Multi-SDR Frequency Direction Finder Setup

~195 minAdvancedNew

Build a phase-coherent multi-channel SDR receiver array for real-time Direction of Arrival using MUSIC and ESPRIT algorithms. Covers KrakenSDR, antenna geometry, calibration, and frequency-scanning DF.

Prerequisites
This is the most advanced workshop — solid foundations required

Hardware

  • •KerberosSDR (~$160) or KrakenSDR (~$400) — required
  • •4–5 identical vertical antennas for target frequency
  • •Equal-length SMA coaxial cables (4 or 5, matched ±1 cm)
  • •Non-conductive circular array base (PVC or fiberglass)
  • •GPS module (optional, for multi-station triangulation)
  • •Power splitter for calibration (e.g., Mini-Circuits ZFSC-4)

Knowledge

  • •Workshop 1 (SDR basics) and Workshop 2 (DF basics) completed
  • •Python 3: numpy, scipy, matplotlib
  • •Complex number arithmetic and FFT concepts
  • •Basic linear algebra (eigenvalues/eigenvectors helpful)
  • •Linux command line proficiency
Step 1 of 813% complete
Step 120 min
Coherent Receiver Array Theory

A coherent receiver array is multiple receivers sharing a common reference clock, enabling meaningful phase comparisons between channels. Phase-based Direction of Arrival (DoA) algorithms can determine bearing with a fixed antenna array — no rotation needed.

Phase difference and angle of arrival

Phase difference (radians): Δφ = (2π × d × sin(θ)) / λ

d = spacing between antennas (meters)

θ = angle of arrival relative to array broadside

λ = wavelength = c/f (meters)

For antennas spaced d = λ/2 apart (half-wavelength spacing), θ = 90° gives Δφ = π radians (180°). Solving for θ: θ = arcsin(Δφ × λ / (2π × d)). This is the fundamental relationship behind all phase-based DF systems.

Phase Interferometry

Measure phase difference between antenna pairs. Fast, simple, but ambiguous beyond λ/2 spacing.

Sensors: 2–4 antennas

MUSIC Algorithm

Uses eigendecomposition of the cross-correlation matrix to find signal subspace. Superresolution — resolves multiple simultaneous signals.

Sensors: N antennas (N > number of sources)

ESPRIT Algorithm

Similar to MUSIC but doesn't require array calibration. Uses shift invariance between sub-arrays.

Sensors: Even number of antennas in matched pairs

Beamforming (Bartlett)

Scan the array steering vector across all angles; peak response gives bearing. Robust but lower resolution than MUSIC.

Sensors: Any number of antennas

The key insight: a wave arriving at angle θ takes different amounts of time to reach different antennas in the array. This time difference (phase difference) directly encodes the Direction of Arrival.

Coherent Receiver Array Theory