Earlier model Harley's did use iso259 and pl259. The 2014 radio changed to TNC for some reason and why they used an fm connection under the seat god only knows. I want to know how many wats the radio actually puts out. It may need to be peaked and tuned once take antenna is at a decent swr.
The HD engineer I talked to last month told me they just "carried over" that connector & antenna from the '13my bikes. They did NO TESTING to verify the SWR or durability of the new design. He did not know there was a corrosion issue in the field regarding that Motorola (fm) connector. I don't think they even understand that fm connector was designed for use with 75 ohm coax, any more than the connectors they're using for the am/fm/wb are designed for 50 ohm nominal impedance.
I have a Astatic SWR Meter. The tiny design is one of its qualities that I find to be the most appealing. It is incredibly portable due to its compact form. I can easily transport it wherever there are CB radio antennae that I need to tune by doing it in this way.