Build a Portable EFHW Antenna for SOTA & POTA
The portable end-fed half-wave (EFHW) is the antenna that transformed SOTA and POTA operating. Thirty-three feet of lightweight wire on a card winder plus a matchbox-sized 49:1 UNUN weighs under 100 grams, fits in a jacket pocket, deploys in three minutes, and covers 40m, 20m, 15m, and 10m without a tuner. This guide builds the complete portable EFHW system from scratch — including the UNUN, choosing the right wire, setting up the winder, adding the counterpoise, and the specific field deployment technique that makes this antenna work reliably on every activation.
Wire Selection — The Biggest Weight Decision
Wire choice is the most impactful weight decision in building a portable EFHW. The options span a 10:1 weight range for the same electrical length, with corresponding trade-offs in durability and handling:
For dedicated SOTA backpacking where every gram matters: #26–28 AWG PTFE-insulated stranded wire. PTFE insulation is the lightest available, has excellent UV resistance, and the stranded construction handles repeated coiling without fatigue. The trade-off is that very thin wire (28 AWG) breaks more easily and requires care not to kink it sharply during deployment.
For POTA and general field operation where durability matters more than gram counts: #22–24 AWG stranded copper or CCS. Easier to handle, more durable, still very light compared to a home-station antenna.
UNUN Core Selection
The 49:1 UNUN is the heart of the EFHW — it transforms the high feedpoint impedance of the end-fed wire to 50Ω for the coax connection. For a portable build, the core choice determines both size and performance:
- FT-82-43 toroid: 0.825" OD — very compact, 5–7g. Handles 5–10W continuously. Ideal for QRP SOTA builds. Too small for 25W+ operation.
- FT-140-43 toroid: 1.400" OD — medium size, 15–20g. Handles 25–50W. Good all-around portable choice.
- FT-240-43 toroid: 2.400" OD — full size, 45–55g. Handles 100W+. Best for fixed installations; heavier than necessary for backpack SOTA.
For most SOTA operators running 5–10W: the FT-82-43 or FT-140-43 is the right core. For POTA operators running up to 100W from a vehicle: the FT-240-43 is better. Building two versions — a tiny QRP core for summit work and a larger core for fixed or vehicle POTA — is a common approach for operators who do both.
Enclosure Options by Weight
The UNUN enclosure protects the toroid and connections from field conditions. Enclosure options for portable builds:
- Altoids tin: ~25g empty, widely available, naturally shielded. Drill holes for SO-239 and wire connection. Fits an FT-82-43 or FT-140-43 easily. The classic SOTA UNUN enclosure.
- 3D-printed ABS box: 8–15g depending on wall thickness. Custom-sized for the core. Many free designs available on Thingiverse. Excellent choice if you have access to a 3D printer.
- Hammond 1590A (smallest die-cast): 45g — weatherproof aluminum, professional appearance. Better for POTA than summit hiking due to weight.
- Heat-shrink wrapped: 2–5g — wrap the wound toroid and connections entirely in several layers of heat-shrink tubing. No enclosure at all — the lightest possible option. Provides basic moisture and mechanical protection. Popular among ultralight SOTA operators.
- Small project box (plastic): 15–25g — ABS plastic box from electronics suppliers. Lighter than metal, inexpensive, adequate for most conditions.
Connector and Coax Choices
For a truly portable system, the coax connection at the UNUN should match the radio's antenna connector. Options:
- BNC connector: the best portable choice — quick-disconnect, lighter than PL-259/SO-239, excellent for field conditions where the connector is connected and disconnected repeatedly. Most portable radios (KX2, KX3, MTR, FT-818) use BNC.
- SMA connector: lightest of all — used on QRP rigs like the Elecraft KX1 and some SDR-based portable stations. Requires careful handling.
- PL-259/SO-239: heavier but universal. Use if the radio uses an SO-239 and you want to avoid an adapter.
