Build a 3-Element 17m Yagi Antenna
This 3-element 17m Yagi scales the same proven W6SAI-derived design used on this site's 10m Yagi up to the 18.118 MHz WARC band, using the site's own documented frequency-ratio scaling method. It's the largest of the four Yagi designs on this site — nearly 6 m of boom and over 8 m of reflector — but delivers the same 7.5 dBd forward gain and 20+ dB front-to-back ratio, on a quiet, contest-free band known for productive late-afternoon DX. 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 17m band using the ratio 28.5/18.118 = 1.5731, 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 18.068–18.168 MHz with optimum SWR at 18.118 MHz — the 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 18.118 MHz over free space — the same figure as every other Yagi in this scaled family, since gain depends on the antenna's proportions relative to its own wavelength.
Front-to-back ratio
Typically 20–25 dB at the design frequency. Combined with 17m's quiet, contest-free character and productive late-day DX window, this directivity is genuinely useful for working weak DX without competing pile-up interference.
SWR bandwidth
SWR ≤ 2:1 across the entire 18.068–18.168 MHz allocation when tuned to the 18.118 MHz center — as on 12m, the narrow 100 kHz band is comfortably covered without any real edge-of-band compromise.
Element Layout
Given the significant size increase over the 10m/12m/15m versions, this design steps up to 22 mm OD centre tube (from the 19 mm used on the smaller Yagis) with 14 mm OD tip tube, for adequate stiffness across the longer elements. The boom steps up to 50 mm square section — using the boom-correction table's K = 0.965 factor rather than the 38 mm/K = 0.97 used on the smaller designs.
Tip tube overlap: The 14 mm tip tubes slide 120 mm inside the 22 mm centre tubes — a slightly longer overlap than the smaller Yagis given the increased leverage on this larger element. Cut tip tubes 120 mm longer than the dimension shown, and secure with two self-tapping screws through both tubes at the overlap point (one more than the smaller designs, for extra security against the increased element weight).
| Element | Position on boom (m) | Total length (m) | Half-length each side (m) | Centre tube (22mm) | Tip tube (14mm) |
|---|---|---|---|---|---|
| Reflector | 0.00 | 8.28 | 4.14 | 2 × 1.89 m | 2 × 2.25 m |
| Driven element | 2.28 | 7.63 | 3.82 | 2 × 1.73 m | 2 × 2.09 m |
| Director | 5.66 | 7.27 | 3.63 | 2 × 1.65 m | 2 × 1.98 m |
| Component | Material | Length | Notes |
|---|---|---|---|
| Main boom | 50×50×3 mm aluminium square section | 5.82 m | Stepped up from 38mm given the larger overall size; 50 mm overhang each end |
| Mast plate | 4 mm aluminium plate, 250×120 mm | – | Bolts to boom centre; clamps to mast with U-bolts |
| Element-to-boom plate | 4 mm aluminium, 100×70 mm | – | 3 required — one per element |
3-Element Yagi Element Length Calculator
This is the same generic Yagi calculator used on the 10m, 12m, and 15m Yagi pages — it already takes frequency and boom correction as inputs, so no new function was needed. Pre-set here for a 3-element 17m Yagi at 18.118 MHz with the larger 50 mm boom correction.
Materials for one complete 3-element 17m Yagi
Why a Gamma Match
As with the smaller Yagis in this family, 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, scaled proportionally larger for 17m's lower frequency and thicker element stock.
| Parameter | Starting value | Adjustment range | Effect of increasing |
|---|---|---|---|
| Gamma rod length | 550 mm | 390–785 mm | Increases resistance transformation ratio |
| Rod-to-element spacing | 79 mm | 63–126 mm | Changes characteristic impedance of gamma section |
| Series capacitor | 39 pF | 24–70 pF | Reduces capacitive reactance in series |
| Gamma rod OD | 12 mm | 10–14 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. Iterate until SWR is below 1.5:1 at 18.118 MHz. Final adjustment is best done with a NanoVNA at the feed point.
Building the 3-Element 17m Yagi
Eight steps from cutting aluminium tube to tuning the gamma match — allow 10–14 hours given this is the largest and heaviest Yagi design on this site.
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 — this is more important than ever given the thicker-wall tube used here.
