Antenna basics learning

Release time: 2017-07-11

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 worse the match is, and the greater 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.

Polarization modes of 1.2 antennas

The so-called polarization of the antenna, refers to the antenna radiation when the formation of electric field intensity direction. When the electric field intensity is perpendicular to the ground, this wave is called vertical polarized wave. When the electric field intensity is parallel to the ground, the wave is called horizontal polarized wave. Because of wave characteristics, determines the signal propagation in horizontal polarization will produce polarization current in the earth surface near the ground, due to earth polarization current impedance affects the production of heat and the electric signal decays rapidly, while the vertical polarization mode is not easy to generate polarization current, so as to avoid the sharp attenuation of energy, to ensure the effective transmission of signal the.

Therefore, in mobile communication systems, vertical polarization is adopted. In addition, with the development of new technologies, a new kind of dual polarized antenna has appeared. Generally speaking, the design is divided into two kinds of vertical and horizontal polarization and 45 degree polarization. Generally speaking, the latter is better than the former, so most of the current use is "+ 45" polarization. Dual polarized antenna combination antenna orthogonal +45 degrees and -45 degrees two polarization direction, and at the same time in the transceiver duplex mode, greatly reduces the number of antennas in each district; at the same time as + 45 degrees for orthogonal polarization, effectively guarantee the good effect of diversity reception. (its polarization diversity gain is about 5dB, which is about 2dB higher than that of single polarization antenna)

1.3 antenna gain

Antenna gain is used to measure the ability of an antenna to transmit and receive signals in a specific direction. It is one of the most important parameters for selecting the base station antenna.

In general, the gain increased by reducing the vertical plane radiation beam width, while keeping the omni-directional radiation performance in the horizontal plane. Antenna gain is very important to the quality of mobile communication systems because it determines the signal level at the edge of the cell. By increasing the gain, the coverage of the network can be increased in a definite direction, or the gain margin will be increased within the determined range. Any cellular system is a two-way process that increases antenna gain while reducing two-way system gain budget margin. In addition, the parameters characterizing the antenna gain are dBd and dBi. DBi is relative to the gain of the point source antenna, the radiation in all directions is uniform; the gain of dBd relative to the symmetrical long antenna is dBi=dBd+2.15. Under the same conditions, the higher the gain, the farther the radio wave travels. Generally, the antenna gain of the GSM directional base station is 18dBi, and the omnidirectional antenna is 11dBi.

The 1.4 antenna beam width

Lobewidth is commonly used in directional antenna is a very important parameter, it refers to the antenna radiation pattern in the 3dB place below the peak angle width (radiation pattern of the antenna is an antenna measurement index, each direction transmitter ability usually graphically expressed as power intensity and angle relationship).

The vertical beam width of the antenna and the antenna generally corresponds to the direction of the covering radius. Therefore, in a certain range, by adjusting the verticality (pitch angle) of the antenna, we can achieve the purpose of improving cell coverage quality, which is also a method we often use in network optimization. Mainly involves two aspects: the level of beam width and beam width of vertical plane. Half power plane angle (HPlane Half Power beamwidth) 45 degrees, 60 degrees, 90 degrees etc.) defines the antenna plane beam width. The larger the angle, the better the coverage at the junction of the sector. However, when the antenna angle is increased, the beam distortion is more likely to occur. The smaller the angle, the worse the coverage at the sector junction. Increasing the antenna tilt can improve the coverage of the sector boundaries in movement, and in contrast, it is not easy to generate coverage over other cells. In the center of the base station because of the station from the small antenna angle should be used, the antenna half power plane angle is small, the plane half power suburban large antenna half power angle; vertical plane angle (V Plane Half Power (beamwidth): 48 degrees, 33 degrees, 15 degrees, 8 degrees) definition