Explanation: A properly adjusted unilateral radio direction finder combines the bidirectional (figure-eight) pattern of a loop antenna with the omnidirectional pattern of a sense antenna. By precisely phasing and summing these two signals, the 180-degree ambiguity inherent in a simple loop antenna is eliminated. The resulting reception pattern is a **cardioid**, which is geometrically a heart-shaped figure. This pattern has a single, sharp null in one direction, allowing the operator to unambiguously determine the bearing to a transmitting station without needing to resolve a reciprocal direction. Therefore, both "a cardioid" and "a heart-shaped figure" correctly describe this pattern. A figure-eight pattern, while characteristic of a simple loop, is bidirectional and lacks the single-direction null required for unilateral reception.