Dipole Antenna Length Calculator
Half-wave dipole feet and meters from frequency with 468 rule.
What it does
Half-wave dipoles cut by one division: 468 over megahertz gives feet, halved per leg, with meters alongside. Fourteen point one seven five MHz needs 33.02 ft total in two 16.51 ft legs.
The tool above takes frequency and returns total feet with leg length and meters beside it. Wire runs about 5% short of free space 492 over f, which is where 468 comes from.
Rules match ARRL construction guidance with university ham references beside them: two quarter-wave wires on three insulators, six inches extra for trim margin, and SWR tuning toward 1.5 to 1 or better. Meters run about 143 over f by the same arithmetic.
Height, ground, and insulation all pull resonance, so the formula starts long on purpose. Trim symmetric inches from both ends while watching the meter, and stop where the band center dips lowest.
How to use
- Type target frequency in MHz.
- Read total feet first, legs beside it.
- Cut 6 inches over for trim margin.
- Hang high, straight, and symmetric.
- Trim both ends for lowest SWR.
- Recheck band edges after trimming.
Formula
- 468
- Wire constant in feet times MHz.
- 492
- Free space half-wave constant behind it.
- SWR
- Standing wave ratio guiding trims.
Five percent shortening covers end effect and wire velocity factor. Inverted V and insulation shift resonance a few percent more.
Worked example
Example: 14.175 MHz
Default frequency: 14.175 MHz (20 m band).
- Total: 468 / 14.175 = 33.02 ft.
- Legs: 33.02 / 2 = 16.51 ft each.
- Meters: 33.02 x 0.3048 = 10.06 m.
- Margin: cut about 33.5 ft, trim down.
- Tune for lowest SWR near band center.
- Tool shows 33.02 ft, 16.51 ft, 10.06 m.
Reference table
| Band | Frequency | Dipole |
|---|---|---|
| 80 m | 3.5 MHz | 133.71 ft |
| 40 m | 7.15 MHz | 65.45 ft |
| 20 m | 14.175 MHz | 33.02 ft |
| 15 m | 21.225 MHz | 22.05 ft |
| 10 m | 28.5 MHz | 16.42 ft |
| 2 m | 146 MHz | 3.21 ft |
Limits
- Starting lengths; trim to resonance.
- Height and ground shift results.
- Insulated wire runs slightly shorter.
- Single band; traps add bands.
Common mistakes
- Cutting exact without trim margin.
- Trimming one leg only.
- Measuring in meters with 468.
- Hanging low between metal.
- Chasing 1:1 SWR band wide.
Try it yourself
Quick checks. Answers sit tucked under each question.
7.15 MHz?
65.45 ft total.
28.5 MHz?
16.42 ft total.
Zero frequency?
Rejected: enter 0.5 to 1000 MHz.
Questions
Default length?
33.02 ft total with 16.51 ft legs.
Why 468?
Free space 492 shortened about 5% for real wire.
Meters rule?
About 143 over MHz by unit conversion.
Trim direction?
SWR falling with frequency means too long; rising means too short.
Legs equal?
Yes: two quarter-wave halves on a center insulator.
Capacitor page versus this?
That stores charge; this cuts wire to wavelength.