Why Quoting Wire Lengths in Ham Radio Is Misleading
Updated August 21, 2025
If you’ve ever browsed forums or ham radio guides, you’ve seen it: “Cut your dipole to 20.35 meters (66.8 ft) and you’re on frequency.” While appealing in its simplicity, this advice is often useless unless you are using exactly the same type of wire as the original author. Wire length alone doesn’t tell the whole story — because wire doesn’t behave like a perfect conductor in free space. Its velocity factor changes how long an electrical wavelength actually is in that piece of wire.
Understanding Velocity Factor
The speed of a signal in a wire is always slower than in free space. This slowdown is expressed as velocity factor (VF), the ratio of wave speed in the conductor to the speed of light.
For antennas, this means the “electrical length” of the wire is not the same as its physical length. Thin wires have a lower VF (wave slows more), while thicker conductors or wires with insulation can raise or lower it further. This is why one ham’s dipole cut from 1.6 mm (#14 AWG) copper may resonate hundreds of kHz away from another ham’s dipole cut from 3 mm (0.12 in) bare aluminum tubing.
Examples: Wire and Tubing Compared
| Material / Diameter | Approx. VF | Impact on 20 m Dipole (14 MHz) |
|---|---|---|
| 0.6 mm enamelled copper (#22 AWG, ~0.024 in) | ~0.94 | Resonance shifts lower, dipole needs to be cut shorter — about 9.8 m (32.1 ft) per leg instead of handbook 10.05 m (33.0 ft) |
| 1.6 mm stranded copper (#14 AWG, ~0.064 in) | ~0.96 | Close to handbook tables — about 9.95 m (32.6 ft) per leg, still requires trimming in place |
| 2.5 mm solid copper (#10 AWG, ~0.098 in) | ~0.97 | Slightly higher resonance — dipole about 10.0 m (32.8 ft) per leg to center on 14.2 MHz |
| 3 mm aluminum tubing (~0.12 in) | ~0.98–0.99 | Resonance shifts higher, dipole must be cut longer — ~10.2 m (33.5 ft) per leg to cover SSB segment |
| Insulated PVC jacketed wire, 1.5 mm² (~#16 AWG) | ~0.90–0.92 | Much shorter electrical length — needs legs only ~9.4 m (30.8 ft) each for resonance |
These differences look small — a few percent in VF — but at HF, a 2% length error on a 40 m dipole means about 40 cm (16 in) per leg. Enough to move resonance from 7.05 MHz to 7.25 MHz, which is the difference between CW and SSB segments of the band.
Why “Cut and Pray” Doesn’t Work
- Different wire thicknesses and insulation types change velocity factor
- Surroundings (height above ground, nearby metal, trees) further detune resonance
- Even the same nominal length wire spool may vary depending on alloy and stranding
This is why professional antenna builders always say: start a little long, measure, and trim. Length tables are a guide, not a recipe.
Mini-FAQ
- Does insulation affect velocity factor? — Yes. PVC-insulated wire usually lowers VF more than enamel or bare wire, meaning you cut it shorter.
- Can I trust online wire length calculators? — Only as starting points. Always measure with an analyzer after installation.
- Why is my dipole shorter than the handbook says? — Likely because of thin, insulated wire lowering VF, or nearby objects adding capacitive loading.
- Is metric or imperial better to use? — Metric is more precise for trimming, but use whatever you can measure accurately — resonance matters, not the units.
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