Wavelength and the Electromagnetic Spectrum
Every radio signal travels as an electromagnetic wave at the speed of light. That wave has a characteristic length — called its wavelength — that is directly linked to its frequency. Understanding wavelength explains why a 40-metre dipole is about 20 metres long, why VHF antennas are shorter than HF antennas, and how the electromagnetic spectrum is organized from the longest radio waves to the shortest gamma rays.
What Is Wavelength
A wave is a disturbance that travels through space. As the electric field oscillates at a given frequency, it creates a repeating pattern of crests and troughs in space. The distance from one crest to the next — or from one zero crossing to the next zero crossing of the same polarity — is the wavelength.
Symbol: λ (Greek letter lambda). Unit: metres (m), centimetres (cm) or millimetres (mm) for shorter wavelengths at microwave frequencies and above.
Wavelength (λ) is the distance between two successive crests — the length of one complete cycle in space.
View LargerThe Speed of Light
All electromagnetic waves — radio, infrared, visible light, X-rays, gamma rays — travel at the same speed in a vacuum:
In air, the speed is about 0.03% slower — close enough to treat as c for all antenna calculations. In cables and transmission lines, however, the wave travels more slowly. The ratio of actual speed to free-space speed is called the velocity factor:
- Typical coaxial cable: 0.66 × c to 0.85 × c
- Foam-dielectric coax: up to 0.95 × c
- Open-wire (ladder) line: 0.95–0.99 × c
Velocity factor is important when calculating antenna lengths and the electrical length of transmission line sections — a physically short cable can be electrically longer than it appears.
The Wavelength-Frequency Formula
Because the wave travels exactly one wavelength in the time of one period (T = 1/f), the relationship is:
λ (metres) = 300 / f (MHz) (radio shortcut)
f (MHz) = 300 / λ (metres) (rearranged)
Working through the key amateur radio examples:
| Band / Signal | Frequency | Wavelength |
|---|---|---|
| 40m amateur band | 7.1 MHz | λ = 300/7.1 = 42.3 m |
| 20m amateur band | 14.2 MHz | λ = 300/14.2 = 21.1 m |
| 2m amateur band | 144 MHz | λ = 300/144 = 2.08 m |
| Wi-Fi (2.4 GHz) | 2400 MHz | λ = 300/2400 = 0.125 m = 12.5 cm |
Wavelength/Frequency Calculator
Wavelength ↔ Frequency Converter
Enter a frequency to calculate the wavelength, or enter a wavelength to calculate the frequency. Uses λ(m) = 300 / f(MHz), i.e. c = 3 × 10&sup8; m/s.
The Electromagnetic Spectrum
Radio waves, visible light, X-rays and gamma rays are all the same phenomenon — electromagnetic radiation — differing only in frequency (and therefore wavelength). The full electromagnetic spectrum, from lowest to highest frequency:
| Region | Frequency Range | Typical Uses |
|---|---|---|
| ELF / VLF | < 30 kHz | Submarine communications, navigation beacons |
| LF / MF | 30 kHz – 3 MHz | AM broadcast, maritime, 160m ham band |
| HF | 3 – 30 MHz | Shortwave, long-distance communication, 80m–10m ham bands |
| VHF | 30 – 300 MHz | FM broadcast, television, 6m and 2m ham bands |
| UHF | 300 MHz – 3 GHz | Mobile phones, Wi-Fi, 70cm and 23cm ham bands |
| SHF | 3 – 30 GHz | Radar, satellite, microwave links |
| EHF | 30 – 300 GHz | Millimetre-wave communications, imaging |
| Infrared / Visible / UV / X-ray / Gamma | > 300 GHz | Heat sensing, optics, medical imaging, nuclear physics |
All of these are the same type of wave — a self-sustaining oscillation of electric and magnetic fields propagating through space. The dividing lines between bands are conventions, not sharp physical boundaries.
The electromagnetic spectrum from radio waves (low frequency, long wavelength) to gamma rays (high frequency, short wavelength).
