Merv's Brain Dump

Chapter 1 — Introduction and Scope

This manual covers variable capacitor designs for antenna tuners, transmatch circuits, and magnetic loop antenna tuning: commercial air-variable capacitors (motorized), homebrew stack-on flat-plate capacitors (DIY from copper foil and polymer sheet), and a modular controller (ESP32 + encoder + OLED) for motorizing any shaft-driven variable capacitor.

Chapter 2 — Theory of Operation

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)

Chapter 3 — Equipment and Materials

Chapter 4 — Construction

4-1 Motorized Air-Variable

1. Couple the DC motor to the capacitor shaft via a flexible coupler. Flexible couplers compensate for shaft misalignment that would otherwise bind the capacitor bearings.
2. Mount two microswitches at the minimum and maximum capacitance positions (use the shaft angular position to trigger them via a cam or lever arm).
3. Couple the 10-turn pot to the capacitor shaft (1:1 ratio) for position feedback. Wire the pot as a voltage divider (0–3.3V range for ESP32 ADC).
4. L298N H-bridge or L293D motor driver provides bidirectional motor control from ESP32 GPIO signals. Use PWM for speed control if needed (slower near endpoints for finer positioning).

4-2 Stack-On Flat-Plate Capacitor Units

1. Cut copper foil to 50×75 mm sheets. Cut polymer dielectric (Kapton, LDPE, or polypropylene) to 50×80 mm (slightly larger to prevent edge breakdown).
2. Assemble: copper foil (top) / polymer / copper foil (bottom) / PVC backing strip. Offset the top and bottom foil tabs so they do not touch when stacking.
3. For parallel stacking: connect all top tabs together (plate A), all bottom tabs together (plate B). Each additional unit adds ∼50–200 pF depending on dielectric thickness and area.
4. Secure units in a stack with velcro strips. The modular design allows capacitance to be added or removed without soldering.

Chapter 5 — Operating Procedures

5-1 Motorized Air-Variable

1. On power-up, the controller moves to the stored last position (from NVS).
2. Enter the desired capacitance on the OLED display using the rotary encoder. The motor drives to the corresponding position (from the calibration table).
3. For magnetic loop antenna tuning: the ESP32 receives SWR data from the SWR bridge and automatically sweeps capacitance until SWR minimum is found.

5-2 Stack-On Capacitor

Stack-on units are passive; simply connect the plate-A lead to one circuit node and the plate-B lead to the other. Add or remove units to increase or decrease capacitance in approximately 100–200 pF steps.

Chapter 6 — Calibration

1. Motorized cap: move the capacitor to 10 equally-spaced pot positions. Measure capacitance at each position with an LCR meter or NanoVNA (use the impedance measurement mode at a known frequency). Store the (ADC_value, pF) table in ESP32 NVS.
2. Stack-on caps: measure each unit individually with an LCR meter at 100 kHz. Label each unit with its measured capacitance. Build a combination table: n units in parallel = n × C_unit.

Chapter 7 — Verification and Acceptance

1. Motorized cap positioning: command each of 5 target capacitances; measure actual capacitance; error must be <5% of target.
2. Limit switch test: command travel past the end limits; controller must stop at the limit switches without mechanical damage to the capacitor.
3. Stack-on units: each unit must measure within 20% of its labeled value (LCR meter verification; flat-plate caps have significant tolerance).
4. HV rating: apply the rated voltage across the capacitor at maximum capacitance setting and hold for 10 seconds. No arc-over or increase in leakage current.
5. Log: date, cap type, calibration table (10 points for motorized), stack-on unit values, positioning accuracy, operator.

Appendix A — Flat-Plate Capacitor Design Formula

C = ε0 × εr × A / d

Example: 50 × 75 mm LDPE sheet (0.05 mm thick):
  A = 0.050 × 0.075 = 0.00375 m²
  d = 0.05 mm = 5×10−5 m
  εr (LDPE) = 2.3
  C = 8.854e-12 × 2.3 × 0.00375 / 5e-5
  C = 8.854e-12 × 172.5 = 1527 pF ≈ 1.5 nF per unit

For 100 pF per unit: reduce area or increase dielectric thickness.

Appendix B — Common Air-Variable Specifications