Build a 3-Element 12m Yagi Antenna
This 3-element 12m Yagi scales the same proven W6SAI-derived design used on this site's 10m Yagi up to the 24.9 MHz WARC band, using the site's own documented frequency-ratio scaling method. The result is a somewhat larger antenna — about 4.2 m of boom and 6 m reflector — but the same 7.5 dBd forward gain and 20+ dB front-to-back ratio, delivered on a quiet, contest-free band. This guide covers the complete build from aluminium stock to first QSO.
Design Overview
This design scales the site's W6SAI-derived 3-element 10m Yagi (built for 28.5 MHz) to the 12m band using the ratio 28.5/24.94 = 1.1427, exactly the method described in that page's own FAQ for adapting the design to other bands. Every element length, tip-tube overlap, and boom position below is the 10m dimension multiplied by this ratio. It targets 24.890–24.990 MHz with optimum SWR at 24.940 MHz — the practical center of this narrow 100 kHz WARC allocation — and uses the same gamma match feed as the 10m version, scaled proportionally.
Forward gain
Approximately 7.5 dBd (9.65 dBi) at 24.94 MHz over free space — Yagi gain is a function of the antenna's proportions relative to its own wavelength, so scaling the whole design preserves the same gain figure as the 10m original.
Front-to-back ratio
Typically 20–25 dB at the design frequency, same as the 10m design. On a quiet WARC band with no contest QRM, this directivity is especially valuable for pulling weak signals out of the noise from the favored direction.
SWR bandwidth
SWR ≤ 2:1 across the entire 24.890–24.990 MHz allocation — given the band is only 100 kHz wide, a gamma match tuned for the center comfortably covers the whole band with room to spare, unlike the wider 10m band where SWR rises more noticeably toward the edges.
Element Layout
All elements use 19 mm OD aluminium tube for the centre sections, with 12 mm OD tube for the outer tips — the same tube stock as the 10m version, since the larger element lengths on 12m don't require thicker tube for adequate stiffness. The boom is 38 mm square aluminium section, matching the 10m design's boom-correction factor (K = 0.97).
Tip tube overlap: As with the 10m version, the 12 mm tip tubes slide 100 mm inside the 19 mm centre tubes. Cut tip tubes 100 mm longer than the dimension shown to account for the overlap, and secure with a self-tapping screw through both tubes at the overlap point.
| Element | Position on boom (m) | Total length (m) | Half-length each side (m) | Centre tube (19mm) | Tip tube (12mm) |
|---|---|---|---|---|---|
| Reflector | 0.00 | 6.01 | 3.01 | 2 × 1.37 m | 2 × 1.63 m |
| Driven element | 1.66 | 5.54 | 2.77 | 2 × 1.26 m | 2 × 1.51 m |
| Director | 4.11 | 5.28 | 2.64 | 2 × 1.20 m | 2 × 1.44 m |
| Component | Material | Length | Notes |
|---|---|---|---|
| Main boom | 38×38×2 mm aluminium square section | 4.23 m | Scaled from the 10m boom (3.70 m × 1.1427); 50 mm overhang each end |
| Mast plate | 3 mm aluminium plate, 200×100 mm | – | Bolts to boom centre; clamps to mast with U-bolts |
| Element-to-boom plate | 3 mm aluminium, 80×60 mm | – | 3 required — one per element |
3-Element Yagi Element Length Calculator
This is the same generic Yagi calculator used on the 10m Yagi page — it already takes frequency as an input, so no new function was needed. Pre-set here for a 3-element 12m Yagi at 24.94 MHz.
Materials for one complete 3-element 12m Yagi
Why a Gamma Match
Exactly as on the 10m version, the driven element's feed point impedance is pulled down from the free-space dipole value of 73 Ω to approximately 25–30 Ω by the presence of the reflector and director. The gamma match transforms this to 50 Ω by tapping the driven element off-centre, with a series capacitor cancelling the resulting inductive reactance.
Because 12m operates at a lower frequency than 10m, the gamma rod and spacing scale up proportionally with the rest of the antenna, but the series capacitor typically needs a somewhat larger value to present the same reactance at the lower frequency — start within the wider range given in the materials list and tune empirically, exactly as described for the 10m version.
| Parameter | Starting value | Adjustment range | Effect of increasing |
|---|---|---|---|
| Gamma rod length | 400 mm | 285–570 mm | Increases resistance transformation ratio |
| Rod-to-element spacing | 57 mm | 45–90 mm | Changes characteristic impedance of gamma section |
| Series capacitor | 28 pF | 18–50 pF | Reduces capacitive reactance in series |
| Gamma rod OD | 10 mm | 8–12 mm | Minor effect on impedance ratio |
Gamma Match Formula & Tuning
Zγ ≈ 276 × log₁₀(2S / √(d₁×d₂))
where S = spacing, d₁ = element OD, d₂ = rod OD (all same units)
Tuning sequence: Adjust the gamma rod length for minimum SWR, then adjust the series capacitor for further improvement. Iterate until SWR is below 1.5:1 at 24.94 MHz. Final adjustment is best done with a NanoVNA at the feed point rather than from the shack end of the feedline.
Building the 3-Element 12m Yagi
Eight steps from cutting aluminium tube to tuning the gamma match — allow 6–10 hours for the complete build, plus extra time for handling the larger element lengths compared to 10m.
Cut all aluminium tube to length
Cut all tube pieces to the lengths in the element dimensions table above. Label each piece: REF-CTR, REF-TIP, DE-CTR, DE-TIP, DIR-CTR, DIR-TIP. Deburr all cut ends with a round file or deburring tool.
