Build a 15m Half-Wave Dipole Antenna
15m sits between the reliable, year-round performance of 20m and the solar-cycle-dependent excitement of 10m and 12m — a genuinely useful all-around DX band that opens well during daylight hours for much of the sunspot cycle. At just over 22 feet total length, a 15m dipole is compact enough for almost any yard while still delivering real worldwide DX performance. This guide covers the complete build from wire cutting to verified resonance.
Choose Your Target Frequency
15m spans 21.000 to 21.450 MHz — 450 kHz wide, roomier than the WARC bands but narrower than 20m in percentage terms. The band splits cleanly into a few operating segments:
- CW and digital (FT8 at 21.074): 21.000–21.200 MHz — cut for 21.100 MHz
- Phone (SSB): 21.200–21.450 MHz — cut for 21.300 MHz
- Best all-around compromise: 21.225 MHz — the exact band center, covering both CW and phone segments well
Because 21.225 MHz is the true mathematical center of the band, it's the natural default design frequency for most builders rather than a true compromise — SWR rises gently and symmetrically toward both band edges from here.
Choose Your Configuration
At 22 feet total, a 15m dipole fits comfortably in most yards with room to spare:
- Flat (horizontal) — full performance, needs about 11.4 feet of clearance per side.
- Inverted-V — a 20–25 foot center mast is plenty; this is the most common installation for 15m.
- Vertical dipole — mounted on a single mast with a low feedpoint, a compact option that favors a low-angle DX pattern.
- Attic or limited-space — the modest length fits in many attics, though nearby wiring and framing will detune it somewhat.
Height above ground matters here as it does on every HF band — aim for at least 0.3–0.5 wavelength (roughly 15–24 feet at 21.225 MHz) for a useful DX takeoff angle, with higher installations favoring lower angles for long-haul contacts.
15m Dipole Calculator
Everything you need to build a complete 15m dipole
Building the 15m Half-Wave Dipole
Follow these steps in order for a properly tuned, weatherproofed 15m dipole.
Mark Out and Cut the Wire
Unroll the wire on a flat surface. Measure 11.4 feet from one end and mark with tape. Cut at this mark — this is your first leg, 3% longer than the calculated 11.02-foot resonant length. Cut a second identical 11.4-foot leg.
Wind the Current Choke
Wind 7 turns of RG-8X coax through the FT-240-31 toroid. Push the coax through the toroid hole, loop around the outside, and back through again for each turn.
Prepare the Coax End
Strip back 2 inches of outer jacket at the antenna end of the coax. Fold the braid back over the jacket without nicking strands. Strip 1 inch of dielectric to expose the center conductor. Tin both the center conductor and folded braid with solder.
Assemble the Feedpoint
If using a commercial dipole center, follow its instructions. If fabricating your own, drill two pairs of holes for 6-32 screws — one per side connects to the coax, the second connects the antenna wire.
Strip 1.5 inches of insulation from each wire leg end, form a loop with round-nose pliers, and secure it under a screw and lock washer before soldering for electrical continuity.
Attach End Insulators and Support Rope
Thread each wire leg through an egg insulator at the far end. Double the wire back about 3 inches, wrap 4–5 times around the main wire, and solder the wrap. Attach at least 18 inches of Dacron rope to each insulator.
Plan the Installation Layout
Walk the site and confirm your support points. A center support of 20–25 feet gives a good DX takeoff angle at 21.225 MHz. Wire ends only need to clear 6–8 feet above ground. Route the coax to the shack and leave a 12-inch drip loop just below the feedpoint.
Raise the Antenna
For an inverted-V, raise the center mast first with the feedpoint attached, then pull each leg out to its end anchor and secure with modest tension. For a flat dipole, raise the center and one end, then the second end, adjusting tension until level.
Initial SWR Sweep
Connect the NanoVNA at the radio end of the coax and sweep 20.5 to 22.0 MHz. With legs cut to 11.4 feet, expect resonance around 20.55–20.75 MHz — below the band due to the long legs.
Trim to Target Frequency
Each 1 inch trimmed from both legs raises resonance by roughly 18 kHz on 15m.
Trim both legs equally in 3–4 inch increments and re-measure after each cut, switching to 1-inch increments as you approach the target.
