Topic: Antennas and Wave Propagation
An antenna has a radiation resistance of 72Ω, a loss resistance of 8Ω and power gain of 16. Calculate its directivity.
- A. 15.8
- B. 16.8
- C. 17.8
- D. 18.7
Answer: C. 17.8
Explanation: The radiation efficiency $\eta_r = \frac{R_{rad}}{R_{rad} + R_{loss}} = \frac{72}{72 + 8} = \frac{72}{80} = 0.9$. The relationship between power gain ($G_p$) and directive gain ($G_d$) is $G_p = \eta_r \times G_d$. Therefore, directivity $G_d = \frac{16}{0.9} = \frac{160}{9} \approx 17.8$.