Build an 80m–10m EFHW Antenna for Fixed Station Use
An 80m end-fed half-wave antenna covers five HF bands — 80m, 40m, 20m, 15m, and 10m — from a single 66-foot wire and one coax feedline. For a fixed station where weight is irrelevant but durability, weatherproofing, and 100W operation matter, the 80m EFHW with a properly built full-size 49:1 UNUN is one of the most effective and convenient multi-band HF antennas available. This guide builds the complete fixed-station system: a 100W-rated UNUN in a weatherproof enclosure, a #14 AWG wire for long service life, a robust counterpoise system, and the installation approach that gets the most from a single-support antenna.
Wire Length and Band Coverage
The 80m EFHW uses a 66-foot wire (versus the 33-foot portable 40m version). This longer wire resonates on 80m as the fundamental, with harmonics falling on 40m, 20m, 15m, and 10m — providing one additional low band compared to the 40m EFHW:
The 30m band is near (but not exactly at) a harmonic resonance of the 80m EFHW — SWR on 30m is typically 3–8:1, requiring a tuner. This is normal and expected. Use a tuner for 30m operation.
Fixed Station vs Portable — Key Design Differences
A fixed-station 80m EFHW is designed for years of continuous outdoor service at up to 100W — very different priorities from a portable 40m EFHW optimized for grams and minutes. The differences affect every component choice:
- Wire gauge: #14 AWG stranded CCS (copper-clad steel) rather than the #24–28 AWG lightweight wire used for portable builds. CCS resists stretching over long outdoor spans.
- UNUN core: FT-240-43 (2.4" diameter) rated for 100W+, versus the tiny FT-82-43 or FT-140-43 used for QRP portable builds. The larger core handles power and heat without saturation.
- Enclosure: Hammond 1590B or 1590BB die-cast aluminum, fully sealed with gasket and weep hole, versus an Altoids tin or plastic box.
- Counterpoise: a longer, permanently installed counterpoise (6–10 feet minimum) rather than the 2-foot portable version.
- Weatherproofing: two-layer tape application on all outdoor connections, annual inspection schedule.
- Support: permanent mast or tree support rather than a portable fishing pole.
| Band | Frequency Range | Harmonic | Wire electrical length | Typical SWR (no tuner) | Tuner needed? | Notes |
|---|---|---|---|---|---|---|
| 80m | 3.5–4.0 MHz | Fundamental | ½λ | 1.2–2.0:1 | No (edges maybe) | Primary resonance; best match |
| 40m | 7.0–7.3 MHz | 2nd | 1λ full wave | 1.3–2.5:1 | No | Very good; often better than 80m match |
| 30m | 10.1–10.15 MHz | Non-harmonic | Non-resonant | 3–8:1 | Yes | WARC band — tuner required |
| 20m | 14.0–14.35 MHz | 4th | 2λ | 1.5–2.5:1 | No (sometimes marginal) | Works well; slight pattern complexity |
| 17m | 18.068–18.168 MHz | Non-harmonic | Non-resonant | 5–15:1 | Yes | Tuner required; may be difficult |
| 15m | 21.0–21.45 MHz | 6th | 3λ | 1.5–3.0:1 | Sometimes | Often marginal — optional capacitor helps |
| 12m | 24.89–24.99 MHz | Non-harmonic | Non-resonant | 5–20:1 | Yes | Very difficult — dedicated antenna better |
| 10m | 28.0–29.7 MHz | 8th | 4λ | 1.5–3.5:1 | Sometimes | Varies with installation height and surroundings |
SWR values are at the UNUN feedpoint and vary with installation height, wire orientation, and counterpoise quality. A tuner handles the WARC bands and occasional SWR excursions on harmonic bands. An antenna tuner in the shack (or remote ATU at the UNUN) allows operation on all HF bands including 30m, 17m, and 12m.
Efhw 80m 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.
FT-240-43 Core — Why Size Matters at 100W
At 100W on 80m, the UNUN must handle significant RF power without saturating the ferrite core. Core saturation occurs when the magnetic field in the core exceeds the material's saturation flux density — once saturated, the core loses its transformer properties and generates heat rather than transferring power efficiently. The FT-240-43's larger cross-sectional area handles 100W continuous duty on 80m with adequate headroom. Smaller cores (FT-140-43 and below) can handle 100W for short transmissions but will overheat during extended RTTY, FT8, or other high-duty-cycle digital modes.
