director

In a Yagi-Uda antenna, the director is a parasitic element, a conductive rod slightly shorter than the driven element, positioned in front of it along the antenna axis. It has no direct electrical connection to the feed line. By being slightly shorter than the driven element's resonant length, the director presents capacitive reactance, which causes it to reradiate energy in a way that reinforces the forward lobe while suppressing radiation to the rear.

A typical Yagi antenna may have one to several directors. Adding more directors increases forward gain and narrows the beamwidth, but with diminishing returns after three or four elements. A common amateur radio design uses a driven element at the center, a reflector behind it (slightly longer, roughly 5 percent longer), and one or two directors ahead. Each director is separated from its neighbors by approximately 0.1 to 0.15 wavelengths of the operating frequency.

Performance and Trade-offs

The term director reflects its function: it directs or steers the antenna pattern forward. As frequency increases, the physical length of each element decreases proportionally, making compact Yagi arrays practical for VHF, UHF, and microwave bands. A 2-meter Yagi director is roughly 0.9 meters long; a 70-centimeter design requires only about 0.3 meters per element. The director's length must be tuned within tight tolerances, typically within 1 to 2 percent of the target resonance, to maintain phase relationships with the driven element.

In real antennas, the director is often made from aluminum rod or tubing because it offers good electrical conductivity while remaining lightweight and resistant to corrosion. The conductor diameter affects bandwidth and impedance characteristics; thicker rods lower loss and broaden the operating bandwidth. Installation spacing errors or mechanical deformation from wind or ice loading can detune the antenna significantly, reducing gain or causing the main lobe to split.

The director is distinct from the reflector, which sits behind the driven element, is longer, and suppresses rearward radiation rather than enhancing forward gain. In television reception antennas, commercial designs often add passive reflectors and multiple directors to achieve gain figures of 8 to 14 dBi over a dipole, concentrating signal pickup in a single direction with high rejection of off-axis interference.