Wave polarisation and its influence on the power available from a radio signal propagated through the ionosphere. Part 1

Expressions are derived for the relative amounts of power in the ordinary and extraordinary waves excited by an h.f. radio wave at its point of entry into the ionosphere and for the power in the downcoming components of signal, polarised in the horizontal and normal directions, at a distant receiving point. Sample calculations are presented for single-hop propagation over a path between Lerwick (Shetland Isles) and Slough (England), which lies nearly in the plane of the magnetic meridian. For angles of elevation giving quasitransverse propagation (over the Slough leg of the path), the fractions of power in the horizontal and normal components of the ordinary and extraordinary waves are unequal. Under such circumstances, the choice of aerial polarisation at both the transmitting and receiving terminals becomes important, particularly during the daytime, when the extraordinary wave is heavily absorbed. For minimum path attenuation, in general, care should be taken to use a transmitting-aerial polarisation which leads to the excitation of a strong ordinary wave, and a receiving-aerial polarisation responding to the larger resolved linear component of the downcoming ordinary wave.