Description

Frequency spectrum is a limited resource. Due to the large number of available appications, it needs to be shared and thus, only a fraction is available for each type of use (e.g. air-ground communications, mobile communications, radio and tv broadcasting, etc.).
On an global level, the ATC allocated frequencies in the VHF band range from 117.975 MHz to 137.000 MHz. In order to avoid interference (i.e. distortion in reception caused by two very closely spaced emissions), the frequencies are spaced in such a way that the closest two of them differ by at least a specified number of Hz (or kHz). The smaller the spacing, the more channels are available, but this requires more sophisticated equipment in order to avoid interference.