The paper deals with the design of direction-finding aerial systems to cover the whole of the h.f. band, to work on the ground wave and to meet the conditions peculiar to H.M. ships. The special problems encountered are intimately connected with the siting of the d.f. aerial. The aerial must be fitted to the top of a mast, and often at a considerable distance from the receiving compartment.The most satisfactory design of d.f. aerial to meet these conditions consists of two fixed crossed loops with an incorporated sense aerial, and a network for injecting test signals into the d.f. and sense-aerial circuits. The aerial system must be connected to the receiving apparatus by means of transmission lines which may be of considerable length.The paper describes the special problems experienced in achieving satisfactory performance in sensitivity, bearing accuracy and resolution of sense ambiguity, the compromises in design that have to be adopted and the performance figures obtained.