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RavenSDR

A lightweight, purpose-built Android app for monitoring radio frequencies with an RTL-SDR USB dongle. RavenSDR provides a live waterfall display, wideband spectrum scanning, FM/AM demodulation, squelch-controlled listening, and signal-alert notifications — all without requiring root access.

Tested on Android 14 and Android 17 beta.

Credits

RavenSDR builds on the work of several open source projects:

Project Author / Maintainer License Use in RavenSDR
librtlsdr Osmocom community GPL-2.0 RTL2832U chip initialisation and IQ sample streaming
libusb libusb contributors LGPL-2.1 USB Host communication on unrooted Android
KissFFT Mark Borgerding BSD-3-Clause FFT for live spectrum and waterfall computation
SDRTrunk Dennis Sheirer GPL-3.0 Waterfall colour palette design

Screenshots

Explore tab — waterfall and band presets    Listen tab — FM demodulation and band monitor

Table of Contents

Features

Explore

  • Live waterfall display — colour-mapped FFT spectrum updated in real time at 2.048 MSPS
  • Band presets — one-tap access to GMRS (CH 1–7, 15–22), GMRS repeater inputs, FRS (CH 8–14), CB (40 channels), APRS 2m, 915 MHz ISM / Meshtastic, 434 MHz FOBS/LoRa, and 650 MHz wireless mics
  • Explore mode — tune anywhere from 500 kHz to 1766 MHz in configurable steps (100 Hz – 1 MHz)
  • Wideband scan — automatically sweeps across all channels in a band; configurable dwell time

Listen

  • NFM and AM demodulation — narrowband FM and amplitude modulation with real-time audio output
  • Squelch control — adjustable threshold silences the receiver between transmissions
  • Band Monitor — listens across all channels in a band simultaneously and locks when a signal is found
  • Playlist — save named frequencies and play them back sequentially or manage them as a list

Alarms

  • Signal alerts — configurable dB threshold triggers an audible radar ping
  • Alert history log — tap any entry to jump directly to that frequency in Listen mode

General

  • Bookmarks — save and load named frequencies, shared between Explore and Listen tabs
  • AGC / manual gain — toggle between automatic gain control and 30 dB fixed gain
  • Auto-launch — app opens automatically when an RTL-SDR dongle is plugged in via USB OTG
  • Supported hardware — RTL2832U family: standard RTL2832U, RTL2838, RTL2831, RTL2833, RTL2837, and RTL-SDR Blog V4

Hardware Requirements

Item Notes
Android device Android 8.0 (API 26) or higher; USB Host support required
RTL-SDR dongle Any RTL2832U-based device; RTL-SDR Blog V4 recommended
USB OTG cable or adapter Micro-USB or USB-C to USB-A female, matching your phone
Antenna SMA antenna appropriate for your target frequency band

No root access is required. The app uses Android's built-in USB Host API.

Development Environment Setup

Prerequisites

Tool Version Notes
Android Studio Ladybug (2024.2) or newer Download
Android NDK r27 LTS or newer Installed via Android Studio SDK Manager
CMake 3.22.1 or newer Installed via Android Studio SDK Manager
JDK 11 Bundled with Android Studio

1. Install Android Studio

Download and install Android Studio from developer.android.com/studio. During first-run setup, accept all SDK licenses when prompted.

2. Install NDK and CMake via SDK Manager

In Android Studio:

  1. Open File → Settings (macOS: Android Studio → Settings)
  2. Navigate to Appearance & Behavior → System Settings → Android SDK
  3. Select the SDK Tools tab
  4. Check NDK (Side by side) — install version r27 LTS or newer
  5. Check CMake — install version 3.22.1 or newer
  6. Click Apply and let the installation finish

Alternatively, install via the command line (adjust the path to your SDK):

# Linux / macOS
sdkmanager "ndk;27.2.12479018" "cmake;3.22.1"

# Windows (run from the SDK tools directory)
sdkmanager.bat "ndk;27.2.12479018" "cmake;3.22.1"

3. Clone the Repository

git clone https://github.com/JamesBurnettHQ/RavenSDR.git
cd RavenSDR

The repository includes the native library sources (libusb, librtlsdr, kissfft) as vendored copies under app/src/main/cpp/. No separate submodule initialisation is needed.

