Fix dollar signs + add descendants section

# [TECH] Radio & Wireless Communication

**Radio** is the technology of transmitting and receiving information via electromagnetic waves, enabling broadcasting, wireless telephony, navigation, and radar.

## Overview

Guglielmo Marconi (1894–1901) demonstrated long-range radio telegraphy, culminating in the first transatlantic signal (1901). Reginald Fessenden broadcast voice (1906). Edwin Armstrong invented FM radio (1933). The post-WWII wireless revolution expanded from AM/FM broadcasting to cellular phones, WiFi, Bluetooth, and 5G — all electromagnetic wave technologies.

Radio also underpins GPS, radar, satellite communications, and radio astronomy. The cellular networks of the late 20th century became the substrate for smartphones and the mobile internet.

## Key Actors

- **Companies**: Marconi Wireless (1897), RCA (1919), AT&T/Bell Labs, Motorola (1928), Nokia, Qualcomm (1985)
- **Inventors**: Guglielmo Marconi (1874–1937), Nikola Tesla (contested), Edwin Armstrong (1890–1954)

## Key Patents

- Marconi, G. UK Patent 12,039 (1896) — wireless telegraphy
- Armstrong, E. US Patent 1,342,885 (1920) — superheterodyne receiver

## Economic Value

Global wireless communications market: **\$1.5 trillion/year** (2023, GSMA). This includes cellular, WiFi, satellite, and broadcasting. The 5G infrastructure investment alone is projected at \$900B globally through 2025.

## Notes

GSMA Intelligence *State of Mobile* 2023. 5G investment: Ericsson Mobility Report 2023.

## What This Enables
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- **[TECH] Radar** — Radar uses pulsed radio waves and measures round-trip time to detect and locate reflecting objects.
- **[TECH] Satellite Communications** — Geostationary satellites relay radio/microwave signals between ground stations worldwide.
- **[TECH] Mobile Phones & Smartphones** — Cellular networks are radio networks exploiting frequency reuse across geographic cells.
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# Parents

* [SCI] Electromagnetic Wave Theory
* [SCI] Electromagnetic Wave Theory
* [TECH] Telegraph & Telephone
* [TECH] Vacuum Tube Electronics