Dipole Antenna Calculator
Dipole Antenna Calculator
Enter frequency in MHz or GHz. Use band presets below for common amateur and commercial frequencies.
Accounts for conductor type and insulation. Default 0.95 suits bare copper wire.
Applies an additional ~3% length correction for insulated wire antennas.
Enter a Frequency to Begin
Select your target frequency and antenna type above. The calculator will instantly show total length, each leg, wavelength reference values, feedpoint impedance, and a comparison chart across all antenna types.
Ham Radio Band Reference Table (Half-wave Dipole)
| Band | Frequency | Total (ft) | Total (m) | Leg (m) |
|---|---|---|---|---|
| 160m | 1.9 MHz | 246.3 | 75.08 | 37.54 |
| 80m | 3.75 MHz | 124.8 | 38.04 | 19.02 |
| 60m | 5.3325 MHz | 87.8 | 26.75 | 13.38 |
| 40m | 7.15 MHz | 65.5 | 19.95 | 9.98 |
| 30m | 10.125 MHz | 46.2 | 14.09 | 7.04 |
| 20m | 14.175 MHz | 33.0 | 10.06 | 5.03 |
| 17m | 18.1 MHz | 25.9 | 7.88 | 3.94 |
| 15m | 21.225 MHz | 22.0 | 6.72 | 3.36 |
| 12m | 24.94 MHz | 18.8 | 5.72 | 2.86 |
| 10m | 28.85 MHz | 16.2 | 4.94 | 2.47 |
| 6m | 51 MHz | 9.2 | 2.80 | 1.40 |
| 2m | 146 MHz | 3.2 | 0.98 | 0.49 |
| 70cm | 435 MHz | 1.1 | 0.33 | 0.16 |
| CB | 27 MHz | 17.3 | 5.28 | 2.64 |
| FM | 98 MHz | 4.8 | 1.46 | 0.73 |
Calculate half-wave, folded, inverted vee, and full-wave dipole lengths from frequency
A dipole antenna is one of the most fundamental and widely used antenna designs in radio communication. Whether you are a licensed amateur radio operator building your first HF antenna, an RF engineer designing a reception element, or a hobbyist experimenting with IoT and LoRa devices, knowing the correct physical length for your dipole is the essential starting point. This dipole antenna calculator gives you precise lengths in both feet and meters based on your target frequency, with support for velocity factor corrections, wire insulation adjustments, and four antenna configurations. The classic half-wave dipole derives its name from the fact that it is cut to resonate at one half of the target wavelength. At resonance, the antenna presents a predominantly resistive load to the feedline, maximizing energy transfer and minimizing reflected power. The standard design formula, 468 divided by frequency in MHz giving length in feet, is a time-tested approximation that accounts for the end-effect — the slight shortening of the resonant length compared to a theoretical free-space half wavelength. This calculator uses the more accurate velocity-factor form of the formula so you can adjust for different conductor materials and insulation. Beyond the basic dipole, this tool supports three important variants. The folded dipole has the same physical length as a standard half-wave dipole but its wire is folded back on itself with two parallel conductors joined at both ends. This construction raises the feedpoint impedance from 73 ohms to approximately 300 ohms, making it ideal for connection via 300-ohm twin-lead or with a 4:1 balun to 75-ohm coax. Folded dipoles are commonly used as the driven element in Yagi arrays and as FM broadcast receive antennas. The inverted vee configuration hangs from a single apex support point with the two legs sloping downward at 45 degrees or more. Because the legs are not horizontal, the effective electrical length is slightly shorter than a horizontal dipole, and the resonant formula uses 449 instead of 468 for feet. The inverted vee offers a more omnidirectional radiation pattern and requires only one tall support — a significant practical advantage. The full-wave dipole is simply double the half-wave length, offering slightly higher gain broadside to the antenna and commonly used for NVIS (Near Vertical Incidence Skywave) operation on the lower HF bands. Velocity factor is a critical parameter when the antenna element is not bare copper wire or aluminum tubing. Insulated wire slows the propagation of the RF wave along the conductor, effectively shortening the electrical wavelength. A velocity factor of 0.95 is standard for bare copper wire, 0.98 for aluminum or copper tubing, 0.93 for insulated hookup wire, and 0.66 for the inner conductor of coaxial cable when coax is used as the antenna element itself. Entering the correct velocity factor for your conductor ensures the calculated length will resonate at your target frequency without the need for extensive trimming. This calculator also displays full wavelength, half-wavelength, and quarter-wavelength reference values for your frequency. These are useful when designing matching sections, planning feed arrangements, or calculating the physical dimensions of related antenna elements like directors and reflectors. The feedpoint impedance and recommended balun type are shown for each antenna configuration, helping you choose the right coaxial cable and impedance-matching device for your installation. Practical tuning guidance is included: always cut the antenna a few percent longer than the calculated value, then trim gradually while monitoring SWR to find the actual resonant point in your specific installation environment.
