49:1 Unun for End-Fed Half-Wave Antennas
A resonant end-fed half-wave (EFHW) antenna presents a very high feedpoint impedance — typically 2200-4500Ω — because it is fed at its high-voltage, high-impedance end rather than its low-impedance center. A 49:1 unun steps that impedance down toward 50Ω so ordinary coax and a transceiver can drive it directly. This guide covers why 49:1 became the standard EFHW ratio, the winding method, a turns calculator, and full troubleshooting.
Why the end of a half-wave wire is a high-impedance point
Feed a dipole at its center and you are sampling the point of maximum current and minimum voltage — roughly 73Ω. Feed the same length of wire at one end instead, and you are now sampling the opposite extreme: maximum voltage, minimum current. Textbook values for a true end-fed half-wave land around 2450-3000Ω in free space, though real installations (height above ground, nearby objects, exact wire length) commonly push the practical range to 2200-4500Ω. A 49:1 transform (50Ω × 49 = 2450Ω) is chosen specifically to land in the middle of that practical range.
How the winding creates the 49:1 ratio
Unlike the 9:1 unun's trifilar autotransformer (where all three wires are the same length and gauge), a 49:1 unun is normally wound as two separate windings sharing the same core: a short primary of a few turns and a much longer secondary of many turns, connected as an autotransformer with a common ground/shield tap at the primary-secondary junction. Impedance scales with the square of the turns ratio, so a 1:7 turns ratio produces the 1:49 impedance ratio.
49:1 still needs a counterpoise
Because the unun's winding is a single tapped autotransformer, not an isolated balun, the coax shield and the antenna's "other half" still need a defined return path even though the antenna itself is resonant. Most EFHW builds use a short (non-resonant, roughly 17-25 ft) counterpoise at the feedpoint rather than a quarter-wave radial system — see the Counterpoise Design guide for sizing. Skipping the counterpoise on an EFHW is one of the most common causes of the "SWR looks fine but I still get RF in the shack" complaint.
Sizing for power
The primary winding sees relatively low turns but comparatively high RF voltage once the antenna is near resonance, so core selection follows the same margin logic as any other transformer winding — see the Core Selection Guide for the flux density math. FT-240-43 is the standard choice for EFHW ununs at any power level above QRP, because the high feedpoint impedance means comparatively high RF voltage across the winding even at modest power.
| Core Configuration | Primary Turns | Secondary Turns | Recommended Power |
|---|---|---|---|
| FT-140-43 (single) | 2 | 14 | Up to 100W (QRP/portable) |
| FT-240-43 (single) | 3 | 21 | 100–400W |
| FT-240-43 (single, extra margin) | 4 | 28 | 400–1000W |
| FT-240-43 (stacked x2) | 4 | 28 | Full legal limit (1.5kW) |
49:1 Unun Turns Calculator
Materials for a two-winding 49:1 unun
A 49:1 unun wound with a 3-turn primary and 21-turn secondary on a single FT-240-43 core, mounted in a weatherproof enclosure.
Building a 49:1 Unun
Budget 1-2 hours including enclosure assembly and testing. Have your EFHW wire length and counterpoise planned before final installation.
Choose core configuration and turns
Match core and turns to your power level using the reference table or the calculator above — a single FT-240-43 with a 3-turn primary and 21-turn secondary is a solid 100-400W starting point; step up turns or stack cores for higher power.
Wind the primary, then the secondary
Wind the primary turns first, spacing them evenly around roughly half the core's circumference. Wind the secondary turns over the remaining circumference in the same rotational direction, leaving a small gap between the two windings' ends where they will be joined. Keep turns snug and evenly spaced — bunched turns reduce coupling and can shift the effective ratio.
Connect the autotransformer topology
Join the primary's end to the secondary's start — this junction is the common tap. Coax center conductor connects to the primary's start. Coax shield connects to the tap junction, and the counterpoise terminal also connects to this same tap junction. The EFHW wire connects to the secondary's far end.
Test with a dummy load before installing
Connect a non-inductive resistor across the antenna and ground terminals matching the design target (2450Ω is standard for testing a 49:1 unun) and sweep SWR at the coax input with a NanoVNA. A correctly wound unun shows SWR under about 1.5:1 to 1.8:1 into that load — a much higher or wildly varying reading points to a winding or connection error, not the antenna.
