Abstract :
The paper contains an analysis of the behaviour of frame aerials consisting of a single turn of conductor. It is assumed that the behaviour of such systems, including the mutual interactions of the various elements, can, to a useful degree of approximation, be represented by the differential equations of classical transmission-line theory. Formulae are obtained for the effective induced e. m. f. and the effective impedance (at the tuning point) of frame aerials the dimensions of which are not small compared with the wavelength, both for symmetrical and asymmetrical systems of tuning. It is found that in the case of symmetrically tuned systems the output voltages across the two equal halves of the tuning impedance will not in general be quite equal. It is shown that the ¿resonance factor¿ of a frame aerial (i.e. the ratio of output voltage to induced e. m. f.) can be determined by the usual method of reactance-variation at a constant frequency, in spite of the non-uniformity of the current distribution along the length of the conductor, but that the same process carried out by variation of frequency will not, in general, be valid. It is shown that for a given total length of conductor the optimum shape of a rectangular frame aerial, with respect to induced e. m. f., is square. In particular, a square frame with side equal to half a wavelength appears, to have useful practical characteristics in respect of sensitivity and symmetry, both for field-strength measurement and direction-finding. The method of applying the formulae to circular loops by a process of integration is given and illustrated by particular cases. It is found that in the case of small closed aerials the magnitude of the induced e. m. f. is not very sensitive to shape.
