The regular decrease of refractive index with height causes ray-bending and hence a defocusing effect at low elevation angles [ITU-R Handbook on Radiometeorology]. The magnitude of the defocusing of the antenna beam has been evaluated by Yokoi et al. [1970] and Crane [1971]. The loss is independent of frequency and is less than 0.4 dB at 3° elevation, even for very high values of refractivity at ground level. Expressions are provided in § 2.3.2 of Recommendation ITU-R P.618 to estimate the defocusing (beam-spreading) losses on paths at very low elevation angles. The loss is implicitly accounted for in the prediction methods for low-angle fading found in §§ 2.4.2 and 2.4.3 of the Recommendation.