Comparison of measured and predicted field strengths of medium frequency sky waves over short distances in Japan

Nighttime sky waves of four medium frequencies were observed at distances of not larger than about 300 km in Japan for one to two years of high and low solar activities. The sporadic E (Es) layer always appeared with high probability. Analyzing these data, the following results were obtained: 1) the measured median field strengths are different by at least more than 7 dB from full wave predicted values for the international reference ionosphere (IRI) without the Es-layer. But these agree fairly well with the full wave calculation for IRI with the Es-layer, 2) the field strength approximates the Rayleigh distribution, 3) according to the data for high solar activity, the sky wave appears rapidly after sunset, and during the two to six hours after sunset all median field strengths are about constant.