Build a 3-Element 10m Yagi Antenna
A 3-element Yagi on 10 metres is one of the most rewarding antenna projects available to the home builder. The 10 m band's short wavelength keeps element and boom lengths manageable — the entire antenna fits in less than 4 metres of boom — while delivering 7–8 dBd of forward gain, a front-to-back ratio of 20 dB or better, and a radiation pattern well suited to DX contacts when the band is open. This guide covers the complete build from aluminium stock to first QSO.
Design Overview
This design is based on the well-proven W6SAI optimised 3-element 10 m Yagi dimensions, adapted for construction from standard UK and European aluminium tube stock. The design targets the 28.0–29.7 MHz range with optimum SWR centred at 28.5 MHz, covering the CW, SSB, and FM portions of the 10 m band. It uses a gamma match for feed point impedance transformation, eliminating the need for a balun transformer while providing a robust all-aluminium feed system.
Forward gain
Approximately 7.5 dBd (9.65 dBi) at 28.5 MHz over free space. Over real ground at 10 m height this translates to approximately 13–15 dBi peak gain including ground reflection — a significant improvement over a dipole at the same height.
Front-to-back ratio
Typically 20–25 dB at the design frequency. This means the antenna is 100–300× more sensitive in the forward direction than directly behind. At contest time this makes the difference between working a pileup and being buried in QRM.
SWR bandwidth
SWR ≤ 2:1 from approximately 28.0–29.5 MHz — covering the entire 10 m band including the FM section. The gamma match allows fine-tuning of the SWR minimum to any point within the band by adjusting the gamma rod length and series capacitor.
Element Layout
All elements are made from 19 mm OD aluminium tube for the centre sections, with 12 mm OD tube for the outer tips. A stepped-diameter element reduces wind loading and cost while maintaining electrical performance. The boom is 38 mm square aluminium section — square boom simplifies element mounting with U-bolts and provides better torsional rigidity than round boom of the same wall thickness.
Tip tube overlap: The 12 mm tip tubes slide 100 mm inside the 19 mm centre tubes. The dimensions below give the electrical length of each element including the overlap. Cut the tip tubes 100 mm longer than the dimension shown to account for the overlap insertion length. 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 | 5.26 | 2.63 | 2 × 1.20 m | 2 × 1.43 m |
| Driven element | 1.45 | 4.85 | 2.425 | 2 × 1.10 m | 2 × 1.325 m |
| Director | 3.60 | 4.62 | 2.31 | 2 × 1.05 m | 2 × 1.26 m |
| Component | Material | Length | Notes |
|---|---|---|---|
| Main boom | 38×38×2 mm aluminium square section | 3.70 m | 50 mm overhang each end beyond outermost element |
| 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
Scales Yagi element dimensions for your chosen design frequency. This page is pre-set for a 3-element 10m Yagi at 28.5 MHz.
Materials for one complete 3-element 10m Yagi
Why a Gamma Match
A 3-element Yagi's driven element, when surrounded by the reflector and director, has its feed point impedance pulled down from the free-space dipole value of 73 Ω to approximately 25–30 Ω. Standard 50 Ω coaxial cable cannot be connected directly to this low impedance without creating an unacceptable SWR. The gamma match solves this by tapping the driven element at a point away from its centre — the tap point can be adjusted to find the position where the feed point resistance equals 50 Ω.
The gamma match consists of three components: the gamma rod (a short aluminium rod running parallel to and approximately 50 mm from the driven element), the series capacitor (which cancels the inductive reactance introduced by the off-centre tap), and the connecting block that joins the coax outer conductor to the element centre and the coax inner conductor to the gamma rod. The entire assembly is mechanically attached to the driven element and electrically part of the feed system.
| Parameter | Starting value | Adjustment range | Effect of increasing |
|---|---|---|---|
| Gamma rod length | 350 mm | 250–500 mm | Increases resistance transformation ratio |
| Rod-to-element spacing | 50 mm | 40–80 mm | Changes characteristic impedance of gamma section |
| Series capacitor | 22 pF | 15–40 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 (the coax inner conductor connection point along the element) for minimum SWR. Then adjust the series capacitor value to achieve minimum reflected power. Iterate between the two until SWR is below 1.5:1. 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 10m Yagi
Eight steps from cutting aluminium tube to tuning the gamma match — allow 6–10 hours for the complete build.
