An RTL-SDR Blog V4 covers 100kHz to 1.75GHz. Here is a practical band-by-band reference to what transmitters occupy this spectrum and what software can decode them.
Below 30 MHz: HF and Medium Wave
Medium Wave (530 to 1700 kHz)
AM broadcast radio. In North America, stations transmit at up to 50kW daytime. At night, the ionosphere refracts MW signals over distances exceeding 1000km, making distant stations receivable. Requires HF direct sampling mode (RTL-SDR V4) or an upconverter.
Shortwave (1.8 to 30 MHz)
International broadcast stations, amateur radio on the 160m through 10m bands, maritime communications, military utility stations, and digital modes including FT8, WSPR, and SSTV. The Airspy HF+ Discovery is the benchmark entry-level receiver for serious shortwave work, offering dynamic range that competes with bench receivers costing significantly more.
30 to 300 MHz: VHF
Aircraft VHF (118 to 136 MHz)
Air traffic control, ATIS automated weather broadcasts, and aircraft voice communications on AM. Every major airport has multiple active frequencies. ATIS broadcasts loop continuously and are an excellent test of receiver setup.
NOAA Weather Satellites (137 to 138 MHz)
APT image transmissions from NOAA-15, -18, and -19. A simple V-dipole antenna receives them well. Capture windows are 10 to 12 minutes per pass. SatDump or WXtoImg decodes the transmissions into visible and infrared images.
Marine VHF and AIS (156 to 174 MHz)
Maritime voice communications and AIS transponders. AIS signals at 161.975 and 162.025 MHz broadcast vessel position, speed, heading, and identification. AIS-catcher software decodes them in real time, displaying vessel traffic on a map.
UHF (300 MHz to 1 GHz)
ADS-B (1090 MHz)
Aircraft position transponders. The highest-value single frequency for RTL-SDR operation. dump1090 decodes position, altitude, speed, heading, callsign, and aircraft type. A rooftop antenna sees aircraft at cruise altitude (10,700m) at ranges exceeding 400km.
ISM Bands (433 MHz, 868 MHz, 915 MHz)
Industrial, scientific, and medical allocations covering an enormous variety of devices: tire pressure sensors, weather stations, keyless entry, LoRa devices, Zigbee, and custom telemetry. rtl_433 software automatically decodes hundreds of device types — run it and see what appears in your neighbourhood within minutes.
A Note on Receiving vs Transmitting
Everything described here is receive-only. Receiving is legal everywhere without a licence. Transmitting requires licensing from the relevant national telecommunications authority: ISED in Canada, FCC in the US, Ofcom in the UK. HackRF and LimeSDR are transmit-capable devices and should only be used for transmission within your jurisdiction's regulations and your licence authorizations.