## How propagation changes with operating frequency

#### The way in which signals propagate though the air varies with the frequency or wavelength. Some microwave frequencies for example are heavily attenuated by water droplets or oxygen molecules in the atmosphere when the size of these particles becomes comparable with the signal wavelength, (60 GHz for water molecule absorbtion). |
| #### In contrast, at very low operating frequencies (<30 MHz) where the wavelength is very large, radio signals can propagate extremely long distances (in fact around the earth) owing to the various layers within the ionosphere acting as giant wave guides reflecting the signal to and fro between the layers. Unfortunately the properties of the ionospheric layers change with time of day, season, temperature, and so on, and the propagation characteristics can be very unpredictable and data transmission rates are often limited to a few kbps. Above 30 MHz, ionospheric reflection begins to pack up and propagation is principally by line-of-sight path. In order to communicate over long distances, high antenna towers are required to combat the curvature of the earth.