Gqrx SDR

Overview

Gqrx is a powerful open-source SDR (Software Defined Radio) receiver application that enables users to receive, demodulate, and analyze radio signals across a wide frequency range. Built on GNU Radio, Gqrx provides a user-friendly graphical interface for spectrum analysis, signal monitoring, and radio communications research. It supports numerous hardware interfaces including RTL-SDR, HackRF, USRP, and others, making it accessible for both hobbyists and professionals.

Key capabilities include FM/AM/SSB/CW demodulation, waterfall visualization, spectrum analysis, signal recording, and frequency scanning. Gqrx is commonly used in penetration testing, spectrum analysis, wireless security research, and amateur radio applications.

Installation

Linux (Debian/Ubuntu)

# Install from repository
sudo apt-get update
sudo apt-get install gqrx-sdr

# Or build from source
sudo apt-get install git cmake g++ libpulse-dev libusb-1.0-0-dev libfftw3-dev
git clone https://github.com/csete/gqrx.git
cd gqrx
mkdir build
cd build
cmake ..
make
sudo make install

macOS

# Using Homebrew
brew install gqrx

# Or download DMG from official releases

Windows

Download pre-built installer from gqrx.dk or build using MinGW environment.

Hardware Support

Hardware	Frequency Range	Bandwidth	Notes
RTL-SDR	25 MHz - 1.7 GHz	2.4 MHz	Most affordable, USB dongle
HackRF	1 MHz - 6 GHz	20 MHz	TX/RX capable, full duplex
USRP	50 MHz - 6 GHz	Varies	Professional grade, expensive
LimeSDR	100 MHz - 3.8 GHz	40 MHz	Full duplex, affordable
PlutoSDR	70 MHz - 6 GHz	56 MHz	Portable, integrated

Basic Usage

Starting Gqrx with RTL-SDR

# Launch with default configuration
gqrx

# Specify device at startup
gqrx -s 0

# Run in headless mode (remote operation)
gqrx --style plastique

Initial Setup

Connect SDR hardware via USB
Launch Gqrx application
Click “Configure I/O devices” button
Select device from dropdown (RTL-SDR, HackRF, etc.)
Set sample rate (2.4 MHz typical for RTL-SDR)
Click “Apply” to initialize

Frequency Management

Scanning Frequencies

# Common frequency ranges to monitor:
# FM Radio: 88-108 MHz
# 2m Amateur: 144-148 MHz
# 70cm Amateur: 430-450 MHz
# ISM Band: 2.4 GHz
# WiFi: 2.4 GHz, 5 GHz
# GSM: 850/900/1800/1900 MHz

Bookmarks

Save frequently monitored frequencies:

Frequency	Mode	Description
101.5 MHz	FM	Local FM station
146.52 MHz	FM	2m repeater
2.4 GHz	LSB	ISM band monitoring
1.09 GHz	USB	Satellite downlink

Demodulation Modes

Supported Modulation Types

Mode	Use Case	Bandwidth	Notes
AM	Broadcast, emergency services	10 kHz	Amplitude modulation
FM	Radio stations, voice	15 kHz	Frequency modulation
LSB	Amateur radio, telephony	3 kHz	Lower sideband
USB	Amateur radio, data	3 kHz	Upper sideband
CW	Morse code, beacons	500 Hz	Continuous wave
WFM	Broadcast FM, high quality	180 kHz	Wideband FM

Switching Demodulation Modes

# Via GUI: Click demod mode dropdown
# Common workflow:
1. Set frequency (MHz)
2. Select demod mode (FM, AM, USB, LSB, CW)
3. Adjust squelch level
4. Fine-tune frequency with arrow keys
5. Monitor signal strength indicator

Signal Analysis

Waterfall Display Controls

# Waterfall Settings:
- Time Scale: Adjust refresh rate (1s to 60s)
- Frequency Scale: Zoom in/out on spectrum
- Intensity: Adjust color scaling
- Zoom: Click and drag to zoom region
- Pan: Scroll horizontally for freq range

Spectrum Analysis Features

# Signal detection:
# - Peak hold: Shows maximum signal levels
# - Average: Smooths spectrum display
# - Decay: Defines signal fade rate
# - Reference level: Adjust dB scaling

Audio Configuration

Speaker Output

# Configure audio device
1. Go to File → Preferences
2. Select "Audio" tab
3. Choose output device from dropdown
4. Set audio gain (0-100%)
5. Enable/disable audio playback

# Typical settings:
- Audio device: PulseAudio default
- Output gain: 50%
- AF filter: 100 Hz - 5 kHz

Recording Audio

# Record received signals
1. File → Preferences → Recording
2. Select output directory
3. Choose file format (WAV, MP3)
4. Click red record button to start
5. Audio saved with timestamp

# Recording command (CLI):
# ALSA recording
arecord -d 60 -f cd recorded_signal.wav

# PulseAudio recording
parec | sox - recording.wav

Advanced Features

Frequency Scanner

# Configure scanner for frequency range
1. Tools → Frequency Scanner
2. Set start frequency (MHz)
3. Set stop frequency (MHz)
4. Set step size (kHz)
5. Set dwell time per frequency (ms)
6. Click "Scan" to begin

