Build an EFHW Antenna for SOTA and POTA
The end-fed half-wave (EFHW) antenna has become the dominant portable HF antenna among SOTA and POTA activators — and for good reason. A single 40m half-wave wire, 66 feet long, is resonant not only on 40m but also on 20m, 10m and other harmonically related bands, all fed from one end through a 49:1 unun transformer that transforms the wire's 2,450 Ω feedpoint impedance down to 50 Ω for direct coax connection. One support point rather than two, one wire rather than a dipole, and coverage of three to five bands without any reconnection or link adjustment — the EFHW achieves multi-band operation with a simplicity that no other portable antenna matches. This guide covers EFHW theory, the 49:1 unun transformer winding on an FT-140-43 toroid, wire length calculations for 40m through 10m coverage, the counterpoise requirement, deployment configurations, and field operating for SOTA and POTA activations.
Why the EFHW Works on Multiple Bands
A half-wave dipole fed at its centre presents approximately 70 Ω. The same dipole fed at its end — where current is zero and voltage is maximum — presents a very high impedance of 2,000–5,000 Ω. The 49:1 unun transformer steps this high impedance down to approximately 50 Ω for coax. The multi-band magic comes from harmonics:
The 49:1 Unun Transformer — Core of the EFHW
The 49:1 unun (unbalanced-to-unbalanced) transformer is what makes the EFHW practical. It transforms the 2,450 Ω end-fed impedance down to 50 Ω. The ratio 49:1 comes from the impedance transformation ratio of a 7:1 voltage transformer — (7)² = 49. Building this transformer correctly is the most critical step in the entire EFHW build:
The Counterpoise — Essential but Often Misunderstood
The EFHW is end-fed, which means the RF return current must flow somewhere. On a centre-fed dipole, return current flows in the other leg of the dipole. On an EFHW, the return current path is the coax outer shield — which means RF flows on the coax into the radio unless a counterpoise provides an alternative path:
EFHW vs Linked Dipole — When to Choose Each
The EFHW and linked dipole are the two most popular SOTA/POTA portable HF antennas. Understanding the trade-offs helps choose the right antenna for each activation:
- EFHW advantages: one support point (feedpoint end only); no band changing required; simpler deployment (one wire, one end); slightly faster to set up and take down; works better when only one tree or one mast is available.
- Linked dipole advantages: no matching transformer to wind or fail; better SWR on all bands (resonant on each band); includes 17m (EFHW does not cover 17m natively); better on bands away from the harmonic fundamentals; no counterpoise required.
- Choose EFHW when: only one support point is available; fastest possible deployment is needed; 17m coverage is not required; you prefer one wire to manage.
- Choose linked dipole when: 17m is important; you want the lowest possible SWR on every band; you have two support points available and want optimal performance; the transformer winding is unappealing.
- Combined approach: many experienced SOTA operators carry an EFHW for quick activations and a linked dipole for extended operations where working 17m or having perfect SWR on all bands matters.
| Configuration | Wire length | Bands covered | Notes |
|---|---|---|---|
| 40m EFHW (standard SOTA) | 66.0 ft (20.1 m) | 40m, 20m, 15m, 10m | Most popular — covers the four main SOTA/POTA bands |
| 40m EFHW (slightly extended) | 67.3 ft (20.5 m) | 40m, 20m, 15m, 10m, 17m* | *17m coverage marginal — SWR 2.5:1–4:1; ATU required for 17m |
| 80m EFHW | 130.0 ft (39.6 m) | 80m, 40m, 20m, 15m, 10m | Long wire for parks; too long for most summits; covers 5 bands |
| 20m EFHW (ultralight SOTA) | 33.0 ft (10.1 m) | 20m, 10m | Very short and light; only two bands; good for high-sun-angle DX |
| Counterpoise (40m EFHW) | 16–33 ft (5–10 m) | All bands | One or more wires; not resonant — any length from 5m useful |
Cut wire 3–5% longer than the values shown and trim to resonance at 7.050 MHz on 40m using NanoVNA. The resonant length depends on wire diameter, height above ground, and nearby objects — always trim in the field. The harmonic bands (20m, 15m, 10m) do not require separate trimming — they track the 40m length automatically.
Materials for a 40m EFHW with 49:1 unun covering 40m, 20m, 15m, and 10m
Building the 40m EFHW with 49:1 Unun
The 49:1 unun winding is the most technically demanding part of this build — but it takes under 30 minutes once the materials are on hand. The wire preparation is identical to any other portable dipole. The complete EFHW is faster to build than a linked dipole because there are no link connectors to solder.
