Design the half-wave dipole — the most fundamental antenna and the reference against which every other antenna is measured. Adjust the frequency, conductor diameter and velocity factor to see the resonant element length, radiation resistance, feed-point SWR and gain update in real time, and design an antenna that resonates.
Parameters
Frequency f
MHz
Operating frequency the antenna should resonate at
Conductor diameter d
mm
Thickness of the conductor (tube or wire) used for the element
Velocity factor (end effect) VF
Shortening ratio from the end effect. Thinner conductors are closer to 1
Feed-line impedance
Sets the SWR against the 73 ohm radiation resistance
Results
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Wavelength λ (m)
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Resonant length (total) (m)
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Single element length (m)
—
Radiation resistance (Ω)
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Feed-point SWR
—
Antenna gain (dBi)
—
Dipole structure and standing-wave current — animation
Two elements with a feed point at the centre. The standing-wave current (maximum at the centre, zero at the tips) oscillates along the conductor, and a doughnut-shaped radiation pattern pulses broadside to the wire.
Resonant length vs frequency
Current distribution — current amplitude along the element
Resonant element length L (total) and free-space wavelength λ. c: speed of light 3×10⁸ m/s, f: frequency, VF: velocity factor. The velocity factor corrects for the end effect; the radiation resistance is about 73 Ω and the gain is 2.15 dBi.
Feed-point SWR for a purely resistive mismatch. R_r: radiation resistance (about 73 Ω), Z₀: feed-line impedance. The closer to 1, the more efficiently power is transferred.
What is the Half-Wave Dipole Antenna Simulator?
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A "dipole antenna" is basically just a straight wire cut in the middle, right? Can something that simple really radiate radio waves properly?
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Yes — that is exactly it. You split one conductor into two halves and connect the feed line at the cut. That's all. But this is the single most important antenna in radio engineering — it is the "reference antenna". Every other antenna's performance is quoted as "how many dB better than a dipole". Its very simplicity is why it became the yardstick.
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I see. But why "half-wave" specifically? The length can't just be anything?
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Good question. When the length is exactly a half wavelength, the antenna resonates. The RF current driven in at the centre runs out along each arm and reflects from the open ends. When the conductor is a half wavelength long, the reflected wave returns exactly in step and a strong, stable standing wave of current builds up — large at the centre, zero at the tips. Move the frequency slider on the left and you will see the resonant element length change.
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What is so good about resonating?
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At resonance the input impedance becomes almost purely resistive — the troublesome reactance disappears. That makes the antenna easy to feed and lets power flow in efficiently. That resistance is the radiation resistance, about 73 Ω for an ideal half-wave dipole. It is not a loss: it represents the power that was successfully launched into space as radio waves. So 73 Ω is actually proof of good radiation.
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But even after cutting to a calculated half wavelength, you shorten it with a "velocity factor 0.95". What is that?
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That is the end effect. At the antenna tips the electric field bulges a little beyond the conductor, so the antenna behaves as if it were slightly longer than it really is. Cut to exactly λ/2, it resonates below the wanted frequency. To resonate at the target frequency you must cut it a bit shorter than λ/2. That shortening ratio is the velocity factor, about 0.95. The thicker the conductor, the larger the end effect and the shorter it must be.
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There is also an SWR number. What does that tell us?
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SWR (standing-wave ratio) measures how well the feed line and antenna impedances match. The antenna is 73 Ω; the common coax cable is 50 Ω. They differ a little, so part of the power you send reflects back at the feed point — SWR = 73/50 ≈ 1.46. The closer to 1, the less reflection and the better the efficiency. Use 75 Ω coax and it becomes 73/75 ≈ 1.0, almost a perfect match. In practice an SWR of 1.5 or below is considered fine.
Frequently Asked Questions
First find the free-space wavelength λ = c / f, where c is the speed of light 3×10^8 m/s and f is the frequency. Ideally the total antenna length would be λ/2, but in practice it must be cut a little shorter to account for the end effect. This is captured by a velocity factor VF (about 0.95), so the resonant element length is L = VF · λ/2. Each of the two arms is half of that. For example, at 144 MHz the wavelength is about 2.083 m, the resonant total length is about 0.990 m and each arm is about 0.495 m.
At each end of the dipole the electric field bulges a little beyond the conductor, so the antenna behaves as if it were slightly longer than its physical length. This is the end effect. As a result, a dipole cut to exactly λ/2 resonates below the wanted frequency, and to resonate at the target frequency it must be cut a little shorter than λ/2. The velocity factor measures this shortening: roughly 0.97 for a thin conductor and around 0.95 for a thicker one.
At resonance the input impedance of an ideal half-wave dipole is almost purely resistive, about 73 ohms. This is the radiation resistance, and it represents power that is launched into space as radio waves rather than lost. If the feed line is 50 ohm coax, the mismatch with 73 ohms gives a standing-wave ratio SWR = 73/50 ≈ 1.46. With 75 ohm coax it is 73/75 ≈ 1.03, an almost perfect match. The closer the SWR is to 1, the more efficiently power is transferred.
