[Radio1950]

Bazz4CQJ,

The coax is curved for physical relief of thermal effects; the little tags are fragile.
Other methods may be better.
Caution these devices have BeO.
The Anaren RFP250N50 datasheet is good for practical mounting advice.
You can also sometimes buy these devices "used" at very low prices on the usual budget website.
I have another low power (5W) load using a Florida RF 50W device; it was bought as "used".
You might find that "used" high power 100 ohm devices (ex Wilkinsons) are cheap; just use in parallel.

I wonder about using a thermistor on such a large heatsink, as there will be considerable time delay with the temp rise for measurement, no matter where it is physically placed.