1. Line of sight propagation of ultra short waves and microwaves

　　Ultra short waves and microwaves have high frequencies and short wavelengths, and their ground waves decay quickly. Therefore, it is not possible to rely on ground waves for long-distance propagation, as they are mainly propagated by space waves. Space waves can generally only propagate in a straight direction to directly visible areas. The propagation area of ultra short waves within a direct viewing distance is commonly referred to as the "illumination area". The ultra short wave receiving device can stably receive signals within a direct viewing distance.

　　The direct viewing distance is related to the height of the transmitting and receiving antennas, and is influenced by the curvature radius of the Earth. From a simple geometric relationship, it can be inferred that

　　AB=3.57 (√ HT+√ HR) (kilometers)

　　Due to the refractive effect of the atmosphere on ultra short waves, the effective propagation distance is

　　AB times=4.12 (√ HT+√ HR) (kilometers)

　　2. Multipath propagation of radio waves

　　In addition to direct propagation, radio waves can also generate reflections when encountering obstacles such as hills, forests, ground or tall buildings. Therefore, the ultra short waves that reach the receiving antenna not only have direct waves but also reflected waves, and this phenomenon is called multipath transmission.

　　Due to multi-channel propagation, the distribution of signal field strength is quite complex and fluctuates greatly; Due to the influence of multipath transmission, the polarization direction of radio waves can change, resulting in an increase in signal field strength in some areas and a decrease in signal field strength in others. In addition, different obstacles have different ability to reflect radio waves. For example, reinforced concrete buildings have a stronger ability to reflect ultra short waves than brick walls. We should try our best to avoid the impact of multipath transmission effects. At the same time, measures such as spatial diversity or polarization diversity can be taken to correspond.

　　3. Diffraction propagation of radio waves

　　When radio waves encounter obstacles in their propagation path, they always try to bypass them and propagate forward. This phenomenon is called diffraction of radio waves. The diffraction ability of ultra short waves is weak, forming a so-called "shadow area" behind tall buildings. The degree to which signal quality is affected is not only related to the distance and height of the receiving antenna from the building, but also to the frequency. For example, if a building has a height of 10 meters, the signal quality received at a distance of 200 meters from the building is almost unaffected, but at a distance of 100 meters from the building, the received signal field strength will be significantly weakened compared to when there is no height. At this point, if the received TV signal is between 216 and 223 megahertz, the received signal field strength will be 16 decibels weaker than when there is no high signal, and when the received TV signal is 670 megahertz, the received signal field strength will be 20 decibels weaker than when there is no high signal. If the height of a building increases to 50 meters, the received signal field strength will be affected within 1000 meters of the building, resulting in varying degrees of attenuation. That is to say, the higher the frequency, the higher and closer the building is, and the greater the impact. On the contrary, the lower the frequency, the shorter and farther the building, and the smaller the impact.

　　Therefore, when selecting a base station site for antenna installation, it is necessary to consider the various adverse factors that may arise from diffraction propagation according to the above principles and make efforts to avoid them.

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