Build a software-defined radio receiver from 500 kHz to 1.7 GHz using RTL-SDR hardware, GNU Radio signal processing, and custom antenna design.
Build a software-defined radio receiver from 500 kHz to 1.7 GHz using RTL-SDR hardware, GNU Radio signal processing, and custom antenna design. This comprehensive guide covers everything from design through implementation, testing, and deployment.
Software-Defined Radio: hardware digitizes a wide slice of spectrum, software processes the digital data to demodulate specific signals. RTL-SDR: RF input → bandpass filter → LNA → mixer (downconversion to IF) → IF amplifier → ADC (28.8 MHz sample rate, 8-bit) → USB → PC. PC software: GNU Radio implements digital tuning (NCO + mixer shifts desired frequency to baseband), demodulation (FM, AM, SSB), decoding. Frequency range: 500 kHz–1.7 GHz. 2.4 MHz bandwidth viewable simultaneously. Useful for: listening to FM radio, tracking aircraft (ADS-B), weather satellite images (NOAA), marine AIS, pager decoding (POCSAG).
10 components required for this project.
| # | Component | Purpose | Qty |
|---|---|---|---|
| 1 | RTL-SDR V3 Dongle (RTL2832U + R820T2) | Wideband SDR receiver | x1 |
| 2 | HackRF One (optional, TX+RX) | Transmit and receive SDR (licensed bands) | x1 |
| 3 | Low Noise Amplifier (LNA4ALL) | Weak signal amplification | x1 |
| 4 | Band-pass filters (FM, ADS-B, weather) | Interference rejection | x3 |
| 5 | Dipole antenna kit (adjustable) | General-purpose reception | x1 |
| 6 | ADS-B antenna (1090 MHz vertical) | Aircraft tracking | x1 |
| 7 | Raspberry Pi 4 (headless SDR server) | SDR host computer | x1 |
| 8 | GNU Radio 3.10 (open source) | Signal processing flowgraph software | x1 |
| 9 | SDR# (Windows SDR software) | General-purpose SDR receiver app | x1 |
| 10 | SMA adapters and coaxial cable (RG-174) | RF connectivity | x1 |
Follow these 5 steps carefully.
Software-Defined Radio: hardware digitizes a wide slice of spectrum, software processes the digital data to demodulate specific signals. RTL-SDR: RF input → bandpass filter → LNA → mixer (downconversion to IF) → IF amplifier → ADC (28.8 MHz sample rate, 8-bit) → USB → PC. PC software: GNU Radio implements digital tuning (NCO + mixer shifts desired frequency to baseband), demodulation (FM, AM, SSB), decoding. Frequency range: 500 kHz–1.7 GHz. 2.4 MHz bandwidth viewable simultaneously. Useful for: listening to FM radio, tracking aircraft (ADS-B), weather satellite images (NOAA), marine AIS, pager decoding (POCSAG).
GNU Radio uses a flowgraph: signal sources → signal processing blocks → sinks. FM Broadcast receiver flowgraph: RTL-SDR Source (center_freq=100.1e6, sample_rate=2.4e6) → Low Pass Filter (cutoff 75kHz) → FM Demodulator (deviation=75kHz) → Audio Sink (sample_rate=48000). Parameters in Python: blocks connect via ports. Spectrum analyzer: Waterfall Sink (shows frequency vs time as color) and FFT Sink (shows frequency spectrum). IQ data: each sample has two components (In-phase and Quadrature) — preserves full frequency information including sign (positive/negative frequency).
ADS-B: Aircraft broadcast their GPS position, altitude, speed, and call sign at 1090 MHz. Antenna: quarter-wave monopole for 1090 MHz = 69mm wire from center conductor, counterpoise radials. Install dump1090-fa (Flightaware's ADS-B decoder). Run: dump1090 --net. Connects at 127.0.0.1:8080 for web map showing all nearby aircraft (50–300 km range). Increase range: add LNA (20dB+ amplification) and coaxial cable with low loss (LMR-400 or Aircell-7). Enhance antenna: build a 1/4 wave radial antenna on SMA plug.
NOAA-15/18/19 transmit analog APT weather satellite images at 137 MHz. Pass duration: 12–15 minutes (LEO orbit). Predict passes: Heavens-Above.com or Gpredict software (knows satellite orbital elements). Antenna: turnstile antenna (two crossed dipoles, 90° phase offset) for circular polarization — satellite transmits RHC polarization. Record: wide FM demodulation (deviation 34kHz). Decode APT image: WXtoIMG or noaa-apt decoder. Result: visible + infrared image of Earth from 800km altitude — location visible if correct local time and cloud cover.
rtl_power: scan 24 MHz to 1.7 GHz in steps, generate spectrum heatmap. Identify signals: CW morse code (narrow spike), FM broadcast (200kHz wide), TETRA (trunked radio, 25kHz carrier), LTE cellular (wide), ISM bands (433, 868, 915 MHz — remote controls, LoRa, Sigfox). Signal identification tool: sigidwiki.com (community database of 500+ signal types by waterfall appearance). Measure: RSSI (signal strength), bandwidth, modulation type. Record IQ samples to disk for later analysis.
Core code for fm_receiver.py:
#!/usr/bin/env python3 # Simple FM Receiver using GNU Radio and RTL-SDR # pip install gnuradio (or install from package manager) from gnuradio import gr, audio, analog, filter, blocks from gnuradio.filter import firdes import osmosdr class FMReceiver(gr.top_block): def __init__(self, freq=100.1e6): super().__init__() # Sample rate and channel config samp_rate = 2400000 # 2.4 MSPS from RTL-SDR audio_rate = 48000 # Audio output sample rate fm_dev = 75000 # FM deviation ±75 kHz # Source: RTL-SDR hardware self.src = osmosdr.source() self.src.set_sample_rate(samp_rate) self.src.set_center_freq(freq) self.src.set_gain(30) # RF gain in dB self.src.set_if_gain(20) self.src.set_bb_gain(20) # Low-pass filter: pass ±100 kHz around center (FM broadcast bandwidth) lp_taps = firdes.low_pass(1, samp_rate, 100000, 25000) self.lpf = filter.fir_filter_ccf(1, lp_taps) # Rational resampler: 2400000 → 240000 (decimate ×10) self.resamp = filter.rational_resampler_ccc(1, 10) # FM demodulator self.fm_demod = analog.fm_demod_cf( channel_rate=240000, audio_decim=5, deviation=fm_dev, audio_pass=15000, audio_stop=16000, gain=1.0, tau=75e-6) # Audio output self.audio_sink = audio.sink(audio_rate, "", True) # Connect flowgraph self.connect(self.src, self.lpf, self.resamp, self.fm_demod, self.audio_sink) if __name__ == '__main__': freq = float(input("Enter FM frequency (MHz): ")) * 1e6 receiver = FMReceiver(freq) print(f"Receiving FM at {freq/1e6:.1f} MHz...") receiver.run()Test RF Transceiver and SDR by verifying each subsystem individually before full integration.
Verify power voltages, check ground connections, use serial monitor for debug.
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