Assemble stepped-diameter elements
Slide each 14 mm tip tube 120 mm into the corresponding 22 mm centre tube. Drill two 5 mm holes through both tubes at the overlap (rather than the single hole used on the smaller Yagis) and fit stainless M5×12 self-tapping screws, filed flush.
Install element-to-boom mounting plates
Cut three 100×70 mm plates from 4 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; the reflector and director need no isolation.
Mount elements on the boom
Mark the element positions: 0.00 m (reflector), 2.28 m (driven element), 5.66 m (director). Clamp each element-to-boom plate at its marked position with two M8 U-bolts, check square, and tighten to finger-tight plus a quarter turn — aluminium still crushes easily even on this larger hardware.
Build the gamma match
Cut a 550 mm length of 12 mm aluminium rod. Build or purchase a gamma match clamp block with an SO-239 mount, sized for the thicker driven element. The gamma rod runs parallel to the driven element, 79 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. This is the largest, heaviest antenna in this Yagi family — a full tower installation with a properly rated rotator is strongly recommended over a simple push-up mast. Raise to at least 12 m for useful DX performance; 18 m or more is preferable given the lower design frequency.
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 the series capacitor. Iterate until SWR is below 1.5:1 at 18.118 MHz. Because 17m is only 100 kHz wide, SWR should stay well below 2:1 across the entire allocation once tuned.
Correcting for the Boom
Unlike the smaller Yagis in this family, this design uses a 50 mm square boom (K = 0.965) rather than 38 mm (K = 0.97), shortening each element by approximately 18 mm per side — already incorporated in the dimensions above.
| 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 |
| 50 mm square | ~18 mm | Use K = 0.965 (default for this design) |
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 — always start elements slightly long and trim to final length, since re-cutting a too-short element on this scale means sourcing new tube stock.
| Frequency (MHz) | Typical SWR | Gain (dBd approx) | F/B (dB approx) |
|---|---|---|---|
| 18.068 | 1.3:1 | 7.3 | 19 |
| 18.100 | 1.2:1 | 7.4 | 21 |
| 18.118 | 1.1:1 | 7.5 | 22 |
| 18.140 | 1.2:1 | 7.4 | 21 |
| 18.168 | 1.3:1 | 7.3 | 19 |
Late-afternoon DX window: 17m frequently stays open into the late afternoon and early evening after 15m and 12m have started to close — a directional Yagi is especially valuable during this window for pulling out the weaker signals typical of a marginal opening, and its rejection of off-axis interference helps on a band that, while quiet compared to 20m, still sees active DX chasing during good openings.
SWR minimum is at wrong frequency
Given the narrow 100 kHz allocation, even a modest length error matters more here than on 10m. Re-measure all elements with a steel tape before assuming a matching problem — the gamma match primarily affects impedance, not resonance.
Cannot achieve SWR below 2:1 regardless of gamma adjustment
Check that the driven element isn't shorted to the boom, and confirm the gamma capacitor is within 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 — 8–10 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 is the 10m design's dimension multiplied by 28.5/18.118 = 1.5731, the same frequency-ratio scaling method described on the 10m Yagi page's own FAQ. Tube diameters were also stepped up for mechanical strength given the significantly larger element lengths.
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, especially given this antenna's larger size and correspondingly longer feedline runs down the mast.
Can I scale this design further, to 12m or 15m?
Yes — for 24.94 MHz (12m): multiply by 18.118/24.94 = 0.7264. For 21.225 MHz (15m): multiply by 18.118/21.225 = 0.8536. Both are also built as dedicated pages on this site, using thinner element stock given their smaller size.
Why does this design use thicker tube than the 10m/12m/15m Yagis?
At this size, the longer, heavier elements need a stiffer centre tube to avoid excessive sag and flex, particularly under wind load. Stepping from 19 mm to 22 mm centre tube (and 38 mm to 50 mm boom) keeps the mechanical margin comparable to the smaller designs.
How high should I mount this antenna?
At 18.118 MHz, a half-wavelength is approximately 8.3 m. Mounting at one full wavelength (roughly 16.5 m) gives the best low-angle DX performance. Minimum useful height is about 12 m.
Is this a beginner-friendly build?
Not as much as the 10m or 12m versions — the larger element sizes, heavier hardware, and more demanding mast/rotator requirements make this the most involved build in the Yagi family on this site. Builders new to Yagi construction may want to start with the 10m version first.