This paper discusses the principles of design of spaced-loop radio direction-finders and describes a practical instrument embodying these principles. The dimensions of the model described are: size of loops, 1 m square; spacing between loops, 3 m. It has been designed for the frequency range within which the spaced-loop direction-finder can have most useful applications, namely 3–15 Mc/s (λ 20–100m). The sensitivity of the instrument is such that for an arc of silence not exceeding ±5 deg, the required field strength, for a ground wave, varies from 1.5 to 4 μV/m throughout the range. The direction-finder is provided with a simple and effective means of determining the sense of the bearing, operating on novel principles.To obtain the high accuracy of which the spaced-loop radio direction-finder is capable, care must be taken that the local surroundings of the aerials do not contain features capable of introducing errors of various types. In particular, attention must be paid to the proper disposition of power or telephone cables.In the Appendices, the effects of the essentially non-uniform character of the current distribution in loop aerials are considered, and it is shown that certain arrangements of spaced-loop aerials may be very inaccurate on account of a type of polarization error thus introduced.