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When an obstacle is encountered in the transmission, which will bypass the obstacles spread, a phenomenon called diffraction of waves. High frequency on VHF, microwave, wave length diffraction of weak signal strength behind the tall buildings to form a so-called "shadow areas". The degree of signal quality is affected, and not only the height of buildings, and the distance between the antenna and the buildings, and frequency. For example, a building whose height is 10 meters, at the back, a distance of 200 metres, the received signal quality is not affected, but in the 100 meters, received signal strength than when there was no building decreased significantly. Note, as above by said of as, weakened degree also and signal frequency about, for 216 ~ 223 MHz of RF signal, received signal field than no buildings Shi low 16 dB, for 670 MHz of RF signal, received signal field than no buildings Shi low 20dB. If buildings height increased to 50 meters Shi, is in from buildings 1000 meters within, received signal of field are will by effect and weakened. In other words, higher frequency, higher buildings, antenna and the buildings closer, affects the quality of the degree, the greater the signal strength and communication; instead, the frequency, the lower the farther away the higher the building, antenna and the buildings, is less affected.