Output transformer for a 20-30W PA

I have lashed out and bought on of those incredibly low cost "45W PA" kits!



These are rated at 45W output, but this is definitely pushing it as far as the safe operating area of the IRF510 output transistors. Anyway I don't want 45W, so I will redesign for 20-30W output at 12V supply.

CALCULATION

The output load for 20W for each transistor is

Vdd^2 / 2 * Pout, or 12^2/2*20 = 3.6R.

Next the output is the usual 50R, so we have to match 3.6 to 50R. The formula for the turns ratio is

Zout/Zin = (tout/tin)^2. or

tout = tin * sqrt(Zout / Zin), or 2 * sqrt(50 / 3.6) = 7.5

So the MOSFET side input turns are 2+2 turns, and the output turns are about 7 turns.

So I need a transformer with this turns ratiot on a suitable binocular ferrite core.

PLAN

My plan is to try to shoehorn this 20W PA and some LPFs (from QRP Labs for 20/30/40m) into a small box similar to those I have used before (see previous posts). It will be a squeeze as it will need also switching repays and a Arduino Nanocomputer for control and an OLED band display, I might even try to squeeze in a power meter.