Analysis of Directivity and Directivity Coefficient of Microwave Antenna

Source: Electronic Theory

●Directivity

※Direction diagram

The basic function of an antenna is to change the electromagnetic wave transmitted by the feeder into an electromagnetic wave propagating in free space. The antenna pattern is to characterize the distribution of electromagnetic wave energy (or field strength) at various points in space when the antenna radiates. It is one of the main parameters describing the antenna One.

The directional pattern of the antenna is a three-dimensional pattern, and its characteristics can be described by the directional patterns in two mutually perpendicular planes (E-plane and H-plane). The antenna directivity diagram can intuitively reflect the concentration of antenna radiation energy. The sharper the directivity diagram, the more concentrated the radiated energy. On the contrary, the energy is scattered. If the antenna radiates electromagnetic energy evenly to the surroundings, the directivity pattern becomes a spherical surface, which is called non-directional, which is the radiation field of an ideal point source in the air. The antenna directivity diagram can be drawn by testing. If the measured power is the power directivity diagram, if the measured field strength is the field strength, then the field strength directivity diagram can be drawn, but the shapes of the two graphics are exactly the same. of. Usually, the pattern directional graph has multiple lobes, of which the lobes with the largest radiation direction are called the main lobes, and the rest are called side lobes (or side lobes). The main lobe information in the directional graph is what we care most about.

(1) Width of the main lobe of the directional map

The width of the main lobe of the directivity diagram refers to the width between the half-power points (the point where the power drops to half of the maximum radiation direction power), which is formed by the connection of two points with the zero point where the maximum value of the main lobe "1" drops to "0.5" The included angle is expressed by 2θ0.5.

(2) The zero angle of the main lobe of the directional graph

The zero-point angle of the main lobe of the directional graph refers to the angle between the zero radiation directions on both sides of the main lobe, which is represented by 2θ0.

(3) Zero level of side lobe of directivity map

The power level of the side lobe of the directional pattern indicates the ratio of the power level of the side lobe to the power level of the main lobe, generally expressed in decibels (dB), namely:

It is generally hoped that the lower the sidelobe level (that is, the greater the negative value), the better.

※Directivity coefficient

Although the directivity diagram reflects the antenna radiation state to a certain extent, it is a relative value. In order to quantitatively describe the degree of antenna concentrated radiation, the concept of directivity coefficient is introduced.

The directivity coefficient is defined as: at the same distance and the same radiation conditions, the radiation power density Smax (or field strength squared Emax) and the radiation power density S0 (or field strength) of a non-directional antenna (point source) at the maximum radiation directivity of an antenna Squared E20) ratio, denoted by D:

It can be seen that the sharper the directivity of the antenna, the larger the D, on the contrary, the smaller the D. If D=1, it is expressed as a non-directional antenna, which is an ideal point source radiation field.

●Antenna efficiency

Generally speaking, the conductor and insulating medium that constitute the antenna have certain energy loss. The power input to the antenna cannot be completely converted into the radiated power of free space electromagnetic waves. We call the ratio of the antenna radiated power Pr to the antenna input power as the antenna efficiency. which is:

ηa=Pr/Pi

Generally, the efficiency of microwave antennas is very high, and ηa is close to 1. In addition, it is worth mentioning that the antenna efficiency defined here does not include the loss caused by the mismatch between the antenna and the feeder transmission system. For example, considering the reflection coefficient Г at the antenna input, Then the total efficiency of the antenna is: ηA=(1-｜Г2｜)x ηa

●Gain coefficient (gain)

The gain coefficient is abbreviated as gain, and its definition is: the radiation power density Smax (or field strength square E2max) of an antenna in the maximum radiation direction under the same distance and the same input power conditions and the non-directional antenna (ideal point source) The ratio of the radiation power density S0 (or the square of the field strength E20) is expressed by G.

That is, G=ηa﹒ D

It can be seen that the gain coefficient is a parameter that comprehensively measures the energy conversion efficiency and directivity of the antenna, and it is the product of the directivity coefficient and the antenna efficiency. In practical applications, the antenna gain coefficient and the directivity coefficient are important parameters, although they are closely related. For microwave surface antennas, their conversion efficiency is very high, ηa=1, so G=D. The analysis proves that for the microwave surface antenna, its gain coefficient has the following relationship with the antenna aperture:

In the formula: S is the actual area of the antenna radiating aperture; ηe is the utilization factor of the aperture, or aperture efficiency, which is mainly determined by the electromagnetic field amplitude distribution and phase distribution on the aperture. When the mouth surface is evenly distributed and in phase, ηe=1, the maximum gain can be obtained, which can be seen from the above formula: Se=S﹒ ηe (Se is called the effective area of the antenna aperture). For a non-directional antenna (ideal point source), G=D=1, and its effective area is:

The gain coefficient is generally expressed in decibels: GdB=10lgG(dB)

●Antenna impedance

Antenna impedance refers to the input impedance of the antenna input port in the direction of the antenna radiation port. It depends on the antenna structure and operating frequency. The conversion efficiency of the antenna is the highest only when the input impedance of the antenna is well matched with the impedance of the feeder. Otherwise, reflections will occur on the input port of the antenna and a standing wave will be formed on the feeder, thereby increasing the transmission loss. The matching of most antenna input impedance is determined by approximate calculation in engineering design, and then through experimental measurement and correction.

●Antenna polarization

Antenna polarization refers to the orientation of the electric field strength (E) vector in the maximum radiation direction of the antenna. Linear polarization is a more commonly used polarization method. Linear polarization can be divided into "vertical polarization and horizontal polarization". The electric field vector of the former is perpendicular to the ground, and the latter is parallel to the ground.

●Bandwidth

All the electrical parameters of the antenna are related to the operating frequency. When the operating frequency deviates from the center frequency f0, the electrical parameters of the antenna will deteriorate. The frequency bandwidth of an antenna refers to the frequency range in which the antenna can work normally. In this range, the antenna's directivity pattern, gain, impedance and other technical parameters are changed within the allowable range of the index.

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