Transhorizon propagation on v.h.f. and u.h.f. radio links in the United Kingdom

An analysis has been made of v.h.f. and u.h.f. transmission data on 65 overland links of up to 400 km in length in the United Kingdom. The analysis reveals that the field strength exceeded for 1% of the time is significantly greater than that predicted by CCIR recommendation 370-1, especially for band III (150¿250 MHz), but also to some extent for band I (41.68 MHz). A detailed analysis is made of several transmission links in which the factors distance, antenna height, terrain irregularities and meteorological condition were very similar, but the operating frequencies were different. The results of the detailed analysis are interpreted in terms of reflection theory using a 4-ray model. The layer heights at which partial reflection gives enhancements of band-III transmissions similar to those observed experimentally are found to correspond approximately to atmospheric layers of maximum lapse rate of refractive index, as computed from meteorological-radiosonde observations.