Verify Across the Band
Once resonance is confirmed near 21.225 MHz, sweep the full 21.000–21.450 MHz range:
- SWR at 21.000 MHz: 1.2–1.5:1
- SWR at 21.225 MHz: close to 1:1 (the resonance minimum)
- SWR at 21.450 MHz: 1.2–1.5:1
Weatherproof the Feedpoint
Wrap self-amalgamating tape upward from below the feedpoint with 50% overlap, covering all connections and the coax entry. Apply a second layer of PVC electrical tape over it for UV protection.
Document and Make First Contact
Record the final leg lengths, installed height, and resonant frequency in your station log. 15m is often open for DX during daylight hours even at moderate solar activity — a good first target for a new build.
A Reliable Daytime DX Band
15m occupies a useful middle ground in the HF spectrum: high enough in frequency to support long-distance F2-layer propagation during daylight, but low enough that it doesn't require the peak solar activity that 10m and 12m depend on. Across most of the solar cycle, 15m offers at least some daytime DX opening, making it one of the more consistently useful higher HF bands.
Harmonic Relationship to 40m
21.225 MHz is not a clean harmonic of a standard 40m dipole design frequency, but many multiband and trap dipole designs on this site (including several fan dipole and trap dipole guides) do cover 15m as a secondary band precisely because the third harmonic of a 40m half-wave dipole falls within the 15m allocation. A dedicated single-band 15m dipole like this one will always outperform a harmonic-fed compromise, particularly for SWR bandwidth and pattern control.
Good Candidate for Multiband Combination
Because 15m's leg length (about 11 feet) is proportionally distinct from 20m, 17m, and 10m, a 15m dipole combines well as one leg of a fan dipole or trapped multiband antenna without significant interaction, provided legs are spaced or oriented per the fan dipole guide's spacing recommendations.
| Symptom | Most likely cause | Diagnosis | Fix |
|---|---|---|---|
| SWR high (3:1+) across entire band | Connection or wiring error | Inspect all feedpoint connections | Verify center conductor and braid are on opposite dipole sides; check for open connections |
| SWR changes when touching the coax | Common-mode current; no choke | Is the current choke installed? | Install or improve the current choke; add snap-on ferrite near the feedpoint |
| Resonance below 21.000 MHz | Wire legs too long | Measure actual leg lengths | Trim both legs equally — 1 inch per side raises resonance ~18 kHz |
| Resonance above 21.450 MHz | Wire legs too short | Measure actual leg lengths | Splice a short wire extension onto each leg end and re-tune |
| Good SWR but weak signal reports | Antenna too low for the propagation mode in use | Check installed height in wavelengths | Raise the antenna; at low heights, expect a higher-angle pattern favoring shorter-range contacts |
| Band open on the cluster but nothing heard locally | Antenna orientation or local terrain shadowing | Check dipole broadside direction versus target DX | Reorient the dipole broadside toward the desired DX path if the pattern allows |
Why cut for 21.225 MHz instead of the CW or phone segment?
21.225 MHz is the exact mathematical center of the 15m band, so a dipole cut here gives symmetric SWR performance toward both the CW/digital segment and the phone segment. If your operating is heavily weighted to one segment, cutting for that segment's center instead is a reasonable alternative.
How does 15m propagation compare to 20m?
15m tends to open later in the morning and close earlier in the evening than 20m, and is more solar-cycle dependent, but during good conditions it often outperforms 20m for long-path DX with less crowding.
Do I need a current choke on a 15m dipole?
Yes — common-mode current on the feedline affects every band the same way. A 7-turn FT-240-31 choke at the feedpoint keeps the pattern clean and SWR readings accurate.
What height gives the best DX performance?
Higher is generally better for low-angle DX — aim for at least 0.5 wavelength (about 24 feet at 21.225 MHz) if DX is the priority. Lower heights favor higher-angle radiation, which is better suited to shorter, regional contacts (NVIS-style behavior is more pronounced on 40m and 80m than on 15m, however).
Can I use my existing 40m dipole's harmonics on 15m instead?
A 40m dipole's third harmonic does fall near 21 MHz, and many multiband designs exploit this. But a dedicated 15m dipole like this one will have cleaner resonance, better SWR bandwidth, and a more predictable pattern than a harmonic-fed 40m antenna.
What wire gauge should I use?
#14 AWG stranded copper-clad steel is standard across this site's dipole guides and works well here — strong enough for outdoor spans of this length while remaining easy to work with by hand.