| With the rapid development of wireless communication technology,terminal equipment has become a necessity in our daily life.Each terminal equipment contains one or more antennas for radiating or receiving electromagnetic energy.However,because the terminal equipment antennas are usually very close to the head and body of users,people's strong attachment to terminal equipment will lead to prolonged exposure to electromagnetic radiation.For the sake of safety,a parameter called specific absorption rate(SAR)is proposed in international standards to measure electromagnetic energy absorbed by biological tissues.The update speed of mobile phone terminal is the most remarkable among mobile terminal devices,and its development places higher requirements for the antennas mounted on it.The fifth-generation communication system is being popularized at present,and the mobile phone terminal needs to support the frequency bands of the fourth-generation and fifth-generation communication systems,that is,the antennas mounted on the mobile phone terminal need to support the 4G and 5G frequency bands,and need to support multiple input multiple output(MIMO)in the 5G frequency band.Therefore,in addition to meeting the basic electrical performances and appearance requirements in the design of mobile phone antennas,there is also a great demand for the theory and technique of designing low SAR antennas.Besides,the development speed of smartwatches in wearable terminal devices is also increasing day by day.Because smartwatches are usually worn on wrists,long-term use will also cause electromagnetic radiation problems.How to design smartwatch antennas with low SAR characteristics in limited space is also a challenging problem.Therefore,this paper studies the theory and applications of low SAR terminal antennas design.Based on the actual demand,three terminal antennas with low SAR characteristics are designed in this paper.1.A low SAR metal-rimmed mobile phone antenna applied in 4G frequency band is proposed.Starting from the theoretical point of low SAR antenna design,the interaction mechanism of the radiation fields of mobile phone antennas with nearby human tissues is analyzed by using boundary conditions of electromagnetics,and the relationship between SAR and electric field is established.Then,the concept of reverse current is put forward to reduce the electric field radiated by antenna at the air-tissue interface,thus decreasing the SAR of human tissues.Finally,a metal-rimmed mobile phone antenna is designed according to the proposed theory.The ground clearance of the proposed antenna is 2 mm.The measured-6 d B impedance bandwidths are 830-1040 MHz and 1670-2730 MHz,respectively.The measured efficiencies are higher than 30% at the lower band and 40% at the higher band.The simulated and measured normalized peak head and body SAR values are lower than 1.0 W/kg.2.A low SAR MIMO antenna array for 5G mobile phone is designed.According to the research of the previous chapter,when the current distribution on the antenna structure is symmetric and reverse,the antenna has low SAR characteristics.By using the theory of characteristic modes,three kinds of antenna structures with symmetric geometry are analyzed and the mode with this current distribution is excited,and then antenna element is obtained.Finally,an eight-port MIMO antenna array with low SAR characteristics is proposed through further optimization.The size of final antenna element is 23.2 mm×5.6mm,the-6 d B impedance bandwidth of antenna elements ranges from 3400 MHz to 3600 MHz,the measured efficiency is above 48%,the isolations between antenna elements are higher than 16 d B,and the ECC is less than 0.10.So,the proposed antenna has good radiation performance and MIMO characteristics.In addition,both 1 g head SAR and 10 g body SAR are less than 1.0 W/kg when the eight-element antennas are co-excited.3.A low SAR smartwatch antenna is proposed.According to the study content in Chapter 2,the SAR value of the antenna will be lower when the current distribution on the antenna is symmetric and reverse.The smartwatch antenna is mainly designed according to the characteristics of low SAR antenna.The modes of smartwatch antenna are analyzed by using characteristic mode theory,and two orthogonal modes whose current distributions on the antenna are symmetrical and reverse are excited in the desired frequency band.Finally,a dual-port low SAR antenna is designed.The size of the proposed antenna is 32.5mm×32.5 mm×7 mm,the-6 d B impedance bandwidth of the antenna element is from 2400 MHz to 2480 MHz,the efficiency in free space of the antenna is above 86%,and the isolation between the antenna element is higher than 14 d B.Besides,when the distance between the antenna and Body is 5 mm,the 1 g and 10 g Body SAR values of the antenna with single-port exciting or dual-port co-exciting are less than 1.5 W/kg. |
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