The decision you should be able to make
Decide whether height, route or frequency must change before detailed loss modelling.
Core engineering
The first Fresnel radius depends on wavelength and the obstruction's distances from both endpoints. It is zero at the antennas and largest near the path midpoint.
ITU-R P.526 identifies 0.6 of the first Fresnel radius in its diffraction-zone and isolated-obstacle criteria. Treat 60% clearance as a useful screening threshold, not a promise of zero loss or target availability.
Clearance must include Earth curvature/refraction and the physical obstruction, including credible vegetation growth and mounting uncertainty.
First Fresnel radius r1 = sqrt(lambda*d1*d2/(d1+d2)), with lambda, d1 and d2 in consistent units.
For a 10 km, 5 GHz path at the midpoint, lambda is about 0.060 m and r1 is about 12.2 m. Sixty per cent is about 7.3 m, before adding terrain, refraction and survey uncertainty.
Equipment and field checks
- Use the antenna phase centre and operating centre frequency in the profile.
- Check clearance at every material obstruction, not just a midpoint marker.
- Save the profile and assumptions with the project record.
Do not progress until
The documented clearance criterion is met along the full path under the selected refraction case, or diffraction is modelled explicitly.
Limits and cautions
- Vegetation and complex rooftops are not reliably represented as a single clean edge.
Primary sources
Sources were checked for this technical review. Recheck regulator and equipment requirements at the point of design or installation.