Electromagnetic Spectrum: Communications

Electromagnetic waves carry information for phones, TV, and the internet. A transmitter changes the wave to carry a code (this change is called modulation). A receiver picks up the wave and decodes the message.

Wireless on Earth: Radio and Microwaves

• Mobile phones and Wi‑Fi use microwaves. Microwaves can pass through some walls and only need a short aerial. Walls and distance weaken the signal (called attenuation).
• Bluetooth uses low‑energy radio waves/microwaves for short distances (room‑sized). It can pass through walls but becomes weaker quickly.

Satellites (Microwaves)

• Main idea: Communication with artificial satellites is mainly by microwaves. The atmosphere lets microwaves pass, and dishes can send narrow beams up and down.
• Geostationary satellites stay above the same spot on Earth. Used for direct broadcast TV and some satellite phones. A dish must point at the satellite. There is a small time delay because the satellite is very far away.
• Low Earth orbit (LEO) satellites are much closer. Some satellite phones use them. They have shorter delay but the satellite moves across the sky, so a network of satellites is needed.

Optical Fibres (Visible/Infrared)

• Cable TV and high‑speed broadband send data as visible light or infrared through thin glass fibres.
• Glass is transparent to visible light and some infrared, so light travels along the fibre, bouncing inside it, and can carry very high data rates.

Analogue and Digital Signals

• Analogue: a smooth wave that varies like the original sound.
• Digital: a series of 1s and 0s (on/off). Sound can be sent as analogue or converted to digital.
• Benefits of digital: can send data faster, and repeaters can regenerate a clean signal, removing noise. This increases range and keeps quality high.

Real‑World Notes and Misconceptions

• Most mobile calls go to a nearby ground mast, not directly to satellites.
• Bluetooth is for short‑range device‑to‑device links; it is not the internet.
• Microwaves used for communication are low power. However, strong microwave exposure can heat body cells, so transmitters are carefully controlled.