Broadband circular polarized patch antenna with harmonic suppression

Background/Objectives: To develop Circular Polarized (CP) patch antenna with improved bandwidth and harmonic suppression for RF energy harvesting system. Methods/Statistical analysis: A λ /4 resonator is introduced and coupled in proximity to the radiating patch for bandwidth enhancement and high order harmonic suppression. Compared with other dual-resonance patch antennas, the proposed antenna has attractive low-profile property and simple structure. The CP can be achieved by putting arc shape slits at edge of circular patch. The structure, working principle and results we are described. Findings/ Applications: The operating bandwidth is from 1.68 GHz to 2.95 GHz (52.91%) with a reflection coefficient less than -10 dB with Axial Ratio bandwidth of 550MHz (2.21 GHz -2.76 GHz) while harmonic suppression over wide frequency range from 1 to 12 GHz is achieved. The experimental and simulated results is good matching except some slight variation. Novelty/Applications: New techniques for both bandwidth enhanced and harmonic suppression and up to 4th higher order harmonics has been suppressed. Proposed structure offers wideband CP Antenna with harmonic suppression.


Introduction
With the development of antenna technology, one of the focusing issues nowadays is integrating antennas and filters. Suppressing the wideband high order harmonic attracts more and more researchers' attention. Integrating filter at input front end of antenna is the common method to suppress high order harmonics (1) . Applying filters can contribute to other issues such as increasing the size, weight, and cost of the wireless device. In this way, some alternative solutions that do not increase cost or complexity for the device can be attractive. Thus, solutions that use the antenna itself to filter these higher-order modes become interesting (2,3) .
Several studies are being developed for design designing antennas able to attenuate or suppress unwanted frequencies. To suppress the second and third harmonics a spurline filter was inserted in the antenna microstrip feed line (4) . Defected ground structures (DGS) was used in patch antenna to suppress higher order harmonics (5)(6)(7)(8) .
https://www.indjst.org/ Transverse slot orthogonal to the microstrip feedline is introduced as the harmonic suppression unit (9) . To improve the both bandwidth and the harmonic suppression long stub used (10) . A pair of λ /4 stepped-impedance resonators (SIRs) in each port which is to enhance bandwidth and suppress harmonics used (11) .
In RF Energy Harvesting System due to Linear polarization of antenna, it does not get maximum ambient RF energy. Thus using Circular Polarized (CP) antenna, we can avoid wrong polarization which results collect maximum RF energy (12)(13)(14) . A circularly polarized (CP) microstrip antenna with embedded slots is designed which efficiently attains harmonics suppression (15) . CP Antennas offers lots of applications in modern-day wireless communication (16,17) .
In this work, we design a new antenna with harmonic rejection operate at 2.45 GHz. The harmonic rejection has been accomplished by using λ /4 resonator with partial gnd and a capacitive feed line. This structure can achieve harmonic suppression function so that harmonics up to 4 th order can be suppressed. The measured results show good agreement with the simulated results

Geometry and design
The structure of the proposed CP patch antenna is shown in Figure 1. This antenna is designed on a substrate with a relative permittivity of 4.4 and thickness of h = 1.5 mm. The size of the resonator is (Lr) that of the l/4 resonator is chosen. The radius of circular patch (a) is calculated by below equations (18) .
The radius of circular patch is given by, Where, We get Wf=2.8mm Feed and resonant length is given by Feed length (L50)=λ /4*sqrt (4.4) Resonant length (Lr)=λ /4 Ls=L+2*6h=28.4+2*6*1.6=49mm Ws=W+2*6h=38+2*6*1.6=59mm For the purpose of bandwidth enhancement, the resonance of the l/4 resonator with partial gnd, which is close to the TM10 resonance of the patch antenna, is introduced. This can efficiently improve the operating bandwidth. In addition, to achieve a better impedance matching, the partial gnd plane is used as shown in [ Figure 1]. The length of the resonator can be tuned to suppress the certain high-order resonance of the patch antenna. Finally, by optimization, the size parameters of this proposed CP patch antenna are as shown in Table 1.

Effect of simulated return loss of the proposed CP antenna and traditional CP antenna
The Simulation results of proposed CP antenna has been carried out in HFSS. [ Figure 4 ] presents the simulated reflection coefficient of the proposed CP antenna, the traditional CP antenna after introducing the capacitive coupling feed, high-order resonant modes at 5.0GHz, 7.1GHz, 9.5GHz, and 11.0 GHz are significantly suppressed.

