Yagi Antenna Calculator | Element Length & Spacing

What a Yagi Antenna Calculator Does

A Yagi antenna calculator takes your target frequency and number of elements, then returns the driven element length, reflector length, director lengths, and inter-element spacing needed for a functional Yagi-Uda array. It also estimates forward gain in dBi and the approximate front-to-back ratio. These outputs let you skip tedious manual iteration and jump straight to a buildable design—whether you are fabricating elements from aluminum tubing or etching them onto an RF PCB for UHF or microwave bands.

Yagi-Uda Fundamentals: How the Array Works

A Yagi-Uda antenna is an endfire array of parallel dipole elements. Only one element—the driven element—is connected to the feedline. The others operate as parasitic elements excited by mutual coupling:

• Reflector – slightly longer than a half-wavelength (typically 0.50 λ–0.52 λ), placed behind the driven element. It reflects energy forward.
• Driven element – resonant near a half-wavelength (≈ 0.47 λ–0.48 λ after accounting for element diameter). This is the feedpoint.
• Directors – progressively shorter elements (starting around 0.44 λ–0.46 λ) placed in front. Each director adds forward gain and narrows the beam.

The spacing between elements is typically 0.15 λ–0.25 λ for the reflector-to-driven gap, and 0.20 λ–0.35 λ between successive directors. Wider director spacing gives broader bandwidth at the cost of some gain; tighter spacing squeezes out more gain but makes the design more frequency-sensitive.

Key Design Equations

The free-space wavelength at frequency f is:

λ = c / f

where c ≈ 299.792 MHz·m (speed of light). For a 435 MHz design, λ ≈ 689 mm. A half-wave driven element before diameter correction is about 345 mm. Real element diameters shorten resonance; a typical correction factor for a 10 mm diameter rod at 435 MHz is roughly 3–5 %, bringing the driven element to ≈ 328–334 mm.

Gain grows roughly logarithmically with boom length. A well-optimized 5-element Yagi achieves about 10–11 dBi; adding elements beyond 10–12 yields diminishing returns. The Yagi-Uda antenna article on Wikipedia provides a thorough treatment of the mutual-impedance theory behind these relationships.

Interactive Yagi Antenna Calculator

Enter your design frequency and the number of elements below. The tool returns each element’s length, the element-to-element spacing, estimated gain, and front-to-back ratio so you can evaluate the design before committing to fabrication.

For a quick sanity check: a 6-element Yagi at 145 MHz (2 m amateur band) should show a driven element near 980 mm, a reflector around 1 030 mm, directors tapering from roughly 940 mm down to 910 mm, and a total boom length of about 2.1 m with an estimated gain near 11 dBi. If your results land in that neighborhood, you are on track. If the gain seems low, try widening director spacing slightly or adding one more director. To cross-check the gain figure against beamwidth, use the antenna gain and beamwidth calculator.

Translating Calculator Output into a Physical Build

Wire and Tube Yagis (VHF/UHF)

At VHF and low-UHF frequencies (30–900 MHz), elements are typically 3–12 mm diameter aluminum rod or tube mounted through a non-conductive boom or insulated from a conductive boom. Key practical points:

• Element diameter matters. Thicker elements broaden bandwidth but shift resonance downward. The calculator’s diameter correction handles this, but always verify with an NEC simulation or VNA measurement.
• Boom correction. A conductive boom that passes through the elements electrically shortens them by roughly 1–3 % depending on boom diameter. Insulated mounting eliminates this.
• Feed impedance. A plain driven dipole in a Yagi typically presents 20–30 Ω, not 50 Ω. A folded dipole driven element raises impedance to roughly 4× (80–120 Ω), which a simple 4:1 balun brings close to 50 Ω. Alternatively, a gamma match or T-match can transform the impedance directly.