Winding Specifications for the 100W UNUN
The 100W-rated UNUN uses heavier wire than the portable version — #18 AWG enameled wire provides lower resistance per turn and better power handling than the #28 AWG used in portable builds:
The winding procedure is identical to the portable version except for the larger core and heavier wire. The FT-240-43 is significantly larger and heavier than the FT-140-43 — plan the enclosure around the larger core dimensions (approximately 2.4" OD × 0.5" height).
Complete materials for a weatherproof 100W 80m–10m EFHW system
Building the Fixed Station 80m–10m EFHW
Build the UNUN first and verify it before cutting any wire. Allow a full afternoon — the UNUN construction is careful work and the initial outdoor tuning session takes time on 80m.
Prepare the FT-240-43 Core
The FT-240-43 is significantly larger than the portable-build cores — approximately 2.4 inches outer diameter and 1.4 inches inner diameter. Inspect the inner hole edges and sand any burrs with fine sandpaper. Wrap the outside of the core with a single layer of electrical tape to cushion the winding wire and prevent abrasion against the sharp ferrite edges. The tape adds negligible thickness but significantly extends the life of the enamel insulation on the winding wire.
Wind the Secondary — 14 Turns of #18 AWG
Cut 60 inches of #18 AWG enameled wire. Begin winding through the core — each complete pass through the center hole counts as one turn. Wind 14 turns, keeping turns snug, closely spaced, and distributed evenly over approximately 270° of the core circumference. Leave the remaining 90° for the primary winding. Leave 4-inch leads at both ends. Mark the start end "S-start" with a paint pen or permanent marker.
The #18 AWG wire is significantly stiffer than the #28 AWG used in portable builds — use needle-nose pliers to help guide the wire through the small core hole for each turn. Work slowly and do not force the wire. Rough handling during winding creates stress fractures in the enamel that can fail under RF voltage.
Wind the Primary — 2 Turns of #18 AWG
Cut 16 inches of #18 AWG enameled wire. Wind 2 turns through the core in the remaining 90° of core space. Wind in the same direction as the secondary — consistent direction is essential. Leave 4-inch leads. Scrape all four wire leads (both primary and both secondary ends) with fine sandpaper until the bare copper is bright, then tin each lead thoroughly with solder.
Complete All Electrical Connections
Prepare the Hammond enclosure: drill or punch holes for the SO-239 (typically 5/8" hole), two binding post terminals (ANT and CP), and a 1/8" weep hole in the bottom of the enclosure. Mount all three hardware items before making any internal wiring connections.
Solder the autotransformer connections:
- P-hot → SO-239 center pin (coax center conductor)
- P-gnd → SO-239 outer shell (chassis ground) AND to the CP binding post
- S-start → P-hot junction (this is the critical autotransformer connection)
- S-end → ANT binding post (antenna wire terminal)
Solder the optional 100–150 pF, 1000V ceramic capacitor across the ANT and S-start/P-hot terminals. Keep all internal wiring short and tidy — in the Hammond enclosure there is limited space.
Verify the UNUN Before Sealing
Before applying RTV or closing the enclosure, test the UNUN. Connect a 2500Ω resistor from the ANT terminal to the CP/chassis terminal. Connect the NanoVNA to the SO-239. Sweep 3–30 MHz. A properly wired UNUN with a 2500Ω load shows SWR below 2:1 across most of the sweep, with the best match between 7 and 21 MHz. SWR rising above 3:1 at 3.5 MHz with this test load is acceptable — the real antenna's impedance is different from the test load and the match improves in practice.
Also verify with an ohmmeter: SO-239 center to ANT terminal should measure low resistance (the secondary winding resistance, approximately 1–5Ω). SO-239 center to SO-239 outer (chassis) should measure infinite (open circuit). Any short between coax center and ground indicates a wiring error — fix before proceeding.
