Q) What device increases the low-power output from a handheld transceiver?
A) An RF power amplifier
An interesting example of a commercial RF power amplifier designed for use with an HT is the Mirage Dual Band Handheld Amplifier, which includes the 144/440 MHz segments. Feeding 1-7 watts in from an HT, it will put out 45/35 watts respectively. It is also capable of full duplex operation, allowing the user to transmit on one band and listen on the other - provided that the handheld you're using is capable of this function.