Experimental QRO magloop slide variable capacitors (3 and 5mm spacing, ~3-90 pF)

NB! This is legacy model, please use OpenSCAD model, it allows sliding gap customization and has worm gear system with bearings https://www.thingiverse.com/thing:4578300

Two experimental magnetic loop antenna variable capacitors. Built to understand if such 3d printed capacitors are feasible and what are their limits with PETG filament for QRO operation.

Included FreeCAD models, feel free to modify them for you needs.

This work is still in progress, capacitor needs better supports for worm gear (preferably with bearings), holes for nuts, better universal support for more widespread stepper motor model.

Print at your own risk, may need some manual tuning with rasp or try to print with 0.1 precision. Was using prototype/test loop built by using some leftovers from construction work and 3d printed center holder.

Both cover ~3-95 pf, maximum power is around 50W (CW) for small capacitor and 100W (CW) for large on 0.9 M loop, tunes continuously from ~9 to 29 MHz depending on the large ring diameter, check in calculator to be in the range. Tested large capacitor at 50W FT8 for multiple iterations on 30M band and it was slightly warm without noticeable SWR degradation. With better spiral coupling matching got about SWR 1.05 at minimum.

Small capacitor contains 11 (5 + 6) 0.5 mm 30x55 mm aluminium plates connected with M3 screw and nuts for each plate. Plate spacing is 3 mm. Large capacitor plate sizes are 50x60 mm and plate spacing is 5 mm.

I also have home made butterfly capacitors, they can sustain 100 W, but they cannot continously cover all required range starting from small capacitance, because their minimum capacitance is rather large for larger maximum capacitance.

Construction time is about 30-40 minutes when models are already printed.

Added models for EM-290 stepper (from old epson printer) holder and shaft for linking between motor and M3 worm gear.

Made multiple successful test CW QSO at 20M and 30M bands with 100W by using large capacitor from inside the apartment. Also run multiple WSPR iterations at 15W on 30M band, was heard by about 40 stations and was hearing about 20 stations ranging from 1000 up to 2000 km during day time.

10.jul.2020 Build second more efficient antenna prototype by using aluminium water pipe with 30mm larger diameter, efficiency should be around 50% on 14 MHz and higher on 14+MHz. Tube costs around 2$ per meter and it sold bent in 1 m diameter.

14.jul.2020 Made more extensive testing on the new loop. Was operating on 30-10M bands in FT8/FT4 with 50W, no problems, capacitor surface temperature was stable at around 45C.