| With the development of the wireless communication technology,electronic devices are gradually moving towards the trends of multi-functions,integration and miniaturization,which put forward more rigorous requirements on the performance of the antennas.On one hand,to cover more bands of operation of wireless systems,wideband antennas are highly demanded.On the other hand,antennas with polarization diversity are capable of reusing frequency,enhancing the systems'capacity and reducing the signal loss caused by the multi-path fading,which attract more attention in both academic and industry fields.The combination of wide band and polarization diversity is one of hot research topics in designing antennas.As well known,the wireless spectrum resources are limited.The external contradiction between the rapid growth of wireless communication and the shortage of wireless spectrum resources is driving the internal revolution of wireless communication standard.Currently,though both time division duplex(TDD)and frequency division duplex(FDD)can effectively improve the utilization efficiency of the spectrum resources,they are still not enough to deal with the issue of the congested spectrum.Co-time co-frequency full duplex(CCFD)system can double the spectrum efficiency when compared with the TDD and FDD systems,greatly improving the throughput and capacity of the wireless communication systems.Therefore,CCFD is considered as the valuable mobile communication technology and one key technique of the future 5 G communication.Based upon the research project and research hot topics,seven antennas are designed in this dissertation.The novelty works are summarized as follows:1.A wideband two-element dipole antenna array is proposed.In order to differentially feed the two-element dipole antenna array,a slot feeding structure is designed.The feeding structure can be considered as one power divider and it can also provide differential signals for dipole antennas.In addition,meandering rings are loaded in the dipole antenna.These parasitic rings are designed to operate at a lower frequency,which broaden the bandwidth and also decrease the lowest frequency of operation of the antenna array,thus achieving the antenna's miniaturization.2.Several wideband high-isolation dual-polarized antennas are deeply studied.(a)A wideband dual-polarized microstrip patch antenna is designed using a hybrid electromagnetical feeding techniques.The electrical feeding structure is composed by two differentially-fed probes.These probes bring in some inductance which worsens the antenna's input impedance.To improve the impedance matching,two circular patches are introduced to compensate for the inductance.The magnetical feeding loop is composed of two symmetrically-positioned arc-shaped narrow matal srips,two shorting pins and the ground plane.To excite the loop,a balun with tapered structure is adopted.(b)A low-profile dual-polarized patch dipole with wide bandwidth utilizing the mutual coupling between the antenna elements is proposed.The antenna consists of four patch-dipole antenna elements and the two face-to-face patch dipoles are differentically excited for low cross polarizations and broadside radiation.To miniaturize the electrical dimensions of the dual-polarized antenna,several methods consisting of enhancing the mutual coupling of antenna elements and adding four stubs are adopted to decrease the lowest frequency of the antenna.(c)Based upon the stepped slot,a low-profile dual-polarized antenna and its array is designed.The stepped slot,as the main radiating structure,is etched along the diagonals of the square metal ground plane and two stepped microstrip feed lines are utilized to excite the stepped slot,thus generating two orthogonally linear polarizations.Compared with the uniform slot antenna,adopting the stepped slot provides more freedom of design and makes the antenna easier to obtain wider impedance bandwidth.Since the slot antenna radiates bi-directionally,a reflector is placed under the slot antenna with a distance of 0.15?_c(?_c is the free-space wavelength at the center frequency of operation)so as to suppress the back radiation,obtaining both high gain and directional patterns.For improving the cross polarization of the dual-polarized antenna,a shorting pin is introduced to connect the square ground and the reflector.Based on the antenna element,a four-element dual-polarized antenna array is developed,which is fed by two Wilkinson power dividers.(d)Combined with the artificial magnetic conductor(AMC),a wideband dual-polarized slot antenna with a low profile is proposed.In the thesis,the influences of both the presence of the AMC surface and the distance between the slot antenna and the AMC surface on the antenna's input impedance are respectively studied.3.The polarization reconfigurable Fabry-perot resonator antenna for 2.4/2.5 GHz WLAN applications is studied and designed.This antenna is capable of switching between the orthogonally linear polarizations,which is achieved through the use of a feeding network.The feeding antenna is designed to have resonant frequencies close to that of the Fabry-perot cavity.The wideband characteristic of the antenna is achieved by merging the resonances of the feeding antenna and the Fabry-perot cavity.For a broadband 3-dB gain bandwidth,the reflection phase of the partially reflecting surface(PRS)has a positive slope over as a wide frequency band as possible.In order to enhance the gain and its stability,two-element patch antenna array are adopted as the feeder.In addition,four vertically positioned metal walls with proper height are built along the four edges of the ground plane.4.A high-isolation,ultra-wideband simultaneous transmit and receive(STAR)antenna with monopole-like radiation patterns for full-duplex wireless system is studied and designed.Firstly,the STAR antenna topology simultaneously satisfying high isolation and omni-directional radiation patterns in the horizontal plane is studied.Also,the potential antenna types(for example,monocone and bent ring antenna)to achieve ultra-wideband characteristic are considered.Secondly,the far-field radiation patterns generated by the array factor of the four-element circular antenna array is studied in detail,which provides theoretical guidance for the design of the STAR antenna.In the thesis,the evolution process of the STAR antenna along with the simulated results is shown.To miniaturize the STAR antenna,a capacitive circular metal patch is loaded on the top of the monocone to reduce its lowest frequency.Furthermore,four T-shaped parasitic elements with lower operating frequency are designed to shift down the STAR antenna's minimum frequency.In order to obtain the omni-directional radiation patterns in the horizontal plane over the whole band of operation,four directors are properly designed according to the theorem of pattern multiplication.For verification,the STAR antenna is fabricated and measured.The experimental results show the antenna achieves ultra-wide frequency band covering from580 MHz to 1750 MHz.The isolation higher than 40 dB over a 2.9:1 band of operation from600 MHz to 1750 MHz.Both transmit and receive antennas have the same vertical polarization and obtain omni-directional radiation patterns with gain variation less than 5 dB over the entire band of operation. |
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