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The far-zone power density radiated by a helical antenna is approximated as:

$${\overrightarrow W_{rad}= \overrightarrow W_{average} \approx \hat {a_r}C_0\frac{1}{r^2}\cos^4\theta}$$

The radiated power density is symmetrical with respect to $\phi$ and exists only in the upper hemisphere: $0\leq\theta\leq\frac{\pi}{2}; \: 0\leq\phi\leq 2\pi$; $C_0$ is a constant. The power radiated by the antenna (in watts) and the maximum directivity of the antenna, respectively, are

  1. $1.5C_0,10dB$
  2. $1.256C_0,10dB$
  3. $1.256C_0,12dB$
  4. $1.5C_0,12dB$
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