Our car radio has two standards, one is FM radio receiver, the working band is 87.5MHz~108MHz, the working wavelength is about 3 meters, and the other is AM radio receiver, the working band is 531KHz~1629KHz, the working wavelength is about 300 meters.
Radio is a one-way passive receiving device, which only receives signals but does not transmit them. FM transmission towers are generally located on the radio and television transmission towers in a city, or on the top of the highest mountain. For example, the transmission towers in Shenzhen are distributed on Wutong Mountain in the east and Fenghuang Mountain in the west. FM radio is a space wave line-of-sight transmission method, with a transmission power of thousands of watts, which can cover an area of nearly 100 kilometers around the transmission tower. Most of them can cover the urban and suburban areas of a city, and the audience is very wide.
AM transmission towers are usually located in open areas of cities. AM broadcast signals are transmitted through sky waves reflected by the ionosphere of the atmosphere. The transmission power is usually tens of kilowatts or even hundreds of kilowatts. At night, the ionosphere rises in height and AM broadcasts can even be transmitted thousands of kilometers. The author has verified that AM radio stations from Wuhan can be received in Shenzhen, and even AM radio signals from the Philippines can be heard.