Wind the 49:1 Unun Transformer
Wind the 49:1 autotransformer on the FT-140-43 toroid core. This is a two-winding autotransformer where the primary (2 turns) and secondary (14 total turns) share the same continuous wire. The toroid's ferrite material provides the magnetic coupling that makes the transformer work:
Verify the Transformer Ratio with NanoVNA
Before installing the transformer in the enclosure, verify the impedance transformation ratio with the NanoVNA. A correctly wound 49:1 unun with 14:2 turns on Mix 43 core should transform a 50 Ω load to approximately 2,450 Ω at the wire port:
Install Transformer in Enclosure and Wire Connections
Install the wound toroid and all external connectors in the weatherproof ABS enclosure. The enclosure protects the transformer from rain and moisture — essential for an antenna used repeatedly in field conditions. Drill holes for the SO-239, wire binding posts, and a small strain-relief hole for the coax:
Attach the EFHW Wire and Counterpoise
Connect the 70-ft EFHW wire to the red binding post and the counterpoise wire(s) to the black binding post. At the far end of the EFHW wire, attach an end insulator with a paracord loop for tree or stake support.
For the counterpoise, cut one or two wires of 16 ft (5 m) each. These lie on or near the ground during operation — they do not need to be elevated or supported. A dedicated counterpoise wire that is part of the antenna kit (rather than improvised in the field) ensures consistent operation across all activations:
Deploy and Trim Wire Resonance on 40m
Deploy the EFHW in a sloper configuration — feedpoint at the mast top (6–7m) and wire sloping down at a 30–45° angle to a ground anchor or low tree branch at the far end. This is the most common SOTA deployment: the unun enclosure hangs from the mast top via a small loop of paracord, the wire slopes away from the mast, and the coax and counterpoise drop from the unun at the base of the mast.
Connect the NanoVNA to the SO-239 and sweep 6.5–7.5 MHz to find the 40m resonance. Trim the wire from the far end in 6-inch increments until the SWR minimum falls at your target frequency:
Wind on Storage Card and Pack
Wind the EFHW wire onto a winding card for tangle-free storage and deployment. Unlike the linked dipole with five separate segments, the EFHW is one continuous wire — winding and unwinding is very fast. The unun enclosure, winding card, coax, and counterpoise all fit in a small pouch:
Sloper vs Inverted-L vs Near-Vertical
The EFHW wire can be deployed in several configurations from a single mast support point. Each has different radiation pattern characteristics:
- Sloper (most common — 30–45° from vertical): feedpoint at mast top, wire slopes down and away to a ground anchor. Good combination of height and horizontal extension. Radiation pattern is asymmetric — more gain in the downhill direction of the slope. For SOTA, pointing the wire toward the direction of most chasers maximises contacts.
- Inverted-L (L-shaped deployment): wire goes straight up from the mast top for 10–15 ft (using a top section of the mast or a short fibreglass pole extension), then bends horizontal. The vertical section provides some low-angle radiation while the horizontal section provides NVIS coverage. Excellent for 40m regional contacts.
- Near-vertical (steep sloper): wire hangs nearly vertically from the mast top with minimal horizontal distance. This creates an almost vertical antenna with low radiation angle — good for DX on 20m and 10m from summits with limited horizontal space. The mast must be quite tall (8–10m) for this to work well on 40m.
- Tie to tree: with a tall nearby tree, the feedpoint end can go on the mast and the wire far end ties high in the tree — giving a near-flat horizontal configuration at 8–10m height. This is the best performing deployment for DX on 20m, 15m, and 10m from wooded POTA locations.
SOTA and POTA Operating with the EFHW
The EFHW's multi-band capability without band changing makes it extremely convenient for SOTA/POTA operating:
- Start on 40m: in the morning from a European or North American summit, 40m provides the most reliable contacts within 300–1,000 km. Self-spot on SOTAwatch or POTA.app and work chasers for 15–20 minutes before moving to higher bands.
- Move to 20m without touching the antenna: simply QSY the radio to 14.xxx MHz. The EFHW is resonant on 20m and the SWR is acceptable — no link changing, no adjustment. This is the EFHW's signature advantage: band changes are instant.
- Check 15m and 10m: during solar maximum, 15m and 10m can be spectacular from summits. The EFHW covers both. Check the bands before leaving the summit — opening conditions change quickly and 10m intercontinental contacts at 5W from a SOTA summit are memorable.
- FT8 across all bands: the EFHW lends itself to FT8 multi-band operation — a 10-second transmit cycle means you can work multiple bands efficiently. With FT8 auto-sequencing and the EFHW covering all four bands without touching the antenna, a single activation can yield contacts on all four bands with minimal operator effort.
- 17m gap: if the 17m band is open and you want to work it, you need an ATU to match the EFHW's off-resonance impedance on 17m. A small QRP ATU (LDG Z-817, YouKits FG01) solves this but adds weight. Most SOTA operators accept the 17m gap rather than carry the extra weight.