Secure Core and Seal Enclosure
Apply a dab of RTV silicone adhesive under and around the wound toroid to secure it to the enclosure floor. Allow 2 hours of cure time before closing the lid. Apply a thin bead of RTV around the lid perimeter — this supplements the die-cast gasket with an additional moisture seal. Do not fully pot the core in RTV — leave the winding accessible in case future repair is needed. Close the lid and tighten all screws.
Apply self-amalgamating tape around the SO-239 connector and the two binding post openings on the exterior of the enclosure — these are the most common moisture entry points on an otherwise well-sealed Hammond box. Wrap from the enclosure body outward onto the connector body, not the other direction.
Build the Separate Current Choke
An 80m EFHW at a fixed station requires a separate current choke on the coax in addition to the UNUN. Common-mode current is a significant problem on 80m where the coax run is long and the band is noisy. Wind 8 turns of the feedline coax through an FT-240-31 toroid core and mount it in its own weatherproof enclosure immediately below the UNUN — before the coax begins its run to the shack.
The choking impedance at 3.5 MHz with 8 turns on FT-240-31 is approximately 3,500Ω — excellent for 80m noise rejection. This choke prevents common-mode RF current from flowing back to the shack on the coax outer shield, keeping 80m receive noise low and eliminating RF in the shack on transmit.
Cut the 80m Wire and Install End Insulator
Cut the antenna wire to 67.0 feet (3% over the 65.0 ft calculated length for 3.600 MHz). Use a steel tape measure — wire this long (67 feet) shows significant measurement error with a cloth tape due to stretch. Strip the feedpoint end 1.5 inches, form a loop, and connect to the ANT binding post with a wing nut or lock nut. At the far end of the wire, thread through an egg insulator, double back 4 inches, wrap 5 times, and solder. Attach 24 inches of Dacron rope to the insulator.
Install the Counterpoise
For a fixed station 80m EFHW, install a permanent counterpoise that is significantly longer than the 2-foot portable version. The counterpoise affects performance on all five bands — a better counterpoise means better SWR and less common-mode current on all covered bands. Options in order of effectiveness:
- Single 16-foot wire: λ/4 at 14 MHz — good compromise covering all bands with one wire
- Multiple wires of different lengths: a 16-foot wire (λ/4 at 20m), a 33-foot wire (λ/4 at 40m), and a 6.5-foot wire (λ/4 at 80m entry) — the "fan counterpoise" approach. Connect all to the CP terminal.
- A single 33-foot wire: λ/4 at 40m — good for 40m and 80m, less optimal for 20m and above
- Connected to the station ground: if the UNUN is close to a good RF ground (buried radial system, ground rod), connecting the CP terminal to ground is effective — essentially using an elevated ground mount
Run the counterpoise wire perpendicular to the antenna wire and perpendicular to the coax run for best isolation between counterpoise and feedline.
Mount the UNUN and Raise the Antenna
Mount the UNUN enclosure at the feedpoint location — this is the point where the vertical section of the wire begins. For best performance, mount the UNUN as high as possible — at the base of a mast, in a tree, or on a wall bracket at the highest practical point. The UNUN does not need to be at maximum height itself, but the wire should begin rising from the UNUN immediately rather than running horizontally at low height.
Attach a support bracket or rope to the top of the Hammond enclosure and secure it to the support structure. Connect the antenna wire to the ANT terminal and run it away from the UNUN at an upward angle toward the far support. The wire should slope upward from the UNUN to the far end if possible — an inverted-L or sloper configuration works well for the 80m EFHW.
Initial SWR Sweep — All Five Bands
Connect the NanoVNA at the shack end of the coax. Sweep each band in order. With the wire cut to 67.0 feet for a 3.600 MHz target, expect initial resonance around 3.55–3.65 MHz. Record the SWR dip frequency and minimum SWR on each harmonic band before any trimming:
- 80m sweep (3.3–4.1 MHz): find the dip — note frequency
- 40m sweep (6.8–7.5 MHz): second harmonic dip — note frequency
- 20m sweep (13.5–15.0 MHz): fourth harmonic dip — note frequency
- 15m sweep (20.5–22.0 MHz): sixth harmonic — note frequency and SWR
- 10m sweep (27–30 MHz): eighth harmonic — note frequency and SWR
Trim to 80m Target Frequency
Trim the wire end at the egg insulator to raise 80m resonance to the target frequency. On an 80m EFHW, each 3 inches trimmed raises resonance approximately 3–5 kHz:
After each trim, lower the wire end, cut the target amount, re-hang, and re-sweep before trimming again. Trim conservatively — 6-inch increments until within 50 kHz of the target, then 3-inch increments for final adjustment.
