Advanced Calculator Inputs
Example Data Table
| Band | Frequency | Directors | Reflector Factor | Driven Factor | First Director Factor | Suggested Spacing |
|---|---|---|---|---|---|---|
| 2 m VHF | 146 MHz | 3 | 1.05 | 1.00 | 0.95 | 0.20λ, 0.15λ, 0.20λ |
| 70 cm UHF | 435 MHz | 5 | 1.04 | 0.99 | 0.94 | 0.18λ, 0.14λ, 0.18λ |
| 6 m | 50.125 MHz | 2 | 1.055 | 1.00 | 0.955 | 0.20λ, 0.16λ, 0.20λ |
Formula Used
Wavelength: λ = c ÷ f
Half-wave reference: Lhalf = λ ÷ 2
Element length: Lelement = Lhalf × velocity factor × length factor × end effect multiplier
Element spacing: Spacing = λ × selected spacing factor
Boom length: Boom = final element position + boom allowance
Quarter-wave transformer impedance: Zt = √(source impedance × estimated feedpoint impedance)
Quarter-wave transformer length: Lmatch = λ ÷ 4 × feedline velocity factor
These formulas give a practical starting design. Final dimensions should be tuned with an analyzer, test range, or trusted antenna model.
How To Use This Calculator
- Enter the target operating frequency in MHz.
- Select the number of director elements.
- Choose the output unit for your workshop measurements.
- Adjust wire velocity factor and end effect if known.
- Change element length factors for narrowband or broadband designs.
- Set spacing values as fractions of wavelength.
- Enter source and feedpoint impedance for matching estimates.
- Press the calculate button and review the element table.
- Download the CSV or PDF for building notes.
Wire Yagi Antenna Design Guide
Why Wire Yagi Geometry Matters
A wire Yagi antenna uses parasitic elements to shape radio energy. The driven element receives power from the feed line. The reflector sits behind it. One or more directors sit in front. Their lengths and spacing control gain, pattern, and impedance.
Element Length Planning
The driven element is normally near one half wavelength. The reflector is slightly longer. Directors are slightly shorter. This difference creates current phase shifts. Those phase shifts push radiation toward the director side. Wire diameter and insulation can shorten the needed physical length. That is why velocity factor and end correction are included.
Spacing And Boom Length
Spacing is measured as a fraction of wavelength. Close spacing can reduce feed impedance. Wider spacing can improve bandwidth. A compact design may be easier to mount. A longer boom can increase gain with more directors. However, extra directors give smaller gains after a point. Mechanical balance also becomes more important.
Matching And Tuning
Many Yagi designs have feedpoint resistance below 50 ohms. A gamma match, hairpin match, folded driven element, or transformer may help. This calculator gives a quarter-wave transformer estimate. It also shows likely gain and front-to-back ratio. These values are planning estimates. They are not a substitute for field testing.
Practical Building Notes
Cut elements slightly long. Mount them straight and level. Keep element centers aligned on the boom. Use nonconductive support when building a wire version. Measure SWR after installation. Then trim the driven element in small steps. Check the pattern by comparing signal strength from front and rear. Record every change. A careful build log makes repeat tuning easier.
FAQs
1. What does a wire Yagi antenna calculator do?
It estimates element lengths, element spacing, boom length, gain, beamwidth, and matching values for a wire Yagi antenna design.
2. Is the calculated design ready to build?
It is a strong starting point. Real antennas need tuning because wire insulation, mounting height, nearby objects, and ground conditions change performance.
3. Why is the reflector longer than the driven element?
The reflector is longer to create a delayed current phase. That helps reduce rear radiation and pushes more energy forward.
4. Why are directors shorter?
Directors are shorter so they act capacitively. They guide the radiation pattern forward and increase gain in the desired direction.
5. What is velocity factor?
Velocity factor adjusts the physical length for wire insulation, construction style, or conductor behavior. A lower value makes the calculated element shorter.
6. What output unit should I select?
Use the unit that matches your workshop tools. Centimeters and inches are practical for most hobby builds and quick field measurements.
7. Can this calculator estimate antenna gain?
Yes. It provides a planning estimate based on director count. Accurate gain needs modeling software or controlled field measurement.
8. Why include a matching transformer calculation?
Many Yagi feedpoints are not exactly 50 ohms. The transformer estimate helps plan a feed system before final SWR tuning.