Doppler T-Hunt at the Gilruth

Four of the JSC Student Interns from this semester attended a Fox Hunt (or T-Hunt) this past Sunday, November 19, held by Jeff AB4ME.  Jeff hid 2  Bynoics  MicroFox 15mW transmitters (146.565 and 146.430 Mhz) for the students to find using direction finding navigation.  The venue was the open Gilruth field area by the shack. 

Each participant was given a 2m Doppler type hand held receiver.  These were based from the popular Handi-Finder type detectors, but enhanced with a clever LED directional indicator developed by Dave, NZ1J.  Dave kindly provided the Gerber Files, PIC program, schematic, and build information for our club to use.  We procured PCBs (via JLCPCB) and parts from Mouser, such that 5 units were built for the fox hunt, and enough extra for a future club DIY project build night in upcoming months.

Jeff shared with the group, that the techniques for DF are learned over time, as one of the foxes gave the team some challenges due to multipath from metal-sided buildings.

Dominic KJ5DAF mentioned that the LED indicators were very helpful.   This is quite an improved feature from the original version of the Handi-Finder which relied on audio nulling only.  Lit Green LEDs tells the user to point left of right.  A Red LED lights up when the orientation is in boresight of the source.


Dave NZ1J’s implementation cleverly compares the phase shift between the reference antenna switching oscillator (at an audio rate) with the receiver’s audio tone resulting from from the difference orientation of the two doppler antennas to the source.  Cleverly, the receiver audio signal is obtained by a pickup coil loop placed over the radio speaker.  The radio’s speaker magnetic coil induces enough magnetic field strengh for the external pickup coil carry that audio modulation back into circuit for phase comparison with the reference switched frequency.

Congrats to Dominic KJ5DAF, Vika KF0OBR, Michael and Chase for finding the foxes and helping us test drive the new doppler DF systems.

Jeff shared a big lesson learned. We were unable to hear the foxes consistently with the DF sets despite them being full quieting on Jeff’s HT. The solution was to set squelch to zero on the DF sets. It was too awkward to hold to squelch cancel button all the time.   

Approaching a Fox!

the Byonics Fox beacons

Here’s more information on Dave NZ1J’s invention

Many thanks to Jeff for hosting the THunt and Dave NZ1J for his gracious support to us.

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