HF Yagi Planning Guide
Why this design matters
A four element HF Yagi gives useful forward gain without a very long boom. It usually has one reflector, one driven element, and two directors. Each element interacts with the others. Small length or spacing changes can move the best match. That is why a calculator is helpful before cutting metal.
What the dimensions mean
The driven element is near a half wavelength after end effect is considered. The reflector is normally longer. It pushes energy forward. The directors are shorter. They pull the main lobe toward the front. Spacing is shown as a fraction of wavelength. Wider spacing may improve bandwidth. Closer spacing can reduce boom length.
Tuning notes
HF antennas are affected by height, ground, nearby wires, mast size, and tubing diameter. Calculated values are starting dimensions, not final field values. Cut elements slightly long when possible. Then trim both sides equally. Check resonance with an analyzer at the planned operating height. Tune the driven element first. Then adjust matching hardware for the desired feed line.
Matching and feed
A compact four element design often has a low feed impedance. Direct coax feed may not give a good standing wave ratio. Many builders use a gamma match, hairpin match, beta match, or driven element split with a balun. Keep feed connections short and weatherproof. Use a choke near the feed point to reduce common mode current.
Build accuracy
Measure from element center to element center on the boom. Keep all elements square with the boom. Use the same unit system for every cut. Mark each half element separately. Include clamps, end caps, and telescoping overlaps in the shop plan. Longer tubing sections can change effective length.
Using the results
The table gives total element length, half element length, and boom positions. Use the boom position column to drill or clamp in order. The reflector starts at zero. The driven element follows. Director one and director two are placed toward the front. Record measured SWR after installation. Those notes help when retuning for another part of the band.
For contests, place the antenna where rotation is clear. Check safe lifting loads. Recheck hardware after wind, rain, and seasonal temperature changes before every serious operating session.