Abstract :
During prolonged anticyclonic weather conditions, it is well known that abnormally high values of field strength sometimes occur during the night on overland transhorizon radio paths, usually returning to normal values after sunrise. The paper discusses simultaneous recordings of field strength and refractive index, made on two occasions when this occurred. On the first occasion, the return to normal values of field strength occurred within 2h, but, on the second, the change was much slower, taking some 6h. Use of a ray-tracing technique shows that the very high field strength on the first occasion was due to the return of the rays to ground by superrefraction in a thick elevated layer. On the second occasion, the layer was insufficiently thick to support refraction, and a partial reflection occurred. The magnitude of this reflection is computed from linear and cosinusoidal layer models, and the former leads to values of field strength agreeing, to within 5dB, with those measured.
