Dipole Antenna Length Calculator: Free Tool + Full Tuning Guide

Q: How much shorter should an inverted V be than a horizontal dipole?

About 2 to 5 percent shorter than a flat horizontal dipole at the same frequency, depending on the angle between the legs. A common shortcut is to use 449 instead of 468 in the formula. 120 degrees between the legs is the commonly recommended target, with 90 degrees as a practical minimum; a 90 degree apex angle actually gives close to the best 50-ohm impedance match of any angle, with only a small reduction in gain. SWR only degrades quickly below roughly 60 degrees.

Q: Do I need a balun on a dipole?

It is strongly recommended. A dipole is a balanced antenna and coax is an unbalanced feedline, and feeding a balanced antenna directly with coax can let RF current flow on the outside of the coax shield. A 1:1 choke balun at the feedpoint suppresses that common-mode current, which helps keep your radiation pattern as designed and avoids RF feedback symptoms in the shack (hot mic, computer interference). A well-built, symmetrical dipole installation may show only minor common-mode issues on its own, but adding a choke balun is cheap insurance and resolves the problem in cases where it does occur.

Q: How high should a dipole be mounted?

Higher is generally better for DX, with a half-wavelength above ground being a common target for serious HF work. In practice, most hams cannot get a dipole that high. A dipole at 25 to 35 feet still performs well for both regional (NVIS-style) and DX contacts, and the most repeated piece of practical wisdom in amateur radio still applies: the best antenna is the one that is actually up in the air.

Q: Will a calculated dipole length actually be resonant when I install it?

Not exactly, and that is normal. The 468/f formula assumes a generic wire diameter and free-space-like conditions. Real installations are affected by wire gauge and insulation, height above ground, nearby metal objects and structures, soil conductivity, and the antenna's actual shape. Cut the wire 3 to 5 percent longer than calculated, hoist it at the planned height, measure SWR across the band, and trim small, equal amounts from both ends until the lowest SWR point lands on your target frequency.

⚡ Instant Calculator

Dipole & Inverted-V Length

Calculates a practical starting length — cut a little long and trim to resonance (see the tuning guide below).

Frequency (MHz)

Configuration

Total Length

—

Each Leg

—

Total (Metric)

—

Cut Length (+4%)

—

Formula used: horizontal dipole = 468 ÷ frequency (MHz) for total length in feet; inverted-V = 449 ÷ frequency (MHz). "Cut Length" adds 4% so you have trimming margin — see how to trim to resonance below.

📋 In This Guide

How the calculator works
Why 468 (and not 492)?
Horizontal dipole vs. inverted-V
How to trim to resonance
Height, feedline & balun basics
Common mistakes that throw off resonance
Other antenna tools on KE6MGB
Frequently asked questions

📐 How the Calculator Works

A half-wave dipole resonates when its total length is close to half the wavelength of your operating frequency. In free space, that half-wavelength works out to 492 ÷ frequency (MHz), in feet. Real wire isn't in free space, though — it has thickness, and the open ends of the wire create a small capacitive effect (called end effect) that makes the antenna behave as if it's slightly longer than it physically is. The practical result: a real dipole resonates at about 95% of the free-space length, which is where the familiar 468 ÷ f formula comes from.

492/f

Free-space half-wave (ft)

468/f

Real horizontal dipole (ft)

449/f

Typical inverted-V (ft)

143/f

Horizontal dipole (m)

This calculator uses those exact constants. It gives you a real, usable starting length — not a guess — but no formula accounts for your specific wire gauge, insulation, height, or what's near the antenna. That's normal, and it's why every dipole still gets trimmed after it goes up. More on that in the tuning section below.

🔍 Why 468 (and Not 492)?

This is the question almost every new ham asks eventually, and the honest answer is more interesting than "it's just the formula." The number 468 first appeared in the 1929 ARRL Handbook, derived from real-world antenna experiments from the mid-1920s rather than a clean theoretical equation — early builders kept finding that wire dipoles resonated a bit shorter than the textbook free-space half-wavelength (492/f) predicted, and 468 became the round number that captured that real-world shortening reasonably well for the 80m/40m antennas most common at the time.

