Antenna height and radio horizon

Both ends contribute to the horizon

For a smooth spherical Earth, an antenna’s tangent distance is √(2Rh+h²). With two antennas, the two distances are added. When heights are small compared with Earth’s radius, distance grows approximately with the square root of height. Four times the height therefore roughly doubles that antenna’s contribution.

Height is above the reference surface assumed by the model, not automatically your site’s elevation above sea level. A mountain summit and an equally high mast in flat terrain have different terrain profiles. The simple calculator contains no terrain data and cannot resolve that difference.

Why use an effective Earth radius

Atmospheric refraction can curve radio paths. A simplified model represents this with an effective Earth radius kR. k = 1 describes pure geometry; 4/3 is a common standard assumption for a particular average refraction. It is not a current weather observation and may be inappropriate in other atmospheric conditions.

At antenna heights of 10 m and 2 m with k = 4/3, the combined horizon is about 18.9 km. This is a geometric model value. Obstacles can prevent a link at a shorter distance, while other propagation mechanisms can support communication beyond it.

Go beyond horizon for a real path

Next consider terrain profile, Fresnel clearance and link budget. Even a visually clear line may leave too little room for the first Fresnel zone. Transmit power and antenna gainGain×Gain describes the ratio of output to input level. In a receiver it can occur before or after digitisation and is often expressed in dB.Example & full explanation → do not enter the horizon calculation: they do not change geometric horizon, though they can affect whether a particular link is usable.