Analysis of ground reflection influences in outfield test

It is often difficult to completely eliminate the influence of ground reflection in the measurement of low-frequency antennas in the outfield. The vector superposition of the reflected wave and the direct wave leads to an uneven distribution of field strength on the aperture of the antenna to be measured, which results in large error. To eliminate or reduce the test error, a ground reflection model is established in this paper. With this model the influence of the height of the transmitting/receiving antennas, the horizontal distance between the transmitting and receiving antennas, and the ground reflection characteristics on the reflected wave or the total field on the aperture of the receiving antenna are analyzed. The calculation formulas of the aperture field distribution of the receiving antenna and the optimal height of the transmitting antenna are deduced based on the ground reflection model. Moreover, the amplitude and phase of the ground reflection coefficient of a real test site are obtained through fitting the measured data, so as to optimize the locations and heights of the antennas to be measured according to the ground reflection model. Based on this model, the ratio of the maximum and minimum field amplitude in the vertical direction of the antenna aperture to be measured is reduced from 5.4 dB to less than 1 dB after optimizing the height of the antenna, which proves the effectiveness of the method. This method can be used in the designs of outfield test schemes and in the test verification of performance for low-frequency large-aperture antennas.