# Example: Scan 88-108 MHz FM band
Start: 88 MHz
Stop: 108 MHz
Step: 100 kHz
Dwell: 500 ms

Noise Blanker

# Reduce impulse noise
1. Tools → Receiver Options
2. Enable "Noise Blanker"
3. Adjust threshold (0-100)
4. Test with strong signals nearby

Remote Control (TCP)

# Enable remote control
File → Preferences → Network
- Enable TCP Server
- Port: 7356 (default)
- Allow Remote Control: checked

# Connect remotely
telnet localhost 7356

# Remote commands:
f 106500000         # Set frequency to 106.5 MHz
m FM                # Set mode to FM
l MAIN 0 100        # Set level
q                   # Quit connection

Command Line Operations

Launch with Preset Configuration

# Start with specific frequency and mode
gqrx --freq=101500000 --mode=FM

# Disable OpenGL (for systems without GPU)
gqrx --style=plastique

# Set device index
gqrx --device=1

Batch Frequency Scanning

# Create script for automated scanning
#!/bin/bash
declare -a frequencies=(88.1 101.5 146.52 430.2)

for freq in "${frequencies[@]}"
do
    echo "Scanning $freq MHz"
    # Send remote commands to Gqrx
    echo "f ${freq}000000" | nc localhost 7356
    sleep 5
done

Signal Detection & Analysis

FM Signal Analysis Workflow

1. Set mode to FM
2. Adjust filter width to 15 kHz
3. Set squelch to -5 dB (prevents noise audio)
4. Monitor signal strength bar
5. Record samples for analysis
6. Export waterfall screenshot

Weak Signal Reception

# Tips for improving weak signal reception:
1. Use antenna tuned to target frequency
2. Disable noise blanker (may reduce sensitivity)
3. Use USB/LSB for CW signals
4. Reduce audio filter bandwidth
5. Position antenna away from interference
6. Use external LNA (Low Noise Amplifier)

Troubleshooting

Common Issues and Solutions

Issue	Cause	Solution
No signal detected	Device not selected	Check I/O devices config
Distorted audio	Sample rate mismatch	Match hardware & software rates
High noise floor	Poor antenna	Use tuned antenna, move away from sources
Device not found	USB permissions	Add user to dialout group
High CPU usage	Waterfall resolution	Reduce waterfall size or refresh rate
No audio output	Output muted	Check system audio settings

Device Not Recognized

# Check USB device
lsusb | grep -i "rtl\|hackrf"

# Add user to dialout group (RTL-SDR)
sudo usermod -a -G dialout $USER
newgrp dialout

# Set udev rules
sudo cp rtl-sdr.rules /etc/udev/rules.d/
sudo udevadm control --reload
sudo udevadm trigger

Resetting Configuration

# Reset Gqrx settings to default
rm ~/.config/gqrx/gqrx.conf

# Launch fresh
gqrx

Performance Optimization

System Requirements

Minimum:
- CPU: Dual-core 2 GHz
- RAM: 2 GB
- Disk: 500 MB free space

Recommended:
- CPU: Quad-core 2.5+ GHz
- RAM: 4+ GB
- Disk: 1 GB free space

Optimization Techniques

# Reduce CPU usage
1. Lower waterfall resolution
2. Disable OpenGL rendering: gqrx --style=plastique
3. Reduce sample rate when possible
4. Disable peak hold / averaging
5. Close background applications

# Monitor system resources
top
htop
ps aux | grep gqrx

Security & Privacy Considerations

Safe Frequency Monitoring

# Legal frequency bands for monitoring (varies by country):
# United States (FCC):
# - 88-108 MHz: FM Broadcast
# - 144-148 MHz: Amateur 2m band
# - 430-450 MHz: Amateur 70cm band
# - 900-950 MHz: ISM band (limited)

# ALWAYS verify local regulations before monitoring
# Check FCC, OFCOM, or local regulatory body

Recording Restrictions

# Important legal considerations:
# - Recording encrypted communications may be illegal
# - Some jurisdictions prohibit SDR operation
# - Amateur bands require valid license in most countries
# - Always obtain proper authorization

Integration with Tools

Exporting Data

# Export waterfall as PNG/JPEG
1. Right-click waterfall display
2. Select "Save Waterfall"
3. Specify file path and format

# Export recorded audio
1. Locate WAV file in recordings directory
2. Convert using ffmpeg if needed:
   ffmpeg -i recording.wav recording.mp3

Piping to External Tools

# Analyze exported WAV with Audacity
audacity recording.wav &

# Convert and analyze with sox
sox recording.wav -n spectrogram -o spectrum.png

# Process with MATLAB/Python
python3 analyze_signal.py recording.wav

Resources and References

Official Website: https://gqrx.dk
GitHub Repository: https://github.com/csete/gqrx
GNU Radio Documentation: https://www.gnuradio.org
RTL-SDR Resources: https://www.rtl-sdr.com
Amateur Radio Frequencies: https://www.arrl.org/frequencies
FCC Frequency Allocation Chart: https://www.fcc.gov/uls/