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In ionospheric scatter communication systems the angle of elevation at which signals should be transmitted and received depends on the circuit length, the height of the ionospheric scattering layer and the refraction which occurs in the troposphere. The effect of variations of these parameters is discussed, and it is shown that probable variations of ionospheric height and refraction are unimportant for circuits up to about 1100 miles in length. The concept of optimum height of aerial above a smooth reflecting foreground is introduced and is shown to be applicable to the partially incoherent wave which results from the scattering process. The additional question of adequate overlap of those regions of the ionosphere visible from the transmitting and receiving aerials is considered, leading to recommendations for circuit design. It is shown that the design of very long circuits presents difficult problems, and that to secure consistent performance on such circuits it might be advantageous to use two aerial systems at different heights.