Analysis And Design Of Antennas With Characteristic Mode Theory(CMT)

Characteristic mode theory(CMT)has been widely used in the analysis and design of many kinds of antennas in the past ten years.It has been indicated by lots of researches that CMT can provide useful guidelines for designers.Beyond existing results,this dissertation aims to perform more in-depth study.The innovative contributions of the dissertation are outlined as follows:1.Based on CMT,a compact circularly polarized(CP)microstrip antenna with low profile is proposed.When a new characteristic mode(CM)is introduced to extend the bandwidth of a CP antenna,not only its resonant frequency(ResF),but also its polarized direction(PD)should be adjusted.Two parasitic pateches are loaded near the driven patch,and hence,a new parasitc CM is introduced.Through adjusting the sizes and rotated angles of the two parasitic patches,ResF and PD of the parasitic CM could be controlled.Compared to the case with no parasitc patches loaded,-10 dB impedance bandwidth is increased from 4.3% to 6.0%,and 3dB axial ratio bandwidth is increased from 1.3% to 3.3%.Comparaed to the traditional case with four sequentially rotated parasitic patches loaded,the size of the proposed antenna is almost one half smaller.Due to the advantages of compactness and low profile,the proposed antenna is very suitable for the application of Chinese navigation satellite system(CNSS).2.Based on combined CMs,two broadband ring slot antennas are proposed.First we discuss the necessity of applying combined CMs rather than purely magnetic ones in the analysis of slot antennas.In the first slot antenna,two slot stubs are load on the ring to achieve a new comined CM.Since this new CM has strong magnetic current on the stubs but weak on the ring,its ResF could be adjusted by changing the lengths of stubs.However,the radiation pattern of the new CM is different from that of the exsiting one,resulting in an unstable radiation pattern of the first antenna.To solve this problem,we turn to using a parasitic ring instead of directly loading stubs to generate a new CM.Owing to the similar geometries,the new CM and the exsiting one have similar radiation patterns now,which leads to a stable radiation pattern of the second antenna.The bandwidths of the two antennas are 18.2% and 24%,both can be applied to LTE 40/41 bands(15.6%).Owing to their bidirectional radiations,these two antennas are very suitable to be used in a long alley or street.And citizens could hardly find them because they have large ground planes and easy to be conformal,much reducing the potential cost of installation.3.In-depth study on the couplings of CMs is performed from the perspective of energy and circuit.First we explain the couplings come from their near fields.Then by using stored energies of CMs,the calculation formula of coupling coefficient(CC)is derived.It is found that by adjusting coupling of two CMs,antenna Q factor could be reduced,and hence,antenna bandwidth is extened.According to this fact,we propose a coupling method to optimize bandwidth.Since only CC and Q factor at a single frequency are needed to calculate,compared to traditional method by sweeping frequency,the proposed method can save much time for optimization.Different from cavity modes of filters,CMs are radiative.Therefore,coupling effects of CMs should be characterized by J/K inverters with complex values,namely,not only coupling capacitance/inductance,but also coupling conductance/resistance are needed.Patch antennas are used as examples to demonstrate this conclusion.4.Two new kinds of modes,XeR/XmR modes,are proposed.Different from CMs,the eigenvalues of XeR/XmR modes are associated with stored electric/magnetic energies rather than net energy.Hence,Q factor could be represented as direct linear superpositions of their eigenvalues.Both advantages and disadvantages of XeR/XmR modes are discussed in detail.Compared to CMs,the eigencurrents of XeR/XmR modes are not well suited for expanding the total current of an antenna with 2D structure.However,their eigenvalues can provid lots of useful information.The lower bounds of the stored electric/magnetic energies of TE/TM modes can be given.And they could indeed help us quickly determine within which range the minimum Q may locate.