arduino 5 volt to 12 volt

Hi Sander, for any battery powered application, the following points are very crutial

1. You have to put your microcontroller into a Deep Sleep Mode, otherwise, the micro will suck the battery like a leach.
2. You have to measure the consumed current and calculate the battery capacity needed for your application. 
3. If you don't have access to wall power you can use a solar panel as debasis parida said.
4. If you are thinking about putting a solar panel, you need to account for the voltage drop in the wire also.
5. You can always use a swappable battery system which, which gives you a low battery indication.

easiest way to bump 5 volts to 12 volts? You need to use a separate 12 volt supply. Tie the Arduino ground to the 12 ground and the transmitter ground then tie the +12V to the transmitter.
In that case a DC to DC convertor will not meet your needs since the power out of DC to DC convertor is less than the power input and the Arduino power output is extremely limited.
Are you trying to increase the signal to the modulation input to the transmitter or are you trying to increase the power line of the transmitter.