Earth bulge

Earth bulge is a term used in telecommunications. It refers to the circular segment of earth profile which blocks off long distance communications.

Distance to horizon

R is the radius of the Earth, h is the height of the transmitter (exaggerated), d is the line of sight distance

Assuming a perfect sphere with no terrain irregularity, the distance to horizon from a high altitude transmitter (i.e. line of sight) can readily be calculated.

Let R be the radius of Earth and h be the altitude of a telecommunication station. Line of sight distance d of this station is given by the Pythagorean theorem;

$d^2=(R+h)^{2}-R^2= 2\cdot R \cdot h +h^2$

Since the altitude of the station is much less than the radius of the Earth,

$d \approx \sqrt{ 2\cdot R \cdot h}$

The mean radius of the earth is about 6,378 kilometres (3,963 mi). (See Earth radius) Using the same units for both the altitude of the station and the radius of the earth,

$d \approx \sqrt{ 2\cdot 6378 \cdot h} \approx 112.9 \cdot \sqrt{h}$

If the height is given in m. and distance in km.

$d \approx 3.57 \cdot \sqrt{h}$

If the height is given in ft. and the distance in miles,

$d \approx 1.23 \cdot \sqrt{h}$

The actual service range

The above analysis doesn’t take the effect of atmosphere on the propagation path of the RF signals into consideration. In fact, the RF signals don’t propagate in straight lines. Because of the canalizing effects of atmospheric layers, the propagation paths are somewhat curved. Thus, the maximum service range of the station, is not equal to the line of sight distance. Usually a factor k is used in the equation above

$d \approx \sqrt{2 \cdot k \cdot R \cdot h}$

k > 1 means geometrically reduced bulge and a longer service range. On the other hand, k < 1 means a shorter service range.

Experience has shown that, under normal weather conditions k is 4/3. That means that, the maximum service range increases by % 15

$d \approx 4.12 \cdot \sqrt{h}$

for h in m. and d in km.

$d \approx 1.41 \cdot\sqrt{h}$

for h in ft. and d in miles ;

But in stormy weather, k may decrease to cause fading in transmission. (In extreme cases k can be less than 1.) That is equivalent to a hypothetical decrease in Earth radius and an increase of Earth bulge.

Example

In normal weather conditions, the service range of a transmitting station at an altitude of 1500 m. with respect to receivers at sea level can be found as,

$d \approx 4.12 \cdot \sqrt{1500} = 160 \mbox { km.}$

See also

• Radial (radio)
• Transmitter
• Field strength in free space
• Line-of-sight propagation

External links

• http://www.wireless-center.net/Cisco-Wireless-Networking/728.html
• http://web.telia.com/~u85920178/data/pathlos.htm#bulges
• http://www.scribd.com/doc/52528816/64/The-Earth-bulge