Research On Near-field Measurement System Of Planar Antenna Based On Equivalent Magnetic Current Method

The performance of the antenna directly affects the quality of the whole communication system,so the accurate antenna measurement is of vital importance.Compared with other antenna measurement methods,near-field measurement has advantages as low cost,high precision etc.In order to improve antenna measurement ability in our school,this thesis designs and develops a set of planar near-field antenna measurement system,as a make-up for the existing compact range microwave anechoic chamber.The main research contents are as follows:1)Firstly,the near field-far field transformation method based on equivalent magnetic current is studied.It computes equivalent magnetic current distribution on the aperture of the test antenna based on the measured electric field distribution in the near field,and then the antenna pattern is calculated from the magnetic current with Green’s function.In order to reduce the sampling data quantity and improve the measurement efficiency,the near-field to far-field transformation based on equivalent magnetic current on orthogonal section is studied.It computes E-plane and H-plane pattern,with the sampling electric field from two corresponding orthogonal near-field slices.The simulation results of parabolic reflector antenna and helix antenna show that the antenna patterns calculated agree with ideal one in the range of 110~o.The equivalent magnetic current method is feasible.2)An antenna scanning system is developed.It uses modular design method,including 2-dgree precise motor controlling system,displacement feeding back system,communication systems etc.The PID double closed-loop algorithm of position and speed is adopted to make sure the high accuracy of frame displacement and motor speed.The near-field scanning experiment shows that the guiding displacement precision reaches 0.01mm,and the measurement displacement accuracy reaches 0.07mm.3)A near-field measurement software is developed including pre-processing and post-processing module.The pre-processing module includes instruction input,guidance parameter input,scanning parameter input,Vector Network acquisition parameter input,probe position monitoring,near-field data acquisition and so on.The post-processing module mainly includes calling Matlab module for near-field-far-field transformation,antenna pattern display,antenna parameters calculation and so on.4)Finally,a microwave anechoic chamber is designed and built,a planar near-field antenna measurement system is built.The near field antenna measurement is carried out with two specimens including horn antenna and double ridged horn antenna.The antenna radiation pattern obtained from the near field measurement is then compared with the results measured in compact range chamber.The results from the two specimens are basically consistent,the E-plane pattern agrees in the range of 70~o,and the H-plane pattern agrees in the range of 40~o.The results show that the designed near-field measurement system is capable of antenna testing preliminarily.