Wave impedance experiment using a wave guide

I am conducting an experiment to reduce wave guide impedance. To do this, I have a wave guide emitting microwaves down the tube then into free space. A oscilloscope is connected to a to a standing wave detector in the wave guide. I’m moving the standing wave detector until the signal produced on the oscilloscope is at a maximum. The signal being measured is volts vs time. Now the scope isn’t accurate enough to see the frequency of microwave i’m using, so what i’m seeing on the scope is just the time average. Then I repeat the measurement with a horn on the end of the wave guide. The graphs produced are slightly different in magnitude. How can I go from a volts vs time graph to am impedance calculation? I have no idea how to do this and after days of searching online I have found nothing.

More on impedance, I don’t have a great idea of what it is. I think its the ratio between magnetic and electric field amplitudes. I think its good to have a low impedance but don’t know why.