Short-Circuited Stubs at the Source End of a Rectangular Waveguide A rectangular waveguide has two short-circuited stubs (each of some length) connected at the source end for tuning. Which statement best describes the nature of the currents in the main (longitudinal) guide and in the stubs?

Introduction / Context:
Waveguide tuners commonly use short-circuited stubs to supply reactive compensation at the input, adjusting the apparent load seen by the source. Understanding where real power flows versus where reactive energy is stored is central to microwave matching theory.

Given Data / Assumptions:

• Two short-circuited stubs are attached at the source end of a rectangular waveguide.
• The stubs are lossless and function as reactive elements (tunable susceptances).
• The main guide carries the forward traveling wave toward the load.

Concept / Approach:

Short-circuited stubs do not deliver average real power because the net time-average power at the short is zero (voltage is zero at the short, current is large and reactive). The stubs exchange energy with the field over an RF cycle (reactive current). The main longitudinal section transports real power toward the load and returns any reflected power toward the source. Thus, longitudinal current is associated with net power flow, whereas the stub currents are predominantly reactive.

Step-by-Step Solution:

Verification / Alternative check:

Smith chart analysis shows shorted stubs present susceptance or reactance; microwave tuners combine them to cancel load reactance without contributing power themselves.

Why Other Options Are Wrong:

Option B swaps roles erroneously; option C is vague and blurs power versus reactive flow; option D contradicts the existence of reactive currents; option E is false since ideal shorted stubs do not supply average power.

Common Pitfalls:

Assuming large currents imply large real power; forgetting that at a short, voltage is zero so average power is zero.

Final Answer:

the longitudinal current is power current and current in stubs is reactive