Design of inclined coupling slot loaded CPW-fed circularly polarized slot antenna for wireless applications

A new structure for wideband circularly polarized (CP) slot antenna is proposed in this article. The designed antenna consists of a circular patch with two rectangular slots, which are parallel to CPW feed line, excites two resonance modes for wideband performance. To achieve CP operation, an inclined coupling slot loaded feed line on the ground is used. Furthermore, the proposed antenna is an enhanced version of earlier wideband CP antennas having a compact structure with a low-profile of 0.023λ_o (λ_o stands for the free space wavelength at 4.46 GHz) and ground size of 30 mm × 30 mm. It operates over the impedance bandwidth of 88.9% (relative to the center frequency of 4.5 GHz) and 3-dB axial ratio bandwidth found for this band is 2.7% (relative to the center frequency of 4.46 GHz). To verify the performance of the designed antenna, the experimental results match well with the simulated results.

Inclined coupling slot; circular polarization; wideband slot antenna

For several practical applications of a low-profile, small-size, and light weight circularly polarized antennas, microstrip patch antennas may be the most suitable candidate because it satisfies these requirements. Many types of microstrip patch antennas have been proposed and investigated. To generate the circularly polarized waves in a CP antenna which is required in mobile satellite communications, a number of feed points necessary are classified as single-fed type or dual-fed type. The coplanar waveguide (CPW) appeared as an alternative solution for feeding microstrip antennas due to their various properties which are reported in Huang and Ling ([3] ), Huang and Wong ([5] ), Laheurte and Papiernik ([10] ), and Menzel and Grabherr ([11] ). In Menzel and Grabherr ([11] ) and Laheurte and Papiernik ([10] ), CPW-fed aperture-coupled microstrip antennas, the metallic plane is used both as a conducting plane for the CPW line and a reflector plane for the radiating element. A number of slot antennas with CPW feeding are reported to get better performances, mostly for wideband and circular-polarization applications. Literatures (Huang and Wong [5] ) and (Huang and Ling, [3] ) have been presented the CPW-fed microstrip antenna is similar to an aperture-coupled microstrip antenna, couples the electromagnetic energy from the feed line to the radiating patch through a coupling slot and, moreover, has a simplified structure with only two metallization levels. An annular slot antenna with cross-shaped feed line has been reported in Huang, Wu, and Wong ([6] ). The proposed CP design in Jang ([7] ) is achieved by choosing a suitable size of the coupling cross slot.

In recent times, a number of applications, like for satellite, radar, mobile communication, and portable wireless devices, would like small-size circularly polarized microstrip antennas where the antenna size could be a main consideration. A number of dual band antennas have been presented for some specific applications (Ansari et al. [1] ; Huang and Yu [4] ; Kumar et al. [8] ; Vishvakarma and Vishvakarma [15] ). By embedding suitable slots or slits in the radiating patch will excite the patch surface current paths and realize lowering of the resonant frequency of the antenna, which corresponds to a reduced antenna size, compared to a standard circularly polarized microstrip antenna at the similar working frequency. For generating circular polarization (CP) radiation using a single feed, many microstrip antenna designs have been reported (Kumar et al. [9] ). The obtained CP bandwidth (3-dB axial-ratio bandwidth) of the proposed antenna is 1.9%. In Sze and Chang ([13] ), a broadband circularly polarized square slot antenna which is fed by an asymmetric CPW from a corner of the slot is proposed. A compact, circularly polarized, CPW-fed printed square slot antenna is reported in Sze and Pan ([14] ). The design characteristics of reported miniaturized antenna achieve the CP operation and impedance matching independently. An annular ring microstrip antenna with L-shaped strip for CP is presented in (Singh, Gangwar, and Kanaujia [12] ). A new single-feed arrowhead-shape slotted microstrip antenna for CP is proposed in Gautam, Kunwar, and Kanaujia ([2] ). However, the CP radiation and antenna size reduction of the designed antenna (Gautam, Kunwar, and Kanaujia [2] ) are achieved by introducing an arrowhead-shaped slot in the first quadrant on the diagonal axes of a square patch.

