The decision you should be able to make
Recognise when a nominally clear path loses clearance under adverse refractivity conditions.
Core engineering
The effective-Earth-radius method represents ray bending by using a radius k times the physical Earth radius. k = 4/3 is a common reference condition, not a guaranteed worst case.
A smaller k increases the apparent Earth bulge and can reduce clearance. Availability design should use the refractivity statistics and clearance procedures appropriate to the path and climate.
The approximation is useful for terrestrial line-of-sight planning, but ducts, multipath and anomalous propagation are not reduced to one constant k for all time.
Approximate bulge above the endpoint chord at distances d1 and d2 is h = d1*d2/(2*k*Re), using consistent units and Re about 6.371 million metres.
At the midpoint of a 40 km path, the approximation gives about 23.5 m for k = 4/3 and about 31.4 m for k = 1. The 7.9 m difference can decide whether a ridge is clear.
Equipment and field checks
- Record the k-factor or refractivity method used by the planning software.
- Do not treat an undocumented default profile as evidence for a high-availability link.
- Where the margin is sensitive to k, compare alternative heights or routes before installation.
Do not progress until
The path remains acceptable under the documented clearance criterion and refractivity assumptions, or the residual risk is explicitly accepted.
Limits and cautions
- This simple bulge equation is a geometry aid. Use the current ITU-R P.530 method for professional availability design.
Primary sources
Sources were checked for this technical review. Recheck regulator and equipment requirements at the point of design or installation.