The polarization of an electromagnetic wave refers to the change of the direction of the electric field vector with time during the propagation of the electromagnetic wave. The propagation of electromagnetic waves can be understood as the propagation in four-dimensional space, space + time. Polarization can be understood as a certain point in fixed space, and the way the sagittal trajectory of the electric field vector at this point changes with time, the propagation direction of electromagnetic waves must be considered here. In general applications, electromagnetic waves that are far away from the source are usually regarded as plane waves. According to the decomposability of the vector, the electric field vector can be decomposed into two components perpendicular to each other in the equal phase plane, and then the phase and amplitude of these two components can be used to judge polarized way.
Animation demonstration of linear polarization, circular polarization, and elliptical polarization
The polarization modes are divided into linear polarization, circular polarization and elliptical polarization. The phase difference between the two vertical electric field components of linear polarization is nπ, and the amplitude is not required. The amplitude of the two vertical electric field components of circular polarization should be equal, and the phase difference is 90 degrees. Elliptical polarization is the most general form, and there is no requirement for amplitude and phase. Polarization and circular polarization are two special cases of elliptical polarization. The most important point is that the polarization mode is closely related to the propagation direction of the electromagnetic wave.
The type of polarization of the antenna
Antenna polarization involves sending and receiving electromagnetic radiation according to its orientation. The polarization of the antenna determines its electromagnetic radiation transceiver performance. The polarization is based on the plane of oscillation of the electric field component of the electromagnetic radiation. If the polarization of the electromagnetic wave is canceled by the rotation of the antenna polarization, the antenna can only capture a part of the electromagnetic wave. Therefore, if the transmit and receive antennas are referenced to the same plane, their polarization directions should be the same for the best efficiency of the communication link.
The way of antenna polarization is shown in the figure above. Although there are many polarization types, they are mainly divided into three types: linear polarization, circular polarization and elliptical polarization. Radio frequency antennas are usually linearly polarized or circularly polarized antennas, while linearly polarized antennas are usually vertically polarized or horizontally polarized antennas, and circularly polarized antennas are left-handed or right-handed circularly polarized antennas. In addition, there is a common elliptically polarized antenna formed by a complex combination of linear polarization and circular polarization. Another example: according to the number of polarizations, it can be divided into single polarization and dual polarization. Dual polarization can be two linear polarizations or two circular polarizations (left-hand circular polarization and right-hand circular polarization).
Different polarizations may cause information transmission to fail. Understanding this can help us analyze which situations are caused by the poor information transmission efficiency caused by polarization in practical applications, and can also quickly select the suitable polarization type antenna products for the application when making the application plan, and determine the installation. Way. Different applications can get better results from different polarizations.
For example, vertically polarized antennas perform better in land mobile communications applications because vertically polarized electromagnetic waves are more likely to traverse uneven terrain than horizontally polarized electromagnetic waves, while horizontally polarized methods rely on the ionosphere and are typically long It performs better in the use of distance communication. In addition, circular polarization is often used in satellite communications because it generally better mitigates the degradation caused by satellite orientation offsets.
(1) Linear polarization describes an electromagnetic wave whose orientation of the electric field vector in space is fixed and is called linear polarization. Sometimes the ground is used as a parameter, the direction of the electric field vector parallel to the ground is called horizontal polarization, and the direction perpendicular to the ground is called vertical polarization. The plane formed by the electric field vector and the direction of propagation is called the polarization plane. The polarization plane of the vertically polarized wave is perpendicular to the ground; the polarization plane of the horizontally polarized wave is perpendicular to the incident plane formed by the incident line, the reflection line and the normal line of the incident point ground. (2) Mathematical analysis of linear polarization In three-dimensional space, the instantaneous electric field of electromagnetic waves propagating along the Z-axis can be written as:
It can be seen that the trajectory of the endpoints of the composite vector is a straight line. Linearly polarized waves are divided into horizontally polarized waves and vertically polarized waves.