- Coax for the pigtail: RG-174 is lightest but highest loss — fine for the 12–18 inch pigtail between UNUN and radio coax. RG-58 for the radio coax run if under 20 feet; RG-8X for longer runs.
Keep the coax pigtail from the UNUN to the first adapter short — 12 to 18 inches is ideal. The radio coax run from the operating position to the antenna base is separate and its length depends on the site layout.
| Component | Ultralight Build | Standard Build | Heavy-Duty Build | Notes |
|---|---|---|---|---|
| Antenna wire (33 ft) | #28 PTFE — 12g | #24 PTFE — 28g | #18 CCS — 90g | Biggest weight variable |
| UNUN toroid core | FT-82-43 — 5g | FT-140-43 — 15g | FT-240-43 — 50g | Size determines power handling |
| UNUN enclosure | Heat-shrink — 3g | Plastic box — 20g | Hammond 1590B — 85g | Altoids tin: ~25g |
| Wire winder | Card winder — 5g | Plastic spool — 15g | Large reel — 40g | 3D printed options: 8g |
| Counterpoise (2 ft) | 24 AWG wire — 3g | 20 AWG wire — 5g | 18 AWG wire — 8g | Short is fine for portable |
| BNC pigtail (18 in) | RG-174 — 8g | RG-58 — 15g | RG-8X — 25g | Short pigtail, keep light |
| Attachment clip | Carabiner — 5g | Carabiner — 10g | Stainless hook — 15g | For attaching to pole tip |
| Total | ~41g | ~98g | ~313g | Ultralight fits in a shirt pocket |
Standard portable EFHW for SOTA and POTA — ~100g total
Building the Portable EFHW System
Build the UNUN first, test it, then cut and tune the wire. Total bench time approximately 2 hours. The result fits in the palm of your hand.
Prepare the Toroid Core
The FT-140-43 toroid is a donut-shaped ferrite core approximately 1.4 inches in outer diameter. It may have sharp edges from the manufacturing process — run a piece of fine sandpaper around the inner and outer edges to remove any burrs that could cut the winding wire. Apply a strip of electrical tape around the outside of the core (not the hole) to cushion the wire and prevent it from being cut by a remaining sharp edge.
Wind the Secondary (14 Turns)
Cut approximately 36 inches of #28 AWG enameled wire for the secondary. Thread one end through the toroid hole and pull through — this is turn 1. Continue passing the wire through the hole and around the outside of the core. Count carefully — 14 complete passes through the hole = 14 turns. Keep each turn snug against the previous one; spread them evenly around no more than 270° of the core (leave the remaining 90° for the primary). Leave 3-inch leads at each end.
Use a permanent marker to mark the start end of the secondary — this end will make the critical connection to the primary. Label it "S-start".
Wind the Primary (2 Turns)
Cut approximately 8 inches of #28 AWG enameled wire for the primary. Pass the wire through the core hole twice — this is 2 turns. Leave 3-inch leads at each end. The primary occupies the remaining 90° of core space not used by the secondary. Keep the primary turns snug and in the same winding direction as the secondary.
Scrape and Tin All Wire Ends
Enameled wire has an insulating lacquer coating that must be removed before soldering. Use fine sandpaper (400 grit) or a sharp knife to scrape the enamel from the last 1 inch of each wire lead — all four leads from the primary and secondary. Tin each scraped end with solder: heat the wire end and touch solder to the wire (not to the iron). A properly tinned end should be bright and shiny with solder flowed fully around the wire circumference.
Make the Key Autotransformer Connection
The 49:1 UNUN is wired as an autotransformer. The connection topology:
- Primary hot end (P-hot) → BNC center conductor (coax side)
- Primary ground end (P-gnd) → BNC outer shell (coax braid / chassis)
- Secondary start (S-start) → connects to P-hot (this is the key junction)
- Secondary end (S-end) → antenna wire terminal (high impedance side)
The S-start to P-hot connection is what creates the autotransformer. Solder this connection cleanly — it is the most electrically critical point in the UNUN. After soldering, verify: BNC center to antenna terminal should read low but measurable resistance (the 14-turn secondary winding); BNC center to BNC outer should read open (infinite resistance — no short through the core).
