1 Introduction
Nowadays, communication vehicles are widely used. However, due to the narrow space of the vehicle body and the large number of bodywork equipment, the electromagnetic compatibility characteristics of communication vehicles are very prominent. The main purpose of studying electromagnetic compatibility is to pursue the coexistence and non-degradation of various systems. According to the characteristics of communication vehicles, the author believes that clarifying the following aspects has practical guiding significance for improving the electromagnetic compatibility index of the entire vehicle.
2 Wire-to-wire interference
The conductor-to-conductor interference is the most common and strongest interference in the communication cabin. The bundling of cables from various systems and groups provides conditions for this interference. According to the structural characteristics of communication vehicles, the mechanism of electromagnetic interference is analyzed and studied, and the use of near field theory to separate capacitive (electric field) interference and inductive (magnetic field) interference to study will be closer to the actual product.
2.l capacitive interference
For two parallel conductors, if conductor 1 is the source of interference, the voltage is V1, and conductor 2 is the disturbed circuit, the interference voltage generated between conductor 2 and the common ground can be expressed as:
In the formula: ω is the signal frequency of the interference source; R is the impedance of the interfered circuit; C12 is the interference capacitance between the two wires.
This is an important formula of wire-to-wire interference, which illustrates the relationship between capacitive interference voltage and various parameters. It is proportional to the signal frequency of the interference source, the impedance R of the interfered circuit, the interference capacitance between conductor 1 and conductor 2, and the interference source voltage.
For a communication vehicle, the voltage and frequency of the interference source cannot be changed. Only by reducing the interference capacitance between the conductor 1 and the conductor 2 and the input impedance R of the interfered object can the interference voltage be reduced. The input impedance is an inherent characteristic of each system and it is static. Therefore, there is only one parameter that can be controlled, namely the interference capacitance. The interference capacitance is a function of area and distance. Therefore, there are many ways to reduce the interference capacitance, such as: shorten the length of the connecting wire, properly arrange the position of the wire or use shielding technology, or make the distance between the two wires as large as possible. If the distance between the two wires increases and the connecting wire shortens, the actual value will decrease, thereby reducing the interference voltage induced on the wire 2.
It can be seen from formula (l) that capacitive interference exists objectively, but it can be quantified and can be controlled to a certain extent.
It can be seen from the above relationship that the two interference conditions are proportional to the frequency, magnetic flux density, the area enclosed by the closed loop, and the mutual inductance between the two circuits. Therefore, the communication vehicle can improve the electromagnetic environment and the electromagnetic compatibility index of the whole vehicle by controlling the area enclosed by the closed loop and the mutual inductance between the two circuits.
2.2 Inductive interference
For two parallel wires, when current flows in a closed loop, a magnetic flux will be generated, and the magnitude of the magnetic flux depends on the current I and the inductance L, that is:
When current flows in the first wire, magnetic flux will be generated in the second wire, so the mutual inductance between the two wires is defined as:
In the formula: It is the magnetic flux produced by the current of wire 1 on wire 2.
It can be seen from the combination of (2) and (3): the greater the inductance, the greater the magnetic flux, and the greater the magnetic flux, the greater the mutual inductance between the two wires.
From Faraday's law, the voltage induced by a magnetic field with a magnetic flux density in a closed loop of area A is:
The basic equations of inductive interference between the two circuits are shown in equations (5) and (6).
If the closed loop is stationary, the magnetic flux density changes sinusoidally with time, but is constant in the closed loop area. The above formula can be simplified as:
This relationship can also be expressed by the mutual inductance M between the two circuits:
In the formula: M is the mutual inductance between the two circuits; it is the current on the interference circuit.
It can be seen from the above relationship: the interference situation is proportional to the frequency, magnetic flux density, the area enclosed by the closed loop, and the mutual inductance between the two circuits. Therefore, the communication vehicle can completely improve the electromagnetic compatibility index of the vehicle by controlling the area enclosed by the closed loop and the mutual inductance between the two circuits without improving the electromagnetic environment.
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