Miguel Angelo Bartie
I usually measure RF power with a diode and metal film resistors.
The diode barrier voltage is important for low power (below 2W).
The graph you get shows problems at 1.6, 7 and 10 MHz, in my view is that you are having problems with the low pass filter that has a lower than the expected frequency cut, thus reducing the power in these bands.
If possible change the value of the inductors downwards (by taking a loop, each inductor, for example).
After this measure the power again
73 from py2ohh miguel