The signal is transmitted in the feeder line, and in addition to the resistive loss of the conductor, there is also the dielectric loss of the insulation material. These two types of losses increase with the increase of feeder length and operating frequency. Therefore, a reasonable layout should be made to shorten the length of the feeder as much as possible. The magnitude of the loss generated per unit length is represented by the attenuation coefficient β, which is measured in dB/m (decibels/meter). The unit used in cable technical specifications is mostly dB/100 m (decibels/hundred meters).

　　Let the power input to the feeder be P1, and the power output from the feeder with a length of L (m) be P2,

　　The transmission loss TL can be expressed as: TL=10 × Lg (P1/P2) (dB)

　　The attenuation coefficient is β=TL/L (dB/m)

　　For example, the NOKIA 7/8-inch low consumption cable has a attenuation coefficient of β=4.1dB/100m at 900MHz, which can also be written as β=3dB/73m. That is to say, the signal power at 900MHz is reduced by half for every 73m long cable. For ordinary non low consumption cables, such as SYV-9-50-1900MHz, the attenuation coefficient is β=20.1dB/100m, which can also be written as β=3dB/15m. That is to say, for a signal power of 900MHz, the power is reduced by half every 15m of this type of cable!

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