A half-wave dipole is the reference antenna of radio: a straight conductor a little under half a wavelength long, fed at the centre, resonant at the frequency where its length matches the standing-wave pattern that fits on it. Almost every other antenna is described by comparison with it, and the gain figure dBd is defined against it.
Arb Digital publishes free physics calculators that state their assumptions plainly. This one computes the published half-wavelength relation with an end-effect correction, and it is written to make one point clearly: the number it produces is where you start cutting wire, not where you stop. Transmitting on most frequencies requires a licence, a point returned to below.
What This Dipole Antenna Calculator Does
You enter a target frequency and an end-effect factor. The tool returns the total element length, the length of each leg, the free-space wavelength for reference, the uncorrected half wavelength so you can see how much the correction removes, and a cut length that includes a trim allowance.
The quarter-wave vertical option exists because a ground-mounted vertical over a radial system is electrically one half of a dipole, with the ground plane supplying the image of the missing half. Its radiator length is therefore one leg of the equivalent dipole, and the same end-effect correction applies.
The end-effect factor is exposed as an editable input because it is the least certain number on the page. Presenting it as a fixed constant would suggest a precision the physics does not have.
How to Use It
- Enter the centre of your intended range. A dipole is resonant at a point, and its usable bandwidth is narrower at low frequencies and with thin wire. Designing for the middle spreads the compromise evenly.
- Leave k at 0.95 unless you have a reason. It is the conventional starting value for thin bare wire in the clear. Insulated wire, thick tubing or a low installation all call for a smaller number, and you will discover the right one by measuring.
- Cut to the trim length, not the calculated length. The trim allowance gives you material to remove. Wire can always be shortened and never lengthened.
- Trim symmetrically. Take the same amount off each leg. An asymmetric dipole puts current on the feedline and radiates from the coax.
- Measure the result and adjust. Sweep the antenna at its final height with an analyser, find where resonance actually landed, and trim toward the target. This is the step that turns the calculation into an antenna.
The Formula: Half-Wave Length With the End-Effect Factor
Start from the wavelength. For a frequency f, the free-space wavelength is λ = c / f, where c is the speed of light in vacuum, exactly 299,792,458 metres per second by the definition of the metre in the present SI, as published by NIST's fundamental physical constants resource. A resonant half-wave element is not quite half of that. It is
L = k × λ / 2 = k × c / (2f)
where k is the end-effect factor, conventionally about 0.95 for thin wire. Each leg is half of L, and a quarter-wave vertical radiator is one leg.
Work the default. At 14.2 MHz the free-space wavelength is 299,792,458 ÷ 14,200,000 = 21.112 m. Half of that is 10.556 m, and applying k = 0.95 gives a total element length of 10.028 m, so each leg is 5.014 m. In feet that total is 32.90 ft, which is close to the traditional imperial rule of thumb, total length in feet = 468 ÷ frequency in MHz, giving 32.96 ft. That familiar 468 constant is nothing more than a free-space half wavelength in feet, 492, multiplied by an end-effect factor of about 0.951, which is why the two agree.
The end effect itself has a physical cause. The current distribution on a real element does not simply stop at the tip: capacitance between the ends of the element and its surroundings allows displacement current to continue past the physical end, so the element behaves electrically as though it were slightly longer than it is. To resonate at the intended frequency, therefore, it must be cut slightly shorter than a free-space half wavelength. That is the whole of the correction.
Why the Real Length Is Never the Calculated Length
This is the part that separates a calculation from an antenna, and it deserves the space.
Conductor diameter. The end-effect factor depends on the ratio of element length to conductor diameter. A thin wire dipole comes out close to 0.95; a fat aluminium tube element on a VHF beam can need 0.94 or less. Thicker elements are also broader in bandwidth, which is why VHF and UHF antennas built from tubing tolerate a wider range than a thin HF wire does.
