Calculate antenna dimensions
Use a shortening factor for insulated wire, loading, or practical conductor behavior.
Formula used
Free-space wavelength: λ = c ÷ f
Adjusted wavelength: λa = λ × VF
Physical radiator length: L = λa × R × (1 − E)
Here, λ is wavelength in metres, c is 299,792,458 metres per second, f is frequency in hertz, VF is the shortening factor as a decimal, R is the electrical fraction, and E is end correction as a decimal.
How to use this calculator
- Enter the intended operating frequency and select its unit.
- Choose the radiator style, such as a dipole, vertical, loop, or custom fraction.
- Set the shortening factor. Use 100 percent for an ideal free-space starting point.
- Add any estimated end correction, then choose metres, feet, or inches.
- Calculate, cut slightly long, install safely, and tune with suitable test equipment.
Example design values
| Frequency | Configuration | Ideal fraction | Typical use |
|---|---|---|---|
| 7.10 MHz | Half-wave dipole | 0.500 λ | HF band operation |
| 14.20 MHz | Quarter-wave vertical | 0.250 λ | Ground-plane system |
| 144.30 MHz | Five-eighth-wave vertical | 0.625 λ | VHF mobile or base use |
| 433.92 MHz | Full-wave loop | 1.000 λ | Compact experimental loop |
Understanding Antenna Length
Antenna length strongly affects resonance, impedance, radiation pattern, and usable bandwidth. A resonant element transfers energy efficiently at its design frequency. The basic wavelength comes from the speed of light divided by frequency. Higher frequencies create shorter wavelengths. Lower frequencies need longer conductors. A calculator prevents repeated manual conversions between hertz, megahertz, metres, feet, and inches.
Frequency and Wavelength
Frequency is the number of cycles each second. Wavelength is one complete radio wave cycle. Their product equals propagation speed in free space. For most design work, use 299,792,458 metres per second. A one megahertz signal has a free-space wavelength near 300 metres. A 100 megahertz signal has a wavelength near three metres. This inverse relationship makes frequency the main starting point for antenna sizing.
Electrical and Physical Length
Electrical length is the selected wavelength fraction. A half-wave dipole uses one half wavelength. A quarter-wave vertical uses one quarter wavelength. A full-wave loop needs one complete wavelength around its perimeter. Physical length can be shorter than electrical length. Wire insulation, conductor diameter, nearby objects, and end effects alter propagation. The shortening factor and end correction inputs estimate those changes. They do not replace field testing.
Common Element Choices
Half-wave dipoles are popular because they are simple and balanced. Quarter-wave verticals need a suitable ground plane or radial system. Five-eighth-wave verticals may provide lower-angle radiation for some installations. Full-wave loops can offer useful multiband behavior through suitable feeding. A custom electrical fraction supports experimental elements, matching stubs, or unusual designs. Choose the option that describes the radiator, not the feed line.
Practical Construction
Measure the calculated wire slightly long before cutting. Use stranded wire where flexibility helps. Keep metal gutters, wiring, fences, and large trees away when possible. Mount the antenna in its intended position before final trimming. A low antenna can interact with soil and nearby conductors. That interaction changes resonant frequency and feed-point impedance. Weatherproof external connections and add strain relief at support points.
Tuning and Verification
Use an antenna analyser or an SWR meter to verify the completed antenna. Test across the operating band at low power. If resonance is too low in frequency, shorten the element gradually. If resonance is too high, lengthen the element or replace it with a longer piece. Adjust both sides equally on a dipole. Record final dimensions, location, height, and measured results. Those notes make future rebuilding faster and more reliable.
Safety and Expectations
Keep clear of power lines, equipment, and transmission areas. Never install or adjust antennas during lightning, rain, or high wind. Check local regulations, property rules, and licensing requirements before transmitting. This calculator estimates dimensions only. It cannot predict exposure levels, mechanical loading, interference, or damage. Use appropriate grounding, surge protection, safe climbing practices, and tools. Treat the calculated length as a starting dimension. Final performance depends on the complete station, installation environment, and measurement.
Frequently asked questions
Use these answers as practical starting guidance.
What does antenna length mean?
It is the physical conductor dimension chosen to approximate an electrical wavelength fraction. The correct value depends on frequency, construction, surrounding objects, insulation, and the antenna style.
Does frequency determine antenna size?
Yes. Frequency and wavelength are inversely related. A higher frequency produces a shorter wavelength, so comparable antenna elements become shorter as operating frequency rises.
Why is there a velocity or shortening factor?
Real conductors do not always behave like ideal free-space radiators. Insulation, conductor diameter, loading, and nearby materials can shorten the practical resonant length.
Can I use this for a half-wave dipole?
Yes. Select Half-Wave Dipole. The result gives total span and individual leg length. Cut both legs slightly long, then trim them equally during testing.
What should I enter for a quarter-wave vertical?
Select Quarter-Wave Vertical, enter your design frequency, and set a realistic shortening factor. The calculated length applies to the vertical radiator, not necessarily every radial.
Does the loop result show perimeter or one side?
For a full-wave loop, the main result is total perimeter. Enter the intended number of equal sides to see an estimated length for each side.
Is end correction always necessary?
No. It is an estimate for practical shortening. Use zero when you want a basic theoretical starting value, then tune the installed antenna with measurements.
Should I cut the wire to the exact result?
No. Cut slightly longer than the displayed value. Installed resonance often changes because of height, nearby metal, feed method, and conductor characteristics.
Can this calculate feed-line length?
It can estimate an electrical line length only when you choose a custom fraction and use the cable velocity factor. Feed-line impedance behavior also requires cable and termination details.
Why can SWR remain high after using the calculation?
Length is only one variable. Feed-point impedance, ground conditions, height, balance, matching networks, and nearby objects can all affect SWR and resonance.
Is antenna installation safe near utility lines?
No installation is safe near power lines. Keep generous clearance, avoid work during poor weather, use proper supports, and follow local safety requirements.