That thing may be able to handle 300 watts (Pep), but only if the mismatch isn't much to start with. If there's much of a mismatch, fairly high SWR, seriously doubt if it would handle much power at all let alone 300 watts.
One of the biggies with an impedance mismatch (high SWR) is the voltages involved when a conversion/transformation is made. That voltage can really get large which means the capacitors and coil used for that impedance transformation have to have a high voltage rating. With variable capacitors that means a wider separation between the plates, and larger plates to start with. Voltage handling capacity determines the 'size' of the capacitor. The amount of capacitance required is typically fairly large and a tiny variable capacitor that would fit inside that 'matcher's case puts a definite limit to how much capacitance there will be and the plate separations. Keep in mind that you'll have to squeeze two of those capacitors in there, AND a coil! Sorry Charley, just ain't gonna happen if there's much SWR/impedance difference, just ain't no room for the components required.
That 'matcher' also appears to not have a variable inductance/coil. That's another limitation on how large a mismatch/SWR it can handle.
All things considered, I'd cut that power handling figure by a factor of '10'. That would get it closer to what it will do. Then it depends on the capacitance present and the inductance. You just ain't gonna do much 'matching' with a shirt pocket sized tuner/matcher.
- 'Doc
