9.5478
m
4.7739
m
31.325
ft
15.662
ft
375.9
in
187.95
in
10.5634
m
9.5
cm
0.955
73
Ω
9.5478
m
4.7739
m
31.325
ft
15.662
ft
375.9
in
187.95
in
10.5634
m
9.5
cm
0.955
73
Ω
The Dipole Antenna Calculator computes the precise physical dimensions for a half-wave dipole antenna at any given frequency. The dipole is arguably the most important and widely used antenna design in radio communications — it serves as the reference antenna for gain measurements (dBd), the fundamental element in Yagi-Uda arrays, phased arrays, and countless other antenna systems, and it is the simplest antenna that can be built and made resonant with readily available materials.
A half-wave dipole consists of two conductors, each approximately one-quarter wavelength long, arranged in a straight line and fed at the center. The theoretical free-space length of a half-wave dipole is given by L = λ/2 = c/(2f), but in practice the physical length is slightly shorter due to the end effect (fringe capacitance at the wire tips) and the velocity factor of the conductor material. The commonly used practical formula is L = 143/f(MHz) meters (equivalent to about 468/f(MHz) feet), which incorporates an effective velocity factor of approximately 0.95.
The dipole's most important electrical characteristic is its feed point impedance. A half-wave dipole in free space presents approximately 73.1 Ω purely resistive at resonance. This is very close to the characteristic impedance of standard 75 Ω coaxial cable, making direct connection straightforward. For 50 Ω systems (more common in amateur radio and professional RF engineering), a simple 1:1 or 4:1 balun and a small SWR of about 1.46:1 is typically acceptable, or a matching section can be used for a perfect match.
The dipole has a figure-8 radiation pattern in the azimuth plane (when oriented horizontally) — maximum radiation is broadside to the wire, with nulls off the ends. When installed horizontally, the dipole is horizontally polarized; when installed vertically, it is vertically polarized. The theoretical gain of a half-wave dipole over an isotropic radiator (dBi) is 2.15 dBi, which is used as the 0 dBd reference for directional antennas.
Dipoles are used across all frequency bands. In the HF range (3–30 MHz), dipoles for amateur radio bands range from about 5 meters long (for the 10-meter band at 28 MHz) to approximately 40 meters long (for the 80-meter band at 3.5 MHz). At VHF and UHF frequencies, dipoles become compact enough for hand-held and mobile applications. Folded dipoles, which consist of two parallel conductors connected at the ends, present four times the feed impedance (approximately 292 Ω) and offer broader bandwidth — they are the standard feed element in Yagi antennas.
This calculator uses the practical formula L = 143/f(MHz) meters and allows adjustment of the velocity factor for different conductor materials and installation environments. All arm lengths are provided for immediate use in construction.
The total dipole length is calculated as L = (143 / f_MHz) × VF meters, where the constant 143 is derived from c/2 = 149.896 m/MHz × 0.9535 (average practical velocity/end-effect factor). Each arm of the dipole is exactly half the total length. The feed impedance of a half-wave dipole in free space is approximately 73 Ω and is shown as a fixed reference value, as it varies only slightly with height above ground and wire diameter.
Cut each arm to the calculated length and add a small extra amount (2–3 cm at HF, a few mm at VHF) for trimming. Connect a balun (balanced-to-unbalanced transformer) at the feed point to prevent feed line radiation, which can distort the antenna pattern and cause RFI issues. Trim one arm at a time while monitoring SWR, aiming for minimum SWR at your target frequency.
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For the 20-meter ham band centered at 14.2 MHz, each arm of the dipole is approximately 4.78 m (15.7 ft) long.
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A 2.4 GHz dipole for WiFi is only 5.66 cm total (2.83 cm per arm) — small enough to integrate on a PCB as a trace antenna.
The exact free-space half-wavelength would be 149.896/f(MHz) meters. The factor is reduced to ~143 to account for the end effect (the fringe capacitance at the wire tips makes the antenna electrically longer than its physical length) and typical velocity factor of about 0.95 for bare wire. The exact factor can vary from about 140 to 148 depending on wire diameter and height above ground.
A balun (balanced-to-unbalanced transformer) is strongly recommended at the dipole feed point. Without a balun, common-mode current flows back along the outer shield of the coax, which effectively makes the feed line part of the antenna. This can cause pattern distortion, RFI to nearby electronics, and unexpected SWR behavior. A simple 1:1 choke balun (several turns of coax through a ferrite core) is sufficient for most applications.
A folded dipole has both ends of the element connected by a second parallel conductor, forming a narrow oval. It presents approximately 292 Ω feed impedance and has 2–3 times the bandwidth of a simple dipole. Folded dipoles are the standard driven elements in Yagi antennas and are also used in FM radio reception antennas (often matched to 300 Ω twin lead).
Height significantly affects the radiation pattern and feed impedance. At λ/2 height, a horizontal dipole has a low-angle radiation lobe suitable for DX (long-distance) communication. At λ/4 height, there is a single high-angle lobe. For local communication, lower heights work well. For HF DX work, higher is better — aim for at least λ/2 above ground. The feed impedance also varies with height, ranging from about 50 to 90 Ω.
A simple dipole is naturally resonant only on its fundamental frequency and odd harmonics. Multi-band operation can be achieved with: a fan dipole (multiple dipoles of different lengths connected at the same feed point), a trap dipole (LC traps inserted to create multiple resonances), or an antenna tuner (ATU) to match the antenna on non-resonant frequencies. Many HF operators use an 80-meter dipole with an ATU for operation on higher bands.
Thicker wire has lower resistance (better efficiency) and a slightly lower velocity factor (slightly longer electrically). For HF antennas, 12–16 AWG (1.6–2.0 mm diameter) copper wire is common. Thicker wire also provides somewhat broader bandwidth (lower Q) compared to thin wire. For permanent outdoor installations, stranded copper-clad steel (copperweld) wire provides better mechanical strength and corrosion resistance.
In the plane perpendicular to the dipole axis (E-plane), the pattern is a figure-8, with maximum radiation broadside to the wire and perfect nulls off the ends. In the plane containing the dipole (H-plane), the pattern is approximately omnidirectional (toroidal in 3D). This means horizontal dipoles favor propagation perpendicular to the wire's orientation.
Metals expand with temperature. For a copper wire dipole, the thermal expansion coefficient is about 16.5 ppm/°C. Over a 50°C temperature swing, a 10-meter dipole changes length by only about 8 mm — a shift of less than 0.1%, negligible for most applications. However, mechanical sag due to thermal expansion and ice loading is an important structural consideration for long HF dipoles.
Roboculator Team
The Roboculator Team explains calculations, planning tools, and practical formulas in clear language for real-life situations.
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