In a software-defined radio, the hardware digitises a slice of spectrum and software does the filtering and demodulation. Practically, that means you see the band instead of tuning blindly across it — and you can change what the radio is by changing the program.
Receive-only, for about $30
An RTL-SDR dongle (RTL2832U with an R820T2 or R828D tuner) covers roughly 500 kHz to 1.7 GHz. With a good antenna it will let you:
- Watch a whole band at once and see where activity is.
- Decode APRS, SSTV, FT8,
- Diagnose your own station — find birdies, harmonics and
- Learn modulation by looking at it. This is the cheapest education in radio there is.
aircraft ADS-B, NOAA weather satellite images and pagers.
Buy from a reputable source (rtl-sdr.com's v3/v4), add an upconverter or use direct sampling for HF, and fit a bandpass filter if you live near broadcast transmitters.
Better receivers
- Airspy HF+ Discovery, SDRplay RSPdx — real HF performance, good dynamic range, $200–400.
- KiwiSDR and the public WebSDR network — free receivers around the world in your browser.
Listen to your own signal from another continent; nothing else tells you as much about your antenna.
Transceivers
- Hermes-Lite 2, HackRF, LimeSDR, ADALM-Pluto — open, experimenter-oriented.
- FlexRadio, Apache ANAN — direct-sampling HF transceivers run entirely from a PC client.
- Most modern mainstream radios (IC-7300, IC-7610, FTdx10, TS-890) are SDRs internally with a
built-in spectrum display; you get the benefits without a computer.
Software
SDR++, SDRangel, GQRX, SDR# and CubicSDR for general use; GNU Radio if you want to build the signal chain yourself. See software.
Practical notes
- Panadapter output from a conventional radio (an IF tap or the IC-7300's USB stream) gives you the
- Keep the dongle away from the transmit antenna, or fit a receive-protection relay — a few watts
- USB noise is real: use a shielded cable, a ferrite choke and, if needed, a powered hub.
waterfall without replacing the radio.
will destroy the front end.