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For an infinitesimally small dipole in free space, the electric field $E_{\theta }$ in the far field is proportional to $e^{-jkr/r}\sin\theta$, where $k=2\pi /\lambda$. A vertical infinitesimally small electric dipole $\left (\delta l \ll \lambda \right )$ is placed at a distance $h\left ( h> 0 \right )$ above an infinite ideal conducting plane, as shown in the figure. The minimum value of $h$, for which one of the maxima in the far field radiation pattern occurs at $\theta =60^{\circ}$, is

1. $\lambda$
2. $0.5\lambda$
3. $0.25\lambda$
4. $0.75\lambda$