Non reciprocal ionospheric propagation in the medium frequency broadcasting band

Measurements made in the U.S.A. suggest that east-west ionospheric propagation in the medium-frequency broadcasting band is non-reciprocal. A possible explanation is put forward in the paper, which draws attention to the fact that the combination of polarization-coupling loss and ground-reflection loss which occurs between hops on multi-hop paths depends, in general, on the direction of propagation.