(1) Description of circular polarization When the angle between the polarization plane of the radio wave and the normal plane of the earth changes periodically from 0 to 360 degrees, that is, the magnitude of the electric field does not change, the direction changes with time, and the trajectory of the end of the electric field vector When the projection on the plane perpendicular to the propagation direction is a circle, it is called circular polarization. Circular polarization can be obtained when the horizontal and vertical components of the electric field are equal in amplitude and 90 degrees or 270 degrees out of phase. Circular polarization, if the polarization plane rotates with time and has a right helical relationship with the propagation direction of the electromagnetic wave, it is called right circular polarization; otherwise, if it has a left helical relationship, it is called left circular polarization. (2) Mathematical analysis of circular polarization
Figure 2 Circular polarization
(1) Description of elliptical polarization If the amplitude and phase difference of Ex and Ey do not satisfy the above conditions, the trajectory of the endpoint of the composite vector is an ellipse. The ratio of the major and minor axes of the elliptical polarized wave is called the axial ratio. When the axial ratio of the ellipse is equal to 1, the elliptically polarized wave is a circularly polarized wave. When the axial ratio is infinite, the polarization of the electric wave is linear polarization. According to the different rotation directions of the electric field, elliptical polarization and circular polarization can be divided into two types: right-handed polarization and left-handed polarization. Viewed along the propagation direction of the wave, the electric field vector rotates clockwise in the cross-section, which is called right-handed polarization, and counterclockwise is called left-handed polarization.
Dual-polarized antennas are polarized in both horizontal and vertical directions, while single-polarized antennas are only horizontal or vertical. Schematic diagram of dual polarized antenna:
Schematic diagram of a single polarized antenna:
Single-polarized antennas use 3 or 2 antennas as a radio group. When there are 3 antennas, two antennas are responsible for receiving, and one antenna is used for transmitting; when there are two antennas, one antenna is used as a pure receiving antenna, and one antenna combines the functions of transmitting and receiving (usually one transmit and two receive). If dual polarized antennas are used, only 1 antenna is required per sector. Antenna polarization loss and application
The polarization loss of the linearly polarized system depends on the angle between the linearly polarized antenna and the polarization vector of the electromagnetic wave, and the maximum polarization loss occurs when the angle between the two is 45 degrees. At a polarization vector deflection angle of 45 degrees, the maximum polarization loss is 0.5 (ie, 3 dB). In the case of circularly polarized or elliptically polarized systems, the calculation of the polarization loss is more complicated, and the maximum polarization loss can be as high as 30 dB. This is why polarization can be used to achieve signal isolation and interference between antenna systems can occur. Antennas that are polarized differently can still receive signals from electromagnetic waves with different polarization types, despite polarization losses. Therefore, there is a limit to the signal isolation effect that polarization can achieve.
Under normal circumstances, the antenna polarization mode can be selected according to the application requirements. Different applications can get better results from different polarizations. For example, vertically polarized antennas perform better in land mobile communications applications because vertically polarized electromagnetic waves are more likely to traverse uneven terrain than horizontally polarized electromagnetic waves, while horizontally polarized methods rely on the ionosphere and are typically long It performs better in the use of distance communication. In addition, circular polarization is often used in satellite communications because it generally better mitigates the degradation caused by satellite orientation offsets.
Comparison of vertical single-polarized antennas and dual-polarized antennas From the perspective of transmission, since vertical and ground mobile phones are easier to match with vertically-polarized signals, vertical single-polarized antennas will provide more coverage than other non-vertically polarized antennas. The effect is better. Especially in open mountainous areas and plain rural areas, it is more obvious. Experiments have shown that the coverage effect of this antenna is better than that of dual-polarized 45 antennas in mountainous areas or plain rural areas in open areas. The metal body easily rotates the polarization, so there is not much difference in coverage capability whether it is a single-polarized or a 45 dual-polarized antenna. The positive and negative 45-degree dual-polarized antennas are used in urban areas because the urban environment is complex. For example, after the transmitted vertical or horizontal polarized electromagnetic waves propagate through multiple reflections, refractions, and diffractions, they reach the receiving end and have changed. The dual-polarized antenna can reduce polarization loss and accurately receive electromagnetic waves.
Reviewing Editor: Tang Zihong