Build a Portable EFHW Antenna for SOTA and POTA
The end-fed half-wave (EFHW) antenna has become the dominant portable HF antenna for SOTA and POTA operators worldwide — and for good reason. A single wire hung from one point, fed through a compact 49:1 impedance transformer, covers 40m, 20m, 15m, and 10m simultaneously with no band changing, no link connectors to manage, and no tuner required on most bands. A full-length 40m EFHW wire at 66 feet long can be sloped from a single tree, deployed as a sloper, or strung at low height as an NVIS antenna — the single-point hang makes it the most terrain-adaptable portable HF antenna available. This guide builds a complete SOTA-ready EFHW system from a hand-wound 49:1 transformer, lightweight wire, and field deployment hardware that packs to under 150g.
End-Fed Half-Wave Fundamentals
A half-wave dipole has a low feedpoint impedance (~75 Ω) at its center and a very high impedance (~2500–5000 Ω) at each end. The EFHW exploits this high end impedance — it feeds the antenna at one end rather than the center, requiring an impedance transformer to match the high end impedance to the 50 Ω coaxial feedline:
The 49:1 Transformer — Why It Must Be Built Correctly
The 49:1 transformer is the heart of the EFHW system. A poorly wound transformer produces high SWR on all bands and can run hot from core losses. The core material selection is the most critical design decision:
Wire Length and Harmonic Coverage
The EFHW wire length sets which band is the fundamental resonance and which bands are covered by harmonics. For a 40m through 10m system, the wire is cut for 40m fundamental:
The Counterpoise — Often Misunderstood
The EFHW transformer's secondary connects to the antenna wire. The primary connects to the coaxial feedline. But the coax shield needs somewhere to connect — it serves as a partial ground reference for the transformer. Some installations work without a counterpoise; most work better with a short one:
- No counterpoise: the coax braid becomes the effective counterpoise, coupling RF to the feedline outer surface and potentially into the shack. SWR may be acceptable but the radiation pattern is compromised and RF can appear in the operating position.
- Short counterpoise wire: a 17-foot (5.2m) wire connected to the transformer ground terminal and hanging or lying on the ground beneath the antenna. This is λ/4 at 14 MHz — adequate counterpoise for 20m through 10m operation. For 40m, 33 feet (10m) is a better counterpoise length.
- Practical SOTA/POTA solution: carry a 17-foot counterpoise wire and clip it to the transformer ground terminal. On 40m, the feedline serves partly as counterpoise. On 20m–10m, the 17-foot counterpoise is very effective. Many commercial EFHW transformers include a terminal for counterpoise connection.
- Ground connection: at a drive-up POTA site with a vehicle, connect the transformer ground terminal to the vehicle chassis — the vehicle body makes an excellent low-impedance counterpoise for all HF bands.
| Core | Primary turns | Secondary turns | Wire (primary) | Wire (secondary) | Max power | Best bands |
|---|---|---|---|---|---|---|
| FT-140-43 | 2 turns | 14 turns | #26 AWG enamel | #26 AWG enamel | 10W QRP | 40m–10m |
| FT-240-43 | 2 turns | 14 turns | #22 AWG enamel | #24 AWG enamel | 100W | 40m–10m |
| FT-240-31 | 3 turns | 21 turns | #22 AWG enamel | #24 AWG enamel | 100W | 80m–10m |
| Two FT-240-43 stacked | 2 turns | 14 turns | #20 AWG enamel | #22 AWG enamel | 200W+ | 40m–10m |
Portable Efhw Calculator
This design has published dimensions for more than one band, or this page has no dedicated dimensions table this script could confidently locate. The default shown below is the most frequently cited frequency on the whole page -- verify it before trusting the result.
Materials for a complete 40m–10m EFHW system with 49:1 transformer, lightweight wire, and field hardware
Building the Portable EFHW System
The EFHW build has two distinct phases: winding and testing the 49:1 transformer, and cutting and trimming the antenna wire. Build and fully verify the transformer before cutting any wire. A faulty transformer is harder to diagnose after the wire is trimmed — verify transformer performance on a known-good resistive load first, then connect the antenna wire.
