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The radiation pattern of an antenna in spherical co-ordinates is given by

$$F(\theta)=\cos^4\theta\:\:\:;\:\:\:0\le \theta\le \frac{\pi}{2}$$

The directivity of the antenna is

1. $10\:dB$
2. $12.6\:dB$
3. $11.5\:dB$
4. $18\:dB$