Understanding Dipole Antennas
What Is a Dipole Antenna?
A dipole antenna consists of two conductive elements — called arms or legs — arranged in a straight line with a feed point at the center where the transmission line connects. The name 'dipole' refers to the two poles or elements extending outward from the feed point. A half-wave dipole has a total length equal to approximately one half of the wavelength at the design frequency. This causes the antenna to resonate, meaning it presents a predominantly resistive impedance at the feedpoint (approximately 73 ohms in free space) and efficiently converts transmitter power into radiated electromagnetic waves. Dipoles radiate a figure-eight or doughnut-shaped pattern perpendicular to the antenna axis, making them useful for point-to-point communication across a broad broadside direction. They are easy to construct from common wire, require minimal hardware, and work well from medium frequencies through UHF.
How Is the Length Calculated?
The standard formula for a half-wave dipole length in feet is 468 divided by frequency in MHz. In metric units the formula is 143 divided by frequency in MHz for total length in meters. These constants derive from the free-space half-wavelength formula (492 / f in feet, or 150 / f in meters) multiplied by an end-effect correction factor of approximately 0.95. The end-effect accounts for the increased capacitance near the wire ends that shortens the resonant length compared to a theoretical wire in perfect free space. When using a non-unity velocity factor k, the formula becomes L(m) = 142.65 × k / f(MHz). Wire insulation further reduces resonant frequency by approximately 3%, so the insulated wire correction multiplies the result by 0.97. The inverted vee uses the constant 449 instead of 468, reflecting the slightly shorter electrical length produced by the sloping leg geometry. Full-wave uses 936 (twice 468).
Why Does Dipole Length Matter?
Antenna length critically determines whether the antenna resonates at your target frequency. A dipole that is too long will appear inductive at the feedpoint, increasing reflected power and raising the Standing Wave Ratio (SWR) on your transmission line. A dipole that is too short will appear capacitive, with the same problematic effects. High SWR causes increased heating of the feedline and transceiver finals, reduced transmitted power, and potential damage to solid-state amplifiers with SWR protection circuits. At resonance, the antenna presents a mostly resistive load — approximately 73 ohms for a half-wave dipole — which closely matches 50-ohm coaxial cable with an SWR of approximately 1.46:1, generally acceptable without a tuner. Getting the length right from the start minimizes trimming time and ensures reliable performance.
Practical Limitations and Tuning Tips
Calculated dipole lengths are starting points, not final answers. Real-world factors that shift the resonant frequency include height above ground (lower antennas resonate lower due to ground proximity effects), conductor diameter (thicker conductors resonate slightly higher), nearby structures and trees, feed line routing, and the dielectric constant of any surrounding insulation or supports. Always build your dipole 5 to 10 percent longer than the calculated value, then trim both legs equally by small amounts while measuring SWR at the target frequency. An antenna analyzer or SWR meter is invaluable for this process. For best balance and rejection of common-mode currents on the feedline, always use a 1:1 current balun (for standard dipoles and inverted vees) or a 4:1 voltage balun (for folded dipoles). Choose wire gauge 14 to 12 AWG for permanent installations requiring mechanical strength over long spans.
Formulas
Total half-wave dipole length in feet, where k is the velocity factor (0.95 for bare copper wire) and f is the frequency in MHz. The constant 468 accounts for the end-effect correction applied to the free-space half-wavelength (492/f).
Total half-wave dipole length in meters, where k is the velocity factor. Each leg is half the total length. The constant 142.65 is derived from 150 × 0.951 (speed of light half-wavelength with end-effect correction).
