Unit 3 — Assembly and Setup

TM-GEAR-009 — 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-009. Read Chapter 4 — Construction and Assembly completely.

Then come back here.

Chapter 4 Content

4-1 Branch-Line Hybrid (144 MHz)

  1. Calculate λ/4 at 144 MHz on FR4 (1.6 mm, εr=4.6): λ/4 = c/(4 × f × √εeff) ≈ 350 mm / 4 = 87.5 mm. Account for effective permittivity (εeff ≈ 3.4): 87.5 / √3.4 = 47.5 mm.
  2. Draw the four-arm square layout: each arm 47.5 mm long. Shunt arms: 2.9 mm wide (50Ω). Series arms: 5.0 mm wide (35.35Ω).
  3. Mount SMA connectors at each of the four corners. The square layout minimizes parasitic coupling between arms.

4-2 Lumped-Element HF Hybrid (7–30 MHz)

  1. Replace each λ/4 line section with an equivalent LC π-network: L = Z0/ω = 50/(2π×14e6) = 568 nH (shunt arms); C = 1/(ω×Z0) = 1/(2π×14e6×50) = 227 pF.
  2. Use 1% tolerance components to maintain amplitude and phase balance.

Assembly Quality

Chapter 4 specifies 5 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