Mastering the Smith Chart in NanoVNA

Impedance of the antenna has both real and imaginary parts. Real part is the resistance while imaginary parts are the reactances. Reactances can be either inductive or capacitive. Both are out of phase with the resistive component of impedance. Between the two, they are 180 degrees out of phase. Inductive reactance is taken as positive and capacitive reactance as negative. If the antenna is shorter than the resonant value of the frequency, it has capacitive reactance. An antenna which is longer has an inductive reactance. Smith chart represents inductive reactance above the central line and capacitive reactance below that.

One end of the horizontal line represents short while the other end represents the open condition. Radiation resistance of the antenna under test is somewhere in between. Center point represents 50 ohms radiation resistance. These are well illustrated during the recall of calibration of the NanoVNA, as discussed earlier.

Each of the circles on the Smith chart represent resistances. When the graph is above the horizontal line, we have inductive reactance. Points below the horizontal line show capacitive reactance. The green curved tracing represents the variation of these parameters according to the frequency range selected. The frequency at which the graph intersects the horizontal line is the perfectly tuned region where there is only resistive component and no inductive or capacitive component. At the top of the NanoVNA, resistive and reactive components of the impedance are displayed in green colour. Corresponding frequency is displayed in white colour. Displayed value of reactance changes from capacitive below the horizontal line to inductive above that line.