SDRs turn radio into software. With the right hardware and DSP blocks you can observe, decode,
and synthesize signals across wide frequency ranges—from pagers to satellites—within legal limits.
Quick-start checklist
Authorization & spectrum awareness: Receive is broadly permitted; transmit is regulated. Keep work to lab setups or authorized bands.
Pick your band: Decide what you’re observing (FM broadcast, ADS-B at 1090 MHz, ISM at 315/433/868/915 MHz, Wi-Fi/BLE at 2.4 GHz, etc.).
Match antenna: Use an antenna resonant (or close) to your target band; poor antennas = poor results.
Sampling plan: Choose sample rate and center frequency; avoid front-end overload with sane gain settings.
Tooling: Start with a visual tuner (GQRX/SDR#) to sanity-check RF environment, then move to analyzers (URH) or flowgraphs (GNU Radio).
SDR primer: IQ, sampling, gain
SDRs digitize the RF spectrum into complex I/Q samples at a chosen sample rate.
You tune a center frequency and capture a slice of spectrum around it; DSP blocks then filter,
demodulate, and decode signals.
Rules of thumb:
Higher sample rates capture wider bandwidth but require more CPU/storage.
Gain too low = weak signals; too high = clipping/overload. Increase until noise floor rises, then back off slightly.
Filtering is everything: apply appropriate LPF/BPF and decimation before demodulation.
Tune to a center frequency, capture bandwidth, then filter/demodulate.
Common research workflows
1) Spectrum recon & signal hunting (RX)
Use a spectrum/waterfall view (GQRX/SDR#) to identify active carriers and their bandwidth.
Switch demodulators: FM/AM/SSB, narrowband FM, or record raw I/Q for later analysis.
Tag frequencies and note time-of-day activity patterns.
2) Digital signal analysis
Import recordings into Universal Radio Hacker (URH) or GNU Radio.
Identify symbol rate, framing, preamble, and payload; test line codes (NRZ, Manchester, etc.).
Build a decoder; validate against multiple captures to confirm robustness.
Log RSSI and message rates; experiment with different antennas (1/4-wave, collinear, LNA).
5) Wi-Fi/BLE observation (RX only)
Observe 2.4 GHz band occupancy and interference patterns; validate channel plans.
For protocol-level Wi-Fi/BLE work, use dedicated radios/sniffers—SDR RF-level views are complementary.
6) Controlled TX in a lab
Generate test waveforms (tones, reference frames) into a shielded or properly attenuated setup.
Validate demod/decoder against known-good signals; never radiate into public spectrum without authorization.
Legal cautions: Many jurisdictions restrict transmission and certain types of replay/jamming/spoofing.
Keep TX work inside shielded boxes, on dummy loads, or in authorized spectrum. When in doubt—don’t transmit.
Analysis: Universal Radio Hacker (URH), Inspectrum, DSpectrum
Flowgraphs: GNU Radio, Pothos, SoapySDR ecosystem
Portable: PortaPack H4M apps on HackRF for stand-alone capture/playback (within legal scope)
Appliance distros: DragonOS (pre-bundled SDR toolset), Raspberry Pi as a headless SDR node
Antennas & RF hygiene
Resonance matters: Use quarter-wave (~𝜆/4) or band-specific antennas for your target freq.
Front-end protection: Inline attenuators/LNAs where appropriate; avoid strong nearby transmitters.
Feedline & placement: Use low-loss coax for higher bands; elevate antennas; keep them away from metal.
Shielding: For TX experiments, use RF shield boxes or dummy loads to avoid radiating.
Blue-team / monitoring notes
Waterfall baselining: Maintain a “known-good” spectrum snapshot for critical areas.
Anomaly detection: Alert on unexpected carriers, bandwidth spikes, or persistent new signals.
Antenna zoning: Physically separate sensitive receivers from public spaces.
Protocol upgrades: Prefer spread-spectrum, encrypted links with authentication; rotate keys.
Troubleshooting
Can’t see expected signals: Check antenna match, gain, and that you’re tuned to the correct frequency (and not an image).
Flat waterfall: Increase gain modestly; confirm drivers are loaded; try a known-good broadcast (FM radio) as a sanity check.
Clipping/overload: Reduce gain; add attenuation; move farther from strong transmitters.
Decode fails: Verify sample rate, correct demodulator, filter bandwidth, symbol timing, and bit encoding assumptions.
FAQ
What makes SDR different from fixed-function radios?
Flexibility. Demodulation and decoding live in software, so you can adapt to new signals without new hardware (within your front-end’s frequency and bandwidth limits).
Can I transmit with any SDR?
No. Many low-cost SDRs are receive-only. Devices like HackRF can transmit, but it’s regulated—use shielded labs or authorized bands only.
Do I need math/DSP to get started?
You can begin with visual tools and recordings. As you progress, some DSP concepts (filters, sampling, modulation) will help you decode more complex signals.
Devices for SDR
Useful references
GNU Radio docs & tutorials; Pothos/SoapySDR ecosystem guides
