Unit 1 — Theory of Operation

TM-TOOL-010 — Open Handout TM Chapter: Chapter 2 ELOs: Understand the operating principle of the SWR METER — CONSTRUCTION AND USE; identify key specifications Estimated time: 20 minutes

Step 1: Read the TM

Open TM-TOOL-010. Read Chapter 2 — Theory of Operation completely.

Then come back here.

Chapter 2 Content

2-1 Tandem-Match Directional Coupler (HF)

Two toroids wound on BN-43-202 or FT-50-43 cores provide −30 dB coupling. The forward arm measures V_fwd (proportional to forward wave voltage); the reflected arm measures V_ref (proportional to reflected wave). Termination resistors (51Ω 1%) on each secondary provide the directionality; directivity is typically >35 dB across 1.8–30 MHz.

2-2 Diode Detector Circuits

1N5711 Schottky diodes detect the coupled RF. In the square-law region (V_in 100 mV): V_dc ∝ V_rf. The meter driver compensates for detector law by adjusting the meter scale or applying a lookup table in firmware.

2-3 SWR Calculation

SWR = (V_fwd + V_ref) / (V_fwd − V_ref) [analog cross-needle meter] SWR = (1 + sqrt(P_ref/P_fwd)) / (1 − sqrt(P_ref/P_fwd)) [digital meter] 2-4 Range Switching Resistive dividers on the detector outputs set the effective full-scale power range. Switching from 100W to 10W range adds 10× attenuation before the panel meter, maintaining accuracy across the power range. Six ranges: 5W, 20W, 50W, 200W, 500W, 1500W (switchable by front-panel rotary).

Why Theory Matters

You cannot use a measurement tool correctly without understanding how it works. Theory tells you: - What the tool measures and how it converts the quantity to a readable output - What the sources of error are — so you can recognize and minimize them - What the valid operating range is — so you stay within its specifications - How to interpret results that don't match expectations

If a measurement looks wrong, theory is where you look first.

Self-Check Questions

SC1-1. In one sentence, state the operating principle of the SWR METER — CONSTRUCTION AND USE as described in Chapter 2.

SC1-2. What does Chapter 2 identify as the primary source(s) of measurement error or uncertainty?

SC1-3. What key specification(s) (accuracy, range, frequency coverage) does the TM state?

SC1-4. What does Chapter 2 say the SWR METER — CONSTRUCTION AND USE cannot do — what are its limitations?

SC1-5. List two formulas or relationships from Chapter 2 that govern the tool's operation.

Answer Key

SC1-1. See TM §2-1. Compare your sentence to the first substantive paragraph of Chapter 2.

SC1-2. See Chapter 2. Look for language about error sources, accuracy limits, parasitic effects, or frequency dependence.

SC1-3. See Chapter 2. Look for numbers with units: %, ppm, Hz, Ω, dB, W.

SC1-4. See Chapter 2 and Chapter 1. Limitations are often stated as frequency range, power limits, or accuracy bounds.

SC1-5. See Chapter 2. Equations or proportionality statements are the relationships that govern the tool.

Checkpoint

Before proceeding, state without looking: - The operating principle of the SWR METER — CONSTRUCTION AND USE - The primary error source(s) - At least one key specification with its value

→ Proceed to Unit 2