The generator injects 144MHz (it can be another value) with 1Vpp, it is assumed that the generator has 50R impedance. Then the Resistive Pi appears, R1 = 96.24R, R2 = 71.15R, R3 = 96.24R, then a 50R load.
In the scope we can see the 1Vpp input signal and the output signal in 316mVpp, then 20 * log (316/1000) will be -10dB
