The GATE Grind

GATE 2014 EC (EC1) – Question 64

Electromagnetics · Transmission Lines · 2 marks · Multiple choice

A parallel-plate transmission line has propagation speed v and characteristic impedance Z. Neglect fringing. Halving plate spacing causes ______.

  1. v to halve, Z unchanged
  2. v unchanged, Z to halve
  3. both v and Z unchanged
  4. both v and Z to halve

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Show answer and explanation

Correct answer: (B) v unchanged, Z to halve

Explanation

For plate width w and spacing d, the per-unit-length parameters are L′=µd/w and C′=εw/d. Thus v=1/√(L′C′)=1/√(µε) is independent of spacing, while Z=√(L′/C′)=√(µ/ε)d/w is proportional to it. Halving d leaves v unchanged and halves Z, **B**.