What feature of a cardioid pattern antenna makes it useful for direction-finding antennas?
Why A cardioid (heart-shaped) pattern has broad response over most directions but one deep, narrow null. Nulls are far sharper than peaks, so you can pinpoint a bearing much more precisely by rotating for minimum signal than for maximum. The cardioid is usually made by combining a small loop with a vertical sense antenna, and because there is only one null, it resolves the 180-degree ambiguity that a plain loop's two opposite nulls leave you with.
Watch out The choice about a very sharp peak describes what a high-gain beam like a long Yagi gives you, and peaks are inherently broader and harder to read than nulls; a cardioid's maximum is quite broad.
Loop alone = two nulls (ambiguous). Loop + sense = cardioid = ONE null = one bearing.