Research On The Loop Antenna Calibration Method Of 9kHz-30MHz

Loop antennas are widely used in the fields of low-frequency radiated emission,radiated disturbance,and radiated immunity.The accuracy of loop antenna calibration will affect test results.The antenna factor is an inherent parameter of the loop antenna.In daily use,the loop antenna will wear and out,resulting in a certain degree of change in the antenna factor.If the antenna factor becomes larger,the field strength measured using the loop antenna will become larger,which affects the accuracy of the measurement of the field strength value.Based on the full analysis and research of the loop antenna calibration methods,this paper proposes three calibration layout for loop antenna calibration methods,which are the standard method based on Transverse Electric and Magnetic field cell(the TEM method),the standard method based on the current probe(the current probe method)and the three-antenna method.The calibration system for the three methods mentioned above is established,and the influencing factors of the calibration results are analyzed through experiments.The standard uncertainty process of the above three methods is given.For the TEM method,the structure and working principle of the TEM cell are analyzed,and the principle of the TEM method to calibrate the loop antenna is studied.In order to solve the problem of different level responses of the passive loop antenna and the active loop antenna,the calibration layout of the passive loop antenna and the calibration layout of the active loop antenna are proposed respectively.The calibration system is established and the influencing factors of the calibration results are analyzed.For loop antennas with a diameter of less than 60 cm,the standard uncertainty analysis process is given.The extended uncertainty of the four selected frequency points in the 9kHz-30 MHz frequency band is less than 1.0912dB(k = 2).It should be pointed out that there is not only a magnetic field component but also an electric field component in the TEM cell,so the TEM method is only suitable for calibrating the loop antenna shielded by electric fields.Aiming at the current probe method,the principle of the current probe method for calibrating a loop antenna is studied.A loop antenna calibration system based on the current probe method is established and the standard uncertainty evaluation process is given.The extended uncertainty of one selected frequency point in the 150kHz-350 kHz frequency band is less than 5.3160dB(k = 2),and the extended uncertainty of three selected frequency points in the 400kHz-30 MHz frequency band is less than 0.9801dB(k = 2).The current probe method is a feasible loop antenna calibration method,but it should be pointed out that the current probe used in the current probe method needs to be accurately calibrated.Therefore,a current probe calibration method for the application of loop antenna calibration is proposed in this paper.A current probe calibration system is established,and the current probe extended uncertainty in the 150kHz-30 MHz frequency band is evaluated to be less than 0.3311dB(k = 2).Aiming at the three-antenna method,this method is an absolute loop antenna calibration method.Based on the analysis and research of the principle of the three-antenna method to calibrate the loop antenna,a three-antenna method loop antenna calibration system is established,and the standard uncertainty evaluation process is given.The extended uncertainty of one selected frequency point in the 150kHz-359 kHz frequency band is less than 3.0264dB(k=2),and the extended uncertainty of three selected frequency points in the 400kHz-30 MHz frequency band is less than 0.8770dB(k=2).It should be noted that before calibrating the loop antenna with the three-antenna method,a suitable paired loop antenna is selected according to the characteristics of the loop antenna to be tested,so the three-antenna method has higher requirements for the paired loop antenna.Finally,the TEM method,the current probe method,and the three-antenna method were used to calibrate the passive loop antenna with a diameter of 30 cm and the active loop antenna with a diameter of 50 cm,respectively.Calibration frequency points and calibration results were given.For the passive loop antenna,the calibration results of the TEM method and the threeantenna method are very close in the 150kHz-30 MHz frequency band.The maximum difference between the two methods is 0.8457 dB.For the active loop antenna,the calibration results of the TEM method and the current probe method in the 9kHz-30 MHz frequency band are relatively close.The maximum difference between the two methods in the 9kHz-200 kHz frequency band is 1.9172 dB,and in the 250kHz-30 MHz frequency band the maximum difference between the two methods is 0.5280 dB.