Research On Wideband Circularly Polarized Patch Antenna Based On Multimode Theory

In high-performance spacecraft,aircraft,satellite,and missile applications,antenna size,weight,cost,performance,ease of installation,and profile are the main limiting factors.Patch antennas are widely used in these applications due to their low profile,conformal with planar and non-planar structures,low cost,and compatibility with hybrid microwave integrated circuits（MMIC）.In addition,in other government and commercial applications with similar requirement,patch antennas are also in great demand.When different patch shapes and resonance modes are selected,the patch antenna is flexible in frequency,impedance,and polarization.However,the main disadvantages of patch antennas are their low efficiency,low power,poor polarization purity,spurious feed radiation and very narrow bandwidth,usually only a few percent.Compared with linearly polarized antennas,circularly polarized antennas have greater flexibility in the alignment of receiving and transmitting antennas,and are more immune to multipath reflections and weather interference.In this dissertation,the high-order mode of the patch antenna is combined with the main mode to improve the bandwidth.The circularly polarized radiation of the antenna is realized by adopting wideband power division and phase-shifting feed network.The main research contents and contributions of this dissertation are as follows:1.By exciting the high-order mode(TM₂₁)of the circular patch and combining it with the main mode(TM₁₁),a low-profile and wideband circularly polarized antenna is designed,which is composed of the circular patch fed by four ports.A wideband property is achieved by utilizing TM₁₁and TM₂₁ mode,as well as the orthogonal mode of TM₁₁.TheΓ-shaped feed is employed to stimulate TM₂₁ mode.Besides due to the parasitic radiation from the face-to-faceΓ-shaped feeds with opposite phase,the radiation peak of the circular patch under TM₂₁ mode is along the broadside direction,which is different from the original radiation mechanism of TM₂₁ mode.By loading shorting pins in the middle area of circular patch,the polarization orthogonal mode of the fundamental mode(TM₁₁)is excited properly,which contributed to bandwidth enhancement.Cooperated with the quadrature phase feed network,a wideband patch antenna with unidirectional radiation and circular polarization properties is designed,fabricated and measured.The size of the circular patch is 0.57λ₀×0.57λ₀ and the height is only 0.053λ₀（λ₀ is the free space wavelength at the center frequency）.The measured results show that the 109%of fractional bandwidth is obtained under the condition of VSWR<2,ranging from 0.94 GHz to 3.18 GHz,and the 3-d B axial ratio fractional bandwidth is 95%,ranging from 1.04 GHz to 2.93 GHz.Besides the realized LHCP gain>3 d Bic from 1.27 GHz to 2.99 GHz.2.By exciting the high-order mode(TM₂₂)of the circular patch and combining it with the main mode(TM₁₁),a low-profile and wideband circularly polarized antenna is designed.It is one kind of four-port antenna implemented by loading five shorting pins on the circular patch.In order to achieve wideband properties,TM₁₁ and TM₂₂ modes are stimulated in circular patch,along with their polarization orthogonal modes(TM₁₁₊and TM₂₂₊modes).The TM₁₁₊mode is excited by theΓ-shaped feed,while the TM₂₂ mode and TM₂₂₊mode are stimulated by four shorting pins in close proximity to theΓ-shaped feed.Due to the strong coupling between shorting pins andΓ-shaped feed,the peak electric field generated on the two opposing shorting pins causes the magnetic current density on the circumference of the patch to be changed,which in turn leads to the realization of broadside radiation in the TM₂₂ and TM₂₂₊modes.In addition,the function of the shorting pin loaded in the center of the patch is to improve the impedance matching when TM₂₂ and TM₂₂₊modes are operated.Cooperated with the devised one-to-four feed network with equal amplitude and progressive phase difference of 90°,the wideband circularly polarized patch antenna is obtained.To verify this design concept,a prototype is fabricated and measured.The diameter of the patch is 0.68λ₀ and the profile is 0.078λ₀（λ₀ is the wavelength in the free space at the center frequency）.The measured relative bandwidth of VSWR<2 is 106%,involving 1.53-4.96 GHz,while that of AR<3 d B is 98%,ranging from 1.54 GHz to 4.48 GHz.Besides the measured LHCP gain>3 d Bic in the frequency band of1.48-4.42 GHz.3.The coupling mechanism between the circular patch and the square patch is investigated,and a wideband circularly polarized stacked patch antenna is designed.The four-port antenna consists of a circular patch and a square patch,where the square patch is located above the circular patch.Taking into account the mutual coupling between four ports,the active reflection coefficient（ARC）is adopted to describe the impedance matching of the antenna,and a simplified form of ARC is derived.The abnormal electric field distribution generated in the square patch is attributed to the superposition of the electric fields of two orthogonal TM₁₀ modes,which is theoretically verified.Due to the coupling between TM₁₁ mode and TM₁₀ mode,a new resonance mode appears in square patch,where the abnormal electric fields on the two diagonals are generated simultaneously.The effect of the square patch on the circular patch is equivalent to a capacitor at the lower frequency end and the higher frequency end of-10d B impedance bandwidth,while it acts as an inductor at the middle frequency band.Besides the horizontal patch ofГ-shaped feed is sandwiched between two radiators to enhance coupling.Finally,a wideband feed network composed of Wilkinson power divider and phase shifter is devised to provide four signals of equal magnitude and quadrature phase for the proposed antenna.To verify this design concept,a prototype is fabricated and measured.The results show that left-hand circularly polarized（LHCP）radiation is achieved.The 3-d B gain variation bandwidth is 44.6%,ranging from 1.48 GHz to 2.33 GHz,in which VSWR<1.5 and AR<1.5 d B.Besides the peak gain is 9.5 d Bic and antenna efficiency is not less than 64%.