Gain measures how much an antenna concentrates its energy in one direction compared with an imaginary isotropic radiator that radiates equally in all directions. A half-wave dipole does not radiate uniformly: it has a doughnut-shaped pattern broadside to the wire, and in that strongest direction it puts out 2.15 dBi more than an isotropic source. This 2.15 dBi is a fixed theoretical value for the half-wave dipole, and it is also used as the reference for converting other antenna gains (dBd to dBi).
Real-World Applications
Amateur radio and FM broadcasting: On the 144 MHz and 430 MHz amateur bands, the half-wave dipole is the simplest homemade antenna. As in this tool, you calculate the resonant element length from the frequency, then trim the elements millimetre by millimetre to lower the SWR. The familiar T-shaped indoor antenna for FM radio reception is also a half-wave dipole. On the long-wavelength HF bands, tens of metres of wire are strung up as a wire dipole.
The driven element of a Yagi-Uda antenna: The Yagi antenna used for TV reception and directional links places several elements in a row to concentrate the signal in one direction. The element that is actually fed — the driven element — is itself a half-wave dipole. Adding directors and a reflector raises the gain, but the starting point of the design is always the dimensions of this half-wave dipole.
The reference for antenna performance (dBd): The gain figures in commercial antenna datasheets come in two flavours: dBi (referenced to an isotropic radiator) and dBd (referenced to a dipole). The 2.15 dBi gain of the half-wave dipole bridges the two, with dBd = dBi − 2.15. The half-wave dipole is the shared yardstick of radio engineering, the reference point for every antenna evaluation.
The reference antenna for propagation and EMC: For radiated-emission measurements in an anechoic chamber and for field-strength calibration, the half-wave dipole — whose characteristics are theoretically known — serves as the reference antenna. Because its resonant length, radiation resistance and directivity can all be predicted by calculation, it is an ideal reference standard that underpins the reliability of measured values. The resonant-length calculation in this tool is useful when building such reference antennas.
Common Misconceptions and Pitfalls
The biggest pitfall is assuming that cutting to exactly λ/2 makes the antenna resonate. Because of the end effect, a dipole cut to the raw calculated half wavelength resonates below the wanted frequency, leaving an inductive reactance at the target frequency and a worse SWR. In practice you must multiply by a velocity factor (about 0.95) and cut it a little shorter. The velocity factor itself changes with the conductor diameter — the thicker the tube, the larger the shortening. Use the velocity-factor slider in this tool to see that effect.
Next, the misconception that the 73 Ω radiation resistance is always constant. The 73 Ω figure applies only to an ideal half-wave dipole isolated in free space with negligible conductor thickness. A real antenna is influenced by the ground, buildings and nearby metal, and its input impedance changes considerably. At low heights above ground in particular, the radiation resistance can fall to a few tens of ohms. The 73 Ω and the 2.15 dBi gain in this tool are theoretical reference values; in a real environment the installation site must be considered separately.
Finally, do not jump to "low SWR = good antenna". SWR only indicates how well the feed line and antenna impedances are matched; it says nothing about how much the antenna actually radiates (its efficiency). Taken to the extreme, connecting a resistor to the feed line gives a perfect SWR of 1.0, yet no radio waves come out at all. Use SWR as a matching check, and judge real communication performance together with radiation efficiency, directivity and height above ground.
How to Use
Enter operating frequency in MHz (freqNum) or select from preset bands using freqRange dropdown
Set conductor diameter in mm (diaNum) or choose standard wire gauge via diaRange
Input velocity factor as decimal (vfNum, typically 0.95–0.99 for copper wire) or use vfRange presets for common materials
Simulator calculates half-wavelength resonance and displays total resonant length, single-element length, radiation resistance at feed point, SWR bandwidth, and directivity gain
Worked Example
Design a half-wave dipole for 145 MHz (2-meter amateur band). Frequency: 145 MHz, conductor diameter: 2 mm copper wire, velocity factor: 0.96. Wavelength λ = 2.07 m. Resonant length (total) = 0.978 m. Single element length = 0.489 m (each arm from feedpoint). Radiation resistance = 73.1 Ω at resonance. Feed-point SWR with 50 Ω coax ≈ 1.46:1. Antenna gain = 2.15 dBi (omnidirectional in azimuth, nulls at ends). Wire diameter affects Q-factor: thicker conductors (3 mm) reduce losses, slightly broadening SWR bandwidth from 3.2 MHz to 3.8 MHz.
Practical Notes
Velocity factor decreases with insulation: bare copper = 0.98, polyethylene jacket = 0.96, foam polyethylene = 0.94—simulator accounts for this in length calculations
Radiation resistance of 73 Ω requires quarter-wave 75 Ω coax stub or impedance transformer for 50 Ω feedlines to minimize reflection loss
End-effect capacitance shortens mechanical length 2–5% below theoretical λ/2; simulator correction uses conductor diameter to predict actual resonant frequency
Higher frequencies (above 500 MHz) demand thinner conductors (0.5 mm) to avoid skin-effect losses; lower VHF bands tolerate 4 mm diameter for mechanical strength