The input impedance of the antenna is the ratio of the input voltage to the input current of the antenna feed end. The best case is that the antenna’s input impedance is the pure resistance and the characteristic impedance of the feeder. The feeder terminal has no power reflection, there is no standing wave on the feeder, and the input impedance of the antenna is relatively slow with the change of frequency. The work of antenna matching is to eliminate the reactance component in the input impedance of the antenna, so that the resistance component can approach the characteristic impedance of the feeder as close as possible. The match is generally measured by four parameters, namely, reflection coefficient, traveling wave coefficient, Bobbi and echo loss. There is a fixed numerical relationship between the four parameters, and the one is used purely out of habit. In our daily maintenance, we use more Bobbi and return loss. The input impedance of the general mobile communication antenna is 50 Omega.
Stationed in Bobbi: it is the reciprocal of the travelling wave coefficient, and its value is between 1 and infinity. Stationed at Bobbi for 1, it is perfectly matched; Bobbi’s total reflection for infinite is complete mismatch. In mobile communication systems, Bobbi is generally less than 1.5, but in practical applications, VSWR should be less than 1.2. Too large Bobbi will reduce the coverage of the base station and cause greater interference within the system, which will affect the service performance of the base station.
Return loss: it is the reciprocal of absolute value of reflection coefficient, expressed by decibel value. The return loss is between 0dB and infinity. The larger the return loss, the worse the matching. The larger the return loss, the better the match. 0 represents total reflection, infinity means complete matching. In mobile communication systems, the return loss is generally greater than 14dB.
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