What is a radio wave?
Key takeaways A radio wave is energy that travels through space as a vibrating electric and magnetic field, moving at the speed of light. It is described by three numbers: frequency (how fast it cycles, in hertz), wavelength (how long one cycle is, in metres), and amplitude (how strong it is). Radio is just the slow, long-wavelength part of the same electromagnetic spectrum that includes light. Get these three numbers straight and every other SDR idea follows.
This is lesson 1 of the path, and it’s the one everything else leans on. By the end you’ll be able to look at “460.025 MHz” and know roughly how long that wave is, why it behaves the way it does, and how it connects to the dials you’ll turn in GopherTrunk.
What exactly is a radio wave?
A radio wave is a ripple of electromagnetic energy. When a transmitter pushes electrical current back and forth in an antenna, it creates a changing electric field, which creates a changing magnetic field, which creates a changing electric field — and that self-sustaining pattern races away from the antenna at the speed of light. No wires, no medium required; radio waves cross empty space happily.
It’s the same physics as visible light, microwaves, and X-rays. The only thing that separates “radio” from “light” is how fast the field vibrates. Radio waves vibrate slowly enough (thousands to billions of times per second) that we can build electronics to generate and detect them directly — which is exactly what your SDR does.
What is frequency, and why is it measured in hertz?
Frequency is how many complete cycles the wave makes every second. The unit is the hertz (Hz) — one cycle per second. Radio frequencies are big numbers, so we use prefixes:
| Unit | Cycles per second | You’ll see it for |
|---|---|---|
| 1 kHz (kilohertz) | 1,000 | Long-wave broadcast, navigation |
| 1 MHz (megahertz) | 1,000,000 | FM radio, public-safety, scanners |
| 1 GHz (gigahertz) | 1,000,000,000 | Wi-Fi, GPS, cellular, satellite |
When you tell GopherTrunk to listen on 851.0125 MHz, you’re telling it to look for a wave cycling about 851 million times every second. Tune to the wrong frequency and you simply won’t hear the signal — it’s like dialing the wrong phone number.
What is wavelength, and how do I calculate it?
Wavelength (the Greek letter λ, “lambda”) is the physical length of one cycle — the distance from one crest to the next. Because all radio waves travel at the same speed, frequency and wavelength are two sides of the same coin: the faster a wave cycles, the less distance it covers per cycle, so the shorter its wavelength.
The exact relationship is wavelength = speed of light ÷ frequency. For everyday radio there’s a shortcut worth memorising:
λ (metres) ≈ 300 ÷ frequency (MHz)
So a 150 MHz signal is about 2 metres long; a 460 MHz signal is about 0.65 m; a 1.2 GHz signal is just 25 cm. This isn’t trivia — wavelength decides how big your antenna needs to be (antennas are sized to a fraction of a wavelength) and how the wave bends around buildings and hills.
Frequency ↔ wavelength calculator
Uses the exact speed of light (299,792,458 m/s). The “300 ÷ MHz” rule is a handy approximation of the same formula.
What is amplitude, and how does it relate to signal strength?
Amplitude is the height of the wave — how much energy it carries. A bigger amplitude means a stronger signal. As a radio wave spreads out from a transmitter and travels past obstacles, its amplitude shrinks, which is why a distant repeater is harder to hear than a nearby one.
Your SDR doesn’t measure amplitude in metres of wave height; it reports it as a power level in decibels (you’ll meet dBm and the noise floor in the next-but-one lesson). For now, hold onto the intuition: amplitude = how loud the signal is at your antenna, and a digital decoder needs enough amplitude above the background noise to recover the data.
How does information ride on a radio wave?
A plain, unchanging wave — a carrier — carries no information by itself. To send something, the transmitter deliberately changes one of the wave’s three properties in step with the message:
- Vary the amplitude → that’s AM (and the basis of many data modes).
- Vary the frequency → that’s FM (and digital cousins like FSK).
- Vary the phase (the wave’s timing) → the basis of PSK and the digital voice modes GopherTrunk decodes.
That deliberate changing is called modulation, and it’s the whole subject of Module 2. For now, just know that the radio wave is the envelope and modulation is the message written on it.
Where do radio waves sit in the bigger picture?
Radio is the low-frequency end of the electromagnetic spectrum — the same family as microwaves, infrared, visible light, and X-rays, just vibrating far more slowly. “Radio” conventionally spans about 3 kHz to 300 GHz. Within that range, governments carve the spectrum into bands and assign them to uses — broadcast, aviation, public safety, amateur radio, satellite. That’s the subject of the next lesson, Frequency, bands & the spectrum.
Quick check: a 900 MHz signal compared with a 150 MHz signal has a…
Recap
- A radio wave is electromagnetic energy travelling at the speed of light.
- Frequency (Hz) is how fast it cycles; wavelength (m) is how long one cycle is; they’re linked by λ ≈ 300 ÷ MHz.
- Amplitude is its strength, which your SDR reports as a power level.
- Modulation changes the wave to carry information.
- Radio is just the slow part of the same spectrum as light.
Next up: how that spectrum is divided into the bands you’ll actually tune to.
Frequently asked questions
What is a radio wave in simple terms?
A radio wave is an invisible ripple of electric and magnetic energy that travels through space at the speed of light. A transmitter shakes electrons in an antenna, and that motion radiates outward as a wave that a distant antenna can pick up. Radio waves carry information — voice, data, video — by varying the wave’s height (amplitude), rate (frequency), or timing (phase).
What is the difference between frequency and wavelength?
Frequency is how many times per second a wave cycles, measured in hertz (Hz). Wavelength is the physical distance the wave covers in one cycle, measured in metres. They are inversely related — higher frequency means shorter wavelength. For radio you can convert between them with wavelength (m) = 300 / frequency (MHz).
How fast do radio waves travel?
Radio waves travel at the speed of light — about 299,792,458 metres per second (roughly 300,000 km/s) in a vacuum, and very slightly slower in air. This is why you can think of “the speed of light” and “the speed of radio” as the same thing.
Why do I need to understand radio waves to use an SDR?
Every setting on a software-defined radio — the frequency you tune to, the bandwidth you capture, the antenna you choose — is a direct consequence of how radio waves behave. Once frequency, wavelength, and amplitude click, the rest of SDR stops feeling like magic and starts feeling like cause and effect.