EMZETT.Radio
In short: The transmission of data or signals via electromagnetic waves through the air, without physical cabling.
In more detail: The basic principle behind practically all wireless technologies — Wi-Fi, Bluetooth, mobile networks, RFID/NFC. Different technologies use different frequency bands and ranges depending on their purpose (short range/little energy with NFC vs. long range with mobile networks).
In Depth
Frequency, transmission power, modulation
Radio technologies differ mainly in three dimensions: frequency band (determines range and penetration — lower frequencies carry further and through walls better because they bend more, while higher frequencies allow higher data rates because more bandwidth is available per unit of time), transmission power (regulated by law, usually in relation to the respective frequency band — radio amateurs and licensed mobile network operators may transmit considerably more than the ISM band used by Wi-Fi/Bluetooth) and modulation method (how the digital zeros and ones are actually imprinted on the radio wave — e.g. by changing the amplitude, frequency or phase of the carrier wave).
Frequency bands and regulation
In Europe, Wi-Fi and Bluetooth largely share the licence-free 2.4 GHz ISM band (originally reserved for industrial/scientific/medical devices, hence the name), which can lead to mutual interference when many devices transmit at once — one reason why modern Wi-Fi additionally uses the much less crowded 5 GHz and now also 6 GHz bands. Mobile networks, on the other hand, run over licensed frequency bands allocated exclusively to network operators, for which they sometimes pay billions at spectrum auctions — in return there’s practically no interference from other devices there (unlike in the crowded ISM band).
Classification by typical purpose
NFC - a few centimetres, very little energy -> contactless payment
Bluetooth - up to ~10 m, little energy -> headphones, wearables
Wi-Fi - up to ~50 m (indoors), medium energy -> home/office network
Mobile - several km, cell tower infrastructure -> nationwide coverage
The fundamental trade-off
Greater range and higher data rates almost always cost more transmission power (and therefore battery life) and often more elaborate infrastructure. That’s why device developers choose the radio technology specifically according to the use case: a battery-powered sensor that is supposed to last months or years on a single coin cell uses Bluetooth Low Energy (BLE) or RFID/NFC, which deliberately trade very low data rates for minimal energy consumption. A smartphone with a daily charging cycle, on the other hand, can afford Wi-Fi and mobile data with a considerably higher energy demand, because it’s recharged daily anyway.
Signal quality and sources of interference
Physical obstacles (walls, especially concrete and metal), distance and interference from other radio sources (microwave ovens, for example, classically disturb the 2.4 GHz band) worsen the signal quality and therefore indirectly also the achievable data rate — the weaker the signal, the more often data packets have to be resent, which lowers the effective speed, even if the technology’s theoretical bandwidth stays unchanged.