View LargerAmateur Radio Bands and Their Names
Amateur radio bands are named after the approximate wavelength of signals in the band — a convention from the early days of radio when wavelength (not frequency) was the standard way to specify a signal. The 40-metre band, for example, has a center wavelength of roughly 40 metres.
| Band Name | Frequency Range | Approximate Wavelength |
|---|---|---|
| 160m | 1.8 – 2.0 MHz | 150–167 m |
| 80m | 3.5 – 4.0 MHz | 75–86 m |
| 40m | 7.0 – 7.3 MHz | 41–43 m |
| 30m | 10.100 – 10.150 MHz | 29.6–29.7 m |
| 20m | 14.0 – 14.35 MHz | 20.9–21.4 m |
| 17m | 18.068 – 18.168 MHz | 16.5–16.6 m |
| 15m | 21.0 – 21.45 MHz | 14.0–14.3 m |
| 12m | 24.89 – 24.99 MHz | 12.0–12.1 m |
| 10m | 28.0 – 29.7 MHz | 10.1–10.7 m |
| 6m | 50 – 54 MHz | 5.6–6.0 m |
| 2m | 144 – 148 MHz | 2.03–2.08 m |
| 70cm | 430 – 440 MHz | 68–70 cm |
| 23cm | 1240 – 1300 MHz | 23–24 cm |
Major amateur radio bands from 160m (1.8 MHz) to 23cm (1240–1300 MHz), showing the relationship between band name and wavelength.
View LargerWhy Wavelength Matters for Antennas
Antenna dimensions are set by wavelength, not by frequency directly. The two most common antenna types are:
- Half-wave dipole: total length = λ/2
- Quarter-wave vertical: length = λ/4
At 144 MHz (2m band): λ/2 = 300 / (144 × 2) = 1.04 m. A handheld 2m dipole fits in your hand.
This is why VHF and UHF antennas are compact and portable while HF antennas are large. A 160m half-wave dipole would be about 80 metres long — the length of a football pitch. Complete antenna design is covered in Module 12.
Frequently Asked Questions
What does 2 metres mean as an amateur band name?
The 2-metre band covers 144–148 MHz (in most countries). At 144 MHz, the wavelength is λ = 300/144 = 2.08 metres — approximately 2 metres. The band is named after the wavelength of signals in it. Similarly, the 70cm band (430–440 MHz) has a wavelength of about 70 cm. The naming convention comes from the early days of radio when wavelength, not frequency, was the more natural way to specify a radio signal.
Does radio travel at exactly the speed of light?
In free space (vacuum), yes — radio waves travel at exactly the speed of light, c = 299,792,458 m/s, because they are the same thing: electromagnetic radiation. In air, the speed is about 0.03% slower — close enough to treat as c for all antenna calculations. In coaxial cables and transmission lines, the wave travels at 0.66c to 0.95c depending on the dielectric material (the velocity factor of the cable). This matters for antenna trimming: a quarter-wave section of 75% velocity factor coax is physically shorter than a quarter-wave in free space.
Why can radio waves travel through walls but light cannot?
Both radio and light are electromagnetic waves — the difference is frequency and therefore wavelength. Radio waves have wavelengths from millimetres to kilometres. Visible light has wavelengths of about 400–700 nanometres (much shorter). The interaction of electromagnetic waves with materials depends heavily on the relationship between the wavelength and the physical structure of the material. Most building materials (brick, concrete, wood) have structures many orders of magnitude larger than radio wavelengths, so radio passes through with some attenuation. Their molecular structure interacts much more strongly with visible light frequencies, absorbing or reflecting it. Metallic structures block all frequencies by conducting the induced currents.
What is the ionosphere and why does it affect radio propagation?
The ionosphere is a layer of the upper atmosphere (roughly 60–1000 km altitude) where solar radiation ionises gas molecules, creating free electrons. This layer acts like a mirror or lens for certain radio frequencies. HF signals (3–30 MHz) can be refracted back to Earth by the ionosphere, allowing communication over thousands of kilometres with very modest power. Lower frequencies (LF/MF) are absorbed. Higher frequencies (VHF and above) generally pass through the ionosphere and are used for satellite communication. The exact behavior depends on the time of day, season, and the 11-year solar cycle. Propagation is covered in detail in Module 13.
Test Your Knowledge
Answer the questions below to check your understanding. Every answer can be found in the lesson above.