Weatherproof, install, and connect the counterpoise
Mount the tested unun in its enclosure with sealed cable entries and a drip loop below the connector. Attach the EFHW wire to the antenna terminal and route it up and away at as steep an angle as the site allows. Connect a counterpoise to the ground terminal — see the Counterpoise Design guide — before transmitting.
| Symptom | Most likely cause | Diagnosis | Fix |
|---|---|---|---|
| SWR is high even with correct EFHW wire length | Winding topology error, or wire length is not actually close to a resonant half-wave | Test the unun alone with a 2450Ω dummy resistor across the antenna/ground terminals; a correctly wound unun shows SWR under 1.8:1 into that load regardless of the antenna | If the dummy-load test fails, re-check winding connections; if it passes, re-verify wire length against the half-wave formula for your design frequency |
| SWR looks fine but RF-in-the-shack symptoms persist | Counterpoise missing or too short for effective common-mode suppression | Clip on a known-good choke temporarily at the shack entry; if symptoms disappear, common-mode current on the shield was the cause | Add or lengthen the counterpoise per the Counterpoise Design guide, and add a permanent 1:1 current choke at the shack entry |
| SWR is good near the design frequency but very narrow bandwidth | Winding Q is too high for the intended multiband use, or core is undersized causing early saturation that narrows the usable bandwidth | Sweep SWR with a NanoVNA across the full band; compare the -3dB SWR bandwidth against typical EFHW unun performance for your core size | Step up to a larger core (FT-240 vs FT-140) or add a turn or two to both windings while keeping the 7:1 ratio |
| Core runs hot or cracks under power | Core undersized for actual power, or SWR excursions from an off-resonance wire length driving up internal RF voltage | Recalculate flux density per the Core Selection Guide using the actual RF voltage seen at the unun, not just rated transmitter power | Step up to a larger core, add a second stacked core, or trim the wire length closer to true resonance to reduce SWR at the unun |
| Antenna resonates on the design band but tunes poorly on harmonics | Normal EFHW behavior — feedpoint impedance on harmonic bands is not always as close to 2450Ω as on the fundamental | Compare measured feedpoint impedance across bands; if a specific harmonic reads far outside 2200-4500Ω, that is expected on some wire lengths | Use an antenna tuner on the affected harmonic band, or consider whether a 64:1 unun better suits that specific band — see the 64:1 Unun guide |
Why 49:1 instead of 9:1 for an EFHW?
A resonant half-wave fed at its end presents roughly 2200-4500Ω, far higher than the ~450Ω a 9:1 unun targets. A 9:1 unun on a true EFHW would leave a large mismatch a tuner may not fully clean up. 49:1 (targeting ~2450Ω) is matched specifically to the high-impedance end-fed condition.
Do I still need a counterpoise on a resonant antenna?
Yes. Resonance affects the antenna's own impedance, not whether the unun's tapped winding needs a return path. Most EFHW builds use a short, non-resonant counterpoise (roughly 17-25 ft) at the feedpoint rather than a full radial system.
Can I use one 49:1 unun on multiple bands?
Yes, as long as the wire length is cut for the lowest band you want as a true half-wave (typically 80m or 40m), since a half-wave wire is also close to resonant on its odd harmonics (3rd, 5th, and so on), giving usable operation on several higher bands from the same wire without an antenna tuner, and tuner-assisted operation on the others.
My SWR is fine on the low bands but bad on 15m — why?
15m is not a clean odd harmonic of an 80m or 40m half-wave the way 20m and 10m often are, so it commonly shows the worst native SWR on an EFHW cut for a lower band. This is a normal characteristic of the antenna, not a sign the unun is faulty — use a tuner on 15m or consider a different overall wire length if 15m performance matters most to you.
How do I know if my finished unun is actually working correctly?
Test it before installing: connect a 2450Ω non-inductive resistor across the antenna and ground terminals and sweep SWR at the coax input with a NanoVNA. SWR under about 1.5:1-1.8:1 confirms the transformer is working; high or erratic SWR into that known-good resistive load points to the unun, not the antenna.