4. Open in Android Studio

  1. Launch Android Studio
  2. Choose File → Open and select the cloned RavenSDR directory (the one containing settings.gradle)
  3. Wait for Gradle sync to complete — Android Studio will download all Java/Kotlin dependencies automatically
  4. If prompted about a missing local NDK path, open File → Project Structure → SDK Location and confirm the NDK path is detected correctly

5. Build and Install

Via Android Studio (recommended)

  1. Connect your Android device via USB and enable USB Debugging in Developer Options
  2. Select your device from the target device dropdown in the toolbar
  3. Click Run ▶ (or press Shift+F10) to build and install the debug APK

Via Gradle command line

# Debug build and install on connected device
./gradlew installDebug

# Release APK — requires KEYSTORE_PATH, KEYSTORE_PASS, KEY_ALIAS, KEY_PASS env vars
./gradlew assembleRelease

# Windows
gradlew.bat installDebug

The first build compiles three native static libraries (libusb, librtlsdr, kissfft) and one JNI shared library (librtlsdr_android.so) for arm64-v8a and x86_64. This takes several minutes. Subsequent builds are incremental.

Project Structure

RavenSDR/
├── app/
│   ├── build.gradle                  # App-level Gradle config, NDK/CMake wiring
│   └── src/main/
│       ├── AndroidManifest.xml       # USB host feature, USB device filter
│       ├── cpp/
│       │   ├── CMakeLists.txt        # Native build: libusb + librtlsdr + kissfft → JNI .so
│       │   ├── rtlsdr_jni.cpp        # JNI bridge: Java ↔ librtlsdr
│       │   ├── libusb/               # libusb 1.x (Android Linux backend, no udev)
│       │   ├── librtlsdr/            # librtlsdr (RTL2832U driver)
│       │   └── kissfft/              # KissFFT (lightweight FFT for spectrum computation)
│       ├── java/com/ravensdr/
│       │   ├── MainActivity.java     # UI, tab routing, USB and DSP lifecycle
│       │   ├── UsbHelper.java        # USB Host permission and device attachment
│       │   ├── RtlSdrDriver.java     # JNI wrapper around librtlsdr
│       │   ├── DspEngine.java        # IQ → FFT pipeline, continuous scan thread
│       │   ├── ListenEngine.java     # NFM/AM demodulation, squelch, channel scanner
│       │   ├── WaterfallView.java    # Custom View: waterfall + channel markers + alert
│       │   ├── SpectrumView.java     # Custom View: spectrum overlay
│       │   ├── GMRSChannels.java     # Frequency tables for all supported bands
│       │   └── ConsoleLogger.java    # In-app debug log (visible on Console tab)
│       └── res/
│           ├── layout/activity_main.xml
│           └── xml/usb_device_filter.xml   # RTL2832U USB VID/PID list
├── screenshots/                      # App screenshots for README
├── build.gradle                      # Root Gradle file (AGP version)
├── settings.gradle                   # Project name, repository declarations
├── gradle.properties                 # JVM heap, AndroidX, parallel builds
└── gradlew / gradlew.bat             # Gradle wrapper scripts

Native Libraries

RavenSDR bundles vendored copies of three C libraries that are compiled from source by the Android NDK:

Library Version License Purpose
libusb 1.x LGPL-2.1 USB Host communication without kernel driver on Android
librtlsdr mainline GPL-2.0 RTL2832U chip initialisation and IQ sample streaming
KissFFT mainline BSD-3-Clause Fast Fourier Transform for spectrum computation

libusb is configured with the Android Linux/usbfs backend (no udev, no netlink) so it works on unrooted devices using Android's UsbManager.openDevice() file descriptor.

Troubleshooting

Symptom Fix
"No RTL-SDR — plug in dongle" Check the OTG cable; try a different USB port or cable
"USB permission denied" Tap OK on the system permission dialog; revoke and re-grant from App Settings if it doesn't appear
"Init failed: …" Unplug the dongle, wait 3 seconds, and plug it back in
Black waterfall Dongle is initialising; wait 2–3 seconds after connecting
Waterfall freezes DSP engine will auto-retry up to 3 times; if it fails, unplug and restart
No audio in Listen mode Raise the phone media volume; check squelch level (start at 5, lower if no audio)
Build error: NDK not found Install NDK via SDK Manager (see step 2 above)
CMake version mismatch Install CMake 3.22.1+ via SDK Manager

Supported Android Versions

Android Version API Level Status
Android 8.0 – 8.1 26 – 27 Minimum supported
Android 9 – 13 28 – 33 Supported
Android 14 34 Tested ✓
Android 17 beta 37 Tested ✓

License

RavenSDR is released under the GNU General Public License v3.0 — see LICENSE for the full text.

The bundled libraries retain their own licenses:

  • librtlsdr: GPL-2.0
  • libusb: LGPL-2.1
  • KissFFT: BSD-3-Clause

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