Antenna Length Calculator
Antenna Length Calculator
Enter your target operating frequency. Use MHz for HF/VHF, or switch to GHz for microwave bands.
Wire or tubing diameter in millimeters. Affects the end-effect correction (k factor). Typical hookup wire is 1–2 mm; aluminum tubing is 6–25 mm.
Enter Frequency to Calculate
Select an antenna type, enter your operating frequency (or click a ham band preset), and the calculator will instantly show element lengths in both feet and meters.
Calculate precise antenna element lengths for any frequency and antenna type
Whether you are building your first dipole for the 40-meter ham band or designing a 3-element Yagi for VHF work, knowing the correct antenna length is the single most important starting point. An antenna cut to the right length resonates at your target frequency, presents a low SWR to your transmitter, and radiates efficiently. An antenna that is too long or too short produces a high SWR, wastes transmitter power, and can even stress your final amplifier stage. This Antenna Length Calculator takes the guesswork out of antenna construction by applying industry-standard formulas used by amateur radio operators, professional RF engineers, and electronics hobbyists worldwide. This calculator supports six of the most commonly built antenna types: the half-wave dipole, the inverted Vee dipole, the quarter-wave vertical monopole, the 5/8-wave vertical, the full-wave loop, and the classic 3-element Yagi beam. Each type has a distinct radiation pattern, feed impedance, and physical configuration. For example, a half-wave dipole exhibits a figure-8 bidirectional pattern with a feed impedance of approximately 73 ohms in free space, while a quarter-wave vertical produces an omnidirectional pattern at a lower feed impedance of roughly 36 ohms and requires a ground plane or buried radials to work correctly. Understanding these differences is essential before you climb the tower or cut the wire. For ham radio operators, this tool includes quick-select presets for every major amateur radio band from 160 meters all the way down to the 70-centimeter UHF band. Simply click a band label such as 40m or 2m and the center frequency of that band is loaded automatically. You can then fine-tune the frequency to match your specific operating interest within the band — CW operators often favor the lower edge while SSB operators use the upper portion. The calculator immediately shows you all lengths in both feet and meters so you can work with whatever measuring tape or wire reel you have on hand. A key differentiating feature of this tool is the conductor diameter correction factor, often called the k factor or end-effect correction. The classic formula L = 468 / f(MHz) for dipoles in feet assumes a particular ratio of conductor length to diameter. In reality, the physical length of a resonant antenna is slightly shorter than a theoretical half-wavelength in free space because the electric field at the ends of the wire is compressed — an effect called end shortening. Thin wire (small conductor diameter) produces a larger end effect and requires a lower k factor, while thick tubing for VHF/UHF antennas or large aluminum rods needs a slightly higher k factor. This calculator accepts your conductor diameter in millimeters and automatically computes the appropriate correction, giving you a more accurate cut length than the simple 468/f rule can provide. For full-wave loop antennas — popular for their low radiation angle and surprisingly simple construction — the calculator also computes the length of the quarter-wave coaxial matching section needed to transform the loop's nominal 100-ohm feed impedance down to the 75-ohm coaxial cable impedance. You can choose between solid polyethylene coax (velocity factor 0.66) or foam dielectric coax (velocity factor 0.80), or enter a custom velocity factor for specialty transmission lines. The 3-element Yagi section produces all element lengths and spacings using the widely accepted fractional wavelength formulas: reflector at 0.495 lambda, driven element at 0.473 lambda, and director at 0.440 lambda, with spacings of 0.125 lambda between elements. The boom length and an estimated gain of approximately 7.8 dBi are also displayed, giving you everything needed to cut aluminum tubing and mark out boom spacing before assembly. The side-by-side comparison chart is one of this tool's most useful features. After you enter a frequency, the bar chart shows the total wire length for all six antenna types simultaneously. This makes it instantly clear how much more wire a full-wave loop requires compared to a simple dipole, or how compact a quarter-wave vertical is relative to a 5/8-wave version. When you are choosing which antenna to build for a constrained space, this visual comparison can save hours of manual calculation. Always remember the golden rule of antenna building: cut your wire 5 to 10 percent longer than the calculated value, hang the antenna at its intended height, and use an antenna analyzer or SWR meter to find the actual resonant point. Then trim small amounts from each end symmetrically until you reach minimum SWR at your target frequency. This approach accounts for real-world variables including nearby structures, soil conductivity, and the exact composition of your wire insulation — none of which appear in the formula but all of which shift the actual resonant frequency slightly from the calculated value.