Wind the Primary Winding (2 Turns)
The primary winding is 2 turns of #22 AWG enamel magnet wire through the FT-240-43 toroid core. The primary connects to the coaxial feedline — the 50 Ω side of the transformer:
Wind the Secondary Winding (14 Turns)
The secondary winding is 14 turns of #24 AWG enamel magnet wire on the same core. It occupies the remaining 3/4 of the toroid circumference. The secondary connects to the high-impedance antenna wire:
Mount Toroid in Enclosure and Connect All Terminals
Mount the wound toroid inside the ABS enclosure and make all electrical connections:
Verify Transformer with Resistive Load Before Connecting Antenna
Before cutting any wire, verify the transformer performance using a resistive dummy load. This confirms the winding is correct before the antenna wire is committed:
Connect Antenna Wire and Counterpoise
Cut the #26 AWG antenna wire to 67 feet. Connect one end to the ANTENNA terminal on the transformer. Connect the 17-foot counterpoise wire to the GND terminal. Both wires attach with the binding posts or terminal screws — no soldering needed if using proper binding post terminals:
Field Deployment and Initial Tuning
Deploy the EFHW as a sloper — transformer end elevated at the highest available point (tree limb, sotabeams mast, or any elevated support), with the antenna wire running upward and then sloping down to a far anchor point at ground level:
Final Verification and Pack-Out Preparation
After trimming, verify SWR on all four bands with the antenna in its final deployed configuration. Record the SWR values and final wire length for future reference:
| Characteristic | EFHW (this guide) | Linked Dipole (previous guide) |
|---|---|---|
| Band changing | No action — all 4 bands always available | Walk to links and disconnect for each band |
| Support points needed | 1 (single-point hang at transformer) | 1 center point + 2 end anchors |
| Tuner required | Usually not (ATU handles residual SWR) | Never — resonant on each band |
| Weight (complete system) | ~130g | ~200g |
| Build complexity | Higher (transformer winding required) | Lower (wire + connectors) |
| Balanced/Unbalanced | Unbalanced — needs counterpoise | Balanced — feedpoint balun required |
| Best deployment | Sloper from single tree | Inverted-V from single tree |
| 15m performance | Slightly lower — 3rd harmonic issue | Resonant — excellent on 15m |
| Build cost | ~$18 | ~$22 |
| Terrain flexibility | Highest — one anchor point | Good — center + two ends |
| Symptom | Most likely cause | Diagnosis | Fix |
|---|---|---|---|
| High SWR on all bands (above 5:1 everywhere) | Windings phased incorrectly — opposite winding directions | Measure with 3300 Ω load — if SWR still very high, phasing is wrong | Reverse one winding by swapping its two terminal connections; re-test with resistive load |
| Transformer runs hot after 1–2 minutes of transmit | Core losses excessive — wrong core type or too few turns | Touch transformer box after 30 seconds of 100W transmit — warm is OK, hot means core loss | Add a second FT-240-43 core (stack them); or switch to type 31 core for better low-frequency performance |
| 40m SWR acceptable but 20m SWR very high (above 5:1) | Antenna wire length not a half-wave multiple on 20m — minor construction error | Verify wire length is 66–67 ft; measure with steel tape | Trim or extend wire to exact calculated length; small deviations produce large SWR on harmonic bands |
| SWR varies dramatically when feedline is moved | No counterpoise — common-mode current on feedline braid | Move feedline during measurement — SWR change confirms common-mode | Connect 17-ft counterpoise wire to GND terminal; lay on ground in any direction |
| Good SWR on 40m and 20m but very poor on 15m | Normal EFHW behavior on 3rd harmonic — transformer presents higher loss here | This is expected — 15m is the least efficient EFHW band | Use radio's built-in ATU for 15m; or add 100–470 pF series capacitor near transformer for 15m improvement |
| RF burns at operating position during transmit | RF on feedline due to missing or inadequate counterpoise | Touch microphone during transmit — if it tingles, RF is in the shack | Improve counterpoise; add ferrite beads to feedline at transformer end; ensure feedline is at least 15 ft long before entering shack |
Does the EFHW really work without a tuner?