In this article, the case of a circular patch with two linear rectangular slots microstrip antenna for wideband and an inclined slot-loaded CPW feed coupling with radiating patch for CP radiation is demonstrated and experimentally investigated. The circular patch is selected to have a radius of 10 mm. The achieved impedance bandwidths for working band are about 400 MHz (4.3-4.7 GHz) for VSWR < 2. The 3-dB AR bandwidths found for operative band is 120 MHz (4.4-4.52 GHz). The proposed wideband slot antenna is circularly polarized and mostly useful in fixed mobile and satellite systems. The design-process strategy of proposed antenna, which is based on a parametric study, is also explained.

The geometry of an inclined coupling slot-loaded CPW-fed circular patch slot antenna is shown in Figure 1. The proposed antenna has overall length L and width W with substrate height h. A circular patch of radius r with a pair of rectangular slot of suitable length ls and width ws is fabricated on the top surface of the easily available dielectric material FR4 (εr = 4.4) introduced for wideband operation. The ground-plane is on the bottom surface of the dielectric material consisting of an inclined coupled slot with CPW feed of width Wf and length Lf.

The geometry of proposed antenna with top, bottom, and side view is represented by Figure 1(a), 1(b), and 1(c), respectively. An inclined coupled slot of length li and width wi is introduced in the CPW feed line with a θinclination angle to the x-axis which is coupled with radiating circular patch. The effect of inclined slot leads to a circularly polarized microstrip antenna configuration which efficiently meander the excited patch surface current densities of the two orthogonal modes.

The proposed wideband CP slot antenna is fed using 50-Ω SMA connector placed along the CPW feed line. The image of the fabricated antenna prototype is shown in Figure 2, where both top view in Figure 2(a) and bottom view in Figure 2(b) are provided.

A parametric study of the proposed antenna was conducted by using Ansoft HFSS v.14 simulation software, to achieve desired operating band with circularly polarized radiation. To know the effect of each antenna parameters in the parametric study, at a time only one parameter was varied, while other parameters were kept constant. The optimized antenna parameters for wideband circularly polarized operation of the proposed antenna are listed in Table 1.

Dimensions of the proposed wideband CP antenna.

These optimized values of proposed antenna provide wideband circularly polarized radiation with proper gain of the antenna over the required frequency band.

The inclined coupled slot in the ground plane is used to produce the circularly polarized radiation of the proposed antenna. The simulated results of the antenna return loss and the axial ratio (AR) with the variation of the inclined slot length, li, is plotted in Figure 3. The value of inclined slot length li was varied from 18.0 mm to 20.0 mm with fixed width wi. At inclined slot length of 18 mm, a shifted upper operating frequency is resonated with linear polarization. After increasing the value to 19 mm, two resonant frequencies create a wideband with circularly polarized radiation of the antenna. The performance of the antenna at 20 mm of the inclined slot length is not up to level for desired band.

The value of inclined coupled slot width, wi also affects the antenna’s performance. The variation of the return loss and the AR with a variation of the inclined slot width, wi, of the antenna are plotted in Figure 4. The value of wi was varied from 1.5 mm to 2.5 mm. At wi = 2 mm, the antenna had good circularly polarized radiation over the desired band.

An inclined coupling slot of length li and width wi is introduced in the CPW feed line with a θ inclination angle to the x-axis, which affects the antenna’s performance. The inclined angle θ is varied to improve the AR characteristic. The return loss characteristics and ARBW for different values of inclined angle θ are shown in Figure 5. It is observed that the impedance bandwidth increases as the inclination angle is increased from 52° to 53°. At θ = 54°, the impedance bandwidth decreases and there is no appreciable change in AR bandwidth. Finally, at θ = 53°, operating band is wide and the CP performance is good.

The radiating circular patch radius, r, was varied from 9 mm to 11 mm. For patch of 10 mm, the antenna had good impedance matching and impedance bandwidth, and good circularly polarized bandwidth, as shown in Figure 6, respectively. The 3 dB axial-ratio bandwidth increased with an increase in r up to 10 mm. At r = 11 mm, AR bandwidth decreases with impedance bandwidth.

