Study Of Active Circuit Matching Techniques And Diversity Techniques Of Antennas

Due to the constraints of bandwidth,efficiency and volume of the passive antennas,it is usually difficult to reduce the antenna size,enhance the antenna bandwidth and increase the antenna efficiency,simutaneously.The reactance of non-Foster circuit reactance has negative slope characteristic.When using the non-Foster circuit in the matching network of antennas,the limit of Bode-Fano rule can be break.Thus,the antenna,especially the electrically small antenna in HF band,can achieve better miniaturization and broadband characteristics.In addition,in the complex indoor communication environment,it is necessary to eliminate the multipath fading for improving the reliability of the channel.Diversity technology is an effective way to combat multipath fading,while the related research has important theoretical and engineering application value.Combining with the research project,this dissertation focuses on the active matching techniques and diversity techniques.The main academic contributions of this dissertation are as follows.1.The broadband active electrically small monopole antenna is studied.Firstly,a passive electrically small monopole with 0.075?₀ electrical length is analyzed.Because of its characteristics of small reactive and large reactance,the antenna only resonant at narrow bandwidth by using traditional passive matching network.Secondly,two kinds of negative impedance converter circuits based on operational amplifier are studied.It is concluded that the input resistance of the inverting input-output type NIC circuit is positive while that of the non-inverting input-output type NIC circuit is negative.By simulating and testing the key parameters of the circuits,the curves with non-Foster characteristic are obtained.On this basis,a active two stage NIC matching network is designed for the electrically small monopole antenna.By improving the network form,the antenna reactance has been largely offset in the short wave band.Thirdly,on the basis of the measured input impedance of the active monopole antenna,the passive matching network is designed with the optimization algorithm,and the bandwidth of the antenna is further widened.The measured-6d B relative impedance bandwidth can reach 127%.Finally,by applying the comparison method,the received power of the active monopole antenna is tested.Experimental results show that the active monopole antenna has a good effect in the whole working frequency band.2.The broadband active half-loop antenna is studied.Firstly,by using characteristic mode theory,a passive loaded half-loop antenna is analyzed.Results show that the antenna has a single narrowband resonant mode,and the distribution and direction of characteristics current reveal parallel resonant characteristics of the antenna.Secondly,by changing the value of the terminal loaded capacitance,the curves of two key characteristic modes is analyzed,and the admittance curve with negative slope characteristic is calculated.By applying co-simulation technique,the ideal non-Foster impedance matching network is designed to meet the requirements.Next,by replacing the Foster capacitor with the non-Foster network,the resonant mode of the active half-loop antenna turns the narrowband resonant mode into a continuous resonant mode with its radiation pattern kept invariant over a wide bandwidth.Thus,both the impedance bandwidth and radiation bandwidth are widen.Finally,based on simulation verification and parameter analysis of the negative impedance converter circuit using transistors,the actual non-Forster impedance matching network is designed and loaded on the half-loop antenna.The active half-loop antenna is fabricated and tested.The results show that the impedance bandwidth of the active half-loop antenna is 6 times passive half loop antenna.3.Three kinds of diversity antennas are studied.Firstly,a pattern diversity antenna is designed.The antenna is composed of a square patch with double probe,by control the current of the probe,the radiation pattern of the antenna can switch between the broadside and the conical radiation pattern.The overlapping relative bandwidth of two mode is21.8%.Secondly,a polarization diversity antenna is designed.The antenna adopts ring slot antenna to achieve vertical polarization,while arc dipole array to achieve horizontal polarization.By placing the dipole array around the ground of ring slot antenna,and designing non-interference feeding networks,the antenna has a low profile of 0.024?₀,while achieving dual polarizations.Thirdly,a pattern and polarization diversity antenna is designed.The antenna consists of a J-pole antenna and two dipole antennas.Since the J-pole antenna requires no DC ground,the dipole antenna and J-pole antenna are combined without affecting each performance.The arms of the dipole adopt a bent structure for size miniaturization.When placed perpendicular to the floor,the proposed antenna achieves available vertical and horizontal polarizations covering the whole horizontal plane,while has pattern diversity characterisitc in the horizontal direction.