QuadRF: A Raspberry Pi 5 Phased-Array SDR for Drone Tracking and RF Visualization

The QuadRF is a phased-array software-defined radio (SDR) that enables users to visualize and track radio frequency (RF) signals in real-time. Built around a Raspberry Pi 5 and an FPGA board with picosecond-level timing, the device can detect WiFi signals through walls and track drones in flight within a specific frequency range of 4.9 to 6 GHz.

High-Bandwidth RF Streaming via Raspberry Pi 5 MIPI

The QuadRF achieves low-latency SDR streaming of I/Q (In-phase/Quadrature) data at rates exceeding 5 Gbps by utilizing the Raspberry Pi 5's MIPI lanes. This approach bypasses traditional USB interfaces in favor of the Pi's camera and display FFC MIPI connectors, which route data through the RP1 chip.

According to the QuadRF documentation, this implementation offers several advantages:

• Reduced Latency: MIPI provides full-duplex data transfer with almost zero hardware cost to the RF board.
• High Sample Rates: The system can sustain hundreds of MSPS (Mega Samples Per Second) of I/Q without sample loss.
• High-Speed Peripherals: By using MIPI for RF streaming, the PCIe connector remains available for high-speed storage or networking.

Real-Time RF Visualization and Augmented Reality

The QuadRF includes an Augmented Reality (AR) visualizer that overlays RF signal "blobs" onto a live camera feed. This allows users to physically locate the source of a RF signal in their environment.

During testing, the device successfully identified 5 GHz WiFi networks (approximately 5.5 GHz) and neighboring networks, appearing as distinct colored blobs on a display. The system also tracked a DJI Mini Pro 4 drone in flight, identifying its signal as it moved through the sky.

As noted by the creator, Martin McCormick, the AR experience is powered by a web app that streams RF points to a browser, which then merges them with the local camera feed to maintain high frame rates and low latency.

Hardware Architecture and Scalability

The QuadRF is designed as a 4x4 MIMO software-defined radio. While the current handheld prototype is 3D printed, the final production version will move to injection-molded enclosures.

One of the most significant architectural goals is scalability. The system is designed to allow licensed operators to daisy-chain multiple QuadRF modules together. This could potentially create a massive antenna array capable of Earth-Moon-Earth (EME) radio experiments and radio astronomy, with a theoretical maximum of 1.15 MW EIRP.

Technical Constraints and Community Insights

While the QuadRF QuadRF is a powerful tool for RF analysis, it has specific technical limitations and users in the community have highlighted several key points:

• Frequency Range: The device is limited to the 4.9-6 GHz range. Consequently, it cannot detect drones or devices operating on 900 MHz or cellular bands.
• UI/UX: Early prototypes have a rough user interface, though the creator has stated that the UI is open source and is being improved based on user feedback.
• Clock Synchronization: Community members have raised questions regarding how a shared, highly precise clock is distributed across daisy-chained modules to maintain phase coherence.
• Security Implications: The ability to see WiFi traffic and locate signal sources has sparked discussion on the privacy implications of such tools, though the author of the source material suggests that these capabilities have long been available to government agencies.

"If the open source community can come up with something like this, just imagine what governments are capable of."

Summary of Specifications

Feature	Specification
Core Processor	Raspberry Pi 5
RF Interface	MIPI (via RP1 chip)
Data Rate	> 5 Gbps
Frequency Range	4.9 - 6 GHz
Antenna Array	4x4 MIMO Phased Array
Target Applications	Drone tracking, WiFi analysis, EME experiments, astronomy