Question E4A08:
Which of the following is used to measure SWR?  

Choices:  
A: Directional wattmeter  
B: Vector network analyzer  
C: Antenna analyzer  
D: All these choices are correct    

Correct Answer is  D: All these choices are correct  

To measure SWR (standing wave ratio), several instruments can be used.

Why D (All these choices are correct) is correct:
- Directional wattmeter: Measures forward power ($P_f$) and reflected power ($P_r$); SWR is computed from these.
- Vector network analyzer (VNA): Measures the reflection coefficient ($S_{11}$) or return loss and computes SWR directly.
- Antenna analyzer: Measures impedance and/or reflection and directly displays SWR.

Why the other choices are “incorrect” by themselves:
- A, B, and C are each valid tools for measuring SWR, but the question asks which of the listed items can be used. Since all three can, the most complete answer is “All these choices are correct.”

Useful formulas and rules of thumb:
- Reflection coefficient:
  $$\Gamma = \frac{Z_L - Z_0}{Z_L + Z_0}$$
- SWR from reflection coefficient:
  $$\text{SWR} = \frac{1 + |\Gamma|}{1 - |\Gamma|}$$
- From forward and reflected power (directional wattmeter):
  $$|\Gamma| = \sqrt{\frac{P_r}{P_f}}, \quad \text{SWR} = \frac{1 + \sqrt{P_r/P_f}}{1 - \sqrt{P_r/P_f}}$$
- From return loss (VNA):
  $$\text{RL (dB)} = -20 \log_{10} |\Gamma|, \quad |\Gamma| = 10^{-\text{RL}/20}$$
- Rules of thumb: SWR of 1:1 is ideal; ≤ 2:1 is usually acceptable for most HF rigs; higher values indicate increasing mismatch and reflected power.