By Mark Connelly, WA1ION
The K9AY loop antenna can provide a cardioid (heart-shaped, single-null) pattern when one end of the antenna is connected to ground through a variable DC resistance. This occurs because of the interaction between electric and magnetic field pick-up patterns.
The drawing provided above are of a K9AY loop tested at sites in Billerica, MA and East Harwich, MA during 1999.
[To get a larger version, click on the drawing above.]