Also known as: radio wave, radio waves
A radio wave is electromagnetic radiation whose frequency lies in the radio range of the electromagnetic spectrum, conventionally about 3 kHz to 300 GHz.1 Radio waves travel at the speed of light and carry information wirelessly when their amplitude, frequency, or phase is varied — a process called modulation. They are the medium every radio system, from an AM broadcast to a trunked police network, uses to move a message through empty space.
How it works
A transmitter drives an alternating current into an antenna. The accelerating charges launch a self-propagating disturbance: an oscillating electric field regenerates a magnetic field at right angles to it, and that magnetic field in turn regenerates the electric field, so the pair detaches from the antenna and radiates outward at the speed of light. The wave carries energy but needs no medium — this is why radio crosses the vacuum of space. A distant antenna, immersed in the passing field, develops a tiny induced current (often a few microvolts) that a receiver amplifies, filters, and decodes.
Three properties fully describe a simple radio wave and are the only things a transmitter can manipulate: amplitude (strength), frequency (cycles per second), and phase (position within the cycle). Modulation deliberately varies one or more of these in step with the information being sent. The wave’s wavelength follows from its frequency by λ = c / f, and its polarization — the orientation of the electric field — must usually match between transmit and receive antennas for good reception.
In practice
How a radio wave behaves between transmitter and receiver depends strongly on its frequency:
- Free-space spreading. Even in a vacuum, a wave’s power density falls with the square of distance (free-space path loss), which is why link budgets matter.
- Interaction with matter. Lower-frequency waves diffract around hills and buildings; higher frequencies travel more like light and are blocked or reflected, producing multipath.
- Noise and interference. The received wave always arrives buried in thermal and man-made noise; the ratio of wanted signal to that floor (SNR) bounds how reliably it can be decoded.
Relevance to SDR
Radio waves are the raw input to any receiver. An SDR does not decode the wave directly; its front end mixes a slice of spectrum down to baseband and its analog-to-digital converter turns the wave into a stream of IQ samples — a complex-number representation that captures both the amplitude and phase of the wave at each instant. From that point on, everything GopherTrunk does — filtering, demodulation, symbol recovery — is arithmetic on those samples. The physical radio wave has become numbers, but every property discussed here survives the conversion and must be tracked to recover the message.
Sources
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Radio wave — Wikipedia, on radio-frequency electromagnetic radiation and its use for wireless communication. ↩