| Symptom | Most likely cause | Diagnosis | Fix |
|---|---|---|---|
| Very high SWR on all bands — NanoVNA shows near-open | EFHW wire not connected to unun secondary (wire binding post open or loose) | Check wire connection at red binding post; measure continuity from SO-239 centre through transformer to wire end | Reconnect wire to binding post; tighten post if loose; re-solder if connection has failed |
| RF feedback in radio audio — RF in shack | Missing or inadequate counterpoise; RF flowing on coax shield | Connect counterpoise and observe whether feedback disappears; touch coax — if tingle is present, common-mode is on the coax | Deploy counterpoise(s) fully; add second counterpoise wire; move counterpoise direction to be roughly opposite wire direction |
| 40m SWR minimum shifted 200+ kHz above target | Wire is too short — was trimmed too aggressively | Measure wire length and compare to original tuned length | Splice additional wire at the tip using a short overlap and solder connection; add enough to bring resonance back to target frequency |
| 40m SWR minimum far below 7 MHz — resonance below band | Wire is too long — common on first deployment before trimming | Expected with un-trimmed wire; trim 6 inches at a time from wire tip while monitoring NanoVNA | Trim progressively from wire tip in 6-inch increments until minimum rises to 7.050–7.150 MHz |
| 20m SWR very high — above 4:1 | 40m wire length is slightly wrong; 20m is sensitive to the exact 40m resonant length | Check 40m resonance first — if 40m SWR is correct, 20m should be acceptable | Optimise 40m resonance to 7.050–7.100 MHz; 20m SWR automatically improves; if still high on 20m, trim 0.5 inch from wire tip and re-measure 40m |
| Unun gets warm during operation | Core saturation from too much power; or wrong core material | Touch the enclosure during operation at 5W — slight warmth is normal; hot means a problem | Verify core is FT-140-43 (Mix 43); the FT-140-43 handles 25W continuous on SOTA QRP easily; at 100W, use a larger core (FT-240-43) |
Is the EFHW as good as a dipole on each band?
On 40m, a well-deployed EFHW is essentially equal to a centre-fed dipole at the same height. On the harmonic bands (20m, 15m, 10m), the radiation pattern and impedance are slightly different from a centre-fed dipole — the EFHW has higher current near the feed end at even harmonics, which shifts the pattern slightly toward vertical rather than horizontal. In practice, the difference is under 1–2 dB and is not operationally significant at QRP power levels. Many SOTA operators have made contacts on all four bands at QRP with an EFHW that they would not expect to be possible with any antenna — the bands and height above ground matter far more than the 1 dB performance difference between antenna designs.
Can I use the EFHW without a counterpoise?
You can, but the results are often poor — RF feedback in the radio audio, unreliable SWR, and transmitted signals that sound distorted at the receiving end. The counterpoise takes 30 seconds to deploy (just lay it on the ground) and costs nothing in terms of added complexity. Omitting the counterpoise to save a few grams is a false economy. The one scenario where it sometimes works without a counterpoise is when the coax run is very long (20+ feet) and the coax itself acts as the return path — but this is inconsistent and not recommended. Always carry and deploy the counterpoise.
What power level can the 49:1 unun handle?
An FT-140-43 based 49:1 unun handles 25W continuous without thermal concern — well above the 5W QRP standard for SOTA. For POTA operations at 100W, use a larger core: the FT-240-43 (same mix, larger diameter) handles 100W+ continuously. At 5W SOTA power, even an FT-82-43 (smaller, lighter) handles the power comfortably, but the FT-140-43 is the standard recommendation because it provides adequate power handling with minimal weight penalty and is available at most ham radio suppliers.
Why is the transformer ratio 49:1 and not some other value?
The 49:1 ratio comes from the impedance at the end of a half-wave wire, which is approximately 2,450 Ω when installed at typical heights above ground. Dividing by 49 gives 50 Ω. The 49:1 ratio is achieved with a 7:1 voltage turns ratio (7² = 49), implemented as a 14:2 winding. Some builders use a 64:1 (8:1 turns, 16:2 winding) which works better on 40m at low heights where the impedance is higher, or a 36:1 (6:1 turns, 12:2 winding) which works better on 20m and 10m. For a general-purpose SOTA EFHW covering 40m through 10m, the 49:1 ratio is the best compromise — validated by thousands of builders and the standard choice.
Can I operate WSPR with an EFHW for propagation monitoring?
Yes — many operators leave a WSPR transmitter running with an EFHW during activations to demonstrate propagation conditions from the summit. WSPR at 200mW from a SOTA summit with an EFHW frequently produces spots on all four bands simultaneously — 40m regional, 20m transatlantic, 15m and 10m intercontinental during solar maximum. WSPR spots from SOTA summits are archived and frequently referenced by propagation researchers. Running WSPR during an activation is worthwhile from a scientific contribution standpoint as well as being personally interesting.
How do I know if my unun is working correctly without a NanoVNA?
The simplest field test: connect the EFHW to the unun, deploy the wire, and measure the SWR from the radio. On 40m, if the SWR reads 1.5:1 to 2.5:1 without an ATU, the unun is working. If SWR is above 5:1 on all bands, the transformer has likely failed or the wire is not connected. If you have a signal generator and a portable receiver, you can check the transformer by injecting a signal at the coax end and measuring it at the wire end — the transformer should pass signal efficiently. The NanoVNA is the best tool for verification, but the radio's built-in SWR meter gives a functional indication that the system is working at the operating frequency.