I bought a Uniden handheld radio, I also have a 7' whip antenna already on the boat. Is there an adapter that I can buy to connect to my radio antenna connector, so I can attach the whip if I wanted to?
I don't know of a commercially available PL259/BNC adapter due to the physical size differences. You may have to inquire at the radio shop. At the very least they can make you one with a 6" piece of RG58a in between the connectors.
I do not know your handheld. If it is a Bnc connector then can get adapter at any radio shop or radio Shack.
If Antenna comes off and it a special connector you will likely have to get adapter from Uniden.