Assemble stepped-diameter elements
Slide each 12 mm tip tube 100 mm into the corresponding 19 mm centre tube. Wrap self-amalgamating tape at the insertion point for a snug fit if needed. Drill a 4 mm hole through both tubes at the overlap midpoint and fit a stainless M4×10 self-tapping screw, filed flush.
Install element-to-boom mounting plates
Cut three 80×60 mm plates from 3 mm aluminium flat stock, drill U-bolt saddle holes and a 20 mm centre hole for the element tube. The reflector and director need no electrical isolation from the boom; the driven element must be electrically isolated using plastic tube or PTFE spacers.
Mount elements on the boom
Mark the element positions on the boom: 0.00 m (reflector), 1.66 m (driven element), 4.11 m (director). Clamp each element-to-boom plate at its marked position with two M6 U-bolts, check square with a builder's square, and tighten to finger-tight plus a quarter turn.
Fit end caps and weatherproofing
Fit PVC or rubber end caps on all open tube ends. Apply self-amalgamating tape over the gamma match assembly, coax connector, and first 150 mm of feedline coax.
Mount on mast and initial elevation
Clamp the boom-to-mast plate at the boom midpoint, slightly forward of centre to balance the weight distribution — this antenna is meaningfully heavier and bulkier than the 10m version, so confirm your mast and rotator are rated accordingly. Raise the antenna to operating height — minimum 8 m for useful DX performance given the lower frequency; 12 m or more is better.
Tune the gamma match
Connect a NanoVNA at the feed point. Transmit at 5–10 W and adjust the gamma rod length for minimum SWR, then adjust the series capacitor value. Iterate until SWR is below 1.5:1 at 24.94 MHz. Record the final gamma rod length and capacitor value. Because 12m is only 100 kHz wide, SWR should stay well below 2:1 across the entire band once tuned to center.
Correcting for the Boom
As with the 10m design, mounting elements on a conductive 38 mm square boom shortens each element by approximately 12 mm per side compared to free-space values — already incorporated in the dimensions above via K = 0.97. If you use a different boom size, adjust as shown below.
| Boom OD/width | Element shortening per side | Effective K-factor adjustment |
|---|---|---|
| 20 mm round | ~5 mm | Use K = 0.98 |
| 25 mm round | ~8 mm | Use K = 0.975 |
| 38 mm square | ~12 mm | Use K = 0.97 (default) |
| 50 mm square | ~18 mm | Use K = 0.965 |
If your SWR minimum ends up lower in frequency than expected after first assembly, shorten all elements by 5–10 mm per side and re-check. If the minimum is higher than expected, the elements need lengthening — always start elements slightly long and trim to final length, since re-cutting a too-short element requires splicing.
| Frequency (MHz) | Typical SWR | Gain (dBd approx) | F/B (dB approx) |
|---|---|---|---|
| 24.890 | 1.3:1 | 7.3 | 19 |
| 24.915 | 1.2:1 | 7.4 | 21 |
| 24.940 | 1.1:1 | 7.5 | 22 |
| 24.965 | 1.2:1 | 7.4 | 21 |
| 24.990 | 1.3:1 | 7.3 | 19 |
A quiet, contest-free band: Unlike 10m, 12m has no FM sub-band or contest activity to plan around — the entire narrow allocation is used for CW, digital, and phone with no internal segmentation requiring a compromise design. This is one of the simplest bands on this site to build a Yagi for, precisely because there's no edge-of-band tradeoff to weigh.
SWR minimum is at wrong frequency
If the SWR minimum is 100 kHz or more away from the design frequency — a larger fraction of this narrow band than the equivalent error would be on 10m — check element lengths first. A 15 mm error on all elements simultaneously produces a meaningful shift on this band. Re-measure all elements with a steel tape. The gamma match primarily controls impedance matching, not the resonant frequency.
Cannot achieve SWR below 2:1 regardless of gamma adjustment
This suggests the driven element is shorted to the boom (check isolation at the boom plate) or the gamma capacitor value is outside the correct range. Disconnect the gamma match, connect a 25 Ω dummy load at the element centre, and verify the feed system presents reasonable impedance.
SWR changes when the beam rotates
Feed point impedance should not change with orientation. If it does, add a coaxial choke — 8 turns of coax on an FT-240-31 toroid — immediately below the boom where the feedline runs down the mast.
How was this design scaled from the 10m Yagi?
Every dimension — element lengths, tip-tube overlaps, and boom positions — is the 10m design's dimension multiplied by 28.5/24.94 = 1.1427, exactly the frequency-ratio scaling method described on the 10m Yagi page's own FAQ.
Do I need a balun with a gamma match?
The gamma match provides some common-mode isolation on its own, but a coaxial choke on the feedline below the boom is still recommended — it costs little and prevents common-mode current from distorting the pattern.
Can I scale this design further, to 15m or 17m?
Yes — the same ratio-scaling method applies. For 21.225 MHz (15m): multiply by 24.94/21.225 = 1.175. For 18.118 MHz (17m): multiply by 24.94/18.118 = 1.377. Both are also built as dedicated pages on this site.
What gain improvement does this give over a dipole?
Approximately 7.5 dBd of forward gain compared to a half-wave dipole in the same plane — equivalent to roughly a 5.6× increase in effective radiated power, plus the pattern's directivity rejecting interference from other directions.
How high should I mount this antenna?
At 24.94 MHz, a half-wavelength is approximately 6.0 m. Mounting at one full wavelength height (roughly 12 m) gives a low take-off angle well suited to DX. Minimum useful height is about 8 m.
Is this antenna significantly bigger than the 10m version?
Yes — about 14% larger in every dimension (boom, elements, gamma match) given the lower design frequency. The wind load and weight increase accordingly, so verify your mast and rotator can handle it before committing to this build.