---
title: "Short note: Immittance chart"
subject: "RF and Microwave Engineering"
chapter: 2
key: "semester_7/rf_and_microwave_engineering:immittance-chart-note"
source: https://www.bctengineer.com/q/semester_7/rf_and_microwave_engineering/immittance-chart-note
site: BCT Engineering
---

# Short note: Immittance chart

RF and Microwave Engineering · Chapter 2: RF and Microwave Transmission Lines

- Asked in: 2081 Baisakh, 4 marks; 2080 Chaitra, 5 marks (asked 2 times)
- Video: [Full Color Smith Chart and Lumped Element Matching - Lesson 6](https://www.youtube.com/watch?v=fIaIcJEnseA) — EMViso
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/rf_and_microwave_engineering/immittance-chart-note
- Whole chapter: https://www.bctengineer.com/md/semester_7/rf_and_microwave_engineering/chapter-2

## Question

Write a short note on immittance chart (with a sketch).

## Short answer

An immittance chart is a Smith chart with impedance and admittance charts superimposed, the Y-chart being the Z-chart rotated 180 degrees. A point gives normalized z = r + jx or y = g + jb directly; series elements move along constant-r circles, shunt elements along constant-g circles, making L-section and stub matching easier.

## Answer

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.

```text
             +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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is an immittance chart?
   A. A Smith chart with Z and Y charts on the same plane, Y rotated 180 degrees from Z
   B. A chart of only impedances
   C. A bar chart of power
   D. A polar chart of S21 only
   Answer: A. It overlays the admittance chart (Z rotated by 180 degrees) on the impedance chart. (Key point: Definition)

2. On an immittance chart, a shunt element moves the point along what?
   A. A constant-r circle
   B. A constant-g circle
   C. The real axis only
   D. A straight line to the centre
   Answer: B. Series elements move along constant-r circles; shunt elements along constant-g circles. (Key point: Shunt/series)

3. Which half of the chart is inductive?
   A. Lower half
   B. Upper half
   C. Left half
   D. Right half
   Answer: B. Upper half: +jx and -jb (inductive); lower half capacitive. (Key point: Regions)

4. What is the key benefit of reading z and y on one chart?
   A. No need to rotate by lambda/4 to convert between z and y
   B. No need for a chart
   C. It removes reflections
   D. It sets Z0 to zero
   Answer: A. Both are read without a lambda/4 rotation, easing L-section and stub matching. (Key point: Advantage)

## Related in RF and Microwave Engineering

- [Immittance chart and duality parameters](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Aimmittance-chart-duality) — RF and Microwave Engineering, chapter 2
- [Immittance chart sketch and scales](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Aimmittance-chart-scales) — RF and Microwave Engineering, chapter 2