Compute half-wave dipole element lengths, quarter-wave monopole height, and shortened dipole dimensions with velocity factor correction — for any frequency from AM to microwave.
The table below lists the most common RF bands that use dipole or dipole-derivative antennas, with pre-calculated half-wave dipole total length, each element length, and quarter-wave monopole height. All lengths assume ideal free-space with an end-effect factor of 0.955 (VF = 1.0). Multiply by your velocity factor for insulated wire or enclosures.
| Band / Application | Frequency | λ/2 Total | Each Element (λ/4) | Monopole (λ/4) | Notes |
|---|---|---|---|---|---|
| AM Broadcast | 1 MHz | 143.3 m | 71.6 m | 71.6 m | Vertical monopoles used in practice |
| 160m Amateur | 1.84 MHz | 77.8 m | 38.9 m | 38.9 m | Often end-fed or shortened |
| 80m Amateur | 3.6 MHz | 39.8 m | 19.9 m | 19.9 m | Popular DX band |
| 40m Amateur | 7.1 MHz | 20.2 m | 10.1 m | 10.1 m | Most popular HF dipole band |
| 20m Amateur | 14.2 MHz | 10.1 m | 5.04 m | 5.04 m | Primary DX & contest band |
| 15m Amateur | 21.2 MHz | 6.76 m | 3.38 m | 3.38 m | Harmonic of 40m |
| 10m Amateur | 28.5 MHz | 5.02 m | 2.51 m | 2.51 m | Excellent DX when solar cycle active |
| 6m Amateur | 50 MHz | 2.86 m | 1.43 m | 1.43 m | "Magic band" — sporadic-E propagation |
| FM Broadcast | 98 MHz | 1.46 m | 730 mm | 730 mm | Horizontal dipoles used at broadcast towers |
| 2m Amateur / APRS | 145 MHz | 987 mm | 494 mm | 494 mm | Most popular VHF amateur band |
| ADS-B Aviation | 1090 MHz | 131 mm | 65.7 mm | 65.7 mm | Aircraft transponder tracking |
| GPS L1 | 1575 MHz | 91.0 mm | 45.5 mm | 45.5 mm | Patch antennas preferred in practice |
| LoRa / ISM 433 | 433 MHz | 330 mm | 165 mm | 165 mm | IoT, remote controls, garage openers |
| LoRa EU 868 / US 915 | 868 MHz | 165 mm | 82.5 mm | 82.5 mm | LPWAN, smart meters, agriculture |
| Wi-Fi / Bluetooth 2.4 GHz | 2400 MHz | 59.7 mm | 29.8 mm | 29.8 mm | PCB trace dipoles common on modules |
| Wi-Fi 5 GHz (802.11ac) | 5500 MHz | 26.1 mm | 13.0 mm | 13.0 mm | Dual-band routers use λ/2 dipoles |
| 5G Sub-6 GHz | 3500 MHz | 40.9 mm | 20.4 mm | 20.4 mm | Dipole arrays in 5G base stations |
The half-wave dipole is the fundamental antenna from which almost all other antenna types derive. It has a predictable radiation pattern (figure-of-eight in the plane containing the element), a feed impedance of approximately 73 Ω in free space, and gain of 2.15 dBi referenced to an isotropic radiator. From the simple centre-fed wire dipole to the driven element in a Yagi, the folded dipole in a log-periodic array, and the printed dipole on a PCB, this antenna is the reference from which all antenna engineering starts.
A theoretical half-wave dipole resonates when it is exactly λ/2 long. In practice, a real conductor has finite thickness which alters the charge distribution near the wire ends, effectively lengthening the electrical length. This means a real dipole must be cut slightly shorter than the theoretical λ/2 to achieve resonance. The standard correction is:
When a dipole is constructed from insulated wire — or when it is close to a surface, enclosure, or PCB ground plane — the surrounding dielectric slows the wave along the conductor. The velocity factor (VF) captures this: VF = 1.0 for bare wire in free space, and typically 0.92–0.97 for thin PVC-insulated wire. Coaxial cable dipoles can have VF as low as 0.66. Multiply the free-space dipole length by VF to get the physical cutting length.
The 0.955 factor is an average. The true factor depends on how thick the conductor is relative to the wavelength: about 0.93 for thick tubing (λ/d ≈ 100), 0.96 for typical wire (λ/d ≈ 1000) and 0.97–0.98 for thin wire at HF. Height above ground and nearby objects shift it further. To get the resonant length for your exact wire, mounting height and soil, use the Dipole & Monopole Antenna Calculator, which solves the antenna with the Method of Moments and also shows its impedance, SWR bandwidth and radiation pattern.
A quarter-wave monopole over a ground plane (radials or counterpoise) is electrically equivalent to half of a centre-fed dipole. The ground plane provides the mirror image of the missing half. The feed impedance is approximately half that of a dipole — about 36.5 Ω — making it a reasonably good match to 50 Ω coaxial cable. Common examples include the rubber-duck antennas on handheld radios, vehicle roof-mount antennas, and PCB whip antennas on IoT modules.
An elevated ground-plane antenna with a few radials is a poorer mirror than a solid plane: with horizontal radials its feed impedance drops to about 22 Ω, and drooping the radials by roughly 40–45° raises it to 50 Ω. The Monopole & Ground-Plane Antenna Calculator solves the radial length and droop angle for a direct 50 Ω feed and shows the gain and takeoff angle over real ground.
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