Short note: Immittance chart

An immittance chart (also called a ZY Smith chart) is a Smith chart with the impedance (Z) chart and the admittance (Y) chart drawn on the same Γ-plane. The admittance chart is the impedance chart rotated by 180°. One point can therefore be read as a normalised impedance z = r + jx or as a normalised admittance y = g + jb, without rotating the point by λ/4.

             +jx (inductive)
           .-~~~~~~~-.
        .'  Z-circles  '.
  SC   /  (r = const)   \  OC
 Γ=-1 o------- 1 --------o Γ=+1
       \ Y-circles      /
        '. (g = const) .'
           '-~~~~~~~-'
             -jx (capacitive)
r-circles touch at OC (right)
g-circles touch at SC (left)

Main features

• Constant-r circles pass through the open-circuit point (right end). Constant-g circles are their mirror images and pass through the short-circuit point (left end).
• Upper half: inductive (+jx and −jb). Lower half: capacitive (−jx and +jb).
• The centre is the matched point (z = y = 1).
• Usually the Z-chart is printed in one colour (red) and the Y-chart in another (blue).

Uses

• Lumped L-section matching: a series element moves the point along a constant-r circle, and a shunt element moves it along a constant-g circle. Both can be read directly on one chart.
• Design of amplifier matching networks and stub tuners where series and shunt elements are mixed.