Where 468 comes from, and why it is not a constant
A radio wave travels at the speed of light, which is 983,571,056 feet per second. Divide that by a frequency in hertz and you have the wavelength in feet. Do the division once with the frequency in megahertz and the constant falls out: 983.571 divided by frequency in MHz gives the full wavelength, and half of that is 491.79 divided by frequency. That is the physics, and it does not vary.
Wire is not free space. The wave on a real conductor travels a little slower than it does in air, so the physical length that behaves like half a wavelength is shorter than 491.79 over frequency. The ratio between the two is the velocity factor. Multiply 491.79 by 0.951 and you get 467.6, which people round to 468 and then treat as though it were a law. It is a measurement of one particular kind of wire in one particular situation that got copied into every handbook.
The practical consequence: at 14.175 MHz, 492 over frequency gives 34.694 feet and 468 over frequency gives 33.016 feet. The difference is 20 inches — far more than the trimming you would do to move an antenna across a band. If your wire is thick, insulated, close to ground, or has anything metal near the ends, the real factor is lower still and the antenna comes out shorter again.
The four shapes on the selector
| Element | Fraction of a wavelength | What the length means |
|---|---|---|
| Half-wave dipole | 0.5 | End to end, fed in the middle, so each leg is half the total |
| Quarter-wave vertical radiator | 0.25 | Base to tip; it needs a ground system that is not part of this length |
| Full-wave loop | 1.0 | Total perimeter, whatever shape the perimeter is bent into |
| Five-eighths wave vertical | 0.625 | Base to tip; it will not present a sensible feedpoint without matching at the base |
The quarter-wave entry is the one that most often surprises people. The element is only half of the antenna. The other half is the ground system, and a vertical cut to exactly the right length over a poor ground system will be beaten by a badly cut one over a good one, because the losses in the ground are in series with the feedpoint and take their share of the power before anything radiates.
Trim allowance and why it is per free end
Every free end of an element is a place you can shorten from. On a dipole there are two, one at the end of each leg, so the allowance is added to each leg. On a vertical there is one, at the top. On a closed loop there are none, which is why the trim field contributes nothing when a loop is selected — a loop is adjusted by moving the feedpoint or by re-terminating it, not by folding an end back.
Six inches per end is enough for the one to two percent of adjustment a cut usually needs, and it lets you fold the excess back along the wire and clamp it rather than cutting. Fold, measure, and only cut when you are finished. Wire that has been cut short cannot be uncut, and splicing an end changes the thing you were adjusting.
What this deliberately does not do
It does not tune anything. It gives a length to start from, and the length that ends up right depends on height above ground, what is under the antenna, the feedline, nearby structures and whether the ends are near anything conductive. That gets settled with an analyser at the feedpoint.
It also has nothing to say about what you are allowed to transmit. Band edges, permitted power, licence class privileges and the RF exposure evaluation a station is required to carry out are regulatory matters with published limits, they differ between countries, and they change. Those questions go to the regulator and to the licensee, not to a calculator, and this page treats your frequency purely as a number to divide with.
If the element is going up a mast or a tower, stop before you raise anything and look at where it can fall. A mast that touches a power line kills, and the radius it can fall through has to be clear of every line in every direction. That is not a calculation, it is a look around the site.
Questions people ask
Should I use 468 or 492 to work out a dipole?
Both, in that order. 491.79 divided by the frequency in megahertz is half a wavelength in free space and it is exact arithmetic from the speed of light. Multiplying that by a velocity factor gives the physical length of a real wire, and 468 is simply 492 times about 0.951 baked into one number. Keeping the two steps apart lets you change the velocity factor when the wire is not thin bare wire in clear air, which is most of the time.
What velocity factor should I put in for wire?
The page defaults to 0.95, which corresponds to the traditional 468 figure and suits thin bare wire well away from other things. Thicker conductors, tubing, insulated wire and elements close to ground or to a metal roof all come out lower, sometimes noticeably. There is no lookup table worth trusting here because the number depends on the whole installation, not on the wire alone, so treat the output as a cutting length with trim on it rather than a final dimension.
How much does one percent of frequency change the length?
One percent, in the other direction. Length is inversely proportional to frequency, so moving the design frequency up by one percent shortens the element by one percent. At 14.175 MHz a full-size half-wave element is 32.959 feet at a velocity factor of 0.95, so one percent is about 3.96 inches off the whole element and 1.98 inches off each leg. That is the scale of the adjustment, and it is why a six inch fold-back at each end covers a useful range.
Why does the trim allowance disappear when I pick a loop?
Because a full-wave loop has no free end. The trim allowance exists so you can fold an end back and clamp it, shortening the element without cutting, and a closed loop offers nowhere to do that. Loops get adjusted by changing the perimeter at the feedpoint or by re-terminating, so the calculator adds nothing and says so rather than quietly padding the number.
Does cutting the element to the right length make the antenna work well?
It makes it resonant at roughly the frequency you aimed at, which is not the same thing. What gets out depends on the height, the ground under it, what is nearby, and above all on the feedline, because loss in the coax is subtracted twice — once on the way out and once on the way back. An element cut two percent long on good low-loss feedline will beat a perfectly cut one on a long lossy run every time.