Dave you should really check out fellow Jeepers webpages. They could answer a few questions for you! Like this page
Look Here .
Well I know for a fact 15 is simplex not duplex. Duplex means that you transmit on one channel and recieve back on another channel, but the two act together as one. Duplex operates through a repeater station for massively increased distances.
BTW look around one the web a bit, there's heaps of info available. And also you're handheld is a UHF not an AM if you were wondering. Hope this helps a bit!