Let us first discuss USE OF “WAVELENGTHS TOWARDS GENERATOR”(WTG) SCALE AND “WAVELENGTHS TOWARDS LOAD”(WTL) SCALE ON SMITH CHART
The outermost scale around the perimeter of Smith chart is called the Wavelengths towards generator(WTG) scale.It has been constructed to denote movement on the transmission line toward the generator,in units of the wavelength,λ.Thus ,l is measured in wavelengths and one complete rotation corresponds to l=λ/2.
In some transmission line problems,it may be necessary to move from some point on the transmission line toward another point closer to the load,in which case the phase is increased and which corresponds to counter clockwise direction.So Smith chart contains one more scale around its perimeter ,in between the θp and WTG scale, for accommodating such a need. This scale is called the wavelengths toward load (WTL) scale.
Why circumference of Smith chart is calibrated for λ/2?
By rotation of λ/4 or 0.25 λ on the Smith chart transform Zn in to Ynor vice versa. The points representing Yn and Zn will be diametrically opposite to each other on the SWR circle. This is the reason that the circumference of Smith chart is calibrated for λ/2.