Unit 1 — Theory of Operation

TM-GEAR-019 — Open Handout TM Chapter: Chapter 2 ELOs: Understand the operating principle of the VARIABLE CAPACITORS — AIR, MOTORIZED, AND STACK-ON FLAT-PLATE; identify key specifications Estimated time: 20 minutes

Step 1: Read the TM

Open TM-GEAR-019. Read Chapter 2 — Theory of Operation completely.

Then come back here.

Chapter 2 Content

2-1 Air-Variable Capacitor

A parallel-plate capacitor with interleaved rotor and stator plates (no dielectric between the plates). Capacitance is proportional to the overlap area between rotor and stator plates: C = ε0 × A / d, where A is the overlap area and d is the plate spacing (typically 1.5–3 mm for HF/VHF use). As the rotor turns, the overlap area changes continuously from zero (minimum capacitance, plates disengaged) to maximum (full overlap).

2-2 Voltage Breakdown

Air breakdown occurs at approximately 30 kV/cm (3 MV/m). For a 3 mm plate spacing: V_breakdown = 3000V × 0.3 cm = 900V peak. At 100W into 50Ω: V_peak = √(2 × 100 × 50) = 100V peak — well within rating. In a transmatch at high transformation ratio, voltages across the capacitors can reach 1–3 kV. Use the capacitor voltage rating appropriate for the transmatch power and impedance.

2-3 Homebrew Stack-On Flat-Plate Capacitor

Copper foil plates separated by a thin polymer dielectric (polyimide, LDPE, or polypropylene film). Stacking multiple units in parallel increases total capacitance. Stacking in series reduces total capacitance and increases voltage rating. Formula:

C = ε0 × εr × A / d [Farads] ε0 = 8.854×10−12 F/m εr = relative permittivity (LDPE: 2.3; polypropylene: 2.2; polyimide: 3.5) A = plate area (m²); d = dielectric thickness (m)

Why Theory Matters

You cannot build or use RF gear correctly without understanding how it works. Theory tells you: - What the component does and how it produces that effect - What the sources of loss, distortion, or error are — so you can recognize and minimize them - What the valid operating range is — frequency, power, impedance — so you stay within specifications - How to interpret results or system behavior that doesn't match expectations

If a component doesn't perform as expected, theory is where you look first.

Self-Check Questions

SC1-1. In one sentence, state the operating principle of the VARIABLE CAPACITORS — AIR, MOTORIZED, AND STACK-ON FLAT-PLATE as described in Chapter 2.

SC1-2. What does Chapter 2 identify as the primary source(s) of loss or degradation in performance?

SC1-3. What key specification(s) (frequency range, power rating, insertion loss, impedance ratio) does the TM state?

SC1-4. What does Chapter 2 say the VARIABLE CAPACITORS — AIR, MOTORIZED, AND STACK-ON FLAT-PLATE cannot do — what are its limitations?

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

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 loss mechanisms, parasitic effects, frequency limits, or power constraints.

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

SC1-4. See Chapter 2 and Chapter 1. Limitations are often stated as frequency range, power handling, or impedance range.

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

Checkpoint

Before proceeding, state without looking: - The operating principle of the VARIABLE CAPACITORS — AIR, MOTORIZED, AND STACK-ON FLAT-PLATE - The primary loss or degradation source(s) - At least one key specification with its value

→ Proceed to Unit 2