Direction Find an RF Source

~150 minIntermediate

Locate radio transmitters using directional antennas, triangulation, and both amplitude and Doppler-based direction finding methods.

Prerequisites
Required before starting this workshop

Hardware

  • •RTL-SDR V4 or any wideband SDR receiver
  • •Directional antenna (Yagi or loop) for target frequency
  • •Compass (magnetic or phone app with declination)
  • •GPS device or smartphone for position marking
  • •Optional: KerberosSDR for Doppler DF (Step 7)

Knowledge

  • •Completed Workshop 1: Monitor a Spectrum Band
  • •Basic map reading skills
  • •Understanding of compass use and magnetic declination
  • •A target signal to locate (local repeater, 433 MHz sensor, etc.)
Step 1 of 714% complete
Step 115 min
Understanding RF Direction Finding Principles

Radio direction finding (RDF or DF) is the process of determining the angular direction toward a radio transmitter. Three fundamental techniques exist, each with different equipment needs.

Amplitude DF

Rotate a directional antenna; the bearing with maximum (or minimum) signal strength points toward the source.

+ Simple hardware− Slow, affected by reflections, requires human rotation

Phase DF

Compare the phase of the signal arriving at two or more antennas. Phase difference reveals angle of arrival.

+ Fast, no moving parts− Requires coherent multi-channel receivers

Time Difference of Arrival (TDOA)

The signal arrives at multiple receivers at slightly different times. Hyperbolic intersections give location.

+ Very accurate over wide areas− Requires GPS-synchronized receivers and networked setup

Key DF terminology

BearingCompass direction toward the transmitter (0–360°)
Line of Bearing (LoB)A line drawn from your position in the bearing direction
Fix / CutIntersection of two or more LoB lines = estimated transmitter location
AmbiguityMany antennas give 180° ambiguity — signal is ahead OR behind
MultipathReflections from buildings/terrain cause false bearings
Sensitivity nullMinimum of antenna pattern — often more precise than maximum

DF accuracy depends heavily on antenna type, multipath reflections from buildings, and how far you are from the transmitter. In urban environments, expect ±10–30° bearing error.

Understanding RF Direction Finding Principles