An SO239 makes a VERY poor antenna base, for a variety of reasons especially for an HF one. Several of the import antenna companies use them which is unfortunate.
If this is a VHF/UHF antenna, you're much better off with an NMO. That said, some of the import NMOs are not directly compatible with a genuine Motorola.
What are you trying to accomplish?
Alan, KØBG
www.k0bg.com