Design Of Low-SAR Antennas Based On The Theory Of Characteristic Modes Of Composite PEC-Lossy Dielectric Structures

With the rapid development of wireless communication technology and the continuous improvement of people’s demand for high-quality life,smart terminal devices are becoming more and more important in people’s daily lives.Since smart terminal devices such as smart phones and smart watches are usually very close to human body when in use,the antenna will generate some electromagnetic radiation to the human body when emitting energy,it may be harmful to humans.The Specific Absorption Rate(SAR)was proposed to measure the electromagnetic energy absorbed by biological tissues.In order to ensure the safety of people when using smart terminal device,the SAR value of each commercial smart terminal device must be lower than the maximum SAR value in the relevant standards.With the gradual popularization of 5G communications,in order to meet the high channel capacity of antennas,multiple input multiple output(MIMO)technology is widely used in 5G antenna design.When the communication frequency is increased and multiple antennas work at the same time,the SAR value is easily larger than the SAR specified in the standard,which poses a huge challenge to the design of smart terminal antennas.Aiming at the low SAR requirements of smart terminal antennas,this paper conducts research on the theory and application of low SAR antennas,the following three jobs are done:1.The theory of characteristic modes of composite perfect electric conductor(PEC)-lossy dielectric target is studied.The corresponding program is written,which can not only calculate the characteristic modes of composite PEC-lossy dielectric target,but also by degenerated into the theory of characteristic modes of PEC target and the theory of characteristic modes of dielectric target according to the theory of characteristic modes of composite PEC-lossy dielectric target,the self-developed characteristic mode program can also calculate the characteristic mode of PEC target,lossless dielectric target and lossy dielectric target.In order to verify the correctness of the self-developed characteristic mode program in calculating characteristic modes for different target,some numerical examples are used to compare the self-developed characteristic mode program with the electromagnetic simulation software FEKO and some eigenmode literature,and the correctness of the self-developed characteristic mode program in calculating perfect electric conductor target,lossless dielectric target,lossy dielectric target and composite perfect electric conductor-lossy dielectric target characteristic modes is verified.2.Based on the theory of characteristic modes of composite perfect electric conductor(PEC)-lossy dielectric structures,a low SAR four-port MIMO mobile phone antenna operating in3.5GHz band is designed.Firstly,the characteristic modes of the composite structure of the antenna(PEC)and lossy dielectric are calculated by the self-developed characteristic mode program,and the four modes with different coupling coefficients are excited by adjusting the feed phases of the four ports.By comparing the coupling coefficient values of the four excitation modes with the actual SAR values,it can be found that the lower SAR value can be obtained with smaller modal coupling coefficient,which proves the theoretical correctness that exciting low coupling mode can make the antenna achieve low SAR characteristics.A low SAR four-port MIMO mobile phone antenna was designed.The-10 d B bandwidth of the antenna can cover 3400MHz-3600 MHz.The measured free space efficiency of the antenna is 81.3%-89.9%,the isolation between antenna elements is better than 15.4d B,and the ECC is less than 0.07.The 10 g Body SAR when the four ports work simultaneously was 0.47W/kg.3.Based on the theory of characteristic modes of composite perfect electric conductor(PEC)-lossy dielectric structures,a three-band low SAR and high on-body efficiency smartwatch antenna is designed.The characteristic modes of the composite structure of antenna(PEC)and lossy media are calculated,and the low coupling modes in GPS L1 band,WLAN2.4GHz/Bluetooth band and 3.5GHz band are selected.By analyzing the mode current distribution of the three low coupling modes,the feed structure is designed to stimulate the low coupling mode in three frequency bands,so that the antenna can achieve high on-body efficiency characteristics in three frequency bands,and achieve low SAR characteristics in WLAN 2.4GHz/ Bluetooth band and 3.5GHz band.The-6d B bandwidth of the antenna can effectively cover the required operating frequency band,and the free space efficiency of the antenna in the three frequency bands is above 80%.When the antenna is placed 1mm above the hand model,the on-body efficiency in GPS L1 band,WLAN 2.4GHz/ Bluetooth band and 3.5GHz band is 10.4%-10.7%,16.2%-16.4% and 22.7%-24.9%,respectively.The 10 g Body SAR at 2440 MHz is 1.19W/kg,and the 10 g Body SAR at 3500 MHz is 1.22W/kg.