Build a 3-Element 15m Yagi Antenna
This 3-element 15m Yagi scales the same proven W6SAI-derived design used on this site's 10m Yagi up to the 21.225 MHz band, using the site's own documented frequency-ratio scaling method. The result is a noticeably larger antenna than the 10m or 12m versions — about 5 m of boom and 7 m reflector — but the same 7.5 dBd forward gain and 20+ dB front-to-back ratio, on a band known for reliable daytime DX across much of the solar cycle. 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 15m band using the ratio 28.5/21.225 = 1.3428, the same 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 21.000–21.450 MHz with optimum SWR at 21.225 MHz — the exact center of the 15m band — and uses the same gamma match feed as the 10m version, scaled proportionally.
Forward gain
Approximately 7.5 dBd (9.65 dBi) at 21.225 MHz over free space — the same figure as the 10m and 12m versions, since Yagi gain depends on the antenna's proportions relative to its own wavelength, not the absolute frequency.
Front-to-back ratio
Typically 20–25 dB at the design frequency. Combined with 15m's good DX propagation during much of the solar cycle, this directivity is a genuine asset for pulling out weak signals and rejecting interference.
SWR bandwidth
SWR ≤ 2:1 across most of the 21.000–21.450 MHz band when tuned to the 21.225 MHz center — comparable coverage to the 10m version relative to its own band width.
Element Layout
All elements use 19 mm OD aluminium tube for the centre sections, with 12 mm OD tube for the outer tips, matching the 10m design. Given the larger element lengths on 15m, consider stepping up to 22 mm centre tube for added stiffness if your local wind exposure is significant — the boom-correction dimensions below assume the standard 19 mm/38 mm combination.
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, 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 | 7.06 | 3.53 | 2 × 1.61 m | 2 × 1.92 m |
| Driven element | 1.95 | 6.51 | 3.26 | 2 × 1.48 m | 2 × 1.78 m |
| Director | 4.83 | 6.20 | 3.10 | 2 × 1.41 m | 2 × 1.69 m |
| Component | Material | Length | Notes |
|---|---|---|---|
| Main boom | 38×38×2 mm aluminium square section | 4.97 m | Scaled from the 10m boom (3.70 m × 1.3428); 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 and 12m Yagi pages — it already takes frequency as an input, so no new function was needed. Pre-set here for a 3-element 15m Yagi at 21.225 MHz.
Materials for one complete 3-element 15m Yagi
Why a Gamma Match
As with the 10m and 12m versions, the driven element's feed point impedance is pulled down from 73 Ω to approximately 25–30 Ω by the reflector and director. The gamma match transforms this to 50 Ω with an off-centre tap and series capacitor.
At 15m's lower frequency, both the gamma rod length and the series capacitor value scale up further from the 12m version — start within the ranges given in the materials list and tune empirically using the same iterative process as the 10m design.
| Parameter | Starting value | Adjustment range | Effect of increasing |
|---|---|---|---|
| Gamma rod length | 470 mm | 335–670 mm | Increases resistance transformation ratio |
| Rod-to-element spacing | 67 mm | 54–107 mm | Changes characteristic impedance of gamma section |
| Series capacitor | 33 pF | 20–60 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 21.225 MHz. Final adjustment is best done with a NanoVNA at the feed point.
Building the 3-Element 15m Yagi
Eight steps from cutting aluminium tube to tuning the gamma match — allow 8–12 hours given the larger element and boom sizes compared to 10m and 12m.
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.
Assemble stepped-diameter elements
Slide each 12 mm tip tube 100 mm into the corresponding 19 mm centre tube, taping the joint 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 centre hole for the element tube. Electrically isolate the driven element from the boom with plastic tube or PTFE spacers; the reflector and director need no isolation.
Mount elements on the boom
Mark the element positions: 0.00 m (reflector), 1.95 m (driven element), 4.83 m (director). Clamp each element-to-boom plate at its marked position with two M6 U-bolts, check square, and tighten to finger-tight plus a quarter turn.
Build the gamma match
Cut a 470 mm length of 10 mm aluminium rod. Build or purchase a gamma match clamp block with an SO-239 mount. The gamma rod runs parallel to the driven element, 67 mm away, extending toward one tip.
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 weight distribution. This antenna is considerably heavier and bulkier than the 10m or 12m versions — use M8 or larger hardware at the mast joint and confirm your mast and rotator are rated for the increased wind load. Raise to at least 10 m for useful DX performance.
Correcting for the Boom
As with the 10m and 12m designs, 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 above via K = 0.97.
| 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, shorten all elements by 5–10 mm per side and re-check. If the minimum is higher than expected, the elements need lengthening — start elements slightly long and trim to final length rather than risking a too-short cut.
| Frequency (MHz) | Typical SWR | Gain (dBd approx) | F/B (dB approx) |
|---|---|---|---|
| 21.000 | 1.7:1 | 7.1 | 16 |
| 21.074 | 1.4:1 | 7.3 | 19 |
| 21.225 | 1.1:1 | 7.5 | 22 |
| 21.300 | 1.3:1 | 7.4 | 20 |
| 21.450 | 1.9:1 | 7.0 | 15 |
CW/digital vs phone segments: Because 21.225 MHz is the exact band center, this design covers both the CW/digital segment (21.000–21.200 MHz) and the phone segment (21.200–21.450 MHz) with reasonable SWR without a tuner. Operators heavily weighted to one segment can shift the design frequency slightly toward that segment's center following the same scaling and trimming process described in the build steps.
SWR minimum is at wrong frequency
Check element lengths first — a 15 mm error on all elements simultaneously produces a noticeable shift on this band. Re-measure all elements with a steel tape. The gamma match primarily controls impedance matching, not resonance.
Cannot achieve SWR below 2:1 regardless of gamma adjustment
Check that the driven element isn't shorted to the boom at the isolation point, and confirm the gamma capacitor value is within the correct range. Disconnect the gamma match and verify with a 25 Ω dummy load at the element centre.
SWR changes when the beam rotates
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, to eliminate common-mode current.
How was this design scaled from the 10m Yagi?
Every dimension is the 10m design's dimension multiplied by 28.5/21.225 = 1.3428, the same 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 below the boom is still recommended for best results.
Can I scale this design further, to 12m or 17m?
Yes — for 24.94 MHz (12m): multiply by 21.225/24.94 = 0.851. For 18.118 MHz (17m): multiply by 21.225/18.118 = 1.171. 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 — equivalent to roughly a 5.6× increase in effective radiated power, plus meaningful interference rejection from the directional pattern.
How high should I mount this antenna?
At 21.225 MHz, a half-wavelength is approximately 7.1 m. Mounting at one full wavelength (roughly 14 m) gives a low take-off angle for DX. Minimum useful height is about 10 m.
How much bigger is this than the 10m version?
About 34% larger in every dimension given the lower design frequency — a meaningfully bigger and heavier antenna requiring a more substantial mast and rotator than the 10m build.