Effect of resonator length (Lr) on return loss
The effect of resonator length (Lr) can be observed in [ Figure 6 ], which indicates that the resonator length is important parameter of the proposed CP patch antenna for controlling resonating frequency. If the resonator length is increasing, then freq shift to at lower side around 2.3GHz (blue graph) and decreasing resonant length the freq shift to higher side at 2.6GHz (green graph). The optimized resonator length Lr = 29mm (red graph), we got desired freq 2.45GHz.

Effect of radius of circular patch (a) on return loss
The effect of radius of circular patch (a) can be observed in [ Figure 7 ], which indicates that the radius of circular patch is important parameter of the proposed CP patch antenna for better impedance matching. If the radius of antenna is increasing, then the return loss is -22.5 dB at 2.40GHz (blue graph) and decreasing radius of antenna the return loss is -36.5dB at 2.5GHz (green graph). The optimized radius of antenna a = 14.5mm (red graph), we got freq minimum return loss -39.3 dB at 2.45GHz. https://www.indjst.org/

Radiation pattern E and H plane of proposed CP antenna
[ Figure 9 ] it is observed that the radiation patterns of proposed CP antenna are bidirectional in E-plane & omnidirectional in H plane at freq 2.45GHz. It can be observed that the gain of the proposed CP antenna is 2.5 dBi at 2.45 GHz. https://www.indjst.org/

Co and cross polarization in E and H plane of proposed CP antenna
[ Figure 10 ] The simulated co-and cross-polarized radiation patterns of proposed CP antenna in E-plane & H-plane at the resonant frequency 2.45 GHz are plotted in [ Figure 10]. It shows that the CP antenna has a bidirectional radiation pattern in E-plane and omnidirectional in H-plane [ Figure 10a], illustrates simulated co-and cross-polarized radiation patterns of proposed antenna in E-plane, at the resonant frequency 2.45 GHz. The cross-polarization level is more than 40 dB below than the co-polarization level. [ Figure 10b], illustrates simulated co-and cross-polarized radiation patterns of proposed antenna in H-plane, at the resonant frequency 2.45 GHz. The cross-polarization level is more than 42 dB below than the co-polarization level. https://www.indjst.org/

Axial ratio of proposed CP antenna
[ Figure 11 ] The proposed CP antenna has axial ratio (AR) bandwidth of 550 MHz i.e. 22.91% covering the frequency range of (2.21GHz -2.76GHz). The antenna has a good performance in term of AR.

Surface current distributions
The current distribution of the Proposed Vs. traditional CP antenna at 5.0 GHz and 9.5GHz is presented in [ Figure 12 ]. Red colour indicates maximum current along the edge of radiating patch. This means that in proposed CP antenna there is no second https://www.indjst.org/ and third harmonics present.

Comparison Table
As seen the [ Table 2 ], To improve the performance of this antenna, a pair of λ /4 microstrip line resonators is introduced. Traditional CP patch antenna, and proposed CP patch antenna are described. It concludes that proposed CP patch antenna technique's both bandwidth & harmonics suppression of antenna are improved.

Measurement results
The Proposed CP Patch antenna has been fabricated and tested using VNA. The Measurement results of proposed circular patch antenna getting bandwidth of 1220MHz at freq 2.47GHz. It shows there is harmonic reduction through entire freq band form 1-10GHz. The simulated Vs measured return loss proposed circular patch antenna has been shown in [ Figure 13 ].
https://www.indjst.org/ The Measured return loss characteristics of the proposed patch antenna in [ Figure 10], indicated that the return loss is below −10 dB in the operating freq band from 1.94GHz-3.16GHz and fractional percentage BW is around 50.83 %.
[ Figure 14 ] shows fabricated Prototype Proposed CP Patch antenna. Comparison of the proposed CP antenna with reported state of art designs are listed in [ Table 3 ]. The proposed CP antenna offers large bandwidth as compared to all other antennas reported table in (1-5) (9) (11) (12) & (16) . The proposed antenna offers Circular Polarization (CP) compared to antenna reported table in (1)(2)(3)(4)(5) . Also, upto 4 th higher order harmonic suppression achieved in proposed antenna.

Conclusion
A Circular Polarized microstrip-fed patch antenna with improved bandwidth and harmonic suppression has been proposed. Compared with the conventional microstrip-fed antenna, a l/4 resonator and partial gnd are introduced, respectively, to enhance the impedance bandwidth and harmonic suppression. The bandwidth of proposed CP antenna around 1260MHz (1.69-2.95GHz) is 18 times higher than traditional CP antenna. The proposed CP antenna has axial ratio (AR) bandwidth of 550 MHz i.e. 22.91% covering the frequency range of (2.21GHz -2.76GHz). The experimental and simulated results offer good matching except some slight variation.