Research On Active And Passive Loading Technology Of Terminal Antenna

With the development of communication technologies,antennas play an important role in wireless communication systems for signal transmission and reception.The terminal antenna is a general term for the antenna used in the handheld device.Antenna miniaturize is a major research subject based on the demand of portability of handheld device.With the development of technology,there have been many methods to achieve miniaturized,and the antenna loading technology is widely used in improving antenna performance and reducing volume.Therefore,this paper mainly studies the miniaturized technology of RFID handheld terminal antenna based on passive loading,and the miniaturized technology of terminal antenna based on active Non-Foster element loading.The main research contents of this paper are as follows:First,a handheld reader terminal antenna working in the RFID China band(920-925MHz)was designed.The antenna guides the current path through a meandering slot.Four radiating elements of central symmetry are formed by four slots.Antenna units are fed by Wilkinson power divider which is used to form equal magnitude and 90 deg phase delay to obtain circular polarization.The power divider and the antenna unit are connected by a feeding wall.The input impedance of the antenna is adjusted by loading a short-circuit wall beside the feeding wall.In addition,a square slot is set in the center of the radiation patch to guide the current path,in order to increasing the length of the current path reduces the antenna size.Finally,the antenna was processed and tested,and the maximum gain of the antenna in the RFID China band is 4 dB.In addition,the paper also studied the monopole terminal antenna loaded with a NonFoster element.The size of antenna is reduced through circuit loading.First,a monopole antenna for the handset was designed and tested.The length of the antenna is 7 cm,and the resonance frequency was tested to be 1.14 GHz.Test the input impedance of the antenna and find that the impedance of the antenna at 400-678 MHz is a small resistance series a large reactance.The reactance of the antenna can be equivalent to a capacitance of 3pF.Therefore,it is necessary to design a Non-Foster circuit which is equivalent to-3pF,and connect it in series with the antenna to offset the capacitive reactance of the antenna,so that to achieve the improvement of matching.The circuit is designed and simulated by ADS(Agilent Advanced Design System),and the S parameter of the antenna is introduced into the circuit for simulation.Finally,the circuit parameters that meet the requirements are obtained through simulation.We can know this circuit is unstable due to the positive feedback characteristics of the Non-Foster circuit,so stable verification and debugging are required.By reducing the magnitude of the loop gain and changing its phase,the loop gain does not satisfy the self-excited condition.Three simulation methods are used in this paper for stable simulation to verify the stability of the circuit.After processing and testing,we can find when the antenna in series Non-Foster element the VSWR is less than 3 in frequency of 400-678 MHz.