Antenna Length Calculator
Cut lengths for dipoles, verticals, J-poles and Yagis, with the end-effect factor explained and coax stub lengths that use the right velocity factor.
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Coax stubs use a different factor entirely
This is the one that catches people. The antenna end-effect factor is about a radiating element in air. A coax velocity factor is about the dielectric inside a cable, and it is much lower — 0.66 for solid polyethylene. Cut a quarter-wave stub to 0.95 instead and it comes out over a third too long.
The formulas are not wavelength
468/f in feet, 234/f for a quarter wave, 142.65/f in metres — none of these is a wavelength. They are wavelength with about 5% already taken off, because a real conductor is electrically longer than its physical length. The field does not stop dead at the end of the wire, so a half-wave dipole resonates a few percent short of a physical half wavelength. That correction is baked into the constants, which is why you will also find 492/f quoted and why the two mean different things.
- Cut long and trim. You can always take wire off. Putting it back means a joint in the middle of an element, which is a lasting nuisance.
- Height above ground moves resonance more than your trimming will. Every published formula assumes free space and thin wire. A dipole a quarter wavelength up behaves quite differently from the same wire a full wavelength up, in both resonant frequency and radiation angle.
- Lowest standing wave ratio is not necessarily resonance. Feedline loss flatters the reading, and a long run of lossy coax can show a comfortable 1.5:1 on an antenna that is nowhere near right. A genuinely flat reading on a long cheap feedline is a warning sign, not a result.
- Insulated wire comes out shorter than bare. The jacket slows the wave along the conductor. If you reuse a bare-wire length on insulated wire you will land resonant low — and the exact factor depends on the jacket, so trim rather than trust the number.
- Yagi dimensions are a starting point, not a design. Element lengths depend strongly on element diameter, on whether elements are insulated from the boom, and on boom diameter. Dimensions that work with 6 mm elements will be wrong with 12 mm. Anything past a first cut wants modelling software.
How to use
- Enter a frequency, or tap a band button.
- Pick the antenna type and what you are building it from.
- Read the cut lengths, then the spacings, then the notes.
- For coax stubs use the section below — it uses the cable velocity factor, not the antenna one.
Frequently asked questions
Why is 468/f not just half a wavelength?
Because a real conductor is electrically longer than its physical length. The field does not stop dead at the end of the wire, so a half-wave dipole resonates a few percent short of a physical half wavelength. The 468 constant has that roughly 5 percent correction already folded in, which is why you will also see 492/f quoted for a true electrical half wave. The two numbers mean different things and mixing them up costs you 5 percent.
How close will my antenna be to the calculated length?
Close enough to start trimming and no closer. Height above ground shifts resonance by more than the trimming you will do — a dipole a quarter wavelength up behaves quite differently from the same wire a full wavelength up. Nearby metal, the feedline, and the wire's own thickness all move it too. Cut long, measure, trim: you can always take wire off, and adding it back means a joint in the middle of an element.
Does insulated wire need a different length?
Yes, shorter. The jacket slows the wave travelling along the conductor, so an insulated dipole resonates lower than a bare one of the same length. The usual figure is around 0.93 against 0.95 for bare wire, but it genuinely depends on the jacket material and thickness, so treat it as a reason to cut long rather than as a number to trust.
Why do coax stubs use a different factor?
Because it is a different phenomenon. The end-effect factor describes a radiating element in air; a cable's velocity factor describes how fast the wave travels through the dielectric inside it. Solid polyethylene coax runs at 0.66, foam at about 0.82. Cut a quarter-wave stub to the antenna's 0.95 instead of the cable's 0.66 and it comes out more than a third too long, which is one of the commonest mistakes in the subject.
Why does my J-pole put RF on the feedline?
Because a J-pole feeds against its own mast and readily excites current on the outside of the coax braid, which turns the feedline into part of the antenna. That distorts the pattern and brings RF back into the shack. A choke at the feedpoint — several turns of coax or a stack of ferrite beads — is not optional if you want the pattern the design promises.
Are the Yagi dimensions ready to build?
They are a first cut, not a design. Element lengths depend strongly on element diameter, on whether the elements are insulated from the boom or bonded to it, and on the boom diameter itself. A set of dimensions that works with 6 mm elements will be wrong with 12 mm. Use these to buy stock and start cutting, then model the actual design if performance matters.
How much gain does adding elements buy?
Less than you would hope. Gain grows with the logarithm of element count, not linearly — every doubling is worth roughly 2.7 dB, so going from eight elements to sixteen gains less than going from two to four did. Past about ten elements, stacking two smaller arrays generally beats lengthening one boom, and it is easier to keep in the air.
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