Add the Optional 100 pF Capacitor
A 100 pF ceramic disc capacitor (rated 500V+) soldered across the high-impedance terminals (antenna wire terminal to the S-start/P-hot junction) improves SWR on 15m and 10m for many EFHW builds. It resonates with the transformer's leakage inductance at higher frequencies, improving the impedance match on the harmonic bands. The capacitor adds approximately 0.5g and costs pennies — worth including in every portable EFHW build.
Mount in Enclosure and Add Connectors
For an Altoids tin or plastic project box: drill or punch holes for the BNC connector (typically 5/16" or 7/16" for BNC), the antenna wire terminal, and the counterpoise wire terminal (second binding post or a small screw terminal). Mount the BNC connector in one hole. Mount two binding posts — one marked "ANT" for the antenna wire, one marked "CP" for the counterpoise.
Place the wound toroid inside the enclosure and solder the leads to the appropriate terminals. Keep all solder joints short and neat — in a small portable enclosure, wire leads touching the metal case can cause problems. Apply a small dab of RTV silicone to secure the toroid to the enclosure floor — this prevents it from rattling and potentially breaking solder joints during transport.
Test the UNUN Before Cutting Wire
Connect a 2500Ω resistor (or five 470Ω resistors in series — approximately correct for a quick test) across the antenna terminals. Connect the NanoVNA to the BNC. Sweep 3–30 MHz. A working UNUN shows SWR below 2:1 from 7 to 30 MHz with the 2500Ω test load. The UNUN does not need to show perfect SWR — the test load is only approximate — but wildly high SWR (above 5:1) across the sweep indicates a wiring error.
If the UNUN passes the load test, remove the test load and move to wire cutting. If it fails, trace back through the wiring — the most common error is the S-start wire connected to the wrong junction (P-gnd instead of P-hot).
Cut the Antenna Wire
For a 40m fundamental EFHW targeting the most popular SOTA/POTA bands (40/20/15/10m):
Strip the feedpoint end of the wire (the end that will connect to the UNUN antenna terminal) by 1.5 inches. Form a small loop with round-nose pliers. Connect to the "ANT" binding post. Secure with a wing nut or lock nut.
Add the Counterpoise Wire
Cut a 2-foot length of #22 AWG wire and connect one end to the "CP" terminal on the UNUN. This is the counterpoise — it provides the RF return path that reduces common-mode current on the coax and dramatically improves EFHW performance. Without a counterpoise, the coax shield carries return current and becomes part of the antenna, causing SWR to change when you touch the radio and RF to appear in the shack.
The counterpoise length of 2 feet is a starting point — some builders find that adjusting counterpoise length improves SWR. A counterpoise of λ/20 at the operating frequency is the theoretical optimum (approximately 6.5 feet for 40m, 3.3 feet for 20m). In practice, 2 feet provides significant improvement over no counterpoise on all bands.
Wind Wire on the Winder — Figure-8 Method
Wind the antenna wire onto the card winder using the figure-8 method — pass the wire in a figure-8 pattern around both ends of the winder rather than in circular loops around a single axis. Figure-8 winding prevents the wire from developing twists and tangles when deployed in the field. Circular-wound wire uncoils in loops that tangle; figure-8-wound wire pays out in a straight line.
Wind from the far end of the wire (the tip, away from the UNUN) first — leave the UNUN connection end on the outside of the spool for easy access in the field. Wind firmly but not tightly — overly tight winding kinks thin wire at the spool edges. Leave 12 inches of the UNUN end free outside the spool for connecting to the UNUN in the field.
