1 Sources of electromagnetic interference in automobiles Electromagnetic interference arises from interference sources. It is an electromagnetic phenomenon that comes from the outside and detracts from useful signals. There are two types of interference from automobiles to on-board electrical equipment. The first type is radiated interference. Electromagnetic waves penetrate directly into electronic equipment through free space and excite the circuit inside the equipment to generate corresponding interference energy on the circuit, causing logical errors with the circuit. Strong enough electromagnetic interference can even directly damage Sensitive electronic devices; the second type is conducted interference. The interference source couples the interference signal to other equipment through cables such as power lines and signal lines, which causes harm to the normal operation of other equipment. For the independently powered 2.4 GHz communication equipment, it is mainly affected by the radiated electromagnetic interference of the car, so this article mainly analyzes and measures the radiated electromagnetic source of the car. According to the theory of electromagnetic wave generation and propagation, as long as electromagnetic oscillation is caused on a linear circuit, alternating equal charges of different signs will appear at both ends of the linear circuit, and such a circuit will emit electromagnetic waves into space. The energy radiated by the electromagnetic wave in a unit time is proportional to the fourth power of the frequency, that is, the higher the oscillation frequency of the circuit, the easier it is to radiate electromagnetic waves outward. There are many circuits on the car that meet this condition, so the car can emit electromagnetic interference of various frequencies. Each time the traffic density doubles, the interference noise power spectral density increases by 3 to 6 dB (A). The strongest source of electromagnetic interference in the automotive electrical system is the ignition system. When the automobile engine is running normally, the transient voltage at the secondary of the ignition coil is very high, and can rise to 35 kV in 50 μs. When the spark plug electrode discharges, it will form strong electromagnetic radiation and propagate to the surrounding free space. This radiated electromagnetic noise contains very high frequency components and is the main source of interference for TV broadcasting. There are many inductive loads on cars, such as various electric motors and solenoid valves. At the instant when the coil of the solenoid valve is opened, a reverse voltage of several dozen times its working voltage will be generated. This reverse voltage continues to resonate in an LC series oscillating circuit formed by inductance and distributed capacitance, thereby generating electromagnetic radiation with very rich harmonics. This is also a very important source of electromagnetic interference. There are also many contact switches on the car. Due to the contact resistance of the contacts, the switch often generates electric sparks when opening and closing. If the current in the circuit is relatively large, the electromagnetic radiation caused by this spark can also interfere with other electrical equipment. When the DC motor is working, the carbon brush and the commutator will also generate strong sparks, causing radiated electromagnetic interference in a wide frequency range. DC motors are commonly used for wiper motors of automobiles, and the interference generated by the outside is also strong. 2 Measurement and analysis of radiated electromagnetic interference of automobiles 2.1 Measurement method In the 2.4 GHz frequency band, the electromagnetic wave power of the environment where the car is located and the electromagnetic wave power when the car is working in the same environment are measured separately. By comparing these two values, the relative intensity of the electromagnetic interference generated by the car in the 2.4 GHz frequency band can be obtained. 2.2 Measurement process The measurement process is as follows: (1) Install a spectrum analyzer. The spectrum analyzer has a recording software and driver running on the Windows operating system. Start the laptop first, connect the spectrum analyzer FR24-SAU to the laptop with a USB cable, install the spectrum analyzer driver after the operating system prompts to find new hardware, and finally install the spectrum analyzer's recording software FRMT . (2) Measuring environmental noise. Put the antenna of the spectrum analyzer in the co-pilot position, start the laptop and run the recording software of the spectrum analyzer, set the parameters of the spectrum analyzer on the recording software, turn on the peak hold function of the spectrum analyzer, and turn off the car The engine and all on-board electrical equipment are continuously measured for 3 minutes, and the measurement result is recorded as "environmental noise". (3) Measure car noise. Similar to the procedure for measuring environmental noise, turn on the peak hold function of the spectrum analyzer. During the measurement period, keep the car engine running at all times. During each 30 s, the throttle and the turn signal are switched on and off, and the measurement is continued for 3 min. The result is recorded as "car noise". In order to obtain a more stable measurement environment and reduce the influence of other interference sources on the measurement results, the measurement time is selected at 11:00 pm, and the measurement location is selected in an open field more than 200 m away from the residential house. During the measurement, the WiFi wireless network function of the laptop must be turned off to prevent it from affecting the measurement results. 　
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