RF direction finding, in all three rigs
One kit, three vehicles
- KrakenSDR (five coherent receivers) and a five-element circular array.
- 4Runner: the array fixed on the Gobi rack, next to the solar panels (mind the charger's RF noise: ferrites, short shielded runs, misra watching the noise floor).
- F100 and Chevy: the same receiver with a magnetic-mount array on the steel roof, which makes a good ground plane. One kit moves between rigs, or a second one later.
- Element spacing about half a wavelength for the band of interest (below).
Bands: 2 m and 70 cm (fox hunting)
Five elements in a circle, ideally about half a wavelength apart:
| Band | Wavelength | Ideal spacing | Circle radius | On a vehicle |
|---|---|---|---|---|
| 70 cm (~440 MHz) | 0.68 m | 0.34 m | ~0.29 m | fits any roof |
| 2 m (~146 MHz) | 2.05 m | 1.03 m | ~0.87 m (1.75 m across) | too wide: use a compromise, ~0.6 m radius (~0.35 λ spacing), some accuracy lost |
Two sets on one rack plate (or two magnetic-mount rings): short 70 cm whips (~17 cm, quarter wave) in a tight circle, 2 m whips (~49 cm) at the compromise spacing. KrakenSDR's own array calculator has the last word on spacing.
Close in, the array stops helping: a near fox overloads any receiver. The final approach is on foot with a tape-measure Yagi and an offset attenuator; the vehicle finds the area. (International ARDF hunts on 2 m and 80 m.)
Toward a search-and-rescue rig
- KrakenSDR covers 24–1766 MHz, so it hears the 121.5 MHz homing signal that personal locator beacons and aircraft ELTs send alongside their 406 MHz satellite bursts (those are digital and decodable, sometimes with a GPS position).
- At 121.5 MHz a full array is ~2.1 m across: vehicle DF is a compromise there too, and the last stretch is a handheld Yagi or Doppler DF.
- APRS on 2 m for team positions, on the same map as the bearings.
- Real SAR work goes through the county SAR team or the Civil Air Patrol: their procedures, training and coordination come first; the rig supports them.
Where the bearings go
The KrakenSDR serves bearings over the network; the vehicle box adds GPS position and heading (gpsd) and logs both, so fixes from several positions triangulate and a good hunt becomes an episode. Any screen can show them:
| Rig | Display |
|---|---|
| 4Runner | our Android Auto app, side-loaded in developer mode (Car App Library, navigation category, bearing lines over the map) |
| 1974 F100 | the same app's phone or tablet layout, on a mount |
| 1949 Chevy | a round, period-styled gauge in the dash: a small round display on an ESP32, a needle pointing at the signal, signal strength as a second hand; on the same CAN bus as the engine gauges (engine-can.md) |
One app, two layouts (Android Auto and phone), built from one shared core; the gauge reads the same bearings off the bus.
Open questions
- One KrakenSDR moving between rigs, or one each
- The Chevy gauge's size and face (a 2" or 2-1/16" round to match the dash)