Quarter Wave Antenna Length Calculator
Calculate the ideal and practical cut length for a quarter-wave antenna element.
What Is a Quarter Wave Antenna?
A quarter-wave antenna is one of the most common antenna lengths used in radio systems. The idea is simple: make the radiator one-quarter of the signal wavelength. At that length, the antenna is much easier to match and usually performs efficiently for a single frequency or narrow band.
You will see quarter-wave designs in ham radio, public safety radios, Wi-Fi projects, telemetry links, LoRa builds, and simple ground-plane antennas. Even when you do not build a pure quarter-wave radiator, this length is often the starting point for tuning.
How This Calculator Works
The calculator first finds wavelength from the frequency, then divides by 4 to get the quarter-wave length. It then applies your velocity factor and optional practical shortening percentage.
- Wavelength: λ = c / f
- Quarter-wave: λ/4 = c / (4f)
- With velocity factor: L = (c / (4f)) × VF
- Practical cut length: Lcut = L × (shortening% / 100)
Where c is the speed of light (299,792,458 m/s), and f is frequency in Hz.
Why Velocity Factor Matters
Real conductors and dielectric materials slow RF propagation compared to free space. That is why two antennas with the same frequency target can require slightly different physical lengths. Velocity factor helps model this effect.
- Wire in free air: typically around 0.95 to 0.98
- Coaxial cable stubs (solid PE): around 0.66
- Foam dielectric coax: around 0.78 to 0.85
Example Calculation
Suppose you want a quarter-wave antenna for 146.52 MHz (2-meter calling frequency), with VF = 0.95.
- Quarter-wave in free space ≈ 0.5117 m
- With VF 0.95, electrical length ≈ 0.4861 m
- That is about 48.6 cm, 19.1 inches
This gives you an excellent starting cut length for a simple monopole element.
Common Frequencies and Approximate Quarter-Wave Lengths
- 27 MHz (CB): ~2.78 m (109.3 in) in free space
- 50 MHz (6 m band): ~1.50 m (59.0 in)
- 146 MHz (2 m band): ~0.51 m (20.2 in)
- 433 MHz (70 cm / ISM): ~0.17 m (6.8 in)
- 915 MHz (ISM): ~0.082 m (3.23 in)
Build and Tuning Tips
1) Ground Plane Quality
A quarter-wave vertical usually depends on a counterpoise (radials, metal roof, vehicle body, or proper ground plane). Without it, feed-point impedance and pattern can change drastically.
2) Keep Connections Clean
Bad solder joints, corroded connectors, and poor cable routing can hurt performance more than a few millimeters of length error.
3) Tune at Final Installation Height
Nearby metal, mounting brackets, and height above ground all affect resonant frequency. Tune where the antenna will actually be used.
Frequently Asked Questions
Is this calculator for transmitting or receiving antennas?
Both. Resonant length matters most for transmit efficiency and safe matching, but receive antennas also benefit from proper electrical length.
Can I use this for dipoles?
Yes, as a guide. A half-wave dipole is two quarter-wave legs, but feed-point behavior and end effects still mean you should trim and tune.
Do I still need an antenna analyzer?
If possible, yes. The calculator gives a reliable first estimate; an analyzer lets you dial in the final dimensions quickly and accurately.