Element Roles
The roles of the Elements are as follows: #1 Reflector, #2 Driven Element, #3 and higher = Directors.
How to mount and feed
Horizontal or vertical polarization is determined by the orientation of the elements.
The driven element is normally a split dipole or a folded dipole. Use a 1:1 current balun (or choke) at the feedpoint to prevent feedline radiation.
Elements can be aluminum tubing or rod. Diameter has a small effect — thicker elements run slightly shorter.
Element Material
Aluminum tubing (1/4" to 1/2" diameter common on VHF/UHF, larger on HF) works well. Keep elements insulated from a metal boom, or apply a small boom-correction if they pass through the boom.
These calculations give good starting lengths. Final trimming of the driven element for lowest SWR is always recommended.
Gain vs Spacing
The gain of a Yagi varies with element spacing and tuning. For any given spacing there is an optimum length that produces maximum forward gain. Maximum front-to-back ratio rarely occurs at the exact same settings that give maximum gain. The driven-element impedance also changes with spacing and tuning, so the antenna should be tuned to its final condition before the final matching is completed.
Warning about long Yagis
Note: Adding more than 8–10 elements often results in diminishing returns for gain, a very long boom, narrower bandwidth, and increased mechanical complexity. A well-built 3 to 6 element Yagi is usually the practical sweet spot for most amateur stations.