Investigation On Design Theory Of Multband Grid Antennas And Key Techniques Of Liquid Crystal Reconfigurable Antennas

Microstrip antennas and arrays have been widely used in the wireless communication system and they are also very suitable for on-chip integration of package systems due to their benefits of low profile,simple fabrication,light weight,easy integration and low cost.Differential input and output design of the radio frequency system required microstrip antenna from the single-ended operation to the differential operation.Differential microstrip antennas can further enhance the miniaturization and integration of systems,and also suppress the common-mode noise to improve system performances.Dual-/multi-band differential antennas have been required for the fifth generation mobile communication,especially in the microwave and millimeter wave frequencies.In addition,to meet the multi-function requirements of fifth generation mobile communication,reconfigurable antennas have caused great attention in recent years.Conventional reconfigurable antennas adopted switch and varactor diodes,which will generate the parasitic effects and frequency limits.In recent years,liquid crystal-based frequency,polarization and radiation pattern reconfigurable antennas have became an important research area due to overcoming these limitations.According to the above application requirements,the extended cavity model of microstrip antenna,dual-band microstrip grid array antennas,and the liquid crystal-based frequency,polarization and radiation pattern reconfigurable antennas are studied in the thesis.Main research content and innovation points of the thesis are summarized as follows:(1)The four-port cavity model is presented to study a differential dual-polarised annular-ring patch antenna operating at higher order mode TM₁₂.Acceptable agreements among calculated,simulated,and measured impedances and radiation patterns are obtained.Theoretical analysis shows that the differential port-to-port coupling impedance is zero,and its isolation is theoretically infinite.The differential input impedance is about four times of the single-ended input impedance at the resonant frequency of higher order mode TM₁₂.The differential operation can suppress the adjacent modes TM₀₂ and TM₂₂ near the operating mode TM₁₂ and lead to a more symmetrical E-plane radiation pattern and lower cross polarisation level.(2)A dual-band differential shifted-feed microstrip grid array antenna(DSF-MGAA)with two parasitic patches is presented.By moving the feed location,a conventional MGAA(C-MGAA)can be turned to a SF-MGAA for dual-band operation.The high frequency band of SF-MGAA corresponds to the operation mode of the C-MGAA,and its low frequency band is similar to that of the patch or meshed patch antenna.To improve its impedance bandwidth,a new differential feeding scheme and two parasitic patches are adopted.The proposed DSF-MGAA can cover dual-band of 3.7 GHz and 28.5 GHz.The measured-10 d B impedance bandwidths of 3.4%and 4.5%,and corresponding to the peak gains of 8.98 d Bi and 19.2 d Bi are obtained in the low and high frequency bands,respectively.The DSF-MGAA can be a potential candidate for the future dual-band wireless communication.(3)A shared-aperture dual-band differentially-fed microstrip array with the microstrip grid and patch antennas is presented.The patch antenna array as high frequency elements is inserted into microstrip grid array antenna(MGAA)operating at low frequency band.The differentially-fed patch array and MGAA can obtain more symmetrical radiation pattern and lower cross polarization level.Impedance bandwidth of MGAA can be enhanced from 4%to 17%due to the coupling with the patch array.The measurement shows the proposed antenna array with feeding network achieves impedance bandwidths of 17%and 15%,and peak realized gains of 16 and 15 d Bi at dual-bands of 28 and 38GHz,respectively.(4)A differential probe fed liquid crystal(LC)-based frequency tunable circular ring patch antenna is presented.Besides,cavity model is extended to analyze the LC-based antenna for the differential operation.According to the cavity model,the permittivity and loss tangent of the LC are extracted from the measured differential reflection coefficients.Acceptable agreements among the measurement,simulation and calculation for differential impedances,reflection coefficients and radiation patterns are obtained and also validate the extraction method and model.More importantly,the extended cavity model can provide a deep physical insight into the LC-based antenna.(5)A LC-based polarization reconfigurable cross-slot fed patch antenna is presented.The LC-based antenna is designed to operate at around 27.6 GHz.The antenna includes four perpendicular patches which are fed by a cross slot.By controlling LC permittivities under these patches,the different resonance frequencies can be obtained.Based on the degenerate mode theory,the frequency shift of the perpendicular patches generates the phase difference of 90°,and then the circular polarization will be achieved.When resonance frequencies of the patches are the same,linear polarization will be obtained.The simulated results shows that the LC-based antenna can have the left/right-hand circular polarization,and linear polarization switchable functionality.(6)A LC-based three-element microstrip parasitic array antenna with the beam switching is presented.A simple model of three-element parasitic array antenna on the ground is given and analysed to understand the beam switching mechanism.In the proposed antenna,LC permittivities under three elements can be changed by bias voltages to control their resonant electrical lengths.Therefore,the LC-based antenna can operate in three modes to obtain three beam switching directions.The measured results show that three beam switching directions(?=59°,-2°,-57°)in H-plane are achieved.