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The required polar diagram depends on four factors, namely the size and shape of the zone in the ionosphere which can be seen from both terminals, the effect of the angle through which the radiation is scattered, the allowable multi-path delay and the interference. Diagrams showing the mutually visible zone are presented for path lengths between 1200 and 1800km. The available information on the scattering properties of the ionosphere suggests that there is not much advantage in reducing the beam width in the horizontal plane below about 30Â° (between 3 dB points). Diagrams showing the delay in transmission for paths off the great circle are presented. The required polar diagram in the horizontal plane is discussed in detail for a path length of 1200km. For the polar diagram in the vertical plane it is important to ensure that the lowest lobes from each terminal overlap well, and that as much power as possible goes into the lowest lobe; rules for achieving this are given. Protection against interference to or from distant stations is discussed. Requirements for the reflecting surface (land or sea) are investigated, and it is shown that horizontal polarization is preferable; the size and allowable roughness of the surface are specified.