Build a 20m Half-Wave Dipole Antenna
The 20m half-wave dipole is the most productive HF antenna you can build in a single afternoon. At 14 MHz, the wire is only 33 feet long — short enough to fit almost any property — yet 20m offers the best combination of propagation, activity, and DX opportunity in amateur radio. This guide takes you from wire cutting to verified resonance, covering every step in enough detail that a new ham can build a working antenna on their first attempt.
Choose Your Target Frequency
The 20m band spans 14.000 to 14.350 MHz. Where you cut the dipole determines where SWR is lowest — you want the resonance point near your primary operating mode:
- CW and digital (FT8/FT4): 14.000–14.100 MHz — cut for 14.050 MHz
- General SSB DX and DXpeditions: 14.150–14.230 MHz — cut for 14.200 MHz
- SSB calling and casual operation: 14.225–14.300 MHz — cut for 14.250 MHz
- Best all-around compromise: 14.200 MHz — covers CW end with tuner, SSB center with low SWR
The 20m band is narrow enough that a dipole cut for 14.200 MHz will show SWR below 2:1 across the entire band — most modern transceivers handle this without an ATU. Cut for your primary operating interest.
Choose Your Configuration
A 20m dipole can be installed in several configurations depending on your available supports. The configuration affects both the radiation pattern and the practical installation difficulty:
- Flat (horizontal) — both legs at the same height. Requires three support points (two ends + center or center mast). Best DX performance when feedpoint is at 30+ feet. Figure-8 pattern broadside to the wire.
- Inverted-V — both legs slope down from a single apex. Requires only one tall support plus two low anchor points. More omnidirectional pattern than flat dipole. Feed impedance closer to 50Ω when apex angle is 90–120°. Best choice for most installations.
- Sloper — one end high, one end lower at an angle. One support needed. Mixes horizontal and vertical polarization — useful for DX. Feed impedance varies with slope angle.
- Attic — runs inside the attic if no outdoor installation is possible. Performance reduced by building materials but provides basic HF operation.
For a first dipole, the inverted-V is the best choice — one tall support, flexible end anchor placement, and naturally good SWR due to the lower feed impedance compared to a flat dipole.
Everything you need to build a complete 20m dipole
Building the 20m Half-Wave Dipole
Follow these steps in order. Each step builds on the previous — read through completely before cutting any wire.
Mark Out and Cut the Wire
Unroll the wire on a flat surface — a driveway or lawn works well. Measure 17.0 feet from one end and mark with a permanent marker or colored tape. Cut at this mark. This is your first leg. Measure and cut a second 17.0-foot piece. You now have two legs, each 3% longer than the calculated 16.48-foot resonant length — this extra length gives you trimming room.
Keep the remaining wire — you may need small trimmings later and it is useful to have the exact same wire type for any extensions if ever needed.
Wind the Current Choke
A 1:1 current choke (current balun) prevents RF from flowing back down the outside of the coax shield — which would cause the feedline to become part of the antenna and produce common-mode problems including RF in the shack, unreliable SWR readings, and distorted radiation pattern.
Wind 8 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 — each pass through counts as one turn. Keep turns tight and consistent. The completed choke should look neat with all turns close together and uniform spacing around the core.
Prepare the Coax End
At the antenna end of the coax run, strip back 2 inches of outer jacket. Carefully fold the braid back over the jacket — do not nick or cut braid strands. Strip 1 inch of inner dielectric to expose the center conductor. Tin both the center conductor and the folded braid with solder — heat the conductor first, then touch solder to it (not to the iron). A shiny, smooth tinned surface indicates a good preparation.
Assemble the Feedpoint
If using a commercial dipole center, follow its instructions — most have two screw terminals on each side (one for the inner conductor, one for the braid). If fabricating your own center from PVC pipe or a plastic project box:
Drill two pairs of small holes, one pair per side of the center. Thread 6-32 stainless machine screws through each pair. One screw on each side connects to the coax — center conductor to one side, braid to the other. The second screw on each side connects the antenna wire.
Strip 1.5 inches of insulation from each wire leg end. Form a loop in the stripped wire using round-nose pliers. Place the loop over the appropriate feedpoint screw and tighten firmly with a nut and lock washer. The connection should be mechanically secure before any solder is applied — the mechanical connection carries the tension; solder only ensures electrical continuity.
Attach End Insulators and Support Rope
At the far end of each wire leg, thread the wire through an egg insulator. Double the wire back on itself about 3 inches and wrap tightly 4–5 times around the main wire. Solder the wrap to prevent unwinding. This creates a secure mechanical attachment to the insulator that will not slip under tension.
Attach a minimum of 18 inches of Dacron rope to each insulator. This rope gap between the insulator and the physical support point (tree, post, stake) is important — it prevents the conductive wire from being detuned by the support material and provides some mechanical isolation from support movement in wind.