Cut all aluminium tube to length
Using a pipe cutter or hacksaw with a fine-tooth blade, cut all tube pieces to the lengths specified in the element dimensions table. Label each piece with masking tape: REF-CTR (reflector centre), REF-TIP, DE-CTR, DE-TIP, DIR-CTR, DIR-TIP. Deburr all cut ends with a round file or deburring tool — sharp edges inside the tubes will prevent clean tip insertion and can cut the self-tapping screws short.
Assemble stepped-diameter elements
Slide each 12 mm tip tube 100 mm into the corresponding 19 mm centre tube. The fit should be tight but not forced — some tip tube ODs are slightly undersized; wrap two layers of self-amalgamating tape around the tip tube at the insertion point if needed for a snug fit. Once positioned correctly, drill a 4 mm hole through both tubes at the overlap midpoint and fit a stainless M4×10 self-tapping screw. The screw head should be countersunk or file-flush to avoid interference when the element passes through the element-to-boom plate.
Install element-to-boom mounting plates
Cut three 80×60 mm plates from 3 mm aluminium flat stock. Drill two 8 mm holes for M6 U-bolt saddles in each plate, spaced to suit your U-bolt size. Drill a 20 mm hole at the plate centre for the element tube to pass through. Deburr all holes. The element passes through the centre hole and is clamped between the U-bolt saddle and the plate. For the reflector and director (which are parasitic — not connected to the coax), no electrical isolation is needed between the element and the boom. The driven element must be electrically isolated from the boom — use plastic tube or PTFE spacers at the boom contact points.
Mount elements on the boom
Mark the element positions on the boom: 0.00 m (reflector), 1.45 m (driven element), 3.60 m (director). Clamp each element-to-boom plate to the boom at its marked position using two M6 U-bolts. The element should be centred on the boom — equal lengths on each side. Check square (element perpendicular to boom) with a builder's square before fully tightening the U-bolts. Tighten to finger-tight plus ¼ turn — aluminium crushes easily; do not overtighten.
Build the gamma match
Cut a 400 mm length of 10 mm aluminium rod or tube for the gamma rod. Fabricate or purchase a gamma match clamp block — a machined aluminium block with two holes (one for the gamma rod, one for the driven element clamp), an SO-239 coax connector mount, and provisions for the series capacitor. The gamma rod runs parallel to the driven element, 50 mm away from its surface, starting at the element centre and extending approximately 350 mm toward one tip. Connect the coax outer to the element at the centre, and the coax inner through the series capacitor to the far end of the gamma rod.
Fit end caps and weatherproofing
Fit PVC or rubber end caps on all open tube ends — these prevent water ingress into the elements and insects nesting inside (a frequent cause of element resonance shifts). Apply self-amalgamating tape over the entire gamma match assembly, the coax connector, and the first 150 mm of feedline coax. Do not seal the tube ends with tape — the cap method allows thermal breathing; tape traps condensation.
Mount on mast and initial elevation
Clamp the boom-to-mast plate at the boom midpoint (approximately 1.80 m from the reflector end — slightly forward of centre to balance the weight distribution). The mast clamp should allow full 360° rotation for beam heading changes. Raise the antenna to operating height — minimum 6 m for useful DX performance; 10 m or more is better. The more you can clear nearby rooftops and obstructions, the more the antenna's pattern performs as modelled.
Tune the gamma match
Connect a NanoVNA or antenna analyser at the feed point — at the gamma match, not at the shack end of the feedline. Transmit at 5–10 W and adjust the gamma rod length (slide the tap point) for minimum SWR. Then adjust the series capacitor value for further improvement. Iterate until SWR is below 1.5:1 at 28.5 MHz. Record the final gamma rod length and capacitor value for future reference. Verify SWR at 28.0 and 29.5 MHz — it should remain below 2:1 across the full band.
Correcting for the Boom
Mounting elements on a conductive aluminium boom introduces a parasitic effect — the boom acts as a short section of transmission line at each element crossing, slightly detuning each element. For a 38 mm square boom, the boom correction factor shortens each element by approximately 10–25 mm compared to the free-space values. This is already incorporated in the element dimensions given above through the K-factor of 0.97. If you use a different boom size, adjust as shown in the table 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 |
The boom correction is most significant for the director — the shortest element — and least significant for the reflector. If your SWR minimum ends up at a lower frequency than expected after first assembly, shorten all elements by 5–10 mm per side (trim from the tips) and re-check. If the SWR minimum is higher than expected, the elements need lengthening — this requires re-cutting, which is why it is always better to start with elements cut 10 mm too long and trim to final length.
