The HAM radios are essentially collectibles - they will also increase your hearts & minds meter for that zone.
You will need to find and activate (press
) 20 of these to obtain this achievement. They show up on your minimap/zone map after you've taken the corresponding strike point.
How soon you unlock this is a matter of play style, but I unlocked it when I needed to increase my H&M meter anyway for the Earlston Yellow Zone intro quest.