Basic knowledge of antennas

JasonYoo

Basic knowledge of antennas [Copy link]

The main parameters that characterize antenna performance include radiation pattern, gain, input impedance, standing wave ratio, polarization mode, etc.

1.1 Antenna input impedance
The input impedance of the antenna is the ratio of the input voltage to the input current at the antenna feed end. The best connection between the antenna and the feed line is that the antenna input impedance is a pure resistor and is equal to the characteristic impedance of the feed line. At this time, there is no power reflection at the feed line terminal, there is no standing wave on the feed line, and the input impedance of the antenna changes relatively slowly with frequency. The matching work of the antenna is to eliminate the reactance component in the antenna input impedance and make the resistance component as close to the characteristic impedance of the feed line as possible. The quality of matching is generally measured by four parameters, namely reflection coefficient, traveling wave coefficient, standing wave ratio and return loss. There is a fixed numerical relationship between the four parameters, and which one is used is purely out of habit. In our daily maintenance, standing wave ratio and return loss are more commonly used. The input impedance of a general mobile communication antenna is 50Ω.

Standing wave ratio: It is the reciprocal of the traveling wave coefficient, and its value is between 1 and infinity. A standing wave ratio of 1 indicates perfect matching; a standing wave ratio of infinity indicates total reflection and complete mismatch. In mobile communication systems, the VSWR is generally required to be less than 1.5, but in actual applications, the VSWR should be less than 1.2. An excessively large VSWR will reduce the coverage of the base station and increase 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 ranges from 0dB to infinity. The larger the return loss, the worse the match, and the larger the return loss, the better the match. 0 represents full reflection, and infinity represents complete matching. In mobile communication systems, the return loss is generally required to be greater than 14dB.



1.2 Polarization of the antenna
The so-called polarization of the antenna refers to the direction of the electric field strength formed when the antenna radiates. When the direction of the electric field strength is perpendicular to the ground, the radio wave is called a vertically polarized wave; when the direction of the electric field strength is parallel to the ground, the radio wave is called a horizontally polarized wave. Due to the characteristics of radio waves, horizontally polarized signals will generate polarized currents on the surface of the earth when they are close to the ground. Polarized currents generate heat energy due to the influence of earth impedance, which causes the electric field signal to decay rapidly. However, vertical polarization is not easy to generate polarized currents, thus avoiding the significant attenuation of energy and ensuring the effective propagation of signals.

Therefore, in mobile communication systems, vertical polarization is generally used. In addition, with the development of new technologies, a dual-polarized antenna has recently emerged. In terms of its design concept, it is generally divided into vertical and horizontal polarization and ±45° polarization. The latter is generally better than the former in terms of performance, so most of them currently use ±45° polarization. The dual-polarized antenna combines antennas with mutually orthogonal polarization directions of +45° and -45°, and works in the duplex mode of transceiver at the same time, which greatly saves the number of antennas in each cell; at the same time, since ±45° is orthogonal polarization, it effectively guarantees the good effect of diversity reception. (Its polarization diversity gain is about 5dB, which is about 2dB higher than that of a single-polarization antenna.)

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

Generally speaking, the increase in gain mainly depends on reducing the beam width of the vertically oriented radiation while maintaining omnidirectional radiation performance in the horizontal plane. Antenna gain is extremely important to the operating quality of mobile communication systems because it determines the signal level at the edge of the cell. Increasing the gain can increase the coverage of the network in a certain direction, or increase the gain margin within a certain range. Any cellular system is a two-way process, and increasing the gain of the antenna can simultaneously reduce the gain budget margin of the two-way system. In addition, the parameters that characterize antenna gain are dBd and dBi. DBi is the gain relative to the point source antenna, and the radiation in all directions is uniform; dBd is relative to the gain of the symmetrical array antenna dBi=dBd+2.15. Under the same conditions, the higher the gain, the farther the radio wave propagates. Generally, the antenna gain of a GSM directional base station is 18dBi, and that of an omnidirectional one is 11dBi.

1.4 Antenna Beam Width
The beam width is a very important parameter commonly used in directional antennas. It refers to the width of the angle formed by the point 3dB below the peak in the antenna's radiation pattern (the antenna's radiation pattern is an indicator of the antenna's ability to send and receive signals in all directions, usually expressed graphically as the relationship between power intensity and angle).

The vertical beam width of the antenna is generally related to the coverage radius in the direction corresponding to the antenna. Therefore, by adjusting the verticality (pitch angle) of the antenna within a certain range, the purpose of improving the coverage quality of the cell can be achieved, which is also a method we often use in network optimization. It mainly involves two aspects: horizontal beam width and vertical plane beam width. The half-power angle of the horizontal plane (H-Plane Half Power beamwidth) ( 45°, 60°, 90°, etc.) defines the beam width of the antenna in the horizontal plane. The larger the angle, the better the coverage at the junction of sectors, but when the antenna tilt angle is increased, the more likely it is that beam distortion will occur, forming cross-area coverage. The smaller the angle, the worse the coverage at the junction of sectors. Increasing the antenna tilt angle can improve the coverage at the intersection of sectors in terms of mobility, and relatively speaking, it is not easy to cause cross-area coverage of other cells. In the city center, due to the small station distance and large antenna tilt angle, antennas with small half-power angles in the horizontal plane should be used, and antennas with large half-power angles in the horizontal plane should be used in the suburbs; the half-power angle of the vertical plane (V-Plane Half Power beamwidth): (48°, 33°, 15°, 8°) defines the beam width of the antenna in the vertical plane. The smaller the half-power angle of the vertical plane, the faster the signal attenuates when deviating from the main beam direction, and the easier it is to accurately control the coverage range by adjusting the antenna tilt angle.

1.5 Front-Back Ratio
indicates the quality of the antenna's back lobe suppression. If an antenna with a low front-to-back ratio is selected, the back lobe of the antenna may cause cross-area coverage, resulting in confusion in the switching relationship and dropped calls. Generally, it is between 25-30dB, and antennas with a front-to-back ratio of 30 should be preferred. [C]