Rich,

What I have done on several of my rigs is to bolt the IRF510 to the rear panel of a metal enclosure with a TO-220 insulating kit to isolate it from ground. I find it runs stone cold at 7-8 watts for extended transmissions. Unfortunately the IRF510 on the BitX40 is not flush with the edge of the PCB, (something I would really like to see if there is ever a rev 2 board produced), so I had to cut a 1/16 inch aluminum spacer to bolt to rear chassis, and then mount the 510 to that, but I would guarantee it would stay plenty cool at 20 watts and you could even bolt a small heat sink on the back panel if additional cooling was needed.?