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Knowledge of Radio

2018-11-15

Which band of radio waves does radio broadcast or television use? How do they spread?

 

At present, AM radio broadcasting is divided into long wave, medium wave and short wave bands, which are transmitted by the corresponding radio waves respectively.

 

There are only two big-band radio broadcasts in China: medium-wave and short-wave. The frequency band of medium wave broadcasting is about 550 kHz-1600kHz, which is mainly by ground wave propagation, and also accompanied by partial sky wave, while the frequency band used in short wave broadcasting is about 2MHz-24MHz, mainly by sky wave propagation, accompanied by ground wave in short distance.

 

FM radio broadcasting uses ultrashort wave (VHF) radio waves to transmit signals, the frequency of which is about 88MHz-108MHz. it is mainly transmitted by space waves.

 

At present, terrestrial radio and television are divided into VHF (very high frequency or meter wave) and UHF (ultra high frequency or decimeter wave) two frequency bands. In our country, the frequency range of TV in VHF band is 48.5 MHz-3MHz. it is divided into 1-12 channels, and the frequency range of UHF band is 470MHz-956MHz. it is divided into: 3-68 channels. They are basically propagated by space waves. There are 6 frequency bands of satellite broadcast TV, the main frequency band is 12kMHz. it also depends on space wave propagation.

 

[AM/FM introduction] what is AM wave? What is FM wave?

 

The modulation mode in which the carrier amplitude is changed according to the modulated signal is called amplitude modulation. An AM wave is called an AM wave. It maintains the frequency characteristic of high frequency carrier, but the shape of envelope is similar to signal waveform. The amplitude of the amplitude modulation wave is determined by the intensity of the modulated signal. Amplitude modulated waves are represented by the English letter AM.

 

The modulation mode that makes the carrier frequency change according to the modulation signal is called frequency modulation. The amplitude of modulated frequency is determined by the size of modulated signal, and the period of variation is determined by the frequency of modulated signal. The amplitude of the modulated wave remains unchanged. The waveforms of FM waves, like a spring that is unevenly compressed, are represented by the letter FM.

 

What is the process of transmitting and receiving radio communications?

 

The transmission of a radio program takes place on a radio station. The sound wave of the broadcast program is converted into an audio frequency signal by an electroacoustic device, and amplified by an audio frequency amplifier, the oscillator generates a high frequency equal amplitude oscillation signal, and the modulator modulates the high frequency equal amplitude oscillation signal by the sound frequency signal. The modulated high-frequency oscillation signal is amplified and sent to the emission line and converted into radio wave radiation.

 

Radio broadcasting is received by radio. The receiving line of the radio receives the radio wave in the air; the tuning circuit selects the signal of the desired frequency; the detector restores the high frequency signal to the audio signal (that is, demodulation); after the demodulation, the audio signal is amplified to obtain sufficient driving power; Finally, the broadcast content is restored by electroacoustic conversion.

 

To sum up, the transmission and reception of radio communication (which also belongs to the category of radio communications) can be summarized as the conversion process of three opposite aspects, namely, transmitting information-low frequency signal, low frequency signal-high frequency signal, High frequency signal an electromagnetic wave.


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