Instant 3-Element Yagi Dimension Calculator
What this 3-Element Yagi Calculator Does
A 3-element Yagi antenna is one of the most popular directional antennas in amateur radio and RF experimentation. It gives useful forward gain, decent front-to-back rejection, and a manageable mechanical size. This calculator provides fast starting dimensions for:
- Reflector length (rear element)
- Driven element length (center element, feedpoint)
- Director length (front element)
- Element spacing along the boom
Design Equations Used
The tool uses practical first-pass formulas that work well for many homebrew builds:
- Wavelength:
λ = 300 / f(MHz) - Driven element:
(150 / f) × velocityFactor - Reflector:
driven × 1.05 - Director:
driven × 0.95 - Reflector-to-driven spacing:
0.20 × λ - Driven-to-director spacing:
0.15 × λ
These values are intentionally conservative and easy to build. Final optimization depends on element diameter, mounting style, nearby objects, boom conductivity, and feed matching network.
How to Use the Results
1) Cut Slightly Long
Cut each element a bit longer than calculated (for example 1-2%). It is much easier to trim metal than to add it back.
2) Assemble and Measure SWR
Install the antenna at least several wavelengths away from large conductive objects when tuning. Use an antenna analyzer if possible; SWR meters are useful but less informative.
3) Trim the Driven Element First
Bring resonance to your target frequency by trimming the driven element in very small, equal increments on both sides. After that, tweak reflector/director if needed for pattern and front-to-back performance.
Typical 3-Element Yagi Performance
- Approximate free-space gain: 6 to 8 dBi
- Front-to-back ratio: 10 to 20 dB (build dependent)
- Feed impedance at driven element: often 20-30 Ω without matching
- Common matching methods: gamma match, hairpin/beta match, or folded dipole feed
Practical Build Tips
Element Materials
Aluminum tubing is common for permanent outdoor builds. Copper wire or rods can work for prototyping. Keep element diameters consistent whenever possible.
Boom and Insulation
A conductive boom can affect resonance; isolated element mounts can reduce that interaction. If you directly bond elements to a metal boom, expect some retuning.
Feedline and Balun
Even a simple choke balun (coax coil or ferrite choke) can improve pattern symmetry by reducing common-mode current.
Important Notes
This calculator is a strong starting point, not a full electromagnetic solver. For precision builds, validate with modeling software (e.g., NEC-based tools) and real-world measurements.