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From the manual:

6.2 Input attenuator
The pi-network consisting of R2, R3 and R4 forms an attenuator. This is
necessary because the output of the QCX will be approximately 5W into 50-
ohms; this is a sinewave with 45V peak-peak. This will be too high for the
Gate-Source voltage specification (+/- 20V) of the IRF510 transistors.
To operate the transistor gates safely, and to correctly load the QCX at
approximately 50-ohms output impedance, and to ensure stability, this “3dB
pad” is a good practice inclusion in the circuit.

I'm not surprised that the input SWR is 2:1.