[Amps] Heatsinks for SSPAs - Newbie Questions and UK source

[Conrad Farlow]

I am just about to try an build my first ever SSPA using a pallet with a 
heat-spreader, only 250W in AB for this first one. I must confess that I 
am utterly clueless when it comes to working out what I need for a 
heatsink.

The device is a BLF278 and there is some thermal data on the datasheet 
as you might expect. This amplifier will be operating on the low VHF 
bands so I expect I'll get an efficiency of around 55% in AB mode. The 
max dissipation of the device is 500W so I suppose that means that I 
have to get rid of 225W of heat. The datasheet then goes on to mention 
thermal resistance from the junction to the mounting base and then the 
thermal resistance from the mounting base to the heatsink. Is the 
mounting base the heat-spreader or is that just the mounting flange of 
the device.

Anyway armed with that data how on earth do I figure out the heat 
sinking requirements.

Assuming someone enlightens me can anyone tell me a good source of 
suitable heatsinks in Europe, even better if it is a UK source? I would 
like to mount a pair of 24v fans on the heatsink relatively easily.

Many thanks

Conrad G0RUZ