What type of radiation pattern is created by a single-turn, terminated loop such as a pennant antenna?
Why A small loop by itself has a figure-8 (bidirectional) pattern, but adding a resistive termination to one side of the loop introduces a traveling-wave component that combines with the loop response. The two components add in one direction and cancel in the other, producing a cardioid: one broad lobe forward and a deep null off the back. Pennants, flags and EWEs are all low-noise receiving antennas that use this trick to null out interference or noise from one direction.
Watch out Bidirectional describes the untuned, unterminated small loop with its two opposite nulls; the whole point of the termination resistor is to break that symmetry and leave a single null.
Terminate the loop and you kill one lobe: figure-8 becomes cardioid (heart-shaped, one null).