Initial SWR Tuning at Home
Set up the antenna in the backyard or similar before the first activation. String the wire as a sloper or inverted-L from a fishing pole or convenient support at approximately the height you expect to use in the field. Connect the counterpoise and the coax to the UNUN. Connect the NanoVNA at the radio end of the coax.
Sweep 7.0–7.3 MHz. Find the 40m SWR minimum. With wire at 33.7 ft, expect resonance around 7.05–7.12 MHz. Trim the wire tip in 1-inch increments, re-hanging the wire after each trim, until resonance reaches 7.150 MHz. Then sweep 14.0–14.35 MHz to confirm 20m resonance near 14.3 MHz, and check 21 and 28 MHz for 15m and 10m.
3-Minute SOTA/POTA Deployment Sequence
Practiced deployment makes the difference between a smooth activation and a frustrating one. Follow this sequence until it becomes automatic.
Raise the Pole First
Before touching the antenna wire, identify wind direction and select a deployment direction that puts the wire downwind from the pole. Extend the fishing pole to full height section by section from the base. Anchor the pole base in a rock crevice, against your pack, or in a ground stake. Guy the pole with 3 lengths of paracord to rock placements if wind is significant. The pole must be stable before attaching the antenna — a falling pole with the antenna attached damages everything.
Attach UNUN to Pole Top
Clip the UNUN carabiner to a short cord loop tied to the pole tip — or directly to the top section if the pole has a ring. The UNUN should hang just below the pole tip, free to swing slightly. Connect the coax from the radio to the UNUN BNC. Connect the counterpoise wire to the UNUN CP terminal — let it hang or clip its far end to a rock at the base of the pole. The radio can now be powered up and warmed up while the wire is deployed.
Deploy the Wire
Hold the wire winder in one hand and walk away from the pole in the chosen direction, allowing the wire to pay out from the winder. Walk the full wire length — approximately 33 feet — in a straight line away from the pole. At the far end, anchor the wire tip at a low point (a rock, stake, or pack — 1–2 feet above ground is fine). The wire should slope from the UNUN at the pole top downward at 30–45 degrees to the anchor point.
The wire does not need to be perfectly straight — a slight curve around rocks or vegetation is fine and has negligible electrical effect at HF frequencies.
Quick SWR Check — Then Operate
If carrying the NanoVNA (recommended for first several activations), do a quick 40m sweep — 30 seconds confirms the antenna is working correctly before starting to call CQ. SWR below 2:1 at 7.150 MHz → start operating. SWR above 3:1 anywhere → check the counterpoise connection and the UNUN-to-wire connection before troubleshooting further.
If you have tuned the antenna at home and know its resonant frequency, you can skip the NanoVNA check and simply monitor the radio's built-in SWR meter on the first transmission. A reading below 2:1 means the antenna is working. Start calling CQ and self-spot on the SOTA or POTA spotter immediately.
High SWR in the Field — Quick Checks
- SWR changes when you touch the radio: counterpoise not connected or floating — reconnect the CP wire to the UNUN CP terminal and clip its free end to something at ground level
- SWR much higher than at home: wire deployed differently than during home tuning — check that the full wire length is deployed and not bunched near the UNUN
- No SWR dip found on 40m sweep: UNUN connection loose or wire not connected — check the wire-to-UNUN terminal connection and the coax-to-UNUN connection
- SWR good on 40m, very high on 20m: wire partially deployed (only half the wire out) — verify the full 33-foot wire is deployed and the tip is anchored at the correct distance
- SWR shifts with wind: wire end not anchored, swinging — secure the wire tip to a fixed point rather than letting it hang free
- SWR on the radio shows higher than NanoVNA: normal — the radio's meter calibration may differ; if NanoVNA shows below 2:1, the antenna is working
Improving Performance on Each Band
- 40m SWR too high: wire is too short (over-trimmed) — splice 3–4 inches onto the wire tip and re-tune; carry a short splice piece in the kit
- 20m SWR marginal: adjust counterpoise length — try 6.5 feet instead of 2 feet; 20m responds more to counterpoise optimization than 40m
- 15m SWR high: add or adjust the 100 pF capacitor across the UNUN high-Z terminals — 15m is the most sensitive band to the optional capacitor
- 10m SWR erratic: 10m is the most sensitive to counterpoise position and length — reposition the counterpoise more perpendicular to the coax run
- Poor signal reports despite good SWR: antenna is too low — raise the pole height or find a higher support; every foot of pole height improves signal, especially on 40m
- Contact rate low despite good signals: self-spot on SOTA/POTA spotter and use FT8 on 20m during marginal propagation — FT8 works 20+ dB below the SSB threshold
Does the wire need to be perfectly straight to work?
