Topic: Antennas and Wave Propagation
The solid angle subtended by the sun as viewed from the earth is $\Omega = 4 \times 10^{-5}$ steradian. A microwave antenna designed to be used for studying the microwave radiation from the sun has a very narrow beam whose equivalent solid angle is approximately equal to that subtended by the sun. What is the approximate directivity, D?
- A. $10^5$
- B. $\pi \times 10^5$
- C. $\pi \times 10^6$
- D. $10^6$
Answer: B. $\pi \times 10^5$
Explanation: The directivity $D$ of an antenna is given by the relation $D = \frac{4\pi}{\Omega_A}$. Given that the solid angle $\Omega_A = 4 \times 10^{-5}$ steradians, substituting this into the equation yields $D = \frac{4\pi}{4 \times 10^{-5}} = \pi \times 10^5$.