Plan the Installation Layout
Before raising anything, walk the site and confirm your three support points (center mast + two ends for flat dipole, or one center mast + two low anchors for inverted-V). For an inverted-V at 14 MHz, target:
- Apex (center): minimum 30 feet, ideally 40–50 feet
- Wire ends: minimum 6 feet above ground to keep clear of people
- Each wire slopes at 30–45° from the apex — this naturally results from a 30-foot apex with ends at 6 feet when the wire is 16.5 feet long
Route the coax to the shack before raising — it is much easier to route coax when everything is at ground level. Leave a drip loop of 12–18 inches in the coax just below the feedpoint — this prevents water from running down the coax and into the connector.
Raise the Antenna
For an inverted-V: raise the center mast first with the feedpoint assembly attached at the top. Run the coax down the mast or alongside it secured with UV ties every 3 feet. Once the center is at height, pull the wire legs out to their end support points and secure. Tension each leg enough that it is taut but not under extreme stress — the wire should not sag significantly but should not be pulled to its breaking strength either.
For a flat dipole: raise the center and one end, then raise the second end. Adjust tension on all three supports until the wire is level and well-supported.
Initial SWR Sweep
With the antenna at final height and the coax run to the shack, connect the NanoVNA at the radio end of the coax. Sweep 13.5 to 15.5 MHz. Look for the frequency of minimum SWR — this is the antenna's actual resonant frequency. Note both the frequency of the minimum and the minimum SWR value.
Expected results with legs cut to 17.0 feet:
- Resonance: 13.8–14.1 MHz (slightly below the target due to long legs)
- Minimum SWR: 1.2 to 1.6:1 (depending on installation height and surroundings)
- SWR at 14.200 MHz: probably 1.5 to 2.5:1 before trimming
Trim to Target Frequency
Each 1 inch trimmed from both legs raises the resonant frequency by approximately 10–15 kHz on 20m. Calculate how much you need to shift resonance and trim accordingly:
Always trim both legs equally. Lower the wire ends enough to safely access them — you do not need to lower the full antenna, just enough to cut and re-secure. After trimming, re-raise the ends and re-measure. Repeat in smaller increments until resonance falls within 25 kHz of the target frequency.
Verify Across the Band
Once resonance is confirmed at the target frequency, sweep the full 20m band from 14.000 to 14.350 MHz. Record SWR at both band edges and at center. A properly tuned 20m dipole at reasonable height should show:
- SWR at 14.000 MHz: 1.5–2.0:1
- SWR at 14.200 MHz: 1.1–1.4:1 (the resonance minimum)
- SWR at 14.350 MHz: 1.5–2.0:1
If any part of the band shows SWR above 2.5:1, the antenna is not resonating correctly — recheck the installation, particularly the current choke and all connections.
Weatherproof the Feedpoint
Moisture is the primary failure mode of outdoor antenna feedpoints. Water wicking into the coax connector or feedpoint connections causes corrosion, increased resistance, and eventually open circuits. Proper weatherproofing:
Starting at the bottom of the feedpoint assembly, wrap self-amalgamating tape upward with 50% layer overlap, covering all exposed connections, the coax entry, and 2 inches of coax jacket below the feedpoint. The tape should be under slight tension as it goes on — this is what causes it to fuse to itself. Apply a second layer of standard PVC electrical tape over the self-amalgamating tape for UV protection.
Document and Make First Contact
Record the final antenna dimensions: exact trimmed leg lengths, installed apex height, end heights, total coax length, resonant frequency, and minimum SWR. Store this in your station logbook. This information is invaluable for future troubleshooting, for rebuilding after storm damage, or for helping other operators build their own version of your antenna.
Connect the radio, confirm SWR on the transceiver's built-in meter, and tune to 14.200 MHz. The 20m band is active around the clock — call CQ or answer a DX station. Your first contact on a home-built antenna is a milestone worth noting in the log.
| Symptom | Likely Cause | First Check | Solution |
|---|---|---|---|
| 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 to coax near feedpoint |
| Resonance too low (below 14.000 MHz) | Wire legs too long | Measure actual leg lengths | Trim both legs equally — 1 inch per side raises resonance ~12 kHz |
| Resonance too high (above 14.350 MHz) | Wire legs too short | Measure actual leg lengths | Splice wire extension onto each leg end and re-tune |
| Minimum SWR not reaching below 1.5:1 | Installation near metal or lossy ground | Is the antenna near metal structure? | Move antenna away from metal; raise apex height; verify choke is working |
| SWR reads differently at different times | Wet or corroded connection | Inspect feedpoint weatherproofing | Re-weatherproof all connections; check for moisture in coax connector |
| RF in shack — mic clicks, hot chassis | Common-mode current on coax | Check current choke quality | Improve choke — more turns, better core material; add second choke at entry to shack |
| Good SWR but no contacts — low signal reports | Antenna too low; wrong direction | What is the apex height? | Raise the apex; orient the wire broadside toward target DX region |
Why Height Matters More Than Almost Anything Else
The single biggest improvement available to most 20m operators is raising the antenna. A dipole at 50 feet significantly outperforms the same dipole at 25 feet for DX operation — the improvement is real, measurable, and has no cost beyond the support structure.
