In a Yagi-Uda antenna, how do the lengths of the reflector and directors compare to the driven element?

• A. The reflector is longer; directors shorter
• B. The reflector shorter; directors longer
• C. All elements same length
• D. Reflector same length; directors vary

Answer: (A)

In a Yagi-Uda antenna the element lengths are chosen to shape the phase of currents and thus the radiation pattern. The driven element is near resonance at the design frequency, serving as the feed point. The reflector, placed behind the driven element, is made slightly longer than the driven element. That extra length makes the reflector respond with a current that lags the drive, effectively acting like a mirror to energy coming from the driven element and boosting radiation in the forward direction.

In front of the driven element, the directors are made shorter than the driven element. Shorter elements resonate at a slightly higher frequency, causing the currents to lead and interact to steer energy forward, enhancing forward gain and shaping the beam.

As a practical rule of thumb, the driven element is about half a wavelength long, the reflector is a bit longer, and the directors are a bit shorter, with progressively shorter lengths for additional directors. This arrangement produces the characteristic directional radiation pattern of a Yagi.

So the reflector is longer; the directors shorter.