No. The EFHW works well with bends, kinks around rocks, and changes of direction — as long as the total wire length is correct and the wire is extended away from the UNUN rather than coiled or doubled back on itself. A wire that curves around a boulder, bends around a tree, and ends up at 45 degrees to the pole performs essentially identically to a perfectly straight wire of the same length. The key dimensions are total wire length and keeping the wire extended rather than bunched.
Can I use the EFHW from a vehicle or balcony?
Yes — the EFHW works well from any elevated support. From a vehicle, string the wire from a mag-mount pole on the roof to a tree or fence stake. From a balcony, hang the wire over the railing and down at an angle. The antenna's performance is essentially the same regardless of how the wire is supported as long as it is extended away from the UNUN and the counterpoise is in place. For fixed installations from a balcony or HOA-restricted location, the EFHW is one of the most effective stealth wire antennas available.
Why do I need a counterpoise if the EFHW is supposed to be simple?
Without a counterpoise, the coax shield becomes the return current path — the coax acts as a conductor connected to the feedpoint, making it part of the antenna. This causes the SWR to depend on how the coax is routed and how long it is, makes SWR change when you touch the radio, and causes RF to appear at the operating position. A 2-foot counterpoise adds 3 grams to the system and eliminates all these problems. On SOTA summits, RF in the shack causes audio clicks, hot mics, and uncomfortable RF burns on the operator's hands. The counterpoise prevents all of these.
What is the best SOTA fishing pole for the EFHW?
A 7-meter carbon fiber telescoping fishing pole (crappie pole style) is the SOTA standard — approximately 180–220 grams collapsed to 65cm. The carbon fiber construction is much lighter than fiberglass at the same length. One critical caveat: carbon fiber is electrically conductive. Keep the antenna wire at least 6 inches away from the carbon fiber pole along its entire length — route the wire off to one side of the pole tip using a short cord loop, not along the pole body. Touching the wire to the carbon fiber changes the antenna's electrical characteristics and can produce erratic SWR.
Can I run 100W through this portable EFHW?
Not with the standard portable build using an FT-82-43 or FT-140-43 core. The FT-82-43 handles approximately 10W continuously; the FT-140-43 handles 25–50W. For 100W operation, use an FT-240-43 core — this adds about 35 grams but handles 100W safely. The wire also matters — #24 AWG wire has adequate current handling for 100W at HF frequencies, but the wire end connections must be secure. Most SOTA operations use 5–25W where the compact portable UNUN is entirely adequate.
How does the EFHW compare to a linked dipole for SOTA?
Both are excellent SOTA antennas — the choice depends on operating style. The EFHW deploys faster (3 minutes vs 6–8 for a linked dipole), requires only one support point, and covers four bands without link changes. The linked dipole provides true balanced-feed performance with no common-mode issues, and changing bands is as simple as clipping or unclipping one connector. For operators who primarily work one or two bands from each summit and want maximum RF performance, the linked dipole is slightly better. For operators who rapid-cycle through multiple bands (40m → 20m → 15m without moving the antenna), the EFHW is more convenient. Many experienced SOTA operators carry both — a linked dipole for primary use and an EFHW as a backup and for quick one-band activations.