The fabrication of antenna is done by standard photolithography process. All the electrical performance of the proposed antenna such as return loss, AR, gain, and radiation pattern are done by an AgilentTM vector network analyzer of PNA-L series. In this article, two rectangular slots on the circular patch are used to generate two resonant modes to achieve wide operative band. The circularly polarized radiation is achieved by generating two orthogonal fields due to coupling of inclined slot-loaded CPW feed to the circular patch.

To understand the operation of CP of the proposed antenna, the vector surface current distribution on the radiating circular patch, varying with phase is examined. Figure 7 shows the simulated surface current distributions of proposed antenna at 4.46 GHz for different time phase of ωt = 0°, ωt = 90°, ωt = 180°, and ωt = 270°. It is observed that the vector surface currents in time phase of 0° and 90° are equal in magnitude and opposite in phase of 180° and 270°, respectively. This suggests that as phase changes, the simulated surface current vectors turn anticlockwise, which means that the antenna can radiate the left-hand circular polarization (LHCP) at operative AR band.

The measured and simulated return loss and AR curves of the wideband circularly polarized antenna are shown in Figure 8. The results of dual resonance modes create a wideband of range 4.3-4.7 GHz. It is observed that the antenna gives a measured 3-dB AR bandwidth of 120 MHz from 4.4 GHz to 4.52 GHz with the center frequency at 4.46 GHz, as required for fixed mobile communication.

The antenna gain variation with frequency is shown in Figure 9 for the proposed antenna. The result gives a maximum simulated gain of 4.8 dBi and the measured gain of 4.9 dBi for desired operating band of the antenna. Figure 10 shows the simulated 3D radiation pattern at operating frequency 4.46 GHz. The measured and simulated radiation patterns of the proposed antenna in xz-plane and yz-plane for this frequency are shown in Figure 11(a) and 11(b), respectively. It can be seen that the designed antenna can radiate the LHCP in the upper-half space at 4.46 GHz. As shown in figures, comparison of all simulated and measured results gives good agreement between them. The analysis of measured and simulated results shows some deviation; this may be due to effect of fabrication imperfections and measurement errors.

The designed circular patch with two linear rectangular slots microstrip antenna for wideband with an inclined coupled slot introducing in CPW feed for CP radiation is experimentally verified in this article. The proposed wideband CP slot antenna having simple structure shows a good quality of CP and is easy to fabricate. The operating impedance bandwidth and 3-dB AR bandwidth are successfully achieved by varying some of the antenna parameters. All the required results obtained by simulation are well satisfied with measured results. Analysis of designed antenna characteristics is showing good radiation properties with LHCP wave at desired ARBW. This antenna has suitable wireless applications in fixed mobile and fixed satellite (space-to-earth) communications.

PHOTO (COLOR): Figure 1. Geometry of proposed antenna: (a) top view (b) bottom view, and (c) side view.

PHOTO (COLOR): Figure 2. Fabricated image of proposed antenna: (a) top view and (b) bottom view.

PHOTO (COLOR): Figure 3. Effect of varying inclined slot length on return loss and axial ratio.

PHOTO (COLOR): Figure 4. Effect of varying inclined slot width on return loss and axial ratio.

PHOTO (COLOR): Figure 5. Effect of varying inclination angle on return loss and axial ratio.

PHOTO (COLOR): Figure 6. Effect of varying patch radius on return loss and axial ratio.

PHOTO (COLOR): Figure 7. Simulated surface current distributions on radiating patch of proposed antenna at 4.46 GHz.

PHOTO (COLOR): Figure 8. Simulated and measured results of return loss and axial ratio of proposed antenna.

PHOTO (COLOR): Figure 9. Simulated and measured results of gain of proposed antenna.

PHOTO (COLOR): Figure 10. 3-D radiation pattern of proposed antenna at 4.46 GHz.

PHOTO (COLOR): Figure 11. Simulated and measured radiation pattern of proposed antenna at 4.46 GHz in (a) xz-plane and (b) yz-plane.