Radio horizon

The radio horizon refers to the places where from a transmitter outbound direct ( ie non-diffracted or reflected ) radio waves run exactly tangential to the earth's surface. He usually runs at a greater distance than the optical horizon. The decreasing with increasing altitude refractive index of the atmosphere causes a refraction of electromagnetic waves back to Earth. This gradient of the refractive index is for radio waves (up to about 100 GHz ) is larger than for the light. Therefore, radio waves have a beyond the optical horizon range. This range is up to the radio horizon approximately

With the Earth's radius and height as the transmitting antenna above ground level.

In a rough approximation can be expected in practice by an enlargement of the radio horizon by about 15 % compared to the optical horizon. Alternatively, this can be interpreted as the apparent increase in the radius of the Earth. Default is then assumed to be greater by a factor of 4/3 earth radius.

Substituting in this formula the mean earth radius of 6370 km multiplied by a factor of 4/3 for a normal atmosphere ( about 8,500 miles), so the distance of the radio horizon in miles calculated by

With the height of the transmitting antenna in meters above ground level.

Has the recipient a relevant height above the ground, then add the two radio horizons. The following formula can then the quasi-optical range can be calculated:

With a radius of the earth, as the height of the transmit antenna and the height of the receive antenna, both on the ground floor.

If, in turn, the increased to 4/3 earth radius of about 8500 km mix, then the quasi-optical range results in km by

With the height of the transmitting antenna in meters and as the height of the receiving antenna in meters.

These formulas take strong purpose of simplicity, that the earth is a sphere. Height differences in the propagation path such as mountains and valleys are not considered. These formulas agree on level ground or on the sea quite good match reality and often serve as a first estimate in the calculation of the propagation path of radio waves.

Formulas for calculating the radio horizon from digitized terrain data can be found for example in corresponding sources of the International Telecommunication Union ( ITU).

Tropospheric propagation effects such as diffraction, scattering and reflection are not considered in the calculation of the radio horizon. They cause a propagation of electromagnetic waves over the radio horizon ( OTH propagation, English: Trans -horizon propagation ).

Radar horizon

Using the same formulas and the radar horizon is calculated. In contrast to an over the horizon radar, the operation of a radar unit is based on a quasi- optical propagation of the radio waves. This is only given when the radio horizon of the radar and the radar horizon of the target overlap or touch just yet.