Understanding Antenna Length Calculations
What Is Antenna Resonance?
A resonant antenna is one whose physical length causes it to appear as a pure resistance to the transmission line at the design frequency, with no reactive (capacitive or inductive) component. At resonance, the standing wave ratio (SWR) on the feed line reaches its minimum, maximum power is transferred from the transmitter to the antenna, and radiation efficiency peaks. The resonant length of a half-wave dipole in free space would be exactly half the wavelength of the operating frequency, but practical antennas in the real world are always slightly shorter due to an electromagnetic effect at the wire tips called end shortening or end effect. The classic 468/f formula for dipoles already incorporates a 5 percent end-effect correction factor built into the constant 468 (which is 95 percent of the theoretical 492 feet per half wavelength).
How Are Antenna Lengths Calculated?
The fundamental relationship is the wavelength formula: lambda (meters) = 300 / f(MHz), or lambda (feet) = 984 / f(MHz). From there, each antenna type applies a specific fraction of the wavelength and an end-effect correction. Dipole: L(feet) = 468 / f, L(meters) = 143 / f. Quarter-wave vertical: L(feet) = 234 / f, L(meters) = 71.5 / f. Five-eighths vertical: L(feet) = 585 / f, L(meters) = 183.8 / f. Full-wave loop: L(feet) = 1005 / f, L(meters) = 306 / f. Inverted Vee legs are 5 percent longer than horizontal dipole legs because the downward angle of the wire changes its effective electrical length. For the 3-element Yagi, each element is expressed as a fraction of lambda: reflector = 0.495 lambda, driven element = 0.473 lambda, director = 0.440 lambda. Conductor diameter correction (k factor) adjusts these values for real-world wire sizes.
Why Does Antenna Length Matter?
An incorrectly cut antenna creates a reactive impedance mismatch between the antenna and the transmission line. This mismatch produces a high SWR (Standing Wave Ratio), which causes two problems: first, some power is reflected back toward the transmitter rather than being radiated; second, the reflected power heats the coaxial cable and can damage the transmitter's output stage if SWR protection is not present. Modern transceivers have built-in SWR protection that reduces output power when SWR exceeds about 2:1, so a poorly cut antenna directly reduces your effective radiated power. Additionally, different antenna types have different feed impedances — a dipole is approximately 73 ohms, a quarter-wave vertical is about 36 ohms, and a full-wave loop is about 100 ohms — and knowing these values is essential for selecting the correct balun, matching network, or coaxial cable type.
Limitations and Real-World Considerations
Antenna length calculators provide an excellent starting point but cannot account for every real-world variable. Height above ground significantly affects the resonant frequency of horizontal antennas — the lower the antenna, the more the ground capacitance detunes it, typically requiring a longer physical length than the formula predicts. Nearby conductive objects such as metal roofs, power lines, towers, and even dense tree foliage interact with the antenna's near field and shift resonance. Wire insulation adds a small amount of capacitance along the wire, effectively lengthening it electrically — insulated wire typically resonates 2 to 5 percent shorter than bare wire of the same physical length. The formulas in this calculator assume an isolated antenna in free space; always plan to tune your antenna empirically with an antenna analyzer or SWR meter after installation. Build 5 to 10 percent longer than calculated and trim to resonance.
Formulas
The classic dipole formula with a built-in 5% end-effect correction. The theoretical free-space half wavelength is 492/f, reduced to 468/f to account for end shortening caused by the electric field compression at the wire tips.
Fundamental relationship between frequency and wavelength. All antenna length formulas derive from this — each antenna type uses a specific fraction of the wavelength multiplied by an end-effect correction factor.
Exactly half the dipole length. A quarter-wave vertical requires a ground plane (radials or conductive surface) to complete the antenna system. Feed impedance is approximately 36 ohms over a perfect ground plane.
Total wire length for a full-wave loop antenna regardless of shape (square, triangle, or circle). Feed impedance is approximately 100 ohms, requiring a matching section to 50-ohm coax.
