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Quarter Wave Antenna Calculator
Calculators / Quarter-Wave Antenna
Quarter-Wave Antenna
Calculator
Calculate the exact physical length of a quarter-wave (λ/4) monopole antenna for any frequency — used in WiFi antennas, RF modules, IoT devices, and PCB designs. Includes ground-plane radial length, velocity factor correction, and conductor diameter shortening.
Monopole Element (λ/4)
Ground Plane / Radials (λ/4)
⚙ Quarter-Wave Engine
Free-space λ = — m
Bare copper/aluminium ≈ 0.95. PCB trace ≈ 0.66–0.85
Affects length via thick-element correction
Material correction applied to VF
PCB / Chassis
Infinite ground
4 Radials
Standard GP
8 Radials
+0.5 dB gain
32+ Radials
Near-perfect
Advanced Parameters
Radials as fraction of λ (standard = 0.25 = λ/4)
°
Drooped radials (−45°) raise Zin toward 50 Ω
Free-standing: bare element in air
m
Height above real ground affects Zin and efficiency
Calculated Results
Antenna Length (L = λ/4)
—mm
Free-space λ—
Radial Length—
Length (inches)—
Length (feet)—
Feedpoint Impedance (Zin)—
Gain (dBi)—
Bandwidth (approx.)—
Thick-element correction—
Effective VF (k × mat)—
Number of radials—
Enter values to calculate.
📖 Theory
A quarter-wave monopole antenna is exactly one-quarter of the operating wavelength. It requires a ground plane (or radials) to act as the “missing” quarter-wave, together forming the equivalent of a half-wave dipole.
Formula
L = (k × c) / (4 × f)
Shorthand: L(mm) = 71,500 × k / f(MHz)
Ground Plane Effect
Over a perfect ground: Zin ≈ 36.5 Ω, Gain = 5.19 dBi.
With 4 radials: Zin ≈ 40–50 Ω (near 50 Ω coax).
Drooped radials (−45°) raise Zin toward 50 Ω exactly.
IoT / PCB Rule-of-Thumb
433 MHz → 163 mm
868 MHz → 82 mm
915 MHz → 78 mm
2.4 GHz → 31 mm
5.8 GHz → 13 mm
📡 Radiation Pattern Viewer
Live Simulation
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◉ Pattern scales with radial count
Plane
Frequency
433 MHz
Radials
4
E-Plane Pattern
3D Elevation View
ℹ
Design Insights
Select a frequency and ground plane configuration to see design insights.
Common Applications
WiFi Antennas
2.4 GHz (31 mm) and 5 GHz (13 mm) PCB quarter-wave whips on routers, access points, and WiFi modules.
RF Modules & IoT
433/868/915 MHz wire antennas on LoRa, Sigfox, Z-Wave, and custom RF transceiver modules.
IoT Devices
Bluetooth, Zigbee (2.4 GHz), NB-IoT, LTE-M antennas for smart home sensors and industrial IoT.
Amateur Radio VHF/UHF
2 m (144 MHz) and 70 cm (433 MHz) ground-plane antennas — the most popular portable VHF antenna.
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