Frequency, wavelength and why bands have names
The 300 ÷ f shortcut, where band names like '2 metres' come from, and how harmonics get you into trouble.
6 minute read
Radio waves travel at the speed of light — about 300,000 kilometres per second, or 3 × 10⁸ metres per second. That constant is what links frequency to wavelength.
The shortcut you must know
wavelength (metres) = 300 ÷ frequency (MHz)That is it. Because the speed of light is roughly 300 million metres per second, working in megahertz makes the number come out in metres directly.
| Frequency | Wavelength | Common name |
|---|---|---|
| 3.6 MHz | ~83 m | 80 metre band |
| 7.1 MHz | ~42 m | 40 metre band |
| 14.2 MHz | ~21 m | 20 metre band |
| 145 MHz | ~2 m | 2 metre band |
| 435 MHz | ~0.7 m | 70 centimetre band |
Band names are the approximate wavelength, rounded to something memorable. When someone says "I was on 20 metres", they mean around 14 MHz.
It works both ways
frequency (MHz) = 300 ÷ wavelength (metres)If a question gives you a wavelength and asks for frequency, the same division works in reverse.
Frequency ranges you should recognise
- LF — 30 to 300 kHz
- MF — 300 kHz to 3 MHz
- HF — 3 to 30 MHz
- VHF — 30 to 300 MHz
- UHF — 300 MHz to 3 GHz
Notice the pattern: each band is a factor of ten. If you can remember that HF is 3–30 MHz, you can derive the rest by moving the decimal point.
Harmonics
A harmonic is an integer multiple of a frequency. Transmit on 7.05 MHz and your second harmonic sits at 14.1 MHz, the third at 21.15 MHz.
This matters because a poorly filtered transmitter radiates harmonics, and those harmonics land in other bands — sometimes in services that are not amateur at all. Harmonic radiation is a common source of interference complaints and is exactly the kind of thing the regulations require you to prevent.
Why this connects to antennas
Antenna dimensions are fractions of a wavelength. A half-wave dipole for 14.2 MHz is about 10 metres long, because 300 ÷ 14.2 ≈ 21 metres and half of that is roughly 10.5 — trimmed slightly shorter in practice. Get comfortable with 300 ÷ f and antenna questions become arithmetic rather than mystery.