What is an antenna pattern?

What is an antenna pattern?

The antenna pattern diagram, also known as a radiation pattern or far-field pattern, does not provide antenna gain; it yields a directivity coefficient. Antenna gain = directivity coefficient × antenna efficiency. Therefore, the directivity coefficient is always greater than the gain.

G=D*N%.

Since antenna efficiency is generally not 100%, G < d. When calculating the antenna directivity D, it is common to use the beamwidth of the main lobe as shown in the radiation pattern, such as the half-power beamwidth—that is, the beamwidth at which the signal level drops by 3 dB.

Antenna Gain

Antenna gain refers to the ratio of the signal power density generated by an actual antenna to that generated by an ideal radiating element at the same point in space, under conditions of equal input power. It quantitatively describes the degree to which an antenna concentrates the radiation of input power. Gain is clearly closely related to the antenna pattern; the narrower the main lobe and the smaller the side lobes, the higher the gain. Antenna gain is used to measure an antenna’s ability to transmit and receive signals in a specific direction; it is one of the most important parameters in selecting base station antennas. Generally, increasing gain primarily relies on reducing the beamwidth in the vertical plane while maintaining omnidirectional radiation performance in the horizontal plane. Antenna gain is critical to the operational quality of mobile communication systems, as it determines the signal level at the cell boundary. Increasing gain can expand network coverage in a specific direction or increase the gain margin within a defined area. Since any cellular system is a bidirectional process, increasing antenna gain simultaneously reduces the gain budget margin for the bidirectional system. Additionally, antenna gain is expressed in dBd and dBi. dBi represents the gain relative to a point-source antenna, which radiates uniformly in all directions; dBd represents the gain relative to a symmetrical dipole antenna, where dBi = dBd + 2.15. Under identical conditions, the higher the gain, the farther the radio waves travel. Generally, the antenna gain for a GSM directional base station is 18 dBi, while that for an omnidirectional base station is 11 dBi.

Characteristics of the antenna pattern parameters:

To facilitate comparisons of the radiation pattern characteristics of various antennas, certain characteristic parameters must be defined. These primarily include: main lobe width, side lobe level, front-to-back ratio, and directivity, among others.
1. Main lobe width: A physical quantity that measures the sharpness of an antenna’s maximum radiation region. It is typically defined as the width between the two half-power points of the main lobe in the antenna’s radiation pattern.
2. Side lobe level: Refers to the level of the first side lobe closest to the main lobe and with the highest level; it is generally expressed in decibels.
3. Forward-to-rear ratio: This is the ratio of the radiation level in the direction of maximum radiation (forward) to that in the opposite direction (rear), typically measured in decibels.
4. Directional coefficient: At a given distance from the antenna, this is the ratio of the radiation power flux density in the direction of maximum radiation to the radiation power flux density of an ideal omnidirectional antenna with the same radiation power at the same distance.

Plotting an Antenna Pattern

An antenna pattern is a graphical representation of the relationship between an antenna’s radiation characteristics (field amplitude, phase, and polarization) and spatial angles. A complete pattern is a three-dimensional spatial figure, as shown in Figure 3.1. It is plotted by measuring the radiation characteristics point by point on a spherical surface with radius r sufficiently large, with the antenna’s phase center as the center of the sphere (the origin of the coordinate system). Measuring the field amplitude yields the field amplitude pattern; measuring power yields a power pattern; measuring polarization yields a polarization pattern; and measuring phase yields a phase pattern. Unless otherwise specified, all references to “patterns” in this book refer to field amplitude patterns. Mapping three-dimensional patterns is quite cumbersome; in practice, it is generally sufficient to measure the patterns in the horizontal and vertical planes (i.e., the XY and XZ planes)

Antenna radiation patterns can be plotted using either polar coordinates or Cartesian coordinates. Polar coordinate plots are intuitive and simple, allowing the spatial distribution characteristics of the antenna’s radiation field to be directly observed from the plot. However, when the main lobe of an antenna pattern is narrow and the side lobe levels are low, the Cartesian coordinate method offers greater advantages. This is because both the horizontal axis (representing angle) and the vertical axis (representing radiation intensity) can be chosen arbitrarily; for example, even a main lobe width of less than 1° can be clearly represented, whereas this is impossible with polar coordinates. Figure 2 shows the two coordinate representations of the same antenna pattern.

Representation of the direction diagram in Figure 2: (a) Polar coordinates; (b) Rectangular coordinates

Polar patterns are generally plotted on a normalized scale, meaning that the radial length (in polar coordinates) or the y-coordinate (in Cartesian coordinates) is expressed as the relative field strength E(θ, φ)/Emax, where E(θ, φ) is the field strength in any given direction and Emax is the field strength in the direction of maximum radiation. Therefore, the normalized maximum is 1. For antennas with extremely low side-lobe levels, the radiation pattern is usually expressed in decibels, with the normalized maximum set to 0 dB. Figure 3 shows the radiation pattern of the same antenna expressed in both normalized field strength and decibels in Cartesian coordinates.