Input Impedance

You’ve heard of characteristic impedance, you’ve heard of load impedance, and you might’ve even heard of wave impedance, but have you heard of input impedance?

I think input impedance is one of the coolest concepts in transmission line theory and is one of the most important concepts on your journey to understanding RF.

Input impedance describes how impedance varies along the transmission line from a particular point (say, the source) to the load. It’s the impedance looking INTO the entire line from a particular point. Remember, since the wavelength for these types of signals is so small, the impedance does not remain constant. It changes.

So, why is it important for practical applications? If there’s a mismatched transmission line (which introduces reflections and results in signal degradation), you can use the input impedance to form a matching network so the source will see the characteristic impedance of the system. This will drive down reflection and allow maximum power transfer to the complex load.

Next week we’ll go over some special cases for different line lengths and see how the input impedance changes based on those line lengths. Pretty cool stuff