Inverted vee total length in feet. The constant 449 (vs 468 for horizontal dipole) reflects the ~4% shorter electrical length caused by the sloping leg geometry. Feedpoint impedance is approximately 52 ohms.
Full wavelength in meters at the target frequency, derived from the speed of light (299,792,458 m/s) divided by frequency. Used as a reference for calculating half-wave, quarter-wave, and matching section dimensions.
Reference Tables
Ham Radio Band Dipole Lengths (Half-Wave, Bare Copper k=0.95)
| Band | Center Freq (MHz) | Total Length (ft) | Total Length (m) | Each Leg (ft) |
|---|---|---|---|---|
| 160 m | 1.9 | 234.0 | 71.3 | 117.0 |
| 80 m | 3.75 | 118.6 | 36.1 | 59.3 |
| 40 m | 7.15 | 62.2 | 19.0 | 31.1 |
| 20 m | 14.175 | 31.4 | 9.57 | 15.7 |
| 15 m | 21.225 | 20.9 | 6.39 | 10.5 |
| 10 m | 28.5 | 15.6 | 4.76 | 7.8 |
| 6 m | 51.0 | 8.7 | 2.66 | 4.4 |
| 2 m | 146.0 | 3.05 | 0.93 | 1.52 |
Velocity Factor by Conductor Type
| Conductor Type | Velocity Factor (k) | Typical Use |
|---|---|---|
| Bare copper wire | 0.95 | Standard dipole antennas |
| Aluminum tubing | 0.98 | Yagi elements, VHF/UHF dipoles |
| Insulated hookup wire | 0.93 | Indoor/stealth antennas |
| Coaxial cable (inner) | 0.66 | Coax collinear, J-pole feed |
| 300-ohm twin-lead | 0.82 | Folded dipole, FM antennas |
Worked Examples
20-Meter Band Half-Wave Dipole
Apply the formula: L(ft) = 468 × k / f(MHz)
L(ft) = 468 × 0.95 / 14.175 = 444.6 / 14.175 = 31.37 ft
Each leg = 31.37 / 2 = 15.68 ft
In meters: L(m) = 142.65 × 0.95 / 14.175 = 9.56 m; each leg = 4.78 m
2-Meter Inverted Vee with Insulated Wire
Apply the inverted vee formula with insulation: L(ft) = 449 × k × 0.97 / f(MHz)
Effective velocity factor = 0.93 × 0.97 = 0.9021
L(ft) = 449 × 0.9021 / 146 = 405.04 / 146 = 2.774 ft
Each leg = 2.774 / 2 = 1.387 ft = 16.6 inches
Folded Dipole for FM Broadcast Reception
Folded dipole uses the same length formula as a half-wave dipole: L(ft) = 468 × k / f(MHz)
L(ft) = 468 × 0.98 / 98 = 458.64 / 98 = 4.68 ft
Each arm = 4.68 / 2 = 2.34 ft = 28.1 inches
Feedpoint impedance ≈ 300 Ω — matches 300-ohm twin-lead directly, or use a 4:1 balun for 75-ohm coax
How to Use This Calculator
Enter Your Target Frequency
Type the frequency in the input field. You can enter values in MHz (e.g., 14.175 for the 20-meter ham band) or switch to GHz for microwave frequencies. Use the quick band preset buttons to instantly load popular amateur radio bands, CB, FM broadcast, or WiFi frequencies.
Select Antenna Type and Velocity Factor
Choose your antenna configuration: Half-wave Dipole (most common), Folded Dipole (300Ω, used with 4:1 balun), Inverted Vee (single support, omnidirectional), or Full-wave Dipole (NVIS/low-angle). Then select the velocity factor matching your conductor — bare copper wire is 0.95, metal tubing is 0.98, insulated wire is 0.93. Enable the Insulated Wire toggle if your wire has a plastic jacket for an additional 3% correction.
Read Your Antenna Dimensions
Results appear instantly showing total antenna length and each leg length in both feet and meters. The visual dipole diagram illustrates the physical layout with the feed point marked at center. Wavelength reference values (full, half, and quarter wavelength) are shown for designing matching sections or related antenna elements.
