Q: What earth curvature value should I use?

A: The effective earth curvature value (used for drawing profile or computing obstruction losses) is the expression of atmospheric refraction of the radio waves

If you wanted to see the actual physical curvature of the earth itself, the value of 1.000 shows this effect:

The profile plot for an effective earth curvature of 1.000 (in other words, the actual radius of the earth) shows the line of sight grazing the peak at about 26 miles.

However, in most radio applications, atmospheric refraction is considered. Typically, the value of 4/3, or 1.333 is represntative of the effect of the atmoshpere. As shown below, the effect of the atmospheric "bending" results in the line of sight effectively clearing of the peak at 26 miles. (Note that the Fresnel zone is still obstructed so this would not be considered a clear path.)

The refractivity of any medium (like the atmoshphere) depends on its density, so changing atmospheric conditions can change the effect on radio waves. For this reason, microwave path planners often consider other effective earth curvature values.

For example, for an earth curvature of 2/3 earth (0.667), the line of sight is blocked:

In this example, changing atmospheric conditions could cause significant variations in the received signal quality. These effects are usually most noticed at microwave frequencies. Therefore, it is often advisable to check paths at both 4/3 and 2/3 earth radius values for path clearance.