Question E9F05:
What is microstrip?  

Choices:  
A: Special shielding material designed for microwave frequencies  
B: Miniature coax used for low power applications  
C: Short lengths of coax mounted on printed circuit boards to minimize time delay between microwave circuits  
D: Precision printed circuit conductors above a ground plane that provide constant impedance interconnects at microwave frequencies    

Correct Answer is  D: Precision printed circuit conductors above a ground plane that provide constant impedance interconnects at microwave frequencies  

Correct answer: D

1) Why D is correct
- Microstrip is a planar transmission line: a precision PCB trace (width w, thickness t) routed on a dielectric substrate (height h, relative permittivity εr) above a continuous ground plane.  
- Its geometry gives a controlled characteristic impedance $Z_0$ and predictable propagation at UHF/microwave frequencies, making it ideal for “constant impedance interconnects.”

Useful relationships
- Effective dielectric constant:
  $$\varepsilon_{\text{eff}} \approx \frac{\varepsilon_r + 1}{2} + \frac{\varepsilon_r - 1}{2}\,\frac{1}{\sqrt{1 + 12h/w}}$$
- Characteristic impedance (common approximation):
  $$Z_0 \approx \frac{60}{\sqrt{\varepsilon_{\text{eff}}}} \ln\!\left(8\frac{h}{w} + 0.25\frac{w}{h}\right)$$
- Guided wavelength and velocity:
  $$\lambda_g = \frac{\lambda_0}{\sqrt{\varepsilon_{\text{eff}}}}, \quad v_p = \frac{c}{\sqrt{\varepsilon_{\text{eff}}}}$$
Rules of thumb
- Wider trace → lower $Z_0$; thicker substrate (larger h) → higher $Z_0$; higher εr → lower $Z_0$.  
- For FR-4 ($\varepsilon_r \approx 4.2$) with $h \approx 1.6\ \text{mm}$, a 50 Ω microstrip is roughly $w \approx 3\ \text{mm}$.

2) Why the other choices are incorrect
- A: “Special shielding material” — Microstrip is a transmission line structure, not a shielding material.
- B: “Miniature coax” — That describes micro-coaxial cable. Microstrip is planar-on-PCB, not coaxial.
- C: “Short lengths of coax on PCBs to minimize time delay” — Microstrip is not coax, and while it can be used as a delay line, its defining feature is controlled impedance planar routing over a ground plane.

Key contrasts
- Microstrip: trace over one ground plane (external PCB layer).  
- Stripline: trace sandwiched between two ground planes (internal PCB layer).