If your apex is currently at 25 feet and you can raise it to 40 feet, that improvement is worth more than any other single station change — including doubling the transmitter power. Make the antenna as high as available supports allow before investing in any other station improvements.
Orientation for DX
A horizontal dipole has its maximum radiation broadside to the wire — perpendicular to the direction the wire runs. The nulls (deep signal minima of 20–30 dB) are off each wire end. This means orientation matters for fixed-direction DX paths:
- To work Europe from the US east coast: orient the wire north-south (broadside = east-west)
- To work Japan from the US west coast: orient the wire north-south (broadside = east-west)
- To work South America from the US: orient the wire east-west (broadside = north-south)
- An inverted-V is more omnidirectional than a flat dipole — orientation matters less but still has some effect
- If you cannot orient the wire for your primary DX direction, an inverted-V at high height is the best practical compromise
For casual multi-direction operation, orientation matters less. The nulls are deep but most DX paths are not exactly in the null direction. A well-installed dipole in any orientation at 40+ feet makes real DX contacts regularly.
What is the best height for a 20m dipole?
Higher is always better for DX on 20m. The practical target is to get the feedpoint as high as available supports allow. At 30 feet, the antenna works well for regional and moderate DX paths. At 50 feet it becomes competitive for long-path DX. At 65 feet (one full wavelength on 20m) the takeoff angle drops to around 10 degrees — excellent for long-path DX. If you can only achieve 20–25 feet, the antenna still works — it simply favors higher-angle (shorter-range) propagation paths. Any height is better than no antenna.
Do I really need a current choke on a dipole?
Yes — strongly recommended. Without a current choke, RF current flows on the outside of the coax shield. This causes the feedline to become part of the antenna, distorting the radiation pattern, making SWR measurements unreliable (SWR changes when you move the coax), and causing RF to appear in the shack — hot mic, clicking audio, potentially affecting equipment. Many operators run without a choke for years and notice the improvement immediately when they add one. An 8-turn FT-240-31 choke at the feedpoint is inexpensive, easy to build, and makes the antenna behave as intended.
Current choke guide →Can I use a 20m dipole on other bands?
Yes, with a tuner. A 20m dipole will resonate naturally on 10m (the 2nd harmonic) — SWR on 10m will be low without a tuner. On 15m, 17m, 12m, and 40m, the SWR will be high and a tuner is needed. If using a shack tuner with coax, expect higher feedline losses on bands where the SWR is high — this is acceptable for casual use but not ideal for regular multi-band operation. For dedicated multi-band use without feedline losses, add ladder line from the feedpoint to a balanced tuner, or consider a fan dipole or OCFD instead.
What wire gauge should I use?
#14 AWG stranded copper-clad steel (CCS) is the best choice for a permanent outdoor dipole. CCS is stronger than pure copper — it resists the stretching and sagging that occurs with heavy copper wire over long spans and temperature cycling. Stranded is more flexible and fatigue-resistant than solid, which becomes brittle with repeated bending at the end support points. For portable or temporary installations, #18–22 AWG pure copper or CCS wire saves weight. Avoid aluminum wire — it corrodes rapidly at the connections and is significantly harder to solder.
Why is my SWR different on 20m vs what the NanoVNA showed?
The NanoVNA and the transceiver's built-in SWR meter can show different readings for several legitimate reasons. NanoVNA readings at the end of a coax run can be affected by the coax length (the impedance transformation effect of the transmission line). If the coax is a specific length relative to the wavelength, it rotates the impedance and the NanoVNA may show a different value than the antenna's actual feedpoint impedance. This is normal — what matters is that the minimum SWR falls within the operating band and the transceiver's built-in meter shows acceptable SWR when transmitting. Most modern transceivers accept 2:1 or better SWR without any protection circuit activation.
How do I know if my antenna is actually working?
The Reverse Beacon Network (RBN) is the best free tool for verifying antenna performance. Transmit a brief CW signal on 20m (even just your callsign twice) and check the RBN website — automated CW receivers worldwide will spot your signal and report the SNR. Compare your SNR reports to other stations being spotted at the same time from the same geographic area. The WSPR network (Weak Signal Propagation Reporter) is another excellent tool — run WSPR for several hours and compare your spot count and average SNR to reference stations. These tools provide objective, quantitative antenna performance data without needing another station to cooperate.
WSPR antenna testing guide →