Directional coupler coupling to load impedance variation

Is it possible to find directional coupler that can perform constant coupling say -20dB no matter what input impedance it is?
I found that most coupler only work well when the input impedance is at 50Ohm. But when it is connected to a device that is not 50, the coupling constant varies. I dont really understand physically why this coupling will change.

you should match your source to the 50 ohm at first, the reason is the mismatch change the input power to the coupler,so the coupler factor should increase

To my understanding, the purpose of using directional coupler is to extract the transmited and reflected component of the wave. If the input signal is perfectly matched to 50, reflected wave would no longer exist and in that case, we dont need it.
As in a network analyzer, coupler is internal built. I am just curious how the calibration do to correct the non-constant coupling factor for accurate measurment.
Any idea?