This is a center structure for building dipole antennas using window line or ladder line as the actual antenna radiator.
I made it to use a BNC panel mount connector for a coax feedline. It could be modified for an SO-239 using most modeling software.
This was made as part of a unique OCF folded dipole antenna detailed in a video on my you tube channel. Search for my call sign, KB9RLW, and OCF to find it on youtube.