This article references parts of the story in my fiction books, The Meadow Protocol and book 2, After The Brush, part of The Continuity Chronicles series. Available in my store for signed paperback and hard copies for from Amazon to include Kindle and Audible.
Radio waves from a transmitter spread outward in all directions, but directional antennas are more sensitive in some directions than others. DF systems exploit this to determine the angle of arrival (AOA).
Key concepts:
- Bearing: The compass direction (azimuth) from your position toward the transmitter, usually in degrees magnetic or true.
- Null vs. Peak: Many simple antennas (like loops) produce a figure-8 pattern—strongest signal edge-on, weakest (null) when the loop plane is perpendicular to the incoming wave. Nulls are often sharper and more accurate than peaks.
- Ambiguity: A single bearing gives a line, not a point—180° ambiguity (front/back) is common with loops. A sense antenna (omnidirectional) resolves this by creating a cardioid pattern (one strong lobe, one null).
- Triangulation: Plot two or more bearings on a map. The intersection is the estimated location. Accuracy improves with wider baseline (distance between DF stations) and crossing angles close to 90°.
Accuracy formula (simplified): Error in position ≈ (baseline distance × bearing error in radians) / sin(crossing angle). For example, with a 3-mile baseline, 5° bearing error, and 90° crossing angle, the fix uncertainty is roughly 400–500 meters—good enough for patrolling or directing a team.
Essential Equipment for DF
You don’t need expensive military gear. Many preppers and hams build effective DF setups for under $200.
- Receiver:
- Handheld VHF/UHF scanner or ham HT (e.g., Baofeng UV-5R, Yaesu FT-60R) tuned to the target frequency (2m/70cm bands common for local threats).
- Shortwave receiver for HF (e.g., 3–30 MHz) if monitoring long-range government or survivor nets.
- Directional Antennas:
- Handheld Tape Measure Yagi (3-element for 2m): Cheap DIY from PVC, tape measure steel, and coax. Sharp forward lobe, easy to build.
- Loop Antenna: Simple wire loop or ferrite rod for HF/VHF null finding. Rotate until signal nulls.
- Adcock Array (four vertical monopoles): Better for skywave rejection on HF, can be portable.
- Doppler or Pseudo-Doppler: Four-antenna switched array + electronics for automatic bearing (KrakenSDR is popular among hams).
- Support Tools:
- Compass or smartphone with offline topo maps (e.g., Gaia GPS, Avenza).
- Laminated maps, grease pencils for plotting.
- Field phone or wired link (TA-312) for sharing bearings silently between stations.
- Attenuator to reduce strong signals and sharpen nulls.
Step-by-Step: How to Perform DF in the Field
- Detect the Signal:
- Scan frequencies (e.g., 146.520 MHz national simplex, MURS/GMRS, known government bands).
- Log time, frequency, signal strength, any audio (voice, burst, tone).
- Take a Bearing (Single Station):
- Hold or mount the directional antenna.
- Rotate slowly while watching signal strength meter or listening.
- Find the null (weakest signal) or peak (strongest).
- Note the compass bearing (add antenna offset if needed).
- Resolve ambiguity with sense antenna or movement test.
- Triangulate:
- Move to a second location (ideally 1–5 miles away, perpendicular to first bearing).
- Take another bearing.
- Plot both lines on a map. Intersection = estimated location.
- Add a third bearing for better accuracy and error estimation.
- Homing (if close):
- Follow the bearing, attenuating the signal as you approach (use attenuator or detune slightly).
- Signal peaks when you’re near; nulls may flip as you pass the source.
- Analyze:
- Apply ACH (Analysis of Competing Hypotheses): Raider probe? Lost survivor? Surveillance? Match to SALUTE reports.
- Log everything for patterns over time.
Practical Applications for Preppers and Survivalists
- Threat Detection: Locate raider comms or jammers before they locate you.
- Search & Rescue: Home in on a missing team member’s emergency beacon.
- OPSEC Enforcement: Find unauthorized transmitters in your area.
- Fox Hunting Practice: Ham radio ARDF events build skills—hide a transmitter, track it down.
- Passive SIGINT: Monitor and locate without ever keying up.
Limitations and Countermeasures
- Multipath/Reflections: Buildings, hills cause false bearings. Mitigate with elevated antennas or multiple fixes.
- Accuracy Limits: 5–10° typical for handheld; 1–3° with good setup. Short bursts hard to catch.
- Counter-DF: Enemies use burst transmissions, frequency hopping, low power. You counter by continuous monitoring and silent triangulation.
- Legal Note: In normal times, DF on licensed bands is fine; avoid interfering.
Direction finding turns the enemy’s need to communicate into your advantage. With practice, basic gear, and patience, a small group can build an intelligence edge that rivals pre-collapse agencies. Start simple—build a tape-measure Yagi, practice on local repeaters or NOAA stations, and run fox hunts with your team. In a crisis, knowing where the signal is coming from can mean the difference between ambush and ambush avoided. Stay silent, stay aware, and stay ahead.
See also SIGINT
See also COMINT