A Coplanar Waveguide Fed Rectangular Reconfigurable Patch Antenna with Switchable Strips for Polarization Diversity

A compact single feed rectangular patch antenna with reconfigurable circular polarization has been proposed in this paper. The basic antenna structure is a rectangular patch of dimensions 10 mm x 15 mm printed on a thin FR4 substrate of thickness 3 mm. Two rectangular strips are connected through switches at sideways of the main patch to get polarization diversity. The polarization of the proposed antenna can be reconfigured between left hand circular polarization (LHCP) and right hand circular polarization (RHCP) by the current path, which is changed by operating the switches in ON and OFF modes. The antenna is designed and simulated using IE3D MoM based electromagnetic simulator. The simulated results show good return loss, radiation pattern, axial ratio and acceptable gain at the operating frequency. The antenna has around 15 per cent effective impedance bandwidth over 4.0 GHz - 4.8 GHz frequency band at a maximum gain of 3.0 dBi with polarization diversity.

Keywords:    Reconfigurable antenna,  polarization diversity,  circular polarization,  patch antenna,  coplanar waveguide feed

In the history of development of wireless communication systems, reconfigurable antennas have been thought of having a key role in today’s radio electronic systems. From the mid-1990s until the present there has been an increasingly rapid advance in the area of reconfigurable antennas. Miniaturization always indicates scientific progress. So efforts are directed towards reducing the antenna size. So far several studies were made to design a single antenna which is used for single application, but today’s wireless environment need a single antenna serving different applications (switch between different frequency bands) and should have the capability of switching between different operating modes, polarizations, appropriate radiation patterns which helps in miniaturizing entire size of the device. The above mentioned purpose is fulfilled by the use of reconfigurable antennas. To configure means to organize elements of something in a particular form or figure. Hence, by definition reconfigurable antenna means the antenna that can intentionally and reversibly change the distribution or character of its performance-governing electromagnetic fields1. This reconfiguration can be achieved through dynamic redistribution of the antenna’s aperture currents which can be achieved through different mechanisms such as switching, material tuning and structural modifications.The switching mechanism uses solid state switches such as PIN diodes, FETs and RF MEMS switches. The later type switches have many advantages than solid state switches234. The reconfigurable antennas are capable of operating at multi band frequencies, with or without polarization diversity, adaptive beam forming by automatically changing to the appropriate radiation pattern, minimize interference or jamming, estimating direction of arrival.

One of the many advantages of circularly polarized antennas is good receiving sensitivity which makes it useful in mobile applications. The other advantages are low fading when used in WLANs, increased security that can be used in military wireless systems and in RFID systems5. Circular polarization (CP) can be achieved by using a symmetrical patch which can be excited by two identical orthogonal modes. It can also be achieved by feeding two orthogonal identical patch antennas in quadrature. A single feed quasi symmetrical patch antenna that supports two degenerated modes also operates in circular polarization. Previously, many researches6789have shown that there are several types of patch antennas with polarization diversity but these antennas have more than one feeding element and parasitic structures, and in some cases they also have several layers below the main radiating patch which makes the design more complex and asymmetric.

The main aim of this paper is to achieve CP diversity by using geometrically symmetrical shaped antenna with small size and less complexity. Such types of antennas are useful in array type of structures, especially in planar array configuration. Hence we propose a simple and compact rectangular patch with single coplanar waveguide (CPW) feed that produces circular polarization diversity. So far, CP antennas are designed with conventional micro strip feeding structures but this paper proposes CPW feed because of its many advantages such as; reduction in radiation loss, improved bandwidth, easy surface mounting of active and passive devices, reduced cross talk effects between adjacent lines because of ground plane between any two adjacent lines, simplified fabrication10.

Figure 1 shows the geometry of the proposed reconfigurable antenna. The antenna is designed on a low cost FR4 substrate which is 3 mm thick and having dielectric constant, εr = 4.3. The other dimensions of the antenna in mm are listed in Table 1. The basic antenna consists of a rectangular patch whose length L and width W for the given frequency of operation ‘fr’ are mentioned below.

Figure 1. Structural geometry of the proposed antenna.

Table 1. Dimensions of the proposed antenna in mm

$W=\frac{c}{2{f}_{r}}\sqrt{\frac{2}{{\epsilon }_{r}+1}}$      (1)

where c = 3e8 m/s and the effective dielectric constant is given by

${\epsilon }_{reff}=\left(\frac{{\epsilon }_{r}+1}{2}\right)+\left(\frac{{\epsilon }_{r}-1}{2}\right){\left[1+12\frac{h}{w}\right]}^{\text{\hspace{0.17em}}-\text{\hspace{0.17em}}\text{\hspace{0.17em}}\frac{1}{2}}$      (2)

where ‘h’ is the thickness of the substrate. The extended length ΔL of the patch is given by

$\Delta L=0.412h\left[\frac{\left({\epsilon }_{reff}+0.3\right)\left(\frac{w}{h}+0.264\right)}{\left({\epsilon }_{reff}-0.258\right)\left(\frac{w}{h}+0.8\right)}\right]$      (3)

The effective length of the patch for the given resonant frequency is given by

${L}_{eff}=\frac{c}{2{f}_{r}\sqrt{{\epsilon }_{reff}}}=L+2\Delta L$      (4)

