Spherical Near-Field Measurement And Spherical Near-Far Field Transform Algorithm Analysis

Antenna measurement is the important tool of auxiliary antenna design and test results. Because of its no specific requirements in the form of antenna under test, the Spherical Near-field antenna measurement become a most reasonable and effective method of measurements. Spherical near-field measurements sampling the two-dimensional data in a sphere in the near field range, and then through the near-field to far-field data transformation algorithm, the far field electrical characteristics of the antenna under test has been tested.Spherical near field to far field transformation algorithm in this article is based on spherical wave mode expansion theory. Since the spherical wave mode expansion formula applies to the near-field and far-field, so for the same antenna, the near field to far field transformation of the mode coefficient of the formula itself remains the same. The spherical two-dimensional data is sampled in the near field zone, based on spherical wave mode expansion theory used these data we can anti-release each coefficients of the wave mode. If the test probe is an ideal probe, the probe compensation is not required. The coefficients of each mode directly substituted into the spherical wave mode expansion formula in a far-field region, and then the far-field pattern of the antenna under test is derived; when using a non-ideal probe, the probe needs to be compensated. For precision and accuracy of the test, the general use is a directional probe, this probe can effectively reduce multiple reflections between the antenna and the probe. When compensation in the mathematics, the probe can be compensated by a mathematical model imputed as differential operators. Then get the spherical wave mode expansion coefficients in the formula under the probe compensation by the differential operators.Based on this theory, the spherical near field to far field transformation algorithm has been programed by MATLAB. In order to verify the effectiveness of the program, we use the commercial software HFSS simulate a S-band standard gain horn, and get the simulation data in its near field sphere, the near-field data is brought into the spherical near field to far field transformation program, and obtain its far-field data, and then compared with the far-field data obtained by the HFSS simulation; in order to verify the breadth and versatility of the program, an eight-element dipole array antenna has been simulated. Compared the data which calculated from the program with the HFSS simulated data, the results of the two models are in good agreement. Prove the validity, accuracy and versatility of the program. And then analyzed the system error by simulation.Finally, this paper describes the hardware subsystems and software subsystems which build up the spherical near-field testing system. The actual spherical near-field test caveats made an exposition, and several sources of error is qualitatively analyzed.