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GATE 2017 EC – Question 65

Electromagnetics · Antennas and Antenna Arrays · 2 marks · Multiple choice

A half wavelength dipole is kept in the $x$-$y$ plane and oriented along $45°$ from the $x$-axis. Determine the direction of null in the radiation pattern for $0 \leq \phi \leq \pi$. Here the angle $\theta$ ($0 \leq \theta \leq \pi$) is measured from the $z$-axis, and the angle $\phi$ ($0 \leq \phi \leq 2\pi$) is measured from the $x$-axis in the $x$-$y$ plane.

  1. $\theta = 90°$, $\phi = 45°$
  2. $\theta = 45°$, $\phi = 90°$
  3. $\theta = 90°$, $\phi = 135°$
  4. $\theta = 45°$, $\phi = 135°$

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Show answer and explanation

Correct answer: (A) $\theta = 90°$, $\phi = 45°$

Explanation

A dipole radiates no power along its own axis. This dipole lies in the $x$-$y$ plane at 45° from the $x$-axis, so its axis points along $\theta = 90°$ and $\phi = 45°$ (and the opposite direction at $\phi = 225°$). The null within $0 \leq \phi \leq \pi$ is at $\theta = 90°$, $\phi = 45°$.