PCB Yagis (UHF/Microwave)

Above 900 MHz, element lengths shrink enough to etch directly onto a PCB substrate. A 2.4 GHz Yagi on FR-4 (Dk ≈ 4.4) has a free-space half-wave of about 62 mm, but the effective wavelength on the substrate is shorter—roughly 42 mm for a microstrip-style element backed by a ground plane, depending on substrate height. For higher-performance designs at 5.8 GHz or millimeter-wave bands, low-loss laminates such as Rogers RO4350B (Dk 3.48, Df 0.0037) keep parasitic losses manageable.

When laying out a PCB Yagi, ground-plane geometry under the elements significantly affects the pattern. A partial ground or no ground beneath the parasitic elements preserves the endfire pattern; a full ground converts the structure into something closer to a patch array. If your radar front-end needs a PCB-based directional feed, the Yagi topology competes with helical antennas for circular polarization or patch arrays for planar integration.

Gain, Bandwidth, and Trade-Off Summary

A 3-element Yagi offers roughly 7–8 dBi gain with usable bandwidth of 5–8 % of center frequency. A 10-element design pushes toward 14 dBi but narrows bandwidth to 2–3 %. For radar applications where you need to estimate detection range from a known Yagi gain, feed that dBi figure into the radar range equation calculator.

DDR Routing on the Yagi Feed Network PCB

In modern radar and software-defined-radio systems, the Yagi antenna connects to a digital back-end whose FPGA or processor uses DDR memory. Maintaining DDR signal integrity on the same board (or a closely connected board) that carries the RF feed network requires careful length matching. Use the assistant below to check your DDR byte-lane length rules and ensure timing margins are met alongside your RF layout constraints.

For a DDR3-1600 interface, typical within-byte-lane skew must stay under ±25 mil (0.64 mm). If your board also carries a 50 Ω microstrip feed to the Yagi’s driven element, verify the trace geometry with a trace width calculator so both the RF path and digital bus meet their impedance targets on the same stackup.

Practical Tuning and Verification Tips

1. Start with the driven element. Trim it for minimum SWR at center frequency before adding parasitic elements.
2. Add the reflector. Confirm the feedpoint impedance drops (it should) and that the pattern shifts toward the director side.
3. Add directors one at a time. Each director should pull the SWR minimum slightly higher in frequency; retrim the driven element if needed.
4. Measure the pattern. Even a crude field-strength meter at several angles reveals whether the front-to-back ratio matches expectations.
5. Use NEC or FEKO. Free tools like 4nec2 let you simulate the full array including real ground effects before cutting metal.

Frequently Asked Questions

How many elements should a Yagi antenna have?

For most practical applications, 3–7 elements offer the best balance of gain (7–12 dBi), manageable boom length, and reasonable bandwidth. Beyond 10 elements, each additional director adds less than 0.5 dB and the design becomes mechanically unwieldy and frequency-sensitive.

Can I build a Yagi antenna on a PCB?

Yes. At frequencies above roughly 900 MHz, element lengths are short enough to etch on standard or low-loss laminates. A 2.4 GHz 5-element PCB Yagi on 0.8 mm FR-4 fits within about 80 × 25 mm. For frequencies above 5 GHz, use a low-Df substrate like RO4350B to keep efficiency above 85 %.

What impedance does a Yagi feedpoint present?

A standard half-wave driven element in a Yagi typically shows 20–35 Ω resistive at resonance. A folded dipole raises this to approximately 100–140 Ω. Matching to 50 Ω is done with a balun, gamma match, or quarter-wave transformer.

Does element diameter affect Yagi performance?

Yes. Thicker elements lower Q, broadening bandwidth but slightly reducing peak gain. They also shift resonant length downward. The calculator accounts for this; always enter your actual element diameter for accurate results.

How do I increase Yagi gain without adding elements?

Optimize director lengths and spacing using NEC simulation, increase boom length (keeping existing elements but spreading them out), or stack two identical Yagis with proper phasing for an additional 2.5–3 dB.