Dipole Calculator
Size a half-wave dipole antenna for any frequency. Enter the frequency in MHz and choose the length factor (468 for a real wire dipole, 492 for the theoretical free-space value) to get the total length and each leg in feet and meters.
Example: with Frequency (MHz) 14.2 · Length factor 468 — real wire dipole (end effect) → Total dipole length: 32.958 ft (32 ft 11 in) total.
- Each leg (half)16.479 ft (16 ft 6 in) per leg
- Total length in meters10.046 m total
Computed by the calculator below using its default values. Change any input to see your own numbers.
A half-wave dipole is close to 468/f(MHz) feet in practice. The theoretical free-space half wavelength is 492/f, but real wire is about 5% shorter because of the end effect, giving the 468 factor hams use.
Why 468 and not 492
A half-wave dipole is one half wavelength long, fed in the middle. In free space that half wavelength in feet is 492/f(MHz), straight from the speed of light. But a real antenna is built from wire with finite thickness, and the fields fringe out at the ends — the so-called end effect — making the wire behave as if it were a bit longer than it is. To resonate at the target frequency you cut it about 5% shorter, which is where the widely used 468/f figure comes from.
So for a 20-meter band dipole at 14.2 MHz, 468/14.2 gives about 32.96 feet total, or 16.48 feet per leg. The 468 factor is a practical starting point, not a law; wire diameter, insulation, and height above ground all shift resonance, so hams cut a little long and trim while watching the SWR. Use the 492 option only when you want the ideal free-space half wavelength.
How it’s calculated
Total length (feet) = factor / f(MHz), with the factor 468 for a real wire half-wave dipole (including the end-effect shortening) or 492 for the theoretical free-space half wavelength. Each leg is half the total. Meters = feet × 0.3048.
A thin-wire half-wave dipole in the clear. The 468 factor is an empirical average; actual resonant length varies with conductor diameter, insulation, and height, so cut long and trim to your measured SWR.
Half-wave dipole length by band (468/f)
| Frequency (band) | Total length | Each leg |
|---|---|---|
| 3.75 MHz (80 m) | 124.8 ft | 62.4 ft |
| 7.15 MHz (40 m) | 65.45 ft | 32.72 ft |
| 14.2 MHz (20 m) | 32.96 ft | 16.48 ft |
| 28.4 MHz (10 m) | 16.48 ft | 8.24 ft |
| 146 MHz (2 m) | 3.21 ft | 1.60 ft |
Computed as 468 / f(MHz); rounded.
Common mistakes
- Using 492 for a real wire dipole — that ignores the end effect and cuts the antenna too long.
- Entering frequency in kHz or Hz instead of MHz, which throws the length off by orders of magnitude.
- Cutting each leg to the total length instead of half of it.
- Expecting the 468 figure to be exact; trim to the measured SWR since height and wire diameter shift resonance.
Frequently asked questions
What is the dipole antenna formula?
Total length in feet ≈ 468 / frequency in MHz for a real half-wave wire dipole. Each leg is half of that. The 492/f version gives the theoretical free-space half wavelength.
Why is the factor 468 and not 492?
492/f is the ideal free-space half wavelength. Real wire resonates about 5% shorter because of the end effect at the tips, so 468/f gives a closer practical length.
How long is each leg of the dipole?
Exactly half the total length, since a dipole is fed at the center. A 32.96-foot total dipole has two 16.48-foot legs.
Do I need to trim the antenna after cutting it?
Usually yes. The 468 factor is an average; wire diameter, insulation, and height above ground shift resonance, so cut slightly long and trim while watching the SWR.