FM stereo radio is developed on the basis of mono FM broadcasting. With the rapid development of science and technology and the improvement of people's living standards, people also put forward higher requirements for the quality of broadcast sound. FM stereo radio has high fidelity, high signal-to-noise ratio, large dynamic range and wide audio response. With distinct layers and strong sense of existence, it has won people's favor. China began to pilot FM stereo radio in the late 1970s, and it quickly became popular. Because FM stereo radio has low transmitting power, high efficiency and strong anti-interference ability. So it developed rapidly as soon as it came out. Developed countries have taken FM stereo radio as the main means to listen to and cover domestic broadcasts. After more than 20 years of development, FM stereo radio has reached its peak. The development of things often goes downhill, reaching the peak and then falling into the trough. This is not the case with FM stereo radio, which has not shown a downward trend yet. Although the current broadcasting forms are diversified, FM broadcasting is challenged by digital broadcasting, network and other multimedia broadcasting. However, from the perspective of the effectiveness of communication, both digital broadcasting and network multimedia broadcasting are limited by factors such as location, transmission distance and equipment cost. At present, it does not pose a great threat to FM stereo radio.
The receiving tool market is the truly competitive broadcasting market. Judging from the development of digital multimedia broadcasting in a prefecture-level city in Guangdong in recent ten years, the key factor restricting the development of digital broadcasting is the cost of receivers. Although the price of the receiver is falling.
But it still needs about one yuan at present. Therefore. The popularization of broadcast receiving tools is the key. The success of FM stereo radio today is determined by many favorable factors: first, it is not limited by geographical space; Secondly, FM stereo radio technology is mature and stable; Thirdly, receiving tools are popular and have a wide audience; Fourthly, the cost of transmitter and receiver is much lower than other digital and multimedia broadcasts. This is also the enduring charm of FM stereo radio.
The competition in the broadcasting industry is extremely fierce, and now there are multiple radio frequencies in every place to grab viewers. Therefore, many radio stations have increased their transmitting power and improved their transmitting antennas to improve their coverage. How to measure the coverage of a radio station? Theoretically speaking, the frequency band used in FM broadcasting is 87.5MHz~ 108MHz, which belongs to ultrashort wave band. The wavelength is 3.5m ~ 2.8m (also called meter band). The ability of ultrashort wave to spread around the ground is very weak. Theoretically, it can only travel in a straight line in space, also known as line-of-sight propagation. According to the concept of equivalent radius of the earth, the sight distance between any transmitting point and receiving point is:
Among them, Ro- line of sight (unit km), AE- equivalent radius of the earth' h- height of transmitting antenna (m) and Z- height of receiving antenna (unit m)l, because the propagation of ultrashort wave is greatly influenced by topography, features and climate, radio waves will be reflected when they meet the ground and obstacles, and refracted when they pass through the water. Many objects have a strong absorption effect on ultrashort waves, such as mountains and depressions. The low-level listening effect in dense building area is not good.
So, what is the reason for being able to monitor from a distance? This is due to the refraction, reflection and scattering effects of the atmosphere and ionosphere on radio waves. Under certain conditions, ultrashort waves can be scattered along the earth's surface and the high-density troposphere and ionosphere. When radio waves hit the ionosphere in the sky, they will be reflected back to the earth. Some people call this phenomenon "skip pass". It can transmit signals thousands of kilometers away. After several reflections and jumps, the signal reached the other side of the earth. The greater the power of the transmitter, the higher the radiating antenna and the farther the signal transmission distance. Its propagation process is shown in the attached figure.
However, this mode of transmission is very random and can only be received in specific weather and time. Generally speaking, the listening effect is better on cloudy days than on sunny days, sunny days than on rainy days, better in the morning and evening than at noon, and better in summer than in winter, so the received signal fluctuates greatly and is not stable enough. At the same time, the same frequency interference or harmonic interference of adjacent stations will make the receiving effect worse. Therefore, increasing the height and gain of the transmitting antenna to improve the coverage is usually more effective than increasing the transmitting power.
Measuring the coverage of a radio station should not be based on subjective listening, but on measuring the field strength at the boundary of the service area. When measuring the field strength, you should choose different time in the same place.
The service area that meets the national requirements is measured many times and averaged.