On 40m and 20m, yes — a properly built 49:1 transformer with the correct wire length produces SWR below 2:1 on both bands, which most HF radios transmit through without any ATU. On 15m the SWR is typically 2–4:1 due to the 3rd harmonic impedance behavior — the radio's internal ATU handles this on most rigs. On 10m performance varies with deployment height and counterpoise effectiveness. For pure QRP operation (5–10W) into any SWR up to 5:1, almost any radio operates without a tuner. For 100W operation, the radio's internal ATU combined with the EFHW covers all four bands reliably in most field conditions without a separate ATU in the kit.
Can I wind the transformer on two stacked toroids?
Yes — stacking two FT-240-43 cores and winding both together as a single core increases power handling capacity from approximately 100W to 200W or more, and slightly reduces core saturation effects at high power on 40m. The winding procedure is identical — simply slide both toroids together and wind through both simultaneously as if they were one core. The total number of turns remains the same (2 primary, 14 secondary). Stacked cores add approximately 30–40g to the transformer weight — a trade-off that is worthwhile for a contest or DX-focused portable station but unnecessary for typical 5–100W SOTA/POTA operation.
What happens if the wire is not exactly the right length?
The EFHW is moderately tolerant of wire length errors on the fundamental (40m) band — a ±2-foot error shifts the 40m resonance by approximately 150–200 kHz, keeping it within the band. The harmonic bands (20m, 15m, 10m) are less tolerant — a wire that is resonant at exactly 7.1 MHz for 40m produces harmonics at exactly 14.2, 21.3, and 28.4 MHz. A wire that is 2% too long produces harmonics that are 2% too low in frequency — on 20m this shifts the resonance to 13.9 MHz (just below the band), producing poor SWR across 14.0–14.35 MHz. The NanoVNA trimming process corrects this — the target is to get the 40m fundamental resonance at or just above 7.0 MHz, which ensures the harmonics fall within the other amateur bands.
Should I buy a commercial EFHW or build my own?
Commercial EFHW transformers (SOTAbeams, PAR, Chelegance, and many others) cost $30–80 and are pre-tested, weatherproofed, and guaranteed to work. The homebrew transformer in this guide costs $8–12 in materials but requires care in winding and verification. For a first EFHW build, the homebrew approach is recommended for the learning value — winding a toroid transformer and understanding why it works is one of the most instructive experiences in amateur antenna building. For a second or third unit, or for a high-reliability backup antenna for a DXpedition or remote operation, buying a commercial transformer and using your homebrew wire is a reasonable combination that gets the best of both approaches.
How is the EFHW different from a random wire antenna?
The EFHW is a resonant antenna — it is cut to a specific length (half-wave on the fundamental band) and the 49:1 transformer matches its high end impedance to 50 Ω coax. Performance on each resonant band is high and predictable. A random wire antenna is any length of wire fed through a general-purpose ATU that matches the random impedance to the radio — it works on any frequency the ATU can match but is never optimally resonant on any specific band. The random wire approach is more flexible (works on any HF band including WARC bands) but requires a bulkier ATU and produces varying efficiency depending on how well the ATU can match the antenna on each frequency. The EFHW is more efficient per watt on its resonant bands and requires no ATU box, making it lighter for SOTA summit pack-in.
Can I use the EFHW for 80m by making the wire longer?
Yes — a 128-foot wire produces a fundamental resonance at 3.65 MHz (80m) with harmonics at 40m, 20m, 15m, and 10m. This 5-band EFHW is very popular for POTA drive-up sites where packing weight is not a concern. For SOTA summit pack-in, 128 feet of wire is manageable in a lightweight coil but represents more than twice the weight and pack volume of the 40m version. Some SOTA operators carry the 40m EFHW wire and a 62-foot 80m extension piece with a connector — connecting the extension when 80m operation is the goal and leaving it off for lighter 40m through 10m operation. This hybrid approach avoids carrying full 80m wire length on every activation.