Input return loss of Amplifiers

Hello ,
I would like to ask a very basic question regarding the input return loss (S11) of any amplifier.
I understand S11 describes how much power is reflected back to port 1 ; therefore , most designers tend to keep the magnitude in dB less than -10 dB (the lower the better) over the desired frequency of operation.
My question is why the return loss drops at certain frequencies before it rises again ? I understand that this might indicate that the amplifier is perfectly matched at those frequencies but it seems like repeated oscillations , so why does this happen? does it repeat every quarter wavelength for example due to the repetitive nature of the wave? I thought about that but separation of frequencies between the oscillations aren't uniform.
Thank you very much
Amr

It will be flat if the amplifier input is purely resistive. Any reactance will vary with frequency. It will dip when the reactance is at the match impedance.

These repeating peaks and dips are coming from the cable of the VNA.
Probably bad quality cables or/and bad calibration.

BigBoss said:

These repeating peaks and dips are coming from the cable of the VNA.
Probably bad quality cables or/and bad calibration.

Click to expand...

Thank you Big boss but I am using ADS to simulate , haven't done any HW measurements yet.

Amr Wael said:

Thank you Big boss but I am using ADS to simulate , haven't done any HW measurements yet.

Click to expand...

This phenomena may appear in simulations too if there are some transmission lines.

BigBoss said:

This phenomena may appear in simulations too if there are some transmission lines.

Click to expand...

Yes I am using microstrip Lines & Coplanar waveguides but I am trying to find out a physical explanation for this Phenomena.

Your question isn't very clear. Are you asking about effects that only occur with a connected transmission line, or also directly at the amplifier ports? In the former case, expect that the transmission line has characteristic impedance different from reference impedance (e.g. 50 ohms).

FvM said:

Your question isn't very clear. Are you asking about effects that only occur with a connected transmission line, or also directly at the amplifier ports? In the former case, expect that the transmission line has characteristic impedance different from reference impedance (e.g. 50 ohms).

Click to expand...

I designed a PCB using ADS Layout
The microstrip lines are impedance matched to the IC of the amplifier.
I performed EM simulation on the PCB and found those dips in the S11.

Amr Wael said:

I designed a PCB using ADS Layout
The microstrip lines are impedance matched to the IC of the amplifier.
I performed EM simulation on the PCB and found those dips in the S11.

Click to expand...

So that's means the Characteristic Impedance of the Transmission Line is not exactly 50 Ohm therefore you see ripples on S11.