analysis of the designed antenna gives the simulated results of impedance bandwidth from 4.2 GHz to 14.9 GHz of 112% relative to the center frequency of 9.55 GHz. Therefore, 3-dB ARBW for the lower band is 39.3% and for the upper band is 2.8% relative to the center frequency of 6.1 GHz and 14.1 GHz, respectively.  This section elaborates the performance-process of the proposed antenna through parametric simulation. A study on the parametric process has been made in order to get optimized values for certain antenna dimensions such as stub width on the feed and L-shaped strip width on the slotted ground. In this process, the optimized value of the parameter is obtained by varying the value of that parameter and keeps all parameters constant at the time of the simulation.

A. Varying stub width 'Ws' (Without L-shaped strip)
The variation in the value of stub width, without L-shaped strip, is used to enhance the S11 characteristic. The S11 characteristics and ARBW for the stub width 'Ws' from 2 mm to 3.

B. Varying L-shaped strip width 'Wl'
The optimization of thickness of the L-shaped strip can be used for the best circularly polarized performance to the antenna. The effect of different values of Wl on S11 characteristics of the proposed antenna illustrated in Fig. 5 (a).
The impedance bandwidth decreases with an increase in the width of the L-shaped strip. At Wl =1 mm S11 covers a wide range of frequencies. The ARBW characteristic for different values of Wl is illustrated in Fig. 5

A. S11 and axial ratio bandwidth
In this section, simulated and measured results for S11 and axial ratio are compared and explained for the proposed antenna. The designed antenna operates over the impedance bandwidth of 10.7 GHz at frequencies from 4.2 GHz to 14.9 GHz. The 3-dB ARBW for the lower band can reach as large as observed between simulated and measured results, which come from fabrication tolerance and soldering of the SMA connector. Table II listed the details of some important properties of the   proposed antenna at a totally different resonance frequency within the operating band. It shows, the   proposed antenna becomes an excellent choice for wideband applications, where a compact CP antenna is preferred.

C. Radiation patterns
In this section, simulated and measured normalized radiation patterns of the proposed antenna at completely different resonant frequencies are plotted. Fig. 11 shows that the radiation patterns of the   [11]. It is observed that the proposed antenna consists of a low profile, small size, wide impedance bandwidth with dual sense circularly polarized radiation for dual-band, and a good ARBW for both bands. This shows that the proposed antenna is better than other listed antennas in Table IV.  Feed Method Impedance