The actual end-effect correction varies with antenna height above ground, wire gauge, and nearby objects. Modern modeling of a 20m dipole at heights from 1/8 to 2 wavelengths has produced ideal constants ranging from about 466.4 to 483.4 — not a single fixed number at all. 468 is simply the most useful general-purpose average, not a fixed physical law. That's exactly why this calculator (and every other dipole calculator) gives you a starting length to cut and trim, rather than a guaranteed final answer.

Worked example — 20m dipole at 14.2 MHz:
Free-space half-wave: 492 ÷ 14.2 = 34.65 ft
Real dipole length: 468 ÷ 14.2 = 32.96 ft (≈ 32 ft 11.6 in)
Each leg: 32.96 ÷ 2 = 16.48 ft (≈ 16 ft 5.8 in)
Recommended cut length (+4% margin): ≈ 34.3 ft total, trimmed down from there to find resonance.

🔻 Horizontal Dipole vs. Inverted-V

An inverted-V is the same antenna electrically, just installed differently: instead of stringing the wire flat between two supports, you hoist the center feedpoint up on a single mast and let both legs slope downward toward the ground. It's a popular choice specifically because it only needs one tall support instead of two, which matters a lot for portable operation, smaller yards, and field day setups.

Sloping the legs down changes the antenna's effective electrical length slightly, which is why an inverted-V resonates a bit shorter than a flat horizontal dipole at the same frequency — typically 2 to 5% shorter, which is where the 449/f shortcut comes from. The wider the angle between the two legs (closer to a flat 180°), the more it behaves like a true horizontal dipole. 120° between the legs is the commonly recommended target, with 90° as a practical minimum — interestingly, a 90° apex angle actually gives close to the best 50Ω impedance match of any angle, with only a small gain reduction compared to a flat dipole. The real danger zone is much tighter: angles below roughly 60° are where feedpoint impedance and SWR start to degrade quickly, so there's more room to work with than many builders assume.

Feature	Horizontal Dipole	Inverted-V
Supports needed	2 (one at each end)	1 (center mast only)
Footprint	Full antenna length, flat	Roughly half the horizontal footprint
Radiation pattern	Cleaner figure-8, more directional	More omnidirectional, slightly lower gain broadside
Typical length formula	468/f	449/f (varies with apex angle)
Best for	Fixed stations with two tall supports	Portable ops, field day, smaller properties

🛠️ How to Trim to Resonance

No formula replaces actually measuring your installed antenna. Here's the standard process:

Cut long, not short

Cut each leg 3–5% longer than the calculator's result. It's easy to trim wire off; it's a hassle to splice more on. This calculator's "Cut Length" result already adds that margin for you.

Install at your planned height

Resonance shifts with height, nearby objects, and ground conductivity — so trim the antenna where it's actually going to live, not lying on the ground or stretched between two chairs in the yard.

Measure SWR across the band

Use an antenna analyzer (a NanoVNA or similar) or your radio's built-in SWR meter to find the frequency where SWR is lowest. That's your antenna's actual resonant point right now.

Trim in small, equal steps

If the resonant point is below your target frequency, the antenna is too long — trim a few inches off both legs equally and re-measure. If it's above your target, the antenna is too short and needs more wire (or you cut too aggressively last time).

Repeat until you're in range

Trim, re-measure, repeat. Most dipoles land within their target band after 2–4 trimming passes. Once SWR is under 2:1 across the portion of the band you actually plan to use, you're done — perfectly flat 1:1 across the entire band isn't realistic or necessary for a simple dipole.

⚠️

Trim Equally From Both Legs

Always remove the same amount of wire from both legs when trimming a center-fed dipole. Cutting unevenly throws off the antenna's symmetry and feedpoint impedance, which can distort your radiation pattern even if SWR happens to look fine.

📡 Height, Feedline & Balun Basics

Length is only one piece of getting a dipole working well. Three other factors matter just as much:

Height: Higher generally means better DX performance, with half a wavelength above ground often cited as a serious target. Most hams can't get there — a dipole at 25–35 feet still performs respectably for both regional and DX work. An antenna that's up and imperfect will always outperform a "perfect" antenna left in the garage.
Feedline: Standard 50-ohm coax is the typical choice for a dipole. Keep runs as short as practical — every foot of coax adds some loss, and that loss gets worse at higher frequencies (a topic that deserves its own dedicated coax-loss calculator, coming soon to this site).
Balun: A dipole is electrically balanced; coax is not. Without a 1:1 choke balun at the feedpoint, RF current can flow on the outside of the coax shield, leading to RF-in-the-shack symptoms, pattern distortion, and inconsistent SWR readings that change when you touch the feedline. A simple choke balun at the feedpoint solves this in most cases.

