| The constantly advancing wireless communication technology has facilitated the development of wireless devices towards high-performance,integration,and multi-functional operation,which has raised higher requirements for antenna designs.As a significant branch of antennas,circularly polarized(CP)antennas have been widely used in a variety of fields such as satellite navigation systems,radio-frequency identification(RFID)because of the outstanding performances.Focusing on these requirements,an intensive research of different types of CP antennas is carried out in this dissertation,including CP antennas with wide axial ratio(AR)beamwidth,miniaturized CP antennas,multiband CP antennas,wideband CP antennas and omnidirectional CP antennas.In view of the tendency towards multi-functional operation,shared-aperture antennas for circular polarization and near-field coupling are also investigated.The main innovations in this dissertation are summarized as follows:1.A compact microstrip antenna with wide axial ratio(AR)beamwidth is proposed.It is found that by etching slots on the patch and loading coupled strips and shorting pins,not only the resonant frequency can be shifted downward,but also the AR beamwidth is broadened greatly.In the entire upper hemisphere,the ARs of the proposed antenna are less than 3 dB,which reveals outstanding CP property.Considering that the AR beamwidth of this antenna is sensitive to frequency,an improved dual-feed design is proposed to broaden the impedance and AR bandwidths.In the operation frequency band,the obtained 3-dB AR beamwidths are wider than 120°.2.A novel miniaturized dual-band CP quadruple inverted-F antenna is proposed for global positioning system application.Two methods contribute to the miniaturization of this design.One is employing the inverted-F antenna as the radiating element to decrease the height of the antenna.Another is turning the inverted-F elements into the form of spiral and meander shapes and introducing mutual coupling between adjacent elements.In addition,a miniaturized 1-to-4 power divider based on back-to-back microstrip line structure is designed.Compared with other reported dual-band quadrifilar helix antennas(QHAs),the proposed antenna shows remarkable superiority in terms of its electrical size and performances.3.In view of the technical challenge that it is extremely difficult to realize omnidirectional CP and unidirectional CP radiation patterns simultaneously,a novel compact dual-band CP antenna is proposed.In this design,a novel capacitively coupled feeding technique is proposed and implemented to feed four sequentially rotated bended monopoles using a square patch to generate omnidirectional CP radiation patterns.Different from conventional omnidirectional CP antennas using feeding networks or shorted monopolar patches,this technique not only leads to good impedance matching,but also makes it possible to generate unidirectional CP patterns simultaneously.Based on this property,an asymmetric U-slot is etched on the square patch to excite unidirectional CP radiation pattern,keeping the omnidirectional CP mode unaffected.The proposed compact antenna features dual-band circular polarization and two different types of radiation pattern with only one feed port.Moreover,the dual-band performances can be tuned separately,which provides the proposed antenna with more design flexibility.4.In order to achieve multi-functional operation at 13.56/900 MHz,a novel shared-aperture,shared-structure antenna for wideband circular polarization and near-field coupling is proposed.This antenna is developed based on a CP gap-loaded loop antenna and then evolved to contain a two-turn coil for inductive coupling.Meanwhile,the impedance and AR bandwidths of this antenna are broadened to 32.2%and 25.6%respectively.Therefore,this antenna can achieve wideband circular polarization for ultra-high frequency(UHF)RFID application and good transmission performance with high stability for near-field communication(NFC)applications,which provides very good universality and compatibility.The superiority of the proposed antenna over reported publications has been validated by the performance comparison.5.Based on the proposed design methodology for circular polarization and near-field coupling shared-aperture antennas,an improved shared structure for global navigation satellite system(GNSS)and NFC applications is proposed.By introducing an equivalent open loop into a CP asymmetric rectangular loop antenna,a new CP mode with opposite sense is excited.As a result,the proposed antenna can satisfy the dual-band resonance and dual-sense circular polarization requirements for GNSS application.In addition,the coil structure has occupied the utmost of the available space to improve the transmission efficiency in NFC applications.The proposed multi-functional shared-aperture antenna features a simple configuration and low profile,and it can be easily fabricated.By integrating the NFC and GNSS functions into a single portfolio,the proposed design can lead to a significant reduction in size,cost and complexity in contrast with that using separate antennas. |
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