I just uploaded a photo here in this group of the core I am making for an antenna based on Everett's work. On a 20 inch long X 1 inch diameter dowel I slid 18 ferrite toroids 2.125 OD X 1 ID X 0.75 inches thick. [13.5 inches long]. At each end is a PVC conduit fitting to hold the cores in place and offer a 1.5 inch diameter to support 1.5 X 3 inch pipe adapters. The core is secured permanently, but the pipe adapters and section of 3 inch PVC are slip fit so different 3 inch pipe sleeves can be wound with different coils and all will use the same ferrite core.
