A Doppler Type 2 Meter Radio Direction Finder

This Doppler type direction finder is an alternative to the Yagi antenna / attenuator method of direction finding. This requires no attenuator and can provide a bearing to a transmitter that is several miles or several feet away.

The magnetic pickup coil allows the unit to work with most models of hand held radios. It is important to place the coil over the HT speaker and to have the correct side facing out. If the coil is reversed, the direction finder will report left as right and right as left.

The circuit board uses a PIC microprocessor to select the antennas and to measure the phase of the audio in the HT speaker. It uses PIN diodes to switch the antennas. Though slightly shortened, the antennas are dipoles that are spaced 12 inches apart.

This unit is an enhancement of the HANDI-Finder https://www.handi-finder.com/ but is not affiliated with HANDI-Finder. The larger antennas and PIN diodes make this unit more sensitive and less susceptible to inter-modulation distortion. The addition of the pickup coil and phase detector resolves all issues of left/right and front/back. The visual display on the LEDs makes it unnecessary to listen for the tone.

As of December 2023, I've got photos, a bill of materials, a schematic, a hex file to program the PIC microprocessor, dimensions for the frame, and assembly instructions available. I'm also ordering printed circuit boards in small quantities. For anyone who'd like to try building one of these, a printed circuit board is $3 and a programmed PIC chip is $3. USPS is $6. Contact me via my email address on QRZ.

A second YouTube video    • Construction Details for Doppler Radi...   shows the steps to build the frame for this radio direction finder.

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Also, Art, N0EX, has repackaged this direction finder with a surface mount circuit board and a case and vinyl overlay. As of January 2023, he has the first of these partially assembled units available for sale. Contact Art at RDFfun (at) gmail.com or through his QRZ page to learn more.
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And, here's the BOM:

Doppler Direction Finder Bill of Materials 11/12/2021

Qty Part Digi-key Part Number References
1 PIC Pocessor PIC16F18324-I/P-ND U1
1 14 Pin Socket ED3114-ND U1
1 78L05 497-7268-ND U2
2 2N3904 2156-2N3904-FS-ND Q1, Q2
2 PIN Diode 568-1922-1-ND D1, D4
4 Green LED C503B-GCN-CY0C0792CT-ND D5, D6, D8, D9
1 Red LED C503B-RCN-CW0Z0AA1-ND D7
4 1k Resistor 1.0KQBK-ND R3, R6, R13, R17
8 4.7k Resistor 4.7KQBK-ND R2, R5, R10, R11, R12, R14, R15, R16
2 470k Resistor 470KQBK-ND R1, R4
1 220pF Cap 399-14053-1-ND C6
1 0.01uF Cap 399-9772-ND C5
3 0.1uF Cap 399-4264-ND C1, C3, C4
1 10uF Cap 493-17435-1-ND C2
1 1uH Inductor 78F 1R0K-TR-RCCT-ND L1
1 Slide Switch 450301014042-ND SW1
1 9V Battery Clip 36-232-ND TP+

Dave NZ1J