What feature of a cardioid pattern antenna makes it useful for direction-finding antennas?

A cardioid antenna pattern is shaped somewhat like a heart, characterized by a single, deep null (an area of minimum signal reception) directly at the rear of the pattern. In radio direction finding, nulls are typically much sharper and more precisely defined than peaks in a radiation pattern. By rotating the antenna to find the exact point where the received signal drops completely out, a direction finder can pinpoint the bearing of the transmitter with high accuracy. Trying to locate the center of a broad peak is far less precise.