The drain of the IRF510 is connected to the tab. You must NOT ground it. Also it is a good idea to minimize its effect as there is RF on the tab.
Hence, some heat-sink the device but without isolation from it, keeping the heat-sink ungrounded. I don't think this is a very good idea as it expands the area for RF radiation.
There must be a metal heat-sink of some kind for the IRF510 (and most if not all final RF transistors for that matter). For 20 watts or so, one of 8-10 sq. in. is probably enough. Mount the tab with a mica or other insulating shield between it and the metal, and a plastic or other insulating grommet for the screw. Use heat-sink grease underneath the mica (lithium auto bearing grease works well; don't overdo it!).
The RF MOSFETS like the RDxxHHF's have the tab connected to the source, and this is usually grounded anyway, so it is not an issue. But they still require metal heat-sinks.
john
AD5YE
