Frequency ⇔ wavelength

Convert frequency to wavelength and wavelength back to frequency. A higher permittivity means a lower propagation speed.

Signal frequency

Wavelength

PCB relative permittivity

εr

SI prefixes accepted (4k7 / 1M / 10m / 220). Upper-case M = mega, lower-case m = milli

Wavelength

1.44673m

Half wavelength0.72336 m

Quarter wavelength0.36168 m

Frequency100.00197 MHz

History

Press Enter in any field to keep a snapshot here.

Formula

Wavelength: λ = c / (f × √εr)
Half wave: λ/2 Quarter wave: λ/4
Frequency → wavelength: λ [m] = 3×10⁸ / (f[Hz] × √εr)
c = 3×10⁸ m/s (speed of light)

Design notes

The relationship between frequency and wavelength underpins antenna resonant lengths and transmission line design on a PCB. In air (εr = 1) a wave travels at the speed of light; on a board the permittivity slows it down.

Typical relative permittivities:
• FR-4: εr ≈ 4.3–4.8
• Rogers 4350B: εr ≈ 3.48 (high-speed and RF work)
• PTFE: εr ≈ 2.1

The quarter-wave length is the one to know: it sets monopole antenna length and λ/4 impedance transformers.

When you need this

For sizing antennas, chasing resonances, and identifying which trace or cable is radiating in an EMI investigation. Board permittivity can be entered, so it covers both free space and on-board wavelengths.

Why half and quarter wavelengths are shown

A conductor radiates most efficiently when its length is a half or a quarter wavelength. EMI work uses this in reverse: look for traces, cables or slots whose length is near the half or quarter wavelength of the offending frequency, and you have usually found the source.

On a board, cables, connectors, shield seams and splits in a ground plane all become antennas. Take the frequency from the emissions plot, compute the lengths here, and look for structures of that size.

Common frequencies

Free space (εr = 1). On a board, divide by √εr.
Frequency	Wavelength	Half wave	Quarter wave
13.56 MHz (NFC)	22.1 m	11.1 m	5.5 m
100 MHz	3.0 m	1.5 m	0.75 m
433 MHz	69.3 cm	34.6 cm	17.3 cm
920 MHz (LPWA)	32.6 cm	16.3 cm	8.1 cm
2.45 GHz (BLE / WiFi)	12.2 cm	6.1 cm	3.1 cm
5.8 GHz (WiFi)	5.2 cm	2.6 cm	1.3 cm

A quarter wave at 2.45GHz is about 31mm in air and roughly 15mm on FR-4 (εr = 4.3). A floating trace or stub of that length will act as an antenna whether you intended it or not.

Using it for EMI

Radiated testing shows a strong peak at 300MHz.
Entering 300MHz with εr = 1 gives a 1.0m wavelength, 0.5m half wave and 0.25m quarter wave.
A 250mm ribbon cable is the obvious suspect, working as a quarter-wave antenna.
Fit a ferrite on the cable, change its length, or improve the ground and re-measure.
To investigate structures inside the board, enter εr = 4.3 and look for traces or slots near the on-board quarter wave (about 121mm).

Frequently asked questions

Which wavelength applies to a PCB antenna?

A trace antenna has air above and laminate below, so the effective permittivity lies between εr and 1 — around 3.0 for FR-4 at εr = 4.3. Exact dimensions come from an electromagnetic solver or from tuning on an evaluation board. This calculator is for getting close.

Why is a quarter-wave antenna efficient?

At a quarter wavelength the ground plane acts as a mirror, making it equivalent to a half-wave dipole, and the feed-point impedance lands at a convenient value of about 37Ω. That makes matching straightforward and radiation efficient.

Can I identify an EMI source from its frequency?

It is a strong clue. Look for structures — cables, slots, stubs, heatsinks — matching the half or quarter wavelength. Bear in mind you may be looking at a harmonic, so check the fundamental too: peaks at integer multiples of a clock frequency point straight at that clock.

Standards and references

CISPR 32 / EN 55032 — Radiated emission limits for multimedia equipment.
IEC 61000-4-3 — Radiated RF immunity testing.

Last updated: 2026-08-29