| Frequency (MHz) | Typical SWR | Gain (dBd approx) | F/B (dB approx) |
|---|---|---|---|
| 28.000 | 1.8:1 | 7.0 | 15 |
| 28.200 | 1.4:1 | 7.3 | 19 |
| 28.500 | 1.1:1 | 7.5 | 22 |
| 28.800 | 1.3:1 | 7.4 | 20 |
| 29.000 | 1.6:1 | 7.2 | 17 |
| 29.500 | 2.2:1 | 6.8 | 12 |
| 29.700 | 2.8:1 | 6.5 | 10 |
For FM and repeater use (29.0–29.7 MHz): Shift the design frequency to 29.3 MHz by shortening all elements by approximately 18 mm per side from the 28.5 MHz dimensions. This centres the SWR minimum in the FM section while keeping the CW and SSB portions below SWR 2:1. Many 10 m operators build separate optimised antennas for CW/SSB and FM rather than compromising one design for both.
SWR minimum is at wrong frequency
If the SWR minimum is 500 kHz or more away from the design frequency, check element lengths first — a 10 mm error on all elements simultaneously shifts resonance by approximately 100 kHz. A 1 MHz shift suggests a systematic measurement error; re-measure all elements with a steel tape. The gamma match does not significantly affect the resonant frequency — it primarily controls the impedance match, not the resonance point.
Cannot achieve SWR below 2:1 regardless of gamma adjustment
This suggests the driven element is either shorted to the boom (check isolation at the boom plate) or the gamma capacitor value is far outside the correct range. Disconnect the gamma match entirely, connect a 25 Ω dummy load at the element centre, and verify the feed system presents reasonable impedance. If the driven element is making DC contact with the boom, the gamma match sees a short and cannot achieve a useful match.
SWR changes when the beam rotates
Feed point impedance should not change with antenna orientation. If it does, the coax is acting as part of the antenna (common-mode current). Add a coaxial choke — 8 turns of coax on an FT-240-31 toroid — immediately below the boom at the point where the feedline runs down the mast. This isolates the antenna from the feedline.
Do I need a balun with a gamma match?
The gamma match itself provides some common-mode isolation because it connects the coax outer conductor to the element at the centre (a current minimum), reducing the drive on the coax braid. However, a coaxial choke on the feedline below the boom is still recommended for best results — it costs virtually nothing and prevents common-mode current from distorting the pattern and causing RF in the shack.
Can I scale this design for 15m or 12m?
Yes — multiply all element lengths and spacings by the frequency ratio. For 21 MHz (15 m) from a 28.5 MHz design: multiply by 28.5/21 = 1.357. For 24.9 MHz (12 m): multiply by 28.5/24.9 = 1.145. The boom becomes proportionally longer and the antenna heavier, so check that your mast and rotator can handle the increased wind load. Aluminium tube OD can stay the same or go slightly larger on 15 m for mechanical strength.
What gain improvement does this give over a dipole?
The 3-element Yagi provides approximately 7.5 dBd of forward gain compared to a half-wave dipole in the same plane. In practical terms this is equivalent to multiplying transmitter power by about 5.6× — going from 100 W to 560 W in effective radiated power. Combined with the pattern's directivity reducing interference from other directions, the operational improvement on a live 10 m band is dramatic.
How high should I mount this antenna?
At 28.5 MHz, a half-wavelength is approximately 5.25 m. Mounting the antenna at one full wavelength height (10.5 m) gives a take-off angle of approximately 15° — well suited for DX contacts. Minimum useful height for DX is about 6 m (just above λ/2). Heights above 15 m continue to improve low-angle radiation but the marginal gain diminishes. A rooftop mount at 8–12 m typically gives excellent 10 m DX performance.
What wind loading does this antenna present?
The approximate frontal wind area is: boom (3.7 m × 0.038 m) + three elements (average 4.9 m × 0.019 m each) ≈ 0.14 + 0.28 = 0.42 m². At 100 km/h wind (Beaufort 10), this generates approximately 300 N (30 kg) of lateral force on the antenna and mounting hardware. Use M8 or larger stainless hardware at the mast-to-boom joint and verify the mast and rotator are rated for this load. In high-wind areas, a streamlined mounting plate and non-conductive rope guys to the outer boom ends significantly reduce mast bending moment.
Can I add a 4th director to increase gain?
Yes — adding a 4th element (a second director) approximately 1.8 m forward of the first director increases gain by approximately 1.5–2.0 dBd. The boom extends to approximately 5.4 m, wind loading increases proportionally, and the SWR bandwidth narrows slightly. A 4-element design is a worthwhile upgrade if your mast and rotator can handle the additional size and weight. The gamma match dimensions remain unchanged for the same driven element design.