One of the problems that a microwave design engineer frequently faces is that of impedance matching of source and load is important to get maximum power transfer. If you have a 75 ohm load, you don't want to drive it with a 50 ohm source, because it is inefficient. Due to these unequal impedances, there will be a reflection of some power of the signal as it traverses from source to load. The greatest amount of engineering time is spent in searching for ways to provide efficient impedance matching, especially to active circuit elements, so it pays to know some of the many useful impedance-matching methods and their limitations. Maximum power transfer theorem should be remembered as it serves as a great reference in resolution of a impedance matching problem. The theorem simply states that the maximum amount of power will be dissipated by a load resistance when that load resistance is equal to the Thevenin/Norton resistance of the network supplying the power. If the load resistance is lower or higher than the Thevenin/Norton resistance of the source network, its dissipated power will be less than maximum.
Microwave instruments for measurement of impedance by way of direct measurement or S-parameters are among the most widely used tools of the microwave engineer. There are many ways to match impedances, some common methods are:
i) Impedance Sections
ii) Transformers
iii) Matching networks
iv) Tapered baluns
etc.
One of the great advantages of tapered baluns is that it provides broadband impedance matching and also performs an additional functionality of converting a single ended port to a double ended port. More detail on this later!
[2] EE246 — Microwave Engineering, Lesson Autumn 1999
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