So, the match is good over HF. Thinking after I hit send: Why not connect
the secondaries together as you did for the S11 measurement and do an S21
measurement to assess loss? That way, both ports are in a 50-ohm system.
I have just removed the 50 ohm resistor off the second transformer and attached CH1 of the NanoVNA instead. The attached image is the S21 plot of the sweep from 1 to 30MHz. At 7.15MHz the gain shows as -22dB.
