English: Animation showing how a Yagi-Uda antenna works. It consists of 4 half-wave dipole antennas in a line; a driven element (E) which is connected to the transmitter and radiates the radio waves, and 3 parasitic elements, two directors (D1, D2) and one reflector (R) which act as resonators, absorbing and reradiating the waves from the driven element with a different phase. The radio waves from all 4 elements combine and interfere, increasing the power radiated in the desired direction (up) and decreasing the power radiated in other directions. The radio waves from each individual element (wavy moving lines) are shown in a different color. The waves in the forward direction are in phase, and interfere constructively, adding together to produce a higher signal strength, while the waves in the reverse direction are out of phase, partially canceling each other to produce lower signal strength in that direction.