Topic: Electromagnetics / Antennas
The radiation resistance of an antenna is 63 ohms and the loss resistance is 7 ohms. If antenna has power gain of 16, then directivity is
- A. 48.26 dB
- B. 12.5 dB
- C. 17.77 dB
- D. 24.7 dB
Answer: B. 12.5 dB
Explanation: Radiation efficiency $e = \frac{R_{rad}}{R_{rad} + R_{loss}} = \frac{63}{63 + 7} = \frac{63}{70} = 0.9$.
The relationship between Gain ($G$) and Directivity ($D$) is $G = e \times D$.
$16 = 0.9 \times D \implies D = \frac{16}{0.9} = 17.77$ (linear scale).
Converting Directivity to dB: $D_{dB} = 10 \log_{10}(17.77) \approx 12.5$ dB.