The input is a 4:1 transformer, loaded by a 22R resistor, this means the input impedance is about 90R, a bit high, but the power levels are low. The transformer serves to reduce the drive to the MOSFET, I have assumed that the input will be about 1W from the driving low power TX. With the MOSFET biased at Id = 0.4 - 0.5A a swing of less than 1V is needed on the input for full swing on the output.
Note that there is series capacitor missing on the input, a 0.22uF capacitor should be put in series with the input connection.
The output is also a 1:4 balun so that the impedance seen by the transistor is 50/4 = 12R. This lines up with the output impedance of the single transistor at 10W power and 13.8V Vcc, which is R = Vcc^2/Pout, or 12R.
The output low pass filter is the same as on the previous push-pull amplifier I have described.
The quoted MOSFET IRF530 is a bit over-kill, and the smaller IRF510 will be quite enough. Even smaller would be better (my idea; would be SOT32 package, 40V, 5A max with lw input and output capacitances… I especially like the look of Vishay's IRFIZ14G in the isolated TO220 package - no insulation washers needed, just bolt it down! Maybe this is one to try.
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