Reference Tables
Ham Radio Band Center Frequencies and Dipole Lengths
| Band | Center Freq (MHz) | Dipole Length (ft) | Dipole Length (m) | Quarter-Wave (ft) |
|---|---|---|---|---|
| 160m | 1.9 | 246.3 | 75.3 | 123.2 |
| 80m | 3.75 | 124.8 | 38.1 | 62.4 |
| 40m | 7.15 | 65.5 | 20.0 | 32.7 |
| 20m | 14.175 | 33.0 | 10.1 | 16.5 |
| 15m | 21.225 | 22.1 | 6.7 | 11.0 |
| 10m | 28.85 | 16.2 | 5.0 | 8.1 |
| 6m | 52.0 | 9.0 | 2.75 | 4.5 |
| 2m | 146.0 | 3.2 | 0.98 | 1.6 |
| 70cm | 440.0 | 1.06 | 0.325 | 0.53 |
Antenna Type Comparison
| Antenna Type | Wire Fraction of λ | Feed Impedance (Ω) | Radiation Pattern | Gain (dBi) |
|---|---|---|---|---|
| Half-Wave Dipole | 0.5λ | ~73 | Bidirectional (figure-8) | 2.15 |
| Inverted Vee | 0.5λ × 1.05 | ~52 | Near-omnidirectional | 2.0 |
| Quarter-Wave Vertical | 0.25λ | ~36 | Omnidirectional | 2.15 + ground gain |
| 5/8-Wave Vertical | 0.625λ | ~35 (needs matching) | Omnidirectional, low angle | 3.2 |
| Full-Wave Loop | 1.0λ | ~100 | Bidirectional, low angle | 3.5 |
| 3-Element Yagi | 0.473λ driven | ~25 (needs matching) | Directional, 15–20 dB F/B | 7.8 |
Worked Examples
40-Meter Band Dipole Antenna
Total length = 468 ÷ 7.15 = 65.45 feet
Each leg = 65.45 ÷ 2 = 32.73 feet
In meters: 143 ÷ 7.15 = 20.0 meters total, 10.0 meters per leg
Add 5–10% extra for tuning: cut at ~69 feet total, then trim
2-Meter VHF Quarter-Wave Vertical
Quarter-wave length = 234 ÷ 146.0 = 1.603 feet = 19.23 inches
In meters: 71.5 ÷ 146.0 = 0.490 meters = 49.0 cm
Cut four radials to the same length (19.23 inches each)
Feed impedance ≈ 36 Ω; angling radials 45° downward raises it to ~50 Ω
20-Meter Full-Wave Loop with Matching Section
Loop circumference = 1005 ÷ 14.175 = 70.9 feet (21.6 m)
Wavelength at 14.175 MHz: λ = 984 ÷ 14.175 = 69.4 feet
Quarter-wave matching section (free space) = 69.4 ÷ 4 = 17.35 feet
Apply velocity factor: 17.35 × 0.66 = 11.45 feet (3.49 m) of RG-59
How to Use This Calculator
Select Your Frequency
Type your target operating frequency in the Frequency field and choose MHz or GHz. Alternatively, click one of the ham band quick-select buttons (160m through 70cm) to load the center frequency of that band automatically.
Choose Your Antenna Type
Click the antenna type that matches what you want to build: Half-Wave Dipole for a basic horizontal wire antenna, Quarter-Wave Vertical for a ground-plane antenna, Full-Wave Loop for a wire loop, Inverted Vee for a center-supported dipole with drooping legs, 5/8-Wave Vertical for higher gain mobile or base antennas, or 3-Element Yagi for a directional beam antenna.
Adjust Advanced Inputs
Enter your conductor diameter in millimeters for a more accurate end-effect correction factor (k). If you are building a full-wave loop, enter the velocity factor of your coaxial matching section (0.66 for standard polyethylene coax, 0.80 for foam coax). The calculator updates all results automatically as you type.
Read Results and Build
Note the total length and element or leg length in both feet and meters. For Yagi antennas, record all element lengths and spacings. Use the comparison chart to see how your chosen type relates to other antenna types at the same frequency. Export to CSV for your logbook, or print the results page before heading to the workshop.
Frequently Asked Questions
Why does the calculator give a different length than the simple 468/f formula?
