Microstrip propagation — normalized wavelength for a substrate with relative permittivity εr = 9 Assuming a typical microstrip where the effective permittivity ε_eff is less than εr but significantly greater than 1, the normalized guided wavelength λ/λ0 is approximately:

Introduction / Context:
In planar transmission lines, the guided wavelength λ is shorter than the free-space wavelength λ0 by a factor related to the effective permittivity ε_eff of the structure. Microstrip has fields partly in air and partly in dielectric, so 1 < ε_eff < εr. This question asks for the approximate normalized wavelength when εr = 9, a high-permittivity substrate common in microwave practice.

Given Data / Assumptions:

• Relative permittivity of substrate εr = 9.
• For microstrip, ε_eff is typically near (εr + 1)/2 for moderate width-to-height ratios, so ε_eff ≈ 5 as a ballpark.
• Normalized wavelength is λ/λ0 = 1 / √ε_eff.

Concept / Approach:
Using ε_eff ≈ 5 for εr = 9 gives λ/λ0 ≈ 1 / √5 ≈ 0.447, which rounds to about 0.45. Among the choices, 0.4 is the nearest practical estimate. The exact value depends on geometry (trace width vs substrate height), but the trend is robust: higher εr → larger ε_eff → shorter guided wavelength relative to free space.

Step-by-Step Solution:

Verification / Alternative check:
Hammerstad-Jensen or Wheeler equations for microstrip ε_eff yield values around 4–6 for many practical geometries on εr = 9 substrates, reinforcing λ/λ0 in the 0.4–0.5 range.

Why Other Options Are Wrong:

• 10, 2: imply λ > λ0, which cannot occur in a passive dielectric-guided structure.
• 1: would require ε_eff ≈ 1 (air line), not a high-εr substrate.

Common Pitfalls:
Using εr instead of ε_eff; forgetting that microstrip fields are partly in air, so λ/λ0 is not as small as 1/√9 ≈ 0.333.

Final Answer:
0.4