天线外场测试中地面反射的影响分析

Analysis of ground reflection influences in outfield test

低频天线外场测试时往往很难彻底消除地面反射的影响,而地面反射波对直达波的矢量叠加导致待测天线口面上场强分布不均匀,引入较大的测试误差。为消除或减小误差,本文通过建立地面反射模型分析了收发天线高度、收发天线水平距离及地面反射特性对待测天线口面上的反射波或合成场的影响,推导了合成场分布、最佳发射天线高度等计算公式。利用该模型基于实测数据拟合得到地面反射系数幅度和相位,从而得到优化后的测试天线布局。本文基于该模型对外场测试天线高度进行优化后,待测天线口面垂直方向上场强幅度最大值与最小值比值由5.4 dB降到1 dB以内,证明了该分析方法的有效性。该方法可用于低频大口径天线的外场测试方案设计及试验验证工作。

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.