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The antenna pattern is also called radiation pattern (radiation pattern) and far-field pattern (far-field pattern). The antenna gain cannot be obtained from the pattern, but the directivity coefficient is obtained from the pattern. Antenna gain = directivity factor * antenna efficiency. Therefore, it is certain that the directional coefficient is greater than the gain.
The antenna gain is mainly manifested through the test of the pattern. There are many kinds of test systems for testing the pattern. That is, the dark room. And the result of the test in the darkroom is only a result of comparison with the ideal symmetric oscillator. It is known that the gain of an ideal symmetrical oscillator is 2.15dB. In this way, the gain of the antenna can be calculated according to the level of the test. G=D*N%. Generally, the efficiency of the antenna is not 100%, so G
Antenna gain:
Antenna gain refers to the ratio of the power density of the signal generated by the actual antenna and the ideal radiating unit at the same point in space under the condition of equal input power. It quantitatively describes the degree to which an antenna concentrates the input power and radiates it. The gain obviously has a close relationship with the antenna pattern. The narrower the main lobe of the pattern and the smaller the side lobe, the higher the 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 a base station antenna. Generally speaking, the increase of gain mainly depends on reducing the lobe width of the vertical plane radiation, while maintaining the omnidirectional radiation performance on the horizontal plane. The antenna gain is extremely important to the operating quality of the mobile communication system 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. Increasing the gain of the antenna can simultaneously reduce the margin of the two-way system gain budget. In addition, the parameters representing the antenna gain are dBd and dBi. DBi is the gain relative to the point source antenna, and the radiation in each direction 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 waves travel. Generally, the antenna gain of the GSM directional base station is 18dBi, and the omnidirectional antenna gain is 11dBi.
Characteristic parameters of antenna pattern:
In order to facilitate the comparison of the pattern characteristics of various antennas, some characteristic parameters need to be specified. Mainly include: main lobe width, side lobe level, front-to-back ratio, direction coefficient, etc. 1. Main lobe width: It is a physical quantity that measures the sharpness of the antenna's maximum radiation area. The width between the two half-power points of the main lobe of the antenna pattern is usually taken. 2. Side lobe level: refers to the level of the first side lobe closest to the main lobe and the highest level, generally expressed in decibels. 3. Front-to-back ratio: refers to the ratio of the maximum radiation direction (forward) level to the opposite direction (backward) level, usually in decibels. 4. Directivity coefficient: at a certain distance from the antenna, the ratio of the radiation power flow density of the antenna in the maximum radiation direction to the radiation power flow density of an ideal non-directional antenna with the same radiation power at the same distance.
Antenna pattern drawing:
The antenna pattern is a graph that characterizes the relationship between antenna radiation characteristics (field strength amplitude, phase, polarization) and spatial angle. The complete pattern is a three-dimensional space figure, as shown in Figure 3.1. It is drawn by measuring the radiation characteristics point by point on a spherical surface with a sufficiently large radius r with the antenna phase center as the sphere center (coordinate origin). Measure the amplitude of the field strength to obtain the field strength pattern; measure the power to get the power pattern; measure the polarization to get the polarization pattern; measure the phase to get the phase pattern. Unless otherwise stated, the pattern in this book refers to the pattern of field strength and amplitude. The surveying and mapping of the three-dimensional spatial pattern is very troublesome. In actual work, it is generally only necessary to measure the pattern of the horizontal plane and the vertical plane (that is, the XY plane and the XZ plane).
Figure 1 Coordinates of the measurement pattern
The antenna pattern can be drawn in polar coordinates or rectangular coordinates. The characteristics of the polar coordinate pattern are intuitive and simple, and the spatial distribution characteristics of the antenna radiation field strength can be directly seen from the pattern. But when the main lobe of the antenna pattern is narrow and the side lobe level is low, the Cartesian coordinate drawing method shows greater advantages. Because the abscissa representing the angle and the ordinate representing the radiation intensity can be arbitrarily selected, for example, even the width of the main lobe of less than 1° can be clearly expressed, but the polar coordinates cannot be drawn. Figure 2 shows two coordinate representations of the same antenna pattern.
Figure 2 Representation of the pattern (a) Polar coordinates (b) Cartesian coordinates
Generally, when drawing a directional diagram, it is normalized, that is, the radial length (polar coordinates) or the ordinate value (rectangular coordinates) is the relative field strength E(θ,φ)/Emax, where E(θ,φ) Is the field strength value in any direction, Emax is the field strength value in the maximum radiation direction. Therefore, the normalized maximum value is 1. For the antenna pattern of very low sidelobe level, most of them are expressed in decibels, and the normalized maximum value is taken as zero decibels. Figure 3 shows the same antenna pattern in rectangular coordinates with normalized field strength and decibel values.
Figure 3 Normalized pattern
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