Unit 3 — Assembly and Setup

TM-GEAR-003 — 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-GEAR-003. Read Chapter 4 — Construction and Assembly completely.

Then come back here.

Chapter 4 Content

4-1 1:1 Choke Balun (FT-240-31)

  1. Thread RG-303 (or RG-58) coaxial cable through the toroid 12 times. Keep turns close-wound; secure with cable ties after every 3 turns.
  2. Connect the input end to an SO-239 or N-type connector (coax center to center pin, shield to shell). Same for the output end.
  3. Measure choking impedance with NanoVNA: connect one port to the coax center, other port to the coax shield, and sweep 1.8–30 MHz. The |Z| should exceed 2000 Ω across most of the HF band. Core type #31 is optimized for 1.8–30 MHz.

4-2 4:1 Guanella Balun

  1. Wind two identical 8-turn bifilar windings on two separate FT-140-43 cores. Use color-coded wire to track polarity (dot convention).
  2. Connect the two windings in series for the balanced (200Ω) port and in parallel for the unbalanced (50Ω) port. Polarity is critical: the start of winding 1 connects to the finish of winding 2 at the balanced port center tap.

  3. Test: connect a 200Ω resistor to the balanced port; measure 50Ω at the unbalanced port (SWR 4-3 9:1 Un-Un (End-Fed Wire)

  4. Wind 9 turns of trifilar #16 AWG on an FT-240-43 core. Three wires wound simultaneously, in the same direction.

  5. Connect winding A in series with winding B; the junction is the 50Ω (unbalanced input) center tap. Winding C is the 450Ω output winding.
  6. Test: connect 450Ω resistor to high-impedance port; measure 50Ω SWR at the coax port (<1.2:1 across 1.8–30 MHz).

Assembly Quality

Chapter 4 specifies 9 construction/assembly steps.

The assembly directly determines RF performance. 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 behavior - Ground loops — multiple ground paths at different potentials cause noise and calibration errors - Ferrite winding errors — wrong turn count or direction reverses transformer polarity or changes impedance ratio - Incorrect winding direction on toroidal transformers — affects phase and common-mode rejection

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, impedance ratio.

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 RF performance

→ Proceed to Unit 4