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A War Office requirement arose during the war for a direction finder suitable for taking bearings on ionosphere-reflected waves which arrive at a small angle to the vertical and which are subject to fading. It was necessary that this equipment should be highly mobile, suitable for transport by air, and capable of withstanding extreme ranges of temperature and humidity. Ease of erection and simplicity of operation were also to be essential features. Notwithstanding all this, a high order of accuracy was to be retained. The paper describes a rotating spaced-loop d.f. station that was developed at the Signals Research and Development Establishment during 1944Â¿45, to comply with these requirements. In order to simplify the taking of bearings under fading conditions, a visual indicator was incorporated which was also suitable for speedily resolving the ambiguities associated with spaced-loop systems.