The air is full of signals. Learn to hear them.

OpenHertz teaches you radio and SDR by doing it, mission by mission — free, open source, and 100% receive-only. Plug in an RTL-SDR or a HackRF, or use the built-in simulator.

The missions

• First contact — Read a spectrum, understand the noise floor
• Catch an FM station — Find a station, see a wideband signal
• Listen to aviation (AM) — Pilots and control towers, live
• ISM 868 band (sensors / LoRa) — Catch IoT bursts in the act
• The 2.4 GHz chaos — WiFi, Bluetooth, microwaves: the crowd
• POCSAG pagers — Paging — a legacy tech that's still alive
• ADS-B radar — Decode aircraft: callsign, altitude, position
• CAPSTONE — Detect a drone — Spot a wideband OFDM video link

The library

• The airband (aviation AM)
• Antennas
• Interesting bands to explore
• The amateur bands
• Welcome to the library
• Noise floor, SNR and sensitivity
• Resonant circuits and filters
• Phonetic alphabet, Q-codes and RST reports
• Electronic components
• Decibels (dB and dBm)
• Detecting vs decoding
• Drones and the spectrum on the modern battlefield
• Sampling, Nyquist and aliasing
• Electricity: the exam basics
• Transmitter and receiver
• Preparing for the amateur-radio exam
• From I/Q to spectrum: the FFT
• Electronic warfare: reading the spectrum as terrain
• The HackRF One
• Callsigns, operating and the logbook
• I/Q samples
• Legal & safety
• Feed lines, SWR and matching
• What hardware should I start with?
• Modulation: writing information onto a wave
• Radio waves
• POCSAG: the paging that won't die
• Wave propagation
• Tuning your antenna, step by step
• Amateur-service regulations
• Weather-satellite images (coming soon)
• What is an SDR?
• Safety and electromagnetic compatibility
• The waterfall (time cascade)
• Using it live: the method