Abstract :
After introducing the terms ?directive efficiency? and ?sharpness of directivity,? the author considers the general conditions for obtaining good sharpness ofdirectivity in a beamsystem. Supposing that the field strength is required to vary rapidly for angular displacements in a certain plane in the neighbourhood of a certain direction, it is found that the antenna system should be spread in the plane under consideration and in a line perpendicular to the direction of maximum radiation. When a plane reflector is used the system should lie in a line which is not quite perpendicular to the direction required, in order to obtain optimum results. If the antenna system is to hold within a fixed volume, one side of the volume should be as large as possible. At the same time a certain relation should hold between the lengths of the sides and the direction in which radiation is required. It will be evident that, since in a parabola the antennae do not lie in a straight line, the parabolic system is not, for given overall dimensions, the best arrangement for obtaining good sharpness of directivity. A good reflector should be at a definite distance from the transmitter in order that the phase of the radiation fields from the reflector and transmitter respectively may be zero in one direction and 180? in the other. Even when this obtains, the reflector will act in this way for only one direction. For instance, a reflector adjusted to neutralize the radiation in a certain direction in the horizontal plane will not do so in any other direction or in any other plane. A method using an inclined antenna system and reflector for the purpose of obtaining maximum sharpness of directivity at any angle to the vertical is described. Finally, the question of excitation is very briefly considered, and it is suggested that antennae not directly connected to the source may be used. A possible improvement on the Franklin antenna is also described.
