What is the approximate length of the director element of a Yagi antenna for 21.1 MHz?

In a Yagi antenna, the director element is placed in front of the driven element to help focus the signal in a specific direction. To act correctly as a director, this element must be slightly shorter (typically about 5% shorter) than the driven half-wave element. First, find the driven element length for 21.1 MHz using the standard half-wave formula: 143 / 21.1 = approximately 6.77 metres. Next, since the director must be roughly 5% shorter than the driven element, we calculate 6.77 x 0.95, which yields approximately 6.43 to 6.44 metres. Choosing a longer length would incorrectly size it as a reflector (which needs to be about 5% longer than the driven element), and choosing lengths near 3 metres would represent a quarter-wave element, which is incorrect for a Yagi.