The transformer is an autotransformer of 3+21 turns single layer close wound on a ‘stack’ of two Fair-rite 5943000601 cores (FT82-43?). Note that these were genuine Fair-rite stock, other #43 mix products in the market place may be significantly different (see Ferrite cored RF chokes in Class-E RF power amplifiers – core material issues for more discussion).
There are a plethora of designs using FT82-43 published on the ‘net, most of them have appalling loss.
Above is a Simsmith model and measurement of the transformer for reconciliation. The blue VSWR curve is the measurement and the magenta curve is the calibrated model, they agree well considering the tolerance of ferrite materials.
Above is a chart from the model, efficiency is worst at 3.5MHz, about 88% which should be acceptable to most users.
Above is a peak magnetic flux calculation for worst case (3.5MHz) @ 10W, it is way below the saturation flux (given as 0.3T, but best to stay below 0.1T). For other than pulse applications, maximum power (and flux) will usually be limited by heating rather than magnetic saturation.
We can estimate the temperature rise due to heat dissipation in free air from the surface area of the core.
40° rise on say 30° ambient is about as much as is compatible with many plastic insulation materials… but enough to burn skin. About 1.5W average power will achieve that rating.
So at frequencies where to efficiency is poorest at 88%, the transformer is suited to continuous or average power input of 12.4W (and for example, 12.4W continuous RTTY, 25W continuous A1 Morse Code).
If we take the average to peak ratio for SSB telephony to be -15dB (see Average power of SSB telephony), then this transformer should be capable of about 370W PEP of unprocessed speech (still below core saturation), perhaps more like 90W PEP of speech with 6dB processing. In practice, allowing for duty cycle and conversation style speed with pauses, these become conservative ratings.
Conclusions
The transformer as built and measured has performance that should be acceptable to most users, quite probably better efficiency than some QRP style ATUs.