Lower Voltage RF Amplifier for 400 MHz
You wont find any 5W RF PA's running at low voltage let alone ultra LV like 1.5V. So you need a Step-up converter. Lots to choose from. 90% but not at 1.5V.. Look harder for a higher supply voltage.

Added:

Is this from a single D cell? I need to know the ESR of the 1.5V sources.
 i.e. the regulation of the voltage with current delta V / delta I.. This is essential. 

Also will you have enough power to drive 6W step up converter that can deliver 5W.

http://www.maxim-ic.com/datasheet/index.mvp/id/1254  This is 90% efficient ,  and will deliver 1 Amp in the range 7~9V so exceeds your requirements. Cost It also has dual output capability.