Insulation. Insulated wire slows the wave along the conductor, an effect analogous to the velocity factor of a coaxial cable. A dipole made from insulated wire commonly comes out two to five per cent shorter than the same antenna in bare wire. If you cut insulated wire to the bare-wire length, it will resonate low and you will have to remove material you did not expect to.
Height above ground. This is usually the largest single influence and the one people forget. Ground is a lossy, imperfect conductor at a finite distance below the antenna, and the mutual coupling between the element and its image shifts both the feedpoint impedance and the resonant frequency. A dipole at a tenth of a wavelength up behaves very differently from the same wire at half a wavelength up. Ground conductivity, soil moisture and anything else nearby — guttering, a metal roof, a fence, another antenna — all pull the resonance around.
End loading and supports. Insulators, the way the ends are tied off, sag in the wire and any nearby object at the high-voltage ends all add capacitance and lower the resonant frequency.
The consequence is that the practical method is not calculation but measurement. Cut long, hoist the antenna to its final height, sweep it with an antenna analyser or a vector network analyser, find the actual resonant frequency, and trim both legs equally toward the target. A common working rule is that the required change in length scales roughly with the fractional change in frequency, so a resonance two per cent low needs about two per cent removed. Lowering the antenna to trim it and re-hoisting it changes the answer slightly each time, which is why it takes a few iterations. Practical construction and measurement guidance for wire antennas of this kind is collected in the ARRL's antennas resource pages.
Feedpoint Impedance, Baluns and What the Length Does Not Tell You
Getting the length right makes the antenna resonant. It does not by itself make it a good match. A half-wave dipole in free space has a feedpoint impedance near 73 ohms resistive at resonance, but that figure moves substantially with height above ground: it falls toward 50 ohms and below at some heights and rises well above 73 at others, oscillating as the height changes by fractions of a wavelength.
A centre-fed dipole is also a balanced antenna and coaxial cable is unbalanced, so feeding one directly with coax allows current to flow on the outside of the braid. The feedline then radiates, the pattern distorts, and radio-frequency energy appears in the shack. A current balun or common-mode choke at the feedpoint is the standard remedy and is part of building the antenna rather than an optional extra.
Resonance and match are not the same thing as performance either. Pattern, take-off angle and efficiency are determined largely by height and by the ground beneath, and a perfectly matched antenna at a very low height radiates most of its energy straight up. A low standing-wave ratio is a necessary condition for a comfortable transmitter, not evidence that the antenna is working well.
Licensing and Power Limits
Building and listening on an antenna is unrestricted almost everywhere. Transmitting is not. In most jurisdictions, transmitting on the amateur bands requires an amateur radio licence, transmitting on commercial or public-safety allocations requires a specific authorisation, and transmitting without one is an offence. Bands, permitted modes, maximum power and permitted antenna installations are all set by the national spectrum regulator — the FCC in the United States, Ofcom in the United Kingdom, and their equivalents elsewhere — and the rules differ between countries and between licence classes. The relevant service rules and licensing requirements are published by each regulator; the United States amateur service, for example, is described on the FCC's Amateur Radio Service pages.
Power limits are expressed in different ways depending on the service, and effective radiated power matters as much as transmitter output because antenna gain multiplies it. If your limit is written in terms of effective isotropic radiated power, the EIRP calculator on this site works out what your transmitter, feedline losses and antenna gain actually add up to, and that is the number a regulator's limit applies to. Radio-frequency exposure limits are a separate obligation again, with their own assessment requirements and their own distances, and they are not satisfied merely by staying under a power limit.
There are physical hazards too. Antenna work involves height, and antennas near power lines have killed people. The high-voltage ends of a transmitting dipole can deliver a serious radio-frequency burn on contact.
Where This Sits Among the Other Radio Tools
This page turns a frequency into a length. The underlying wavelength arithmetic is handled generally by the wavelength calculator and the frequency converter. For the transmit side, the EIRP calculator combines power, feedline loss and antenna gain, the dBm to watts converter moves between logarithmic and linear power, and the decibel calculator and attenuation calculator deal with gains and losses along the path. Tuned circuits behind the radio are covered by the LC resonant frequency calculator and the RLC impedance calculator.
