The basic concept of antenna direction map

The antenna pattern is also called the Radiation Direction map (radiation pattern), the far-field direction map (Far-field pattern). The antenna gain is not obtained from the pattern above, and the directional coefficients are obtained from the direction graph. Antenna gain = directional coefficient * antenna efficiency. So the direction coefficient is greater than the gain is positive.

The antenna gain is mainly demonstrated by the test of the direction map. There are a lot of test systems for the test direction map. That is, the darkroom. And the results of the test in the darkroom, It is also a result of comparison with the ideal symmetrical oscillator. Both know that the gain of the ideal symmetric oscillator is 2.15dB. This allows the antenna gain to be calculated based on the level of the test.
g=d*n%.
And the efficiency of the antenna is generally not hundred percent, so g<d. In the calculation of the antenna's direction coefficient d is, usually used is based on the direction of the main lobe shown above the width of the lobe calculation, such as the half-power lobe width, that is, the level of 3dB is the width of the lobe.

Antenna gain:

Antenna gain is the ratio of the power density of the signal generated by the actual antenna to the ideal radiation unit at the same point in space as the input power is equal. It quantitatively describes the degree to which an antenna concentrates the input power in the radiation. The gain is obviously closely related to the antenna pattern, the narrower the main lobe, the smaller the sidelobe, the higher the gain. Antenna gain is a measure of the antenna's ability to transmit and receive signals in a particular direction, which is one of the most important parameters for selecting a base station antenna. Generally speaking, the increase of gain mainly relies on decreasing the width of the lobe which is perpendicular to the radiation, and maintaining omni-direction radiation performance on the horizontal plane. Antenna gain is extremely important for the operational quality of a mobile communication system because it determines the signal level of the cellular edge. Increasing the gain can increase the coverage of the network in a certain direction, or increase the gain margin within the defined range. Any cellular system is a bidirectional process that increases the gain of the antenna while reducing the bi-directional system gain budget margin. In addition, the parameters representing the antenna gain are DBD and DBI. The DBI is relative to the gain of the point source antenna, the radiation in all directions is uniform, and the dBd is relative to the dbi=dbd+2.15 of the symmetric array antenna. Under the same conditions, the higher the gain, the farther away the wave travels. Generally, the antenna gain of GSM directional base station is 18dBi, and the omnidirectional is 11dBi.

Characteristic parameters of antenna pattern:

In order to facilitate the comparison of the directional map characteristics of various antennas, it is necessary to specify some characteristic parameters. Mainly include: main lobe width, sidelobe level, front-to-back ratio, directional coefficient, etc.
1. Main lobe width: is the amount of physical quantity that measures the sharp degree of the antenna's maximum radiation area. The width of the main lobe of the antenna direction diagram is usually two half power points.
2. Sidelobe level: Refers to the level of the first sidelobe closest to the main lobe and the highest level, generally expressed in decibels.
3. Before and after: refers to the maximum radiation direction (forward) level and its opposite direction (backward) level ratio, usually in decibels.
4. Direction coefficient: At a certain distance from the antenna, the antenna in the maximum radiation direction of the radiation power flow density and the same radiation power of the ideal non-directional antenna at the same distance of the radiation power flow density ratio.

The pattern of the antenna is plotted:

The pattern of the antenna is a graph characterizing the radiation characteristics (amplitude, phase, polarization) and spatial angle of the antenna. The complete direction map is a three-dimensional spatial graph, as shown in Figure 3.1. It is based on the center of the antenna phase centers (coordinates origin), in the radius r is large enough of the spherical surface, measured by the point of its radiation characteristics plotted. Measuring the amplitude of the field strength, the direction of the electric field is obtained, the power direction map is obtained, the polarization direction map is obtained, and the phase direction map is obtained by measuring the polarization. If not added, this book says that the direction map refers to the field intensity amplitude direction map. The mapping of three-dimensional spatial direction map is very troublesome, in the actual work, generally only need to measure the horizontal plane and the vertical plane (that is, XY plane and XZ plane) of the direction map on the line.

Figure 1 Measuring the coordinates of the direction map

The antenna pattern can be plotted in polar coordinates or in rectangular coordinate. The polar coordinate pattern is characterized by its intuition and simplicity, and the spatial distribution characteristics of the radiated field intensity of the antenna can be directly seen from the direction map. But when the main lobe of the antenna pattern is narrow and the sidelobe level is low, the rectangular coordinate rendering method shows a greater advantage. Because the horizontal axis of the representation angle and the ordinate of the radiation intensity can be arbitrarily selected, for example, even if the main lobe width of 1° can be clearly expressed, and the polar coordinates can not be drawn. Figure 2 shows two coordinate representations of the same antenna direction map.

Figure 2 Representation of the direction graph (a) polar coordinates (b) Rectangular coordinate

Generally, the direction map is normalized, that is, the radial length (polar coordinate) or ordinate value (rectangular coordinates) is the relative field strength E (θ,φ)/emax, where E (θ,φ) is any direction of the field strength value, Emax is the maximum radiation direction of the field strength value. Therefore, the normalized maximum value is 1. For very low sidelobe level antenna, most of the directional map is expressed in decibels, and the normalized maximum value is 0 decibels. Figure 3 shows the same-day line direction map in rectangular coordinates with normalized field strength and decibel values.
Figure

Figure 3 Normalization of the direction map