Example Data Table
| Band Example |
Frequency |
Directors |
Reflector Spacing |
Director Spacing |
Typical Use |
| VHF Two Meter |
144 MHz |
3 |
20% λ |
15% λ |
Portable directional work |
| UHF Seventy Centimeter |
432 MHz |
5 |
18% λ |
14% λ |
Compact high gain array |
| HF Ten Meter |
28.4 MHz |
2 |
20% λ |
16% λ |
Fixed station beam |
Formula Used
Wavelength: λ = c / f. Here, c is 299,792,458 meters per second. Frequency is converted into hertz.
Half wave: half wave = λ / 2.
Driven element: length = half wave × velocity factor × driven factor × diameter correction.
Reflector: length = half wave × velocity factor × reflector factor × diameter correction.
Directors: length = half wave × velocity factor × director factor × taper × diameter correction.
Spacing: spacing = λ × selected spacing percentage.
Boom length: boom length = last element position + two end margins.
VSWR estimate: reflection coefficient = |Zfeed - Zline| / |Zfeed + Zline|. VSWR = (1 + coefficient) / (1 - coefficient).
How To Use This Calculator
Enter the center frequency of your planned antenna. Choose the frequency unit and preferred output unit. Add the number of directors you want. More directors usually mean more gain and a longer boom.
Adjust element factors only when you understand the design goal. The default values are practical starting points. Use trim allowance when you plan to cut elements long for tuning.
Press the calculate button. Review the summary first. Then use the element table for cutting and boom marking. Download the CSV or PDF for workshop use.
Yagi Antenna Planning Guide
A Yagi antenna is a directional array. It uses a driven element, a reflector, and one or more directors. These parts work together to focus radio energy. The focused pattern increases gain in the forward direction. It also reduces signals from the rear and sides.
Why Dimensions Matter
Small length errors can change resonance. Spacing errors can change gain and front to back ratio. A calculator helps you begin with sensible dimensions before field tuning. It does not replace measuring tools. Real materials, mounts, nearby metal, and weatherproofing can shift the final result.
Main Design Choices
Start with the operating frequency. The calculator converts it into wavelength. Element lengths are then based on a half wave section. The driven element is usually near one half wavelength. The reflector is longer. Directors are shorter. Each additional director can raise forward gain, but only up to practical limits. Extra directors also make the boom longer and narrower in bandwidth.
Spacing and Boom Layout
Common reflector spacing is near 0.20 wavelength. Common director spacing is near 0.15 wavelength. Wider spacing can improve bandwidth, but it may reduce compactness. Tight spacing can make matching harder. A steady boom layout is easier to build. A tapered layout can refine gain and pattern shape.
Feed and Matching Notes
Yagi feed impedance varies with element diameter, spacing, and count. A split dipole may need a gamma match, hairpin, or beta match. A folded dipole often gives a higher impedance. Use the shown mismatch results as a planning guide. Final adjustment should be made with an antenna analyzer.
Practical Building Tips
Cut elements slightly long. Then trim in small steps. Keep both sides equal from the boom center. Use strong insulators when the design needs isolated elements. Seal coax joints against water. Mount the antenna in the clear when testing. Ground the support structure according to local safety practice. Record final dimensions after tuning. Those notes make future repairs easier.
Best Use
This tool is best for preliminary layouts, comparison work, and build sheets. It gives consistent numbers fast. For critical stations, verify the design with modeling software and outdoor measurements.
Careful notes also help compare prototypes across nearby future operating bands.
FAQs
What is a Yagi antenna?
A Yagi antenna is a directional antenna. It uses a driven element, reflector, and directors. The layout focuses radio energy in one main direction.
Is this calculator enough for final construction?
It gives strong starting values. Final construction should include trimming, analyzer testing, and outdoor checks. Nearby objects can change tuning.
Why is the reflector longer?
The reflector is usually longer than the driven element. This helps push the main signal pattern forward and reduce rear radiation.
Why are directors shorter?
Directors are shorter than the driven element. They guide energy forward and increase directional gain when spaced correctly on the boom.
What does velocity factor change?
Velocity factor adjusts element length for material and construction effects. A lower value shortens calculated elements. Use measured tuning for final accuracy.
Should I cut elements exactly to the result?
Cut slightly long when possible. Then trim small amounts during tuning. It is easier to remove material than to add it back.
What is boom margin?
Boom margin is extra boom length beyond the outer elements. It helps with mounting, clamps, and mechanical clearance.
Why does impedance matter?
Impedance affects matching to coax or feed line. Poor matching can raise VSWR and reduce delivered power. A matching network may be needed.