# Impedance matching with unknown load

1. I somewhat know the answer to this question but just want to clarify. In order to design the impedance matching, I will need to know the load impedance and the source impedance to begin with. Correct?

2. Let's say I have a circuit designed which is the "load" circuit of my setup. How would I go about measuring its input impedance since this is what is necessary to design the impedance matching circuit?

3. I would also like to know of any easy-to-obtain software that may be useful for impedance matching. I am presently trying to get my hands on ADS from my university but if there are any other software which are helpful that you know off, that would be great.

• I'm confused about what you're trying to do. Could you provide more info about what you're trying to match? – curtis Aug 11 '14 at 18:17
• Are you trying to impedance match to a cable or impedance match to get maximum power transfer? – Andy aka Aug 11 '14 at 18:51
• What are you doing? Coupling between sections of an RF amplifier? Wide band or narrow band? – George Herold Apr 6 '16 at 14:25

In order to design the impedance matching, I will need to know the load impedance and the source impedance to begin with. Correct?

In most situations you need to know the load impedance and the system characteristic impedance, or the characteristic impedance of the line connecting the source to the load.

If the source isn't matched to the characteristic impedance, you might need matching networks at both ends of the connection.

If the interconnect is very short, you might just want to match the source directly to the load, like you say.

How would I go about measuring its input impedance since this is what is necessary to design the impedance matching circuit?

If you designed the load, you might be able to get its characteristics from simulations rather than measurements.

If you do need to measure the load impedance, you can use a vector network analyzer (VNA) to do that.

I would also like to know of any easy-to-obtain software that may be useful for impedance matching. I am presently trying to get my hands on ADS from my university but if there are any other software which are helpful that you know off, that would be great.

The math for most narrowband matching circuits is not that difficult. You could probably write Octave, Scilab, or Python code to do the calculations within an hour or two (Just naming free options).

Often input and output impedances are known (or given by the circuit and can be derived), so you do not neccessarily have to measure them.

If they are not known you also can find the matching network by experiment. This works if you can measure the SWR between a transmitter and the load (e.g. antenna, amplifier input).

I recommend very much the chapter about impedance matching in "RF Circuit Design" by Chris Bowick.

Especially the use of Smith charts (also explained in that chapter) is helpful. With Smith charts you can use a quite simple graphical method (no special software needed) for determining the matching component values if input and output impedances are known.