The classic 468/f formula for dipoles in feet already incorporates an end-effect correction of approximately 5 percent, reducing the theoretical free-space half-wavelength of 492/f to 468/f. This calculator goes one step further by adjusting the correction factor based on your conductor diameter. Thin wire has a slightly higher ratio of length to diameter, which increases the end effect and may require a lower k factor (and therefore a slightly shorter length), while thick aluminum tubing used in VHF arrays has a lower ratio and needs less correction. For most HF wire antennas using standard hookup wire (1–2 mm), the difference from the classic formula is small — typically less than 1 percent — but for precision VHF or UHF work it can be meaningful.
What is the difference between a dipole and an inverted Vee?
Both are half-wave center-fed antennas, but while a dipole has both legs horizontal, an inverted Vee has both legs sloping downward from a central apex, forming a V shape when viewed end-on. The downward angle of the legs changes their effective electrical length — the horizontal component of the wire interacts with the ground differently than a purely horizontal wire. As a result, inverted Vee legs need to be about 5 percent longer than dipole legs to achieve the same resonant frequency. The feed impedance also drops from about 73 ohms (free-space dipole) to approximately 52 ohms for an inverted Vee, making it a much better direct match to 50-ohm coaxial cable without a balun.
How many radials does a quarter-wave vertical need, and how long should they be?
A quarter-wave vertical requires a ground plane to complete the antenna circuit. The most common approach is to install at least four radial wires, each cut to a quarter wavelength, at the base of the vertical element. More radials improve efficiency: 16 buried radials give a significant improvement over 4, and professional broadcast towers use 120 radials. The radials do not need to be elevated — burying them 5 to 10 cm underground works well. For mobile installations on a vehicle, the metal vehicle body itself serves as the ground plane, which is why a quarter-wave whip on a car trunk lid works without any additional radials. The feed impedance of an ideal quarter-wave vertical over a perfect ground plane is approximately 36 ohms; real-world installations typically measure 50 ohms or slightly above, making them a good match to standard 50-ohm coax.
What are the advantages of a full-wave loop antenna?
A full-wave loop has several practical advantages over a simple dipole. Its total wire length (approximately 1005/f in feet) is the same regardless of whether you form it as a square, circle, or triangle, giving you flexibility to fit available space. The radiation pattern has a lower angle of radiation than a dipole at the same height, which generally means better DX (long-distance) performance on HF. The feed impedance is approximately 100 ohms, requiring a matching transformer or a quarter-wave coaxial section to feed with 50-ohm coax — this calculator computes the matching section length for you. Loop antennas also tend to be less sensitive to nearby objects than dipoles, making them practical for restricted installation spaces like small backyards or rooftops.
What gain does a 3-element Yagi provide, and is it better than a dipole?
A properly built 3-element Yagi provides approximately 7 to 9 dBi of forward gain, which is equivalent to 5 to 7 dBd (decibels over a dipole reference). In practical terms, this means your transmitted signal appears roughly 3 to 5 times stronger in the forward direction compared to a dipole running the same power. Equally important is the front-to-back ratio: a well-designed 3-element Yagi suppresses signals arriving from behind by 15 to 20 dB, which dramatically reduces interference from stations off the back of the beam. The trade-off is physical size — a 3-element Yagi for the 20-meter band has a boom around 3 to 4 meters long and requires a rotator to point it toward desired stations. For the 2-meter VHF band, the same design fits in a compact package less than a meter in length, making it ideal for satellite work or weak-signal EME (moonbounce) communication.
Why should I build the antenna longer and then trim it down?
Every real-world installation differs from the free-space model assumed by antenna length formulas. The height of the antenna above ground, soil conductivity at your location, nearby metal structures, tree foliage, and the presence of wire insulation all shift the actual resonant frequency from the calculated value. These effects almost always make the antenna resonate lower in frequency than predicted, meaning the antenna is effectively electrically too long. Starting longer guarantees you can trim the antenna down to the correct resonant frequency. If you start at exactly the calculated length and your antenna turns out to resonate too high in frequency (uncommon but possible at certain heights), you would need to splice in additional wire — a much harder repair. The 5 to 10 percent margin is a well-established rule of thumb in amateur radio construction practice, recommended by the ARRL Antenna Handbook and every major antenna building guide.