Op-amp input signal problem, mismatched

My signal generator drives a 50 ohm coaxial cable, terminated with a 50 ohm to ground at the input of a voltage amplifier.

However, when I look at my input signal over frequency, it doesn’t behave as I expected.

When I remove the connection to the amplifier, then it is all fine. I get an almost a straight line, with some +/- 5 mdB curves. This leads me to conclude that the problem comes from the input impedance of the amplifer, changing with frequency, which again leads to an impedance mismatch of the cable.

The frequency range I am interested in goes up to 50 MHz, where there is already a change of 0.5 dB.

The cable has a length of 1 m. The gain of the amplifer is 10. The generator also has a 50 ohm series resistor (Rser.)

What can I do about that, or what is the general procedure for this kind of problem?

See the simulation in LTSpice.

Many thanks!

EDIT: The generator has a DC offset of 300 mV (see SINE(300m) and an AC amplitude of 250 mV.

The parameters for the cable are specified for 1 meter. The only thing to change is the parameter "len" for the length of the cable, the other parameters remains the same.

EDIT 2:
See the following image: I reduced the length of the cable to 0.1 m. Of course, now at f=50 MHz, I only have approx 200 mdB of change.

The next image shows how it behaves when the cable is disconnected from the Input of the Amplifier at a length of 1 m. Only a minimal change of some mdB, which are negligable