Magnetic loop antenna capacitor controller

Magnetic loop antenna capacitor controller based on

• OpenSCAD project to generate custom sliding variable high voltage capacitors with two predefined configurations
• Arduino Nano
• L298N stepper driver
• EM-290 stepper from Epson printer
• HC-06 bluetooth serial controller for remote control using Bluetooth console application from the phone or PC
• Generated high voltage 3d printed slide capacitor printed with PETG plastic (up to ~15kV and 130pF)
  • Plate sizes are ~70x55 mm
  • Capacitor dimensions are 60x70x80 mm
  • Plate spacing 5 mm
  • 14 plates
  • Worm gear is M4
  • Can handle 100W CW without arcing (tested with 0.9m loop) and continous 50W operation in FT8
  • Capacitance range ~5-130 pF
  • Can tune ~9-29 MHz with 0.9m loop
• Second test generated medium voltage 3d printed slide capacitor up to ~6 kV and higher 400pF capacitance

Serial commands

On successful command execution "ok" is returned when operation was successfully completed or "err" if command was unsuccessful. Next commands are supported:

• Movements
  • p: park to initial 0 position, automatically performed when powered on
  • u, uu, uuu: go/tune up to higher frequency (5/50/500kHz steps)
  • d, dd, ddd: go/tune down to lower frequency (5/50/500kHz steps)
  • freqkhz, frequency in kHz where to move capacitor, should be used after calibration procedure completion
• Calibration
  • t: print calibration table
  • c[freqkhz]: store current minimum swr position into given calibration frequency cell
  • s: save calibration table

Configuration parameters (capctrl.h)

• ConfigSpeed: stepper speed, default is 64
• ConfigStep(5|50|500)kHz: number of steps for tune up/down commands for 5/50/500kHz steps
• ConfigMaxPos: end position of capacitor, maximum number of steps, default is 5200
• ConfigCalPoints: number of calibration points, default is 23
• ConfigCalAddr: EEPROM start address where calibration data is stored, default is 0x0
• ConfigStepCompensate: number of steps for reverse compensation when changing direction

Photos

• Large capacitor 15kV/130pF

• Medium size capacitor with lower maximum voltage, but larger capacitance 6kV/400pF

• Test magnetic loop antenna, 1m loop diameter, 40mm tube diameter, fed using inductive coupling from smaller loop and 1:1 balun