What is software-defined radio (SDR)?

How does an SDR work?

An SDR turns radio into numbers as early as it can and does the rest on a computer. A fast analog-to-digital converter samples the signal in two parts, I and Q, and software filters, demodulates and displays it. The sample rate sets how wide a slice you see: 2.4 million samples a second shows about 2.4 MHz at once.123

• I/Q samples. The signal is sampled twice, a quarter-cycle apart, and stored as complex numbers. That lets the software see frequencies on both sides of where you tuned.1
• Waterfall. The screen stacks spectrum snapshots over time: frequency across, time running down. A voice or data signal shows up as a stripe.4
• Bits. More bits in the converter means a wider range between the weakest and strongest signal it can handle. In theory 8 bits give about 50 dB, 12 bits 74 dB and 14 bits 86 dB; real converters do a little worse.5
• The antenna end still matters. A low-noise amplifier and filters sit before the converter, because real converters cannot handle tiny antenna signals on their own.2

The name goes back to Joseph Mitola, who described the "software radio" in a 1992 IEEE paper. The ITU defines SDR as a radio whose frequency range, modulation or output power can be set or changed by software.267

What is an RTL-SDR?

A cheap TV-tuner stick turned into a receiver. USB dongles for DVB-T television built on the Realtek RTL2832U chip can pass raw I/Q samples to a computer. Eric Fry found this, and in 2012 the Osmocom project released an open-source driver, first written by Steve Markgraf. Purpose-built versions now cost about $30–40.8910

The popular RTL-SDR Blog V4, launched in August 2023 at $29.95, went out of production in August 2026, when its R828D tuner chips ran out and the remaining stock on the market proved counterfeit. Its successor, the V4L, uses the R828S tuner.1314 RTL-SDR Blog warns that dongles in blue or green cases, square silver cases with four screws, or sold as "Pro" are not its products.12

SDR hardware compared

More money buys more bits and a wider view. Only one of the common receivers below can transmit: the HackRF.1510

Airspy and HackRF prices are from sellers the makers list; SDRplay and KiwiSDR prices are the makers' own.2526 Great Scott Gadgets now calls the HackRF Pro, 100 kHz–6 GHz, its current board.27

Transmitting needs a license: in the US no one may operate a radio transmitter without one, and the UK's Wireless Telegraphy Act says the same. With a HackRF that means, for example, a ham license and ham bands.2829

What software do you need?

Free software covers almost everything. Two common choices are SDR# on Windows and SDR++ on any system; specialist decoders add aircraft, trunked and digital radio.3031

Can you use an SDR online?

Yes, without buying anything. Owners connect SDRs to the internet and anyone can tune them in a browser. WebSDR began at the University of Twente in 2007 and lists over 100 receivers; KiwiSDR receivers each take up to eight listeners.383923

• WebSDR. Many people listen to one receiver at once, each tuned to a different frequency. Created by PA3FWM.3938
• KiwiSDR. A shortwave receiver, 0–30 MHz, that its owner can open to the public; the public ones are on a world map.4023
• OpenWebRX. Free software for running your own browser receiver; Receiverbook lists the public ones.41

What you can hear on shortwave this way is on What is shortwave radio?.

What can you listen to with an SDR?

Aircraft, ships, satellites, hams, shortwave and unencrypted two-way radio. Aircraft are within reach of the cheapest dongle: FlightAware says a home ADS-B station costs under $100.4243

Can an SDR hear walkie-talkies?

Analog ones, yes: PMR446, FRS and GMRS radios send plain FM that any receiver on the channel can hear. Digital radios are heard only when they are not encrypted. A PoC radio is a different case: its voice travels as data inside the mobile network, not on a channel an SDR can simply tune.53543755

If privacy matters, check what your radios use: see PMR446, DMR, TETRA and PoC vs PMR446.

Scanner or SDR?

A scanner is a ready-made receiver that hops through stored channels and follows trunked police and fire systems out of the box. An SDR with free software such as SDRTrunk can follow the same systems but takes setting up. Neither can play encrypted traffic.636436

• Trunked systems. P25 is a digital standard for public-safety radio: Phase I uses one 12.5 kHz channel per call, Phase II fits two calls into the same width.65
• A scanner. The Uniden SDS100 follows P25 Phase I and II, Motorola, EDACS and other trunked systems; $699.99 on October 18, 2026.64
• An SDR. SDRTrunk, free for Windows, Linux and Mac, decodes P25 Phase I and II and follows the trunked channels with an SDR.36
• Going quiet. Some agencies encrypt: Minneapolis police since April 2025, the Hennepin County Sheriff since 2019.66

How police, fire and ambulance radio is organized is on public safety radio.

Is it legal to listen with an SDR?

Usually, for open signals; passing on what you heard is another matter. In the US, unencrypted police, fire, ham, CB, GMRS, marine and aviation radio counts as readily accessible to the public. Encrypted traffic and phone calls do not, and you may not divulge or use what you intercept.676869

The same rules apply to shortwave listening.

Questions

Sources

1. IQ sampling (PySDR) · PySDR, Marc Lichtman
2. Software-defined radio · Wikipedia
3. About GNU Radio · GNU Radio project
4. Frequency domain (PySDR) · PySDR, Marc Lichtman
5. Effective number of bits · Wikipedia
6. Software radios: survey, critical evaluation and future directions (NTC-92), DOI record · Crossref, IEEE
7. Report ITU-R SM.2152: definitions of software defined radio and cognitive radio system (2009) · ITU (web archive copy)
8. rtl-sdr wiki (copy of December 31, 2023) · Osmocom (web archive copy)
9. History and discovery of RTLSDR (2013 copy) · rtlsdr.org (web archive copy)
10. About RTL-SDR · RTL-SDR Blog