What is the function of a sense antenna?

In radio direction finding (DF), a standard loop antenna produces a figure-eight radiation pattern. This pattern features two very sharp nulls located exactly 180 degrees apart, making it impossible to tell if a signal is coming from the front or the back. To resolve this 180-degree ambiguity, a vertical omnidirectional antenna—known as a sense antenna—is added to the system. When the signal from the sense antenna is electronically combined in phase with the signal from the loop antenna, the resulting radiation pattern changes dramatically. The combined pattern becomes a cardioid (heart shape). A cardioid pattern has only a single deep null, allowing the operator to pinpoint the exact, unambiguous direction of the transmitting station.