Dipole Antenna Length Calculator

Frequently Asked Questions

How does a dipole length change with frequency?

Dipole length is inversely proportional to frequency: double the frequency and the antenna is half as long. At 7 MHz (40-meter amateur band), a half-wave dipole is about 66 feet total. At 14 MHz (20-meter band), the same calculation gives 33 feet. At 144 MHz (2-meter VHF band), the dipole shrinks to about 39 inches. This inverse relationship is why HF antennas must be very large structures while microwave antennas can be tiny. The wavelength at 100 MHz is about 3 meters; at 10 GHz it is only 3 centimeters.

Why does my VSWR improve when I trim the dipole?

When a dipole is too long for its operating frequency, it presents an inductive reactance at the feed point in addition to its radiation resistance. This reactance causes a mismatch with the 50 Ω or 75 Ω feed system, producing a high VSWR. Trimming each leg removes some of this inductance. As you approach the resonant length, the reactive component drops toward zero, leaving only the radiation resistance (around 73 Ω) at the feed point, and the VSWR improves accordingly. Conversely, a dipole that is too short appears capacitive, and you would need to add length or add a loading coil to correct it. This is why the standard advice is to cut slightly long and trim - you can always remove wire but you cannot add it back.

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How This Calculator Works

A half-wave dipole is the most common antenna in amateur radio. Its overall length is roughly half the wavelength of the signal it transmits or receives. In free space a half wave is 492 divided by the frequency in megahertz (in feet), but a real wire mounted near the ground behaves as if slightly longer, so the widely used practical constant is 468.

This tool divides 468 by your frequency for the total length, divides 234 by the frequency for each leg, and converts the total to meters.

Trimming To Resonance

Cut your wire slightly long, raise the antenna to its operating height, and trim small amounts from each end while checking the standing wave ratio. Tune for the lowest SWR at your target frequency. Trimming both legs equally keeps the feed point balanced.

Tips

Always measure twice and cut a few inches long; you can trim but not add.

Height above ground shifts resonance, so tune in place.

Use a feedline choke to keep RF off the coax shield.

Dipole Variants and Their Uses

The basic half-wave dipole spawns a large family of related antennas, each optimized for specific requirements:

Half-wave dipole: The foundation. 2.15 dBi gain, 73 Ω feed impedance at resonance, figure-8 radiation pattern in the broadside direction. Used from HF through UHF.

Folded dipole: Both ends of the dipole are connected to form a closed loop with the feedpoint in the middle of one conductor. Feed impedance is approximately 300 Ω, which matches twin-lead transmission line directly. This is the classic indoor TV antenna element and the driven element in many Yagi designs.

Sleeve dipole: Fed via a coaxial cable with the outer conductor forming one half and a sleeve over the coax forming the other. The coaxial feed makes it practical for vertical mounting without a balun. Common in commercial VHF and UHF installations.

J-pole and Slim Jim: End-fed half-wave variants that require no ground plane, making them popular for portable VHF/UHF operation and vertical mounting on non-conductive masts. The matching section is built into the antenna itself.

Collinear array: Multiple half-wave dipoles stacked vertically and fed in phase. Each additional element adds roughly 3 dB of gain while maintaining the omnidirectional horizontal pattern. A four-element collinear achieves around 6-7 dBi. Standard on VHF/UHF commercial and amateur repeater sites.

LPDA (log-periodic dipole array): A broadband array of dipoles of increasing length, fed alternately out of phase. Each frequency is dominated by the dipole that is closest to half-wave resonance at that frequency, giving wideband coverage. Used for TV reception from 54-806 MHz and for HF communication antennas covering multiple amateur bands.

Matching and Feeding the Dipole

A half-wave dipole has a feed impedance of approximately 73 Ω at resonance - close to but not exactly the 50 Ω characteristic impedance of standard coaxial cable. The resulting VSWR of about 1.46:1 is acceptable for many applications, but purists use a matching network for the best efficiency. More important than the impedance mismatch is the balanced-to-unbalanced transition: the dipole is a balanced antenna (both legs at equal and opposite voltages relative to ground) while coaxial cable is unbalanced (the outer conductor is at ground potential). Connecting coax directly to a dipole without a balun allows common-mode current to flow on the outside of the coax shield, which distorts the antenna pattern and can cause RF to appear on equipment connected to the other end of the coax.

A 1:1 current balun (choke balun) is the simplest solution: it presents a high impedance to common-mode currents while passing differential (balanced) signals. It can be made by winding several turns of coax through a ferrite toroid or by coiling the coax into a ferrite-loaded coil. A 4:1 impedance balun transforms the 73 Ω balanced dipole feed to approximately 290 Ω, which after a short matching section can give a good match to 50 Ω coax. A lambda/4 matching transformer using 75 Ω coax provides a 50 Ω to 73 Ω match at a single frequency.

Practical Installation

Orientation matters. Horizontal polarization is used for HF broadcasting and amateur radio because the ionosphere preserves horizontal polarization over long paths. Vertical polarization dominates mobile and marine communication because a vertical whip antenna on a vehicle naturally radiates vertically and because vertical polarization experiences less ground reflection loss at low elevation angles. Height above ground has a significant effect: as the antenna height changes from 0.1 to 0.5 wavelengths, the feed impedance swings between 45 and 95 Ω and the radiation pattern shape changes. For HF work, a height of 0.5 wavelengths is a good target for a single dipole. Mounting a dipole over a metal roof or along a conductive surface detunes it by changing the effective electrical length, often requiring shortening the wire to restore resonance.

Dipole in Real-World Applications

The dipole's combination of simplicity, predictable performance, and manufacturability makes it ubiquitous across all of radio history:

AM broadcast: AM stations use phased arrays of vertical antennas (electrically equivalent to monopoles, which are half a dipole over a ground plane) to shape their coverage patterns and protect co-channel stations in adjacent markets. The FCC mandates specific directional patterns, achieved by adjusting the phase and amplitude fed to each tower.

VHF/UHF television: The classic rabbit-ear antenna on a set-top TV is an adjustable dipole for VHF channels and a separate loop or bow-tie element for UHF. The folded dipole with 300 Ω matching was standard in outdoor Yagi TV antennas for decades.

HF amateur radio: The dipole hung between two supports is often the first antenna a new amateur builds, and many experienced operators return to it because a well-built dipole at a good height outperforms many more complex designs. Bands from 80 meters (3.5 MHz, 133 ft dipole) to 10 meters (28 MHz, 16 ft dipole) all use this basic design.

Direction finding: An Adcock antenna consists of two dipole pairs arranged in a cross, used to determine the direction of arrival of a radio signal. This technique is used in search and rescue, spectrum enforcement, and navigation.

Yagi driven element: The Yagi-Uda directional antenna uses a dipole (often folded) as its driven element, with parasitic reflector and director elements that modify the pattern without being directly connected to the feedline.

Dipole Antenna Length Calculator

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