Yagi-Uda – Basic Starting Dimensions
Simplified proportional starting dimensions only (not a full optimized design). Element diameter, boom and matching are not modelled. Use as a first cut, then model or prune.
How the elements create the Yagi pattern
A Yagi-Uda antenna uses a driven element together with parasitic elements, normally a longer reflector and one or more shorter directors mounted along a boom. The diagram on this page shows that relationship clearly and also indicates the preferred direction of radiation. The calculator is intended for basic starting dimensions, not a substitute for a full electromagnetic optimisation program. Even a small change in element length or spacing can alter the impedance and pattern.
The starting dimensions are derived from wavelength and typical proportions for a simple Yagi. Real designs depend on element diameter, boom material, element mounting method, insulation and the number of directors. A metal boom can interact with the elements if the construction does not account for it. Folded or insulated elements may require different corrections. For multi-band or high-performance antennas, modelling is strongly recommended before cutting expensive tubing.
Mechanical accuracy matters. Keep the elements square to the boom, centred correctly and secure against wind movement. The reflector should normally be the longest parasitic element and the directors progressively shorter, but the exact dimensions should come from the selected design. Feed the driven element with the intended matching arrangement and route the coax away from the driven element before it follows the boom. After assembly, measure SWR and, where possible, verify the forward/backward behaviour with a controlled test. If trimming is necessary, change one element at a time and record the effect. The image is deliberately schematic because the calculator's numbers describe the element relationships; a generic photograph of a random beam antenna would not communicate those relationships accurately.
Design
Yagi Basics
Reflector is slightly longer than resonance, directors shorter. Gain rises with more elements and boom length; beamwidth narrows. Driven element is often a folded dipole or gamma-matched dipole for 50 Ω.
Typical proportions (λ)
- Reflector ≈ 0.49–0.50 λ
- Driven ≈ 0.46–0.48 λ
- Directors ≈ 0.42–0.45 λ (tapering)
- Spacing ≈ 0.15–0.25 λ
Where the simple formulas stop being enough
These element lengths and spacings are classic starting points (often derived from early Yagi literature and simplified tables). Real boom correction, element diameter, mounting plate effects and mutual coupling mean a finished beam almost always needs adjustment on an analyser or in a modelling program.
- Boom diameter and element-through-boom mounting change the electrical length of each element.
- Director and reflector lengths interact; changing one moves the others.
- Feed-point impedance of a classic 3-element Yagi is often well below 50 Ω — a matching method (beta match, hairpin, gamma, etc.) is normally required.
Assumptions built into this calculator
- Free-space element lengths with a simple diameter correction; no full moment-method model.
- No boom-correction factor applied automatically.
- Results are design starting points, not final dimensions.
Next step in the workflow
Use the calculation as a starting point, then move to the tool that answers the next practical question.