Impedance Matching – Single Stub Tuning

One way to avoid impedance matching a transmission line to a load with lumped circuit elements is to implement Stub matching. Stubs are sections of transmission line that are terminated by either an open circuit or a short circuit. They can be connected in series or in parallel to the transmission line a certain distance from the load. For microstrip and stripline circuits, parallel configurations are preferred whereas series configurations are preferred for slotline or coplanar waveguides.

Two variables when designing a stub are: the reactance of the stub and the distance between the stub and the load. The idea is that at a specific distance, the susceptance or (reciprocally) the reactance of the load should be cancelled out by the stub. This leads to the cancellation of reflection from the load. This reactance/susceptance value is determine by the length of the stub. The difference in length between a short and open stub is a quarter of the wavelength (which can be confirmed by the Smith Chart).

The Smith Chart can be used to identify the length required for the stub admittance to equal 1+jb by traveling from the right hand side of the chart clockwise until the reactive parts are equal and opposite. You can also draw an SWR circle using the load impedance/admittance and find where the circle intersects the 1+jb circle. Then the same process for finding the stub length can be used. On a Smith Chart, lengths are always a function of the wavelength.

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