Check Feedpoint Info and Build Longer
Review the feedpoint impedance, recommended balun type, and coaxial cable recommendation for your antenna type. When cutting wire, always add 5–10% extra length and trim gradually while measuring SWR. Export your results to CSV for field notes, or print the page for reference during antenna construction.
Frequently Asked Questions
Why is the dipole formula 468/f instead of 492/f?
The theoretical free-space half-wavelength of a wire antenna is 492 divided by frequency in MHz (in feet). However, real wires experience an 'end effect' caused by increased capacitance near the wire tips, which shortens the physical length needed for resonance compared to the free-space ideal. Multiplying 492 by a typical end-effect correction factor of 0.95 gives approximately 468. This empirical constant was established through decades of practical antenna measurement and is accurate for thin wire antennas (14–18 AWG) at typical installation heights. Thicker conductors like aluminum tubing use a slightly higher factor (0.97–0.98), while heavily insulated wire may need values as low as 0.93.
What is velocity factor and why does it matter for antenna length?
Velocity factor (k) describes how fast an electromagnetic wave travels along a conductor relative to the speed of light in free space. Bare copper wire has a velocity factor of approximately 0.95 — meaning RF travels at 95% of the free-space speed. Insulated wire is slower (0.93) because the dielectric material of the insulation slows the wave. Coaxial cable inner conductor can be as low as 0.66. Since the physical antenna length for resonance equals the electrical half-wavelength multiplied by the velocity factor, using an incorrect value results in an antenna that is too long or too short, producing elevated SWR. Always select the velocity factor that matches your actual conductor type.
What is the difference between a folded dipole and a standard dipole?
A standard half-wave dipole consists of a single wire fed at the center, presenting approximately 73 ohms at the feedpoint. A folded dipole uses the same physical wire length but the wire is bent back on itself to form a narrow loop with two parallel conductors connected at both ends. This parallel current path multiplies the feedpoint impedance by a factor of four, producing approximately 300 ohms. This impedance is a perfect match for 300-ohm twin-lead transmission line, which was once the standard for TV and FM antennas. With a 4:1 balun, it can also feed 75-ohm coax. Folded dipoles are also commonly used as the driven element in Yagi-Uda directional antennas due to their broader impedance bandwidth.
Why is an inverted vee shorter than a horizontal dipole?
An inverted vee hangs its two legs downward from a central apex point at an angle, rather than running horizontally. When legs are not horizontal, their effective electrical length is reduced because the component of the electromagnetic field coupling changes with the angle. The commonly used formula constant for an inverted vee is 449 (versus 468 for a horizontal dipole), representing approximately a 4% reduction. Additionally, the inverted vee's legs being closer to ground means ground proximity effects are stronger, which also shifts resonance slightly. The feed-point impedance of an inverted vee is approximately 52 ohms — a much closer match to 50-ohm coax than the 73-ohm horizontal dipole, making it possible to operate without a balun in some installations.
Do I need a balun on my dipole antenna?
A current balun (1:1) is strongly recommended for dipole antennas even though the antenna can operate without one. Without a balun, RF current can flow on the outside of the coaxial cable shield back toward the transmitter, causing common-mode interference, RF in the shack, erratic SWR readings, and potential touch-hazard. A 1:1 current balun placed at the feedpoint prevents this by forcing equal and opposite currents in each dipole leg. For folded dipoles, a 4:1 voltage balun transforms the 300-ohm feedpoint impedance down to 75 ohms for coax matching. Choose a balun rated for at least twice your transmitter output power. Commercial ferrite choke baluns are compact and effective; DIY coax-wound choke baluns using RG-8X or similar are also popular among amateur radio operators.
How does antenna height above ground affect resonant length?
Height above ground significantly affects dipole resonant frequency and feedpoint impedance. At very low heights (less than 0.1 wavelength above ground), ground coupling dramatically lowers the resonant frequency and changes feedpoint impedance, often requiring a shorter antenna than the formula predicts. At typical amateur radio heights of 0.25 to 0.5 wavelengths, the 468/f formula gives a reasonable starting point. Above about one wavelength, free-space values are approached closely. Ground conductivity also plays a role — dry sandy soil is a poor reflector while salt water is an excellent one, each producing different impedance and resonant frequency shifts. For HF operation, the general advice is to install the antenna as high as practical, measure SWR with an antenna analyzer, and trim to resonance at your actual installation height.