What is effective earth radius factor?

Answer 1

Light and radio signals seldom travel in straight lines through air, because the

pressure, temperature, and density of air change with altitude. Light and radio

signals almost always curve vertically, and usually downward.

If you were to draw a picture of the real earth's surface, with the real path of a

light beam or radio signal traveling above it, the surface of the earth would curve

downward on the drawing, and the light or radio signal would also curve above it.

The distance between the surface and the light or radio signal would change across

the drawing, depending on how strongly the signal curved, and in which direction

(up or down).

It would be possible to distort the drawing in such a way as to make the light

or radio signal appear to be a straight line. In order to do that, you'd have to

change the curvature of the earth's surface on the drawing, and maintain the

original spacing between the surface and the radio signal at every point.

The number by which you have to multiply the true earth radius in order to

make the light or radio signal's path appear straight on the drawing, is the

"effective earth radius factor". It's a characteristic of atmospheric conditions

(specifically, the vertical gradient of the atmosphere's index of refraction),

and it's used in designing the optimum physical configuration of point-to-point

radio links, i.e. how high above the ground to mount the antennas at each end

of the link.