there is disclosed herein an antenna for generating a rotating pattern . the pattern is generated electronically , and no physically moving parts are required . an exemplary embodiment of the antenna radiates a cardioid pattern which rotates at a 15hz rate for use as a tacan beacon . the structure of the antenna is in the form of a conic monopole above a ground plane . a plurality of vertical modulator fins , providing controllable shorting elements , and a plurality of suppressor posts are added . the modulator fins have associated diodes to which are applied a bias current along with modulating signals which cause the resistance of the diodes to vary with respect to the flow of rf current at the frequencies of interest . other details of the physical structure of the antenna are shown and described herein , as well as an electronic system for controlling the diodes . in a typical application , the antenna is mounted on an airplane to enable the airplane to serve as a beacon . in this instance , the exemplary system as shown and described herein is adapted to be coupled with the synchro system of the airplane compass for providing a reference with respect to magnetic north so as to properly electronically relate the rotating pattern of the antenna to magnetic north while the airplane upon which the antenna is mounted changes its flight direction .