Spherical Near-field Measurements And Diagnosis Of Phased Array Antennas

Near-field measurement technology is one of the important methods of antenna measurement,and spherical near field measurement technology is commonly used in near field measurement.In this thesis,the Helmholtz equation is deduced from Maxwell’s equations and the equation of spherical wave expansion of the field is obtained by solving Helmholtz equation,thus the formulas of spherical near-field to far-field transformation are obtained.In order to verify the validity of spherical near-field to far-field transformation algorithm,with the array antenna made up of half-wavelength dipole as the theoretical model of the phased array antenna under test,the spherical mear-field measurements on phased array antennas are studied by the method of computer simulation.Using the Fast Fourier Transform(FFT)algorithm,the far-field pattern of the antenna under test is obtained by means of the spherical near-field to far-field transformation algorithm and compared with the theoretical far-field pattern.At the same time,in order to verify the validity of the far-field to near-field transformation,the plane wave spectrum of the antenna under test is obtained from the antenna far field data.The near field of the antenna under test can be calculated by the plane wave spectrum.The calculation results show that the near-field data obtained by far-field to near-field transformation based on the theory of plane wave expansion fit well with the theoretical near field data.With the continuous improvement of measurement theory,antenna diagnosis theory has also been greatly improved.Due to the advantages of high accuracy and wide scanning range of spherical near field antenna measurement,spherical near field diagnosis has been applied more widely in diagnostic technology.In this thesis,the relationship between the spherical sampling data and the current distribution of the planar phased array antenna is established by using the method of moments,and the matrix equation is solved to obtain the current distribution of the antenna under test,so as to diagnose whether the phased array antenna has faults.Furthermore the equivalent magnetic current method is compared with the plane spectrum expansion method to analyze the advantages and disadvantages of the two methods.At the same time,with the development of the Times,there are more and more kinds of antennas,and curved phased array antennas have been widely used,so the diagnosis of curved phased array antennas has gradually become the direction of people’s attention.In this thesis,the plane wave spectrum is obtained from the far-field pattern of the ideal curved phased array antenna,and the current distribution of the elements of the curved phased array antenna is obtained from the plane wave spectrum.After verification of the above results,it can be seen that the wavefront current of planar and curved phased array antennas is basically consistent with the theoretical value,thus verifying the validity of the diagnostic method.In the anechoic chamber,according to the above measurement methods,there will be some errors,because the walls around the anechoic chamber can not fully absorb the scattered waves,so we need other methods to reduce the errors.In this thesis,the mathematical absorber reflection suppression(MARS)algorithm is adopted.With the ideal planar phased array antenna as the antenna under test,a noise source is set around the antenna.The field generated by the antenna under test and the field of noise source is added to the field generated by the antenna under test to expand the spherical wave mode.The appropriate filtering function is selected to filter out the high-order mode of the spherical wave spectrum.Finally,the far-field pattern is obtained from the filtered spectrum.The effectiveness of the algorithm is verified by simulation.Then put the antenna under test into the anechoic chamber,calculate the anechoic chamber scattering by the geometric optics method,and repeat the above steps after the superposition of the near-field data generated by the phased array antenna with the anechoic chamber scattering.The simulation results show that the Mars algorithm is effective in suppressing the multipath effect of the anechoic chamber.