VDF Detection RNAV
On older systems, a loop aerial like those found in the ADFADF —Automatic Direction Finder is used. This aerial setup has a point of no reception (a "null") which is physically rotated until no signal is received. On modern systems, the aerial is a series of dipoles arranged in a circle.
Because each of these dipoles will receive the signal at a very slightly different time, by comparing the phase difference it will know which direction the signal came from. This can be thought of as two people standing in the sea — if one is closer to the beach, they will receive each peak and trough a little later than someone standing further out to sea.