Waveguide discontinuities – Reduced narrow dimension (H-plane step) equivalent circuit A rectangular waveguide has a discontinuity in the form of a locally reduced narrow dimension (height b). In an equivalent circuit, this H-plane step is best represented as:

Introduction:
Waveguide irises and steps are modeled with lumped reactive elements to ease filter and matching design. Whether a discontinuity is “inductive” or “capacitive” depends on its orientation: H-plane versus E-plane. Recognizing the correct equivalent is essential for quick, accurate synthesis.

Given Data / Assumptions:

• Rectangular waveguide supporting TE10 near the discontinuity.
• Narrow dimension b is locally reduced (H-plane step/iris).
• Small discontinuity relative to wavelength so a lumped model is meaningful.

Concept / Approach:

An H-plane iris or step concentrates magnetic field (H) and behaves inductively; the equivalent circuit is a shunt inductance across the guide. Conversely, an E-plane iris (perturbation in the broad wall along E) behaves capacitively. Therefore, reducing the narrow dimension (H-plane perturbation) is modeled as a shunt inductance.

Step-by-Step Solution:

Verification / Alternative check:

Filter handbooks tabulate susceptance of H-plane irises as positive imaginary (inductive), while E-plane irises show capacitive susceptance.

Why Other Options Are Wrong:

Shunt capacitance/series capacitance correspond to E-plane features; shunt resistance implies loss, not pure reactive discontinuity.

Common Pitfalls:

Confusing E-plane with H-plane; forgetting that “inductive” maps to magnetic field concentration.

Final Answer:

a shunt inductance at the discontinuity.