Hence the actual length of the patch is

$L=\frac{c}{2{f}_{r}\sqrt{{\epsilon }_{reff}}}-2\Delta L$      (5)

The optimized dimensions of the patch for the operating frequency of 4.8 GHz are 10 mm x 15 mm. The overall dimensions of the antenna are 35 mm x 30 mm which include patch and CPW ground. The structure of this antenna is modified to get circular polarization. The two orthogonal nearly degenerated resonant modes for CP in single feed CPW patch antenna are obtained by exciting two rectangular strips on both sides of the patch. Two switches are introduced in the structure namely S1 and S2. Switch S1 connects left rectangular strip to the patch, and S2 connects the right rectangular strip. The two switches are realized in the simulation as short copper paths between the main patch and the strips. Presence of the short is treated as ON and the absence of the short as OFF. Different switch configurations which make the proposed antenna to operate in different CP diversity cases are shown inTable 2.

Table 2. Switch configurations for the proposed antenna

The return loss curve of the basic patch antenna of 10 mm x 15 mm is shown in Fig 2. The antenna operates at 4.8 GHz with a return loss of -21 dB.

Figure 2. Return loss curve of the basic rectangular patch antenna.

For reconfigurable operation, in case 1, when both the switches are in ON state, the antenna exhibits linear polarization because of the symmetry in the antenna structure. In case 2, when S1 is ON and S2 is OFF, due to the quasi symmetric structural configuration two orthogonal degenerate modes are generated which makes the antenna to excite in circular polarization. Thus when the antenna is reconfigured from case 1 to case 2, the polarization switches from linear to circular. The current distributions of the reconfigurable antenna in case 2 are shown in Fig. 3. This figure clearly shows the rotation of current vector in the clockwise direction at the position of switch S1 at time instances t = 0, t = T/4, t = T/2, and t = 3T/4. This implies antenna operates in LHCP. Similarly in case 3 i.e. when S1 is OFF and S2 is ON, it exhibits RHCP. In case 4, the antenna radiates through center rectangular patch in linear polarization.

Figure 3. Surface electric current distribution in Case 2.

The return loss curves of the proposed antenna in all the cases are shown in Fig. 4. It shows that antenna in case 1 operates at a frequency of 4.23 GHz. When both S1and S2 are ON, the electrical length of the antenna is larger than the other cases and say lH. In Case 2 and Case 3 the return loss curves are same (because of same electrical length say l), and resonates at slightly higher frequency of 4.47 GHz and 5.76 GHz when compared to case 1. Return loss of -29 dB is obtained at 4.47 GHz and -22 dB at 5.76 GHz. In case 4, when both S1 and S2 are OFF the electrical length of the antenna (lL) is smaller and the operating frequency would be higher than the above cases. The antenna operates at a frequency of 4.66GHz with reduced return loss and gain compared to above cases because of mutual coupling that exists between central patch and side strips.

Figure 4. Return loss curves of the proposed reconfigurable antenna in all modes.

A reconfigurable antenna with polarization diversity must have the same operating frequency with varied polarization. The proposed reconfigurable antenna satisfies this condition with the same operating frequency in both RHCP and LHCP. The radiation pattern curves of the reconfigurable antenna in all cases i.e., Eφ polarization patterns in the elevation cuts (y–z plane and x–z plane) at their corresponding resonant frequencies are shown in Fig. 5. The radiation efficiency is found to be more than 70 per cent in both LHCP and RHCP.

Figure 5. Eφ polarization patterns of the proposed reconfigurable antenna in elevation cut for (a) Case 1, (b) Case 2, (c) Case 3, and (d) Case 4.

The axial ratio of the proposed reconfigurable antenna in all states is less than -3 dB over the entire operating frequency band. The gain of the antenna is also above 2 dBi for all the reconfigurable states. The different parameters of the proposed antenna are shown and compared in Table 3. The effective impedance bandwidth (-10dB return loss) of more than 15 per cent is achieved. The impedance value at resonant frequencies is around 51 Ω which is close to ideal value 50 Ω. A commercial Method of Moments based IE3D CAD tool was used for simulation.

Table 3. Results

This paper gives an account of achieving circular polarization agility using a simple and compact rectangular shaped patch antenna with CPW feed. The miniaturized size of the antenna is 35 mm x 30 mm. The use of CPW feed to improve the overall impedance bandwidth has been demonstrated using single layer and single feeding element to achieve circular polarization. The mode of CP is changed by the use of only one of the two switches at a time. For different polarizations the proposed antenna operates at the same frequency and it is suitable for wireless communication applications.

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 Ch. Sulakshana received BTech (ECE) from G. Narayanamma Inst. of Technology and Science, in 2007. She received MTech (Communication Systems) from in National Institute of Technology Tiruchirappalli, India in 2010 and currently pursuing PhD degree from National Institute of Technology Warangal, India. Her field of study is reconfigurable antennas and other areas of interest are microwave engineering and antenna wave propagation. She has published 12 research papers in international journals and conferences. Dr L. Anjaneyulu obtained his BTech (ECE) in 1989, MTech in 1991 and PhD in 2010 from N.I.T, Warangal, India. He has 22 papers to his credit in national and international conferences and journals. His areas of interest include computer networks, electromagnetic field theory, microwave and radar engineering, and Neural networks and Fuzzy logic systems.