Question E4B05:
What three test loads are used to calibrate an RF vector network analyzer?  

Choices:  
A: 50 ohms, 75 ohms, and 90 ohms  
B: Short circuit, open circuit, and 50 ohms  
C: Short circuit, open circuit, and resonant circuit  
D: 50 ohms through 1/8 wavelength, 1/4 wavelength, and 1/2 wavelength of coaxial cable    

Correct Answer is  B: Short circuit, open circuit, and 50 ohms  

Correct answer: B — Short circuit, open circuit, and 50 ohms

1) Why this is correct
- A one-port VNA calibration uses the SOL method:
  - Short: known reflection of $Γ = -1$
  - Open: known reflection of $Γ = +1$
  - Load (50 Ω): matched termination with $Γ = 0$
- These three standards span the extremes of reflection and the match point, allowing the VNA to solve for systematic errors (directivity, source match, and reflection tracking) at the reference plane.

Reflection coefficient:
$$
\Gamma = \frac{Z_L - Z_0}{Z_L + Z_0}
$$
- For a short: $Z_L = 0 \Rightarrow \Gamma = -1$
- For an open: $Z_L \to \infty \Rightarrow \Gamma = +1$
- For a matched load: $Z_L = Z_0 \Rightarrow \Gamma = 0$

2) Why the other choices are incorrect
- A: 50 Ω, 75 Ω, and 90 Ω  
  - These are all loads with partial mismatches; they do not provide the ideal extremes ($\Gamma = \pm 1$) or a perfect match ($\Gamma = 0$) necessary to solve the calibration error terms cleanly.

- C: Short circuit, open circuit, and resonant circuit  
  - A “resonant circuit” is not a standard with a simple, broadband, known $Γ$. Calibration requires well-modeled, broadband standards (short, open, load).

- D: 50 Ω through various lengths of coax  
  - Cable lengths do not provide the required known reflection standards at the port. Lengths of line are part of other methods (e.g., TRL: Thru-Reflect-Line), not the basic one-port SOL used here.

3) Useful formulas and rules of thumb
- Reflection coefficient:
  $$
  \Gamma = \frac{Z_L - Z_0}{Z_L + Z_0}
  $$
- VSWR:
  $$
  \text{VSWR} = \frac{1 + |\Gamma|}{1 - |\Gamma|}
  $$
- Rule of thumb: 
  - One-port cal: SOL (Short, Open, Load).
  - Two-port cal: SOLT (Short, Open, Load, Thru) or TRL in advanced cases.