AE7TO Portable NVIS Loop Antenna Capacitor Mounting Disks

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These 3d models are designed specifically for mounting MFJ-23 or MFJ-19 butterfly loop tuning capacitors into 4 inch or 6 inch (nominal) Schedule 40 ABS or PVC Drain, Waste, Vent pipes. Both capacitor models can be used with either the 4 inch or the 6 inch pipe centering disks, but you must cut off the little fiberglass ears at both ends of the capacitor to fit it into a 4 inch plastic pipe. The models with filenames that begin with NVIS were created for a 6 inch pipe housing and for the 12.25 foot diameter (38.5 foot circumference) 80-40m (3.5 to 7.3 MHz) portable NVIS loop antenna described in the provided PDF file. The filenames that begin with DX were created for a smaller 40-20m RG-11 loop (20 ft 2 in circumference) built later for higher frequencies using a smaller 4 inch pipe housing. You can use either pipe size and either set of centering disks for either capacitor if you are willing to cut off the capacitor side ears. Also, note that the MFJ-19 tuning range is more limited. Don't expect to tune from 3.5 to 7.3 MHz or from 7.0 to 14.35 MHz with the smaller MFJ-19 capacitor. The FreeCAD source files are also provided, allowing you to check all dimensions and adjust if needed before printing. DWV pipe dimensions vary a bit, and exact roundness is not guaranteed. If you plan to use your antenna in a hot climate, consider printing these parts using ABS or at least painting the housing white. Heat can soften PLA, and UV will shorten its life. Note that only the coax loop's shield is connected to the capacitor. I used standard SO239 panel connectors on either side of the capacitor housing with the ground/shield connection from each connector connected to one side of the variable capacitor using the shortest possible length of heavy copper wire soldered at both ends. The center conductor of the coax loop can be left disconnected at both ends. For the coupling transformer, I started with about 10 turns of THHN #14 copper house wire wound through the toroid and then removed one turn at a time until the SWR was good enough. One end of the THHN is connected to the shield/ground side of another panel connector for the feed line, and the other side is soldered to the center terminal. More details can be found on my project page at oregonham.epizy.com/Portable_Loop_for_NVIS-by_AE7TO.htm