Verify All Bands and Document
Once 80m resonance is at the target, verify all five harmonic bands. On 40m and 20m, SWR below 2:1 without a tuner is the goal. On 15m and 10m, accept up to 3:1 — a tuner handles this easily and these bands benefit the most from the optional 100 pF capacitor. If 15m SWR is above 3:1, add the capacitor across the UNUN high-Z terminals (requires opening the enclosure) and re-verify.
Document the final system: wire length (after all trimming), UNUN core and winding details, resonant frequency and SWR on each band, counterpoise configuration, installation height and configuration (sloper, horizontal, inverted-L), and installation date. Photograph the UNUN enclosure, the wire installation, and the counterpoise routing. Store with station records. Plan annual inspection of the enclosure weatherproofing and all outdoor connections.
Horizontal — Flat or Sloping
The simplest installation: the UNUN at one end at height, the wire running horizontally (or with a slight slope) to a far anchor. For an 80m EFHW, the far anchor needs to be 65+ feet from the UNUN. This configuration produces horizontal polarization with the familiar figure-8 radiation pattern broadside to the wire.
- UNUN at 15–20 feet height — accessible for maintenance while providing clearance from ground effects
- Wire rises slightly from UNUN to far anchor or runs level — avoid a configuration where the wire slopes back down toward the UNUN
- Far anchor: tree branch, fence post, or low mast at the 65-foot distance
- On 80m, a horizontal wire at 20 feet is primarily a NVIS antenna — good for regional emergency communications, not for DX
- For DX performance on 80m, height above 50 feet is needed for horizontal configuration
Inverted-L — Best for DX
The inverted-L uses a vertical section rising from the UNUN base to a mast top, then a horizontal section extending to a far anchor. This is the best configuration for DX on 80m because the vertical section carries the most current and produces low-angle radiation:
- UNUN at ground level or low elevation — easy access for maintenance
- Wire runs vertically up a mast or alongside a tree to maximum available height
- Wire bends horizontally at the mast top and runs to a far anchor
- Vertical section: as long as the mast allows — target 30+ feet of vertical wire
- Horizontal section: (total wire length − vertical section) needed to reach the far anchor
- For a 50-foot mast: 50 ft vertical + 15 ft horizontal = 65 ft total (slightly short — add 5 ft at the far horizontal end)
- The horizontal section direction slightly biases the radiation pattern toward that direction — point it toward the primary DX region
Sloper — One Support, Directional Pattern
A sloper runs the EFHW wire from a high feedpoint (at a tower or mast top) downward at an angle to a low anchor. The sloping wire produces a mix of horizontal and vertical polarization and creates a directional pattern favoring the direction the wire slopes toward.
- UNUN at 40–60 feet height on a mast or tower — requires a tall support
- Wire slopes downward at 30–45° to a far ground-level anchor
- The radiation pattern favors the direction down the slope — point the wire toward your primary DX target region
- Feed impedance with a sloper may be slightly different from a horizontal EFHW — the counterpoise and UNUN position relative to the mast affects the match
- The metallic mast structure near the UNUN can interact with the antenna — maintain at least 2 feet of separation between the wire and the mast for the first several feet below the UNUN
Stealth EFHW — HOA and Restricted Properties
The 80m EFHW's single feedpoint and single wire make it one of the most effective stealth HF antennas for restricted properties:
- Roofline installation: run #26 AWG dark-colored wire along the roof edge from one corner, up and over the peak, and down the opposite slope. The thin wire is nearly invisible from street level. At 66 feet, the wire wraps around a typical house perimeter with room to spare.
- Fence line: use thin insulated wire on small standoff insulators along a wooden fence. At 66 feet, this covers most residential fence lines completely.
- Attic installation: an 80m EFHW inside an attic operates at reduced efficiency (roofing materials add loss) but covers all five bands from an invisible antenna. Avoid metal roofs and foil-backed insulation which dramatically reduce performance.
