Predicting medium-frequency skywave field strength in North America

Medium-frequency propagation is a very complicated matter, and therefore, it has been very difficult to predict the skywave field strength accurately. The existing prediction method in the FCC rules offers reasonable accuracy only when the sunspot number is low, hence, it essentially is a worst-case estimate. A more recent method proposed by the International Radio Consultative Committee (CCIR) works well for other parts of the world but displayed certain limitations when applied to propagation paths in North America. A new method developed by this author mathematically takes care of the uniqueness of North American paths and, consequently, better results have been obtained. This correspondence presents the new prediction method together with a quantitative comparison of the three methods now available.