❌ Common Mistakes That Throw Off Resonance

Trimming the wrong amount per pass: Big trims overshoot the target. Trim a few inches at a time near the end of the process, not a foot at once.

Forgetting insulation thickness: Thick PVC-jacketed wire behaves slightly differently than bare copper — it's a small effect, but it's part of why your real-world number won't match the formula exactly.

Tuning on the ground, then hoisting it up: Resonance measured at 3 feet off the ground will shift once the antenna goes up to 30 feet. Always do your final trimming pass at the real installed height.

Ignoring nearby metal: Gutters, metal roofing, power lines, and chain-link fences near the antenna can all pull resonance away from what the math predicts.

Skipping the balun, then blaming the antenna: RF-in-the-shack and "noisy" SWR readings are very often a missing-balun problem, not a length problem.

← Browse all antenna calculators & build guides

This dipole calculator is the first in a growing set of antenna-building tools on this site. More are on the way:

❓ Frequently Asked Questions

What is the formula for dipole antenna length?

The standard formula is 468 divided by the frequency in MHz, giving total length in feet (each leg is half that). In meters, divide 143 by frequency in MHz. It's an approximation built from a free-space half-wavelength (492/f) adjusted for real-world end effect — a great starting point to cut and trim, not an exact final answer for every installation.

Why is the dipole formula 468 and not 492?

492/f is the true half-wavelength in free space. A real wire antenna behaves as if it's slightly longer than its physical length because of end effect — capacitance that builds up at the open ends of the wire. That effect shortens the physical length needed for resonance by roughly 5%, which is where 468 comes from. It's a useful average derived from real measurements, not an exact physical constant — which is exactly why real dipoles still need trimming.

How much shorter should an inverted-V be than a horizontal dipole?

About 2–5% shorter than a flat horizontal dipole at the same frequency, depending on the angle between the legs — a common shortcut is to use 449 instead of 468. Wider angles (closer to 180°, flatter) behave closer to a true horizontal dipole. 120° is the commonly recommended target, with 90° as a practical minimum — a 90° apex angle actually gives close to the best 50Ω impedance match, with only a small gain reduction. SWR really only degrades quickly below about 60°.

Do I need a balun on a dipole?

It's strongly recommended. A dipole is a balanced antenna and coax is an unbalanced feedline, and feeding a balanced antenna directly with coax can let RF current flow on the outside of the coax shield. A 1:1 choke balun at the feedpoint suppresses that common-mode current, helping preserve your radiation pattern and avoiding RF feedback symptoms in the shack (hot mic, computer interference). A well-built, symmetrical dipole may show only minor issues on its own, but a choke balun is cheap insurance and resolves the problem in cases where it does show up.

How high should a dipole be mounted?

Higher is generally better for DX, with half a wavelength above ground being a common serious target. In practice, most hams can't get a dipole that high — a dipole at 25–35 feet still performs well for both regional and DX work. The most repeated piece of practical wisdom in amateur radio still applies: the best antenna is the one that's actually up in the air.

Will a calculated dipole length actually be resonant when I install it?

Not exactly, and that's completely normal. The 468/f formula assumes a generic wire diameter and free-space-like conditions. Real installations are affected by wire gauge and insulation, height above ground, nearby metal objects, soil conductivity, and the antenna's exact shape. Cut the wire 3–5% longer than calculated, hoist it at the planned height, measure SWR across the band, and trim small, equal amounts from both ends until the lowest SWR point lands on your target frequency.

Can I use this calculator for VHF or UHF antennas?

The 468/f formula works for any frequency mathematically, but it was derived from and is most accurate for HF wire dipoles. At VHF/UHF, wire diameter relative to wavelength becomes a much bigger factor, and most VHF/UHF antennas (J-poles, ground planes) use different practical designs entirely. Treat results above roughly 30 MHz as a rough starting point only.