The language of the universe — how information travels through space at the speed of light
A signal is any physical variation that carries information from one point to another. In radio, we talk about electromagnetic waves: coupled oscillations of electric and magnetic fields that travel at the speed of light, with no need for any material medium.
Imagine throwing a stone into a pond. The concentric ripples carry energy outwards. Radio waves work the same way, but in the electromagnetic field and crossing the vacuum of space at 299,792 km/s.
All communication —voice, satellite data, GPS, WiFi— comes down to signals that change over time and encode information in that change.
Number of complete cycles per second. It is measured in hertz (Hz). The higher the frequency, the more energy the wave carries and the shorter its wavelength.
The time the wave takes to complete one full cycle. It is the inverse of the frequency.
The "height" of the wave. Larger amplitude = stronger signal. In radio it is measured in watts (W) or dBm.
Wavelength is the distance the wave travels during one complete cycle. It is directly related to the frequency and to the speed of light. This relationship is fundamental for antenna design: an efficient antenna must have a size proportional to the wavelength of the signal it works with.
Enter any frequency to calculate its wavelength, optimal antenna size, period and band. Try 436.888 MHz (UNNE-1B).
The electromagnetic spectrum ranges from radio waves (frequencies from Hz to hundreds of MHz) to gamma rays (frequencies of 10²⁰ Hz). Visible light is just a narrow strip. They are all the same physical phenomenon — the difference is the frequency.
Hover over each band to enlarge it and see its description:
| Band | Frequency | Wavelength λ | Use in space |
|---|---|---|---|
| VHF 2 metres | 144–146 MHz | ~2 m | Amateur satellite uplinks, ISS on 145.800 MHz |
| UHF 70 cm | 430–440 MHz | ~70 cm | UNNE-1B: 436.888 MHz. The king band of CubeSats |
| S-band | 2–4 GHz | 7–15 cm | Imaging satellites, high data rates |
| X-band | 8–12 GHz | 2.5–4 cm | Weather satellites, military communications |
| Ka-band | 26–40 GHz | 7.5–11 mm | Satellite internet (Starlink, HughesNet) |
A pure radio wave (the carrier) carries no useful information. To send data the wave must be modulated: one of its properties —amplitude, frequency or phase— is varied in a controlled way to encode the information.
The amplitude of the carrier varies in proportion to the information signal. The frequency stays constant. It is the oldest and simplest modulation.
Choose "AM modulation" in the visualiser above and watch how the envelope of the wave changes over time — that envelope is the information.
The frequency of the carrier varies with the signal. The amplitude stays constant, which makes it much more resistant to electrical noise.
UNNE-1B uses narrow FM (NFM) for its voice repeater. If you have an RTL-SDR, tune to 436.888 MHz in NFM mode during a pass and you will hear the satellite.
The digital version of FM: the frequency jumps between two fixed values to represent 0 and 1. The AX.25 protocol —the standard for amateur satellites— uses 1200 bps AFSK (Bell 202).
UNNE-1B sends its telemetry in FSK at 200 bps, with its two tones 1125 Hz apart. AMSAT-EA publishes the ground software to decode it; see the FSK knowledge pill.
It modulates the phase of the carrier. BPSK uses 2 phases (0° and 180°), QPSK uses 4. Very spectrally efficient: it sends more bits per Hz of bandwidth.
The next generations of CubeSats use GMSK and BPSK to reach 9600 bps or more while keeping the bandwidth small.