- Across property: if the property is large enough, the wire can run across the back yard at low height — 8–10 feet on garden posts is below the visual horizon from a street. Not ideal for DX but excellent for NVIS regional operation on 80m and 40m.
Why does the 80m EFHW need a larger UNUN than the portable 40m version?
Two reasons: operating frequency and power level. The FT-240-43 is larger because 80m operation at 3.5 MHz stresses ferrite cores more heavily than 40m operation at 7 MHz — the permeability of type 43 ferrite provides maximum performance in the 3–30 MHz range, but the lower 80m frequencies require more core cross-section to avoid saturation at 100W. The FT-82-43 and FT-140-43 used in portable builds are too small to handle 100W continuous duty at 3.5 MHz without overheating. For 80m operation at 5–25W QRP, the FT-140-43 is borderline adequate but the FT-240-43 is strongly preferred for any extended digital mode operation.
Can I use this 80m EFHW on 30m, 17m, and 12m?
Yes, with a tuner. The WARC bands are not harmonic resonances of the 80m EFHW, so SWR is high without a tuner (3–20:1 depending on the band). However, with a good antenna tuner and the ladder line approach — or simply using the radio's internal ATU if SWR is below 5:1 at the feedpoint — WARC band operation is feasible. On 30m (10.1 MHz) the antenna is typically close enough to a resonant condition that most internal ATUs can find a match. On 17m and 12m, the impedance is more challenging — a remote ATU at the UNUN feedpoint is the best solution for convenient multi-band WARC band operation.
How long is the counterpoise for the 80m EFHW?
For a fixed station, a multi-wire counterpoise system is recommended rather than a single short wire. Use three wires: a 6.5-foot wire (λ/20 at 80m), a 16-foot wire (λ/4 at 20m), and a 33-foot wire (λ/4 at 40m) — all connected to the CP terminal and running in different directions from the UNUN. This fan counterpoise provides a useful ground reference on all five primary bands. Alternatively, connecting the CP terminal to a good RF ground (a ground rod with 8+ buried radials) is very effective for a ground-mounted or low-mounted UNUN installation.
Is the 80m EFHW competitive with a full-size 80m dipole?
On the harmonic bands (40m, 20m, 15m, 10m), yes — the EFHW performs identically to a dipole of the same wire length at the same height and orientation. On 80m specifically, the comparison is more nuanced. If both antennas are at the same height and same orientation (both horizontal at 50 feet, for example), performance is essentially identical on 80m — both are half-wave antennas resonating at the same frequency. The EFHW's advantage is convenience: one feedpoint, one coax, one support if needed. The dipole's advantage is the balanced feed which inherently reduces common-mode current issues. For fixed station use, the dipole's RF cleanliness advantage often outweighs the EFHW's installation convenience advantage.
My 80m SWR is good but 40m SWR is very high — what is wrong?
This suggests the wire length is correct for 80m but the harmonic resonance is not falling in the 40m band — the 40m resonance may be falling significantly above or below 7.0–7.3 MHz. This can happen if the wire length is cut for a frequency at one end of the 80m band, shifting the 2nd harmonic significantly. For example, wire cut for 3.900 MHz produces a 2nd harmonic at 7.800 MHz — completely outside the 40m band. The solution is to cut the wire for a primary frequency near 3.600–3.650 MHz, which places the 2nd harmonic at 7.200–7.300 MHz — squarely in the 40m band. Re-cut or extend the wire to achieve the lower 80m resonance frequency.
Can I run 1500W through this antenna?
Not through the standard FT-240-43 UNUN described in this guide. The FT-240-43 handles 100W continuous duty and approximately 200W on SSB (50% duty cycle) with good margin. For legal limit operation (1500W), the UNUN must be re-designed with a larger core — typically two FT-240-43 cores stacked, or a specialty core designed for high-power operation. The antenna wire (#14 CCS) handles 1500W at HF frequencies without issue. The limiting factor is the UNUN core and the voltage rating of the optional 100 pF capacitor. For 1500W builds, use a ceramic capacitor rated 5kV minimum and two stacked FT-240-43 cores with heavier wire (#14 AWG enameled for the winding).