Ionospheric propagation at medium frequencies depends almost entirely on the ordinary wave, which is, in general, elliptically polarised. This paper describes coupling losses which occur when this wave is transmitted and received with vertical aerials, it also discusses additional losses caused by changes of polarisation when intermediate earth reflections occur. Formulas are given, permitting the various losses to be estimated; these are necessarily approximate, because they are based on the simple ray-theory approach. By way of example, curves are given showing the calculated losses for one typical frequency and angle of incidence.