Unit 3 — Assembly and Setup

TM-TOOL-001 — Open Handout TM Chapter: Chapter 4 ELOs: Execute assembly steps in the correct sequence; verify build quality before operation Estimated time: 20 minutes

Step 1: Read the TM

Open TM-TOOL-001. Read Chapter 4 — Construction and Assembly completely.

Then come back here.

Chapter 4 Content

4-1 Watson-Watt Loop Construction

  1. Wind two identical shielded loops: 30 cm diameter, 3 turns #18 AWG, Faraday shield (gap at top, not a closed loop).
  2. Mount loops orthogonally on a common center mast; orient one loop N-S, other E-W.
  3. Run equal-length coax (within 5 mm) from each loop to the receiver switching point.
  4. Connect sense antenna (vertical whip, λ/4 at center frequency) at same switching point.

4-2 Butler Matrix PCB

  1. Fabricate 4-port 3 dB 90° hybrid couplers on FR4 (Z0=35.4Ω microstrip, λ/4 length at design frequency).
  2. Connect hybrids per the Butler Matrix topology: E1,E2 → Hybrid #1,#2; outputs of Hybrid #1,#2 → Hybrid #3,#4 with a 45° fixed phase shifter between Hybrid #2 output and Hybrid #4 input.
  3. Verify phase relationships with NanoVNA S21 phase measurements before installing array elements.
  4. Mount array elements at corners of a square, λ/4 side length, connected to matrix element ports E1–E4.

4-3 Doppler Commutator

  1. Mount 4 or 8 vertical elements equally spaced on a circle of radius λ/4 at operating frequency.
  2. Wire each element through an RF relay (PIN diode or mechanical) to a common output coax.
  3. Drive relay sequence from ESP32 GPIO at rotation rate f_rot (start at 200 Hz for HF).
  4. Connect common output to receiver; extract FM component in software (GNU Radio or custom DSP).

Assembly Quality

Chapter 4 specifies 12 construction/assembly steps.

The assembly directly determines measurement quality. Common errors: - RF leads too long — lead inductance raises SWR and limits high-frequency performance - Cold solder joints on RF nodes — high resistance causes signal loss and intermittent readings - Ground loops — multiple ground paths at different potentials cause noise and calibration errors - Ferrite winding errors — wrong turn count or direction reverses transformer polarity

If Chapter 4 specifies a verification step after assembly (e.g., "verify DC resistance = X before proceeding"), do it. Those checks exist because they are the most common failure points.

Self-Check Questions

SC3-1. How many assembly steps does Chapter 4 specify?

SC3-2. What is the first assembly step? State it exactly from the TM.

SC3-3. Does Chapter 4 specify maximum lead length anywhere? If so, what is the limit and why?

SC3-4. Does Chapter 4 require a bench verification after assembly? What does it check?

SC3-5. What would you do if a winding resistance measurement came out wrong during assembly verification?

Answer Key

SC3-1. Count the numbered steps in Chapter 4.

SC3-2. See Chapter 4, step 1. Copy it exactly.

SC3-3. RF lead length limits are typically 10–15 mm for HF circuits. Longer leads add ~1–2 nH per mm, raising inductive reactance at high frequencies.

SC3-4. Scan Chapter 4 for verification steps. Common checks: DC resistance, winding balance, null depth on test signal.

SC3-5. Stop assembly. Diagnose before proceeding — a winding error found before completion is much easier to fix than one discovered after the unit is boxed.

Checkpoint

Before proceeding: - [ ] You have read Chapter 4 completely - [ ] You can state the number of assembly steps and the first and last steps - [ ] You understand how assembly quality affects measurement accuracy

→ Proceed to Unit 4