Question E9G07:
On the Smith chart shown in Figure E9-3, what is the only straight line shown?  

Choices:  
A: The reactance axis  
B: The current axis  
C: The voltage axis  
D: The resistance axis    

Correct Answer is  D: The resistance axis  

Correct answer: D — The resistance axis

Why D is correct:
- A Smith chart plots normalized impedance $z = r + jx$ mapped to reflection coefficient $\Gamma$.
- The only straight line on the chart is the horizontal real axis where reactance is zero ($x = 0$). This is the resistance axis (purely resistive impedances).
- Along this line:
  - Far left: $r = 0$ (short, $\Gamma = -1$)
  - Center: $r = 1$ (match, $\Gamma = 0$)
  - Far right: $r \to \infty$ (open, $\Gamma = +1$)

Why the others are incorrect:
- A: Reactance axis — There is no straight “reactance axis.” Constant reactance ($x$) values plot as circular arcs above (inductive, $x>0$) and below (capacitive, $x<0$) the resistance axis, tangent at the rightmost edge.
- B: Current axis — The Smith chart is not a current–voltage plot; it’s an impedance/reflection coefficient map. No “current axis” exists.
- C: Voltage axis — Similarly, there is no “voltage axis” on a Smith chart.

Useful formulas and rules:
- Normalize impedance:
  $$z = \frac{Z}{Z_0} = r + jx$$
- Reflection coefficient:
  $$\Gamma = \frac{z - 1}{z + 1}$$
- VSWR from $|\Gamma|$:
  $$\text{SWR} = \frac{1 + |\Gamma|}{1 - |\Gamma|}$$
- Rules of thumb:
  - Constant resistance ($r$) traces are circles that intersect the resistance axis.
  - Constant reactance ($x$) traces are arcs tangent at the rightmost edge.
  - Moving along a lossless line stays on a constant-$|\Gamma|$ (SWR) circle centered at the chart’s center.