What this antenna length calculator does
A half-wave dipole is two straight wires fed at the center. Its length depends on the frequency you want to receive or transmit. In free space, a half wavelength is 492 / f feet when f is in MHz. Real wire is not in free space. The wire has thickness and the ends have an effect that makes the electrical length a little longer than the physical length. We shorten the physical wire by a velocity factor to compensate. The default here is 0.95, which is common for thin wire. That gives the amateur-radio rule 468 / f feet. This calculator runs that math for you and shows the total dipole length, each leg, a quarter-wave vertical, and a full-wave antenna in meters, centimeters, feet, and inches.
The formulas behind the numbers
The basic link is v = f x lambda. In a vacuum, lambda = c / f, where c = 299,792,458 m/s. For an antenna, you care about half of that wavelength. The free-space half-wave length is (c / f) / 2. Then you multiply by the velocity factor to get the physical wire length. Each leg of a center-fed dipole is half the total. A quarter-wave vertical is half the dipole length. A full-wave antenna is twice the dipole length.
Here is the worked example for 146 MHz, a common 2 m amateur band frequency:
- Free-space half wave: (299,792,458 / 146,000,000) / 2 = 1.027 m
- With velocity factor 0.95: 1.027 x 0.95 = 0.976 m total (3.20 ft)
- Each leg: 0.976 / 2 = 0.488 m
- Textbook rule: 468 / 146 = 3.21 ft
The small difference between 3.20 ft and 3.21 ft comes from rounding. Both are close enough to start with. You will trim the antenna on site anyway.
Why the velocity factor matters
The velocity factor is a number below 1 that accounts for how fast a wave travels along a real conductor compared to free space. For a thin wire in air, 0.95 is a good starting point. That value comes from the end effect: the wave does not stop exactly at the wire tip, so the antenna behaves as if it is slightly longer than it is. Thick elements or insulated wire need a lower factor, often 0.90 to 0.93. If you use a lower factor, the calculator gives you a shorter antenna. That is correct because the thicker or insulated wire already adds electrical length.
You should treat the calculated length as a starting point. Cut your wire a little long, then trim it while watching an SWR meter. SWR stands for standing wave ratio, a measure of how well the antenna matches the feed line. A low SWR means more power leaves the radio instead of reflecting back. The exact length that gives the lowest SWR depends on height above ground, nearby objects, and the feed line. That is why real antennas are trimmed on air, not in a workshop.
Common questions about dipole and quarter-wave lengths
What is the 468 rule?
The 468 rule is a shortcut for the total length of a half-wave dipole in feet: 468 / f, where f is in MHz. It comes from the free-space half wavelength (492 / f) multiplied by a velocity factor of about 0.95. That factor accounts for the end effect in thin wire. The result is close to what you get from the full formula.
How do I use this for a quarter-wave vertical?
A quarter-wave vertical is half the length of a half-wave dipole. If the dipole total is 0.976 m at 146 MHz, the quarter-wave vertical is 0.488 m. That is the same as one leg of the dipole. You can build it as a single wire above a ground plane.
Why is my SWR high after cutting to the calculated length?
The calculation assumes free space and a thin wire. Your antenna is near the ground, near trees, or inside a building. Those objects change the electrical length. The feed line and connectors also affect the match. Trim the antenna in small steps, maybe 1 cm at a time, and check the SWR after each cut.
Does the velocity factor change with frequency?
For thin wire in air, the velocity factor stays close to 0.95 across a wide frequency range. For thick elements or insulated wire, it can be lower and may vary a little with frequency. If you build for a narrow band, one value is fine. For wideband use, you may need to compromise or model the antenna.
Can I use this for a full-wave loop?
Yes, the full-wave length is twice the half-wave dipole length. But a full-wave loop has different feeding and impedance than a dipole. The length is a starting point. You will still need to tune it with an SWR meter or an antenna analyzer.
Units and rounding
The calculator shows results in meters, centimeters, feet, and inches. All values come from the same formula, so they agree with each other. Small rounding differences are normal. For example, 0.976 m is 3.20 ft, and 3.20 ft is 38.4 inches. If you work in inches, cut to the nearest 1/8 inch and then trim.
Remember that frequency is the input you control. If you change frequency, the length changes in inverse proportion. Double the frequency and the antenna is half as long. That is why a 146 MHz antenna is about 3.2 ft, while a 100 MHz FM antenna is about 4.7 ft. The math is the same.