天线的基础知识（Basic knowledge of antennas）.doc

天线的基础知识（Basic knowledge of antennas） First, the basics of antennas The main parameters characterizing the antenna include directional pattern, gain, input impedance, stationary Bobbi, polarization, and so on. Input impedance of 1.1 antenna The input impedance of the antenna is the ratio of the input voltage to the input current at the antenna feed end. Connect the antenna and feeder, the best case is the input impedance of the antenna is pure resistance and is equal to the characteristic impedance of the feeder, the feeder terminal has no power reflection, not standing on the feeder, the input impedance of the antenna with frequency changes gently. The matching of the antenna is to eliminate the reactance component in the input impedance of the antenna, so that the resistance component is close to the characteristic impedance of the feeder as close as possible. The matching is usually measured by four parameters, namely, the reflection coefficient, the traveling wave coefficient, the stationary Bobbi and the echo loss. There is a constant numerical relationship between the four parameters. In our daily maintenance, more are used in Bobbi and echo loss. The input impedance of a general mobile communications antenna is 50 ohms. Stationed in Bobbi: it is the reciprocal of the traveling wave coefficient, whose value is between 1 and infinity. Stationed in Bobbi is 1, indicating complete matching; the Bobbi for infinity means total reflection and complete mismatch. In mobile communication systems, Bobbi is generally required to be less than 1.5, but VSWR should be less than 1.2 in practical applications. Too large to stay in Bobbi will reduce the base station coverage, and cause greater interference within the system, affecting the service performance of the base station. Return loss: it is the reciprocal of the absolute value of the reflection coefficient, expressed in decibels. The value of return loss is between 0dB and infinity. The larger the return loss is, the wor