Arb Digital builds free tools like this one because useful pages earn attention. If you want tools, calculators or content built for your own audience, we can help.
Browse All Free Tools Talk to Arb DigitalCommon Mistakes to Avoid
- Cutting to the calculated length exactly — wire shortens easily and lengthens with difficulty. Cut long, hoist, measure and trim.
- Ignoring insulation — insulated wire typically resonates a few per cent low compared with bare wire of the same length, because the insulation slows the wave along the conductor.
- Tuning at the wrong height — ground proximity shifts both resonance and feedpoint impedance, so an antenna trimmed at head height will not be right once it is up.
- Trimming one leg only — an asymmetric dipole unbalances the feedpoint and puts common-mode current on the feedline, which then radiates.
- Assuming a low SWR means a good antenna — a resonant, well-matched dipole at a very low height still radiates most of its energy upward. Match and performance are different questions.
Related Free Tools From Arb Digital
Convert between frequency and wavelength with the wavelength calculator and the frequency converter. Work the power budget with the EIRP calculator, the dBm to watts converter, the decibel calculator and the attenuation calculator. Design the tuned circuits behind it with the LC resonant frequency calculator and the RLC impedance calculator. Every free tool Arb Digital publishes is listed on the free online tools hub.
Frequently Asked Questions
Total length equals the end-effect factor times the speed of light divided by twice the frequency, which is the same as the factor times half the free-space wavelength. Each leg is half the total. In imperial units the traditional rule is total length in feet equals 468 divided by the frequency in megahertz.
Because of the end effect. Capacitance between the ends of the element and its surroundings lets displacement current continue past the physical tip, so the element behaves electrically as though it were slightly longer than it is. To resonate at the intended frequency it must therefore be cut slightly short, typically to about 95 per cent.
About 0.95 as a starting point for thin bare wire well clear of ground. Thicker conductors need a slightly smaller value, insulated wire smaller again, and a low installation smaller still. The right value for a specific antenna is discovered by measurement rather than looked up.
Enough to matter. Ground is a lossy conductor at a finite distance below the antenna, and coupling between the element and its image shifts both resonance and feedpoint impedance, with the effect changing as height changes by fractions of a wavelength. An antenna trimmed near the ground will not be resonant once it is at working height.
Cut it long, install it at its final height, sweep it with an antenna analyser to find where it is actually resonant, then remove equal amounts from both legs. The fractional change in length needed is roughly the fractional change in frequency required, and it usually takes a few iterations.
Close to 73 ohms resistive at resonance in free space, but the real figure depends strongly on height above ground and can be well below or well above that. A balun or common-mode choke is also needed, because a balanced antenna fed directly with unbalanced coax lets current flow on the outside of the braid and the feedline then radiates.
To receive, generally no. To transmit, yes in most jurisdictions. Amateur bands require an amateur licence, other allocations require specific authorisation, and transmitting without one is an offence. Permitted bands, modes, power and installations are set by the national spectrum regulator and differ between countries and licence classes.
Electrically, close to it. A quarter-wave vertical over a ground system relies on the image in the ground to supply the missing half, so its radiator length is one leg of the equivalent dipole. In practice its performance depends heavily on the quality of the ground or radial system, which has no counterpart in a dipole.
This tool is provided for educational use only. It gives a starting length from a published relation and is not a substitute for measurement: the real resonant length of an installed antenna depends on conductor diameter, insulation, height above ground and surroundings, and must be found with an analyser. Transmitting requires a licence in most jurisdictions and is governed by your national spectrum regulator, whose rules on bands, power, antenna installations and radio-frequency exposure you are responsible for meeting. Antenna work involves working at height and near conductors; keep antennas well clear of power lines.
