EFHW Wire Length Calculator
Enter your target frequency and wire details to estimate an End-Fed Half-Wave (EFHW) radiator length.
What is an EFHW antenna?
An End-Fed Half-Wave (EFHW) antenna is a resonant half-wave wire that is fed near one end through a matching transformer, most commonly a 49:1 unun. It is popular with portable operators, POTA/SOTA enthusiasts, and home stations because it can be lightweight, efficient, and easy to deploy as a sloper, inverted-L, or mostly horizontal wire.
The “half-wave” part matters: the total wire length is approximately one-half wavelength at your chosen fundamental frequency. Because a half-wave radiator exhibits high feedpoint impedance at the end, a transformer is typically used to transform that impedance down toward something your coax and transceiver can handle.
How this calculator works
Core length formula
This page uses a common starting formula:
Length (feet) = 468 ÷ Frequency (MHz) × Velocity Factor
Then an optional trim percentage is applied so you can pre-compensate for your building style or expected tuning outcome.
Why velocity factor matters
Insulated wire often appears electrically longer than the same physical length of bare wire. That means your physical cut length may need to be shorter when insulation is present. Velocity factor helps compensate for this. Real-world environment still dominates final tuning, so treat the number as a reliable starting point—not final truth.
Recommended tuning workflow
- Choose your center operating frequency for the primary band (for example, 7.100 MHz for 40m CW/data or 7.200+ for SSB preference).
- Calculate a starting wire length.
- Install the antenna in its intended final shape and height before serious trimming.
- Measure resonance/SWR with the transformer, feedline, and choke configuration you will actually use.
- Trim a little at a time (1–3 cm / 0.5–1 in), then re-check.
Quick reference frequencies
| Band | Typical Center (MHz) | Use Case |
|---|---|---|
| 80m | 3.600 | Regional contacts, NVIS and nighttime work |
| 40m | 7.100 | Versatile day/night DX and portable operation |
| 20m | 14.200 | Strong daytime DX band |
| 17m | 18.100 | Lower contest congestion, great propagation |
| 10m | 28.400 | Excellent when solar conditions are good |
Installation and matching considerations
Transformer and common-mode control
A 49:1 transformer is the classic choice for many EFHW builds, though 64:1 is also seen in some designs and operating goals. Coax shield current can become part of the antenna system, so adding an effective common-mode choke near the feedpoint often improves consistency and reduces RF in the shack.
Height and nearby objects
Ground conductivity, roof material, trees, gutters, and nearby structures all influence resonance and radiation pattern. If you tune at 6 meters high and later raise the antenna to 10 meters, expect the resonance point to shift.
Counterpoise or no counterpoise?
Many EFHW systems work “without” a dedicated counterpoise, but current always returns somehow—often on the coax. A short, intentional counterpoise can improve repeatability. This calculator displays a short 0.05λ reference value, which can be used as an experimental starting point.
Troubleshooting high SWR
- Verify transformer wiring and turns ratio.
- Check for poor solder joints and ring-terminal corrosion.
- Confirm feedline/choke placement hasn’t changed between measurements.
- Move the wire away from metallic gutters and power lines.
- Re-check analyzer calibration and measurement setup.
Safety reminders
- Keep antennas well clear of power lines at all times.
- Use proper strain relief and weatherproof feedpoint connections.
- Follow local regulations and amateur radio licensing rules.
- At higher power levels, evaluate RF exposure and station grounding carefully.
Bottom line
A good EFHW build is equal parts math and field adjustment. Use this calculator for a strong initial cut, then validate with measurements in the final installed environment. With a decent transformer, clean feedline management, and patient trimming, an EFHW can be an excellent multiband antenna system for both home and portable stations.