How much gain does an antenna have compared to a half-wavelength dipole if it has 6 dB gain over an isotropic radiator?
The correct answer is A: 3.85 dB. An antenna with 6 dB gain over an isotropic radiator has 3.85 dB gain compared to a half-wavelength dipole. This is because a dipole itself has about 2.15 dBi gain (2.15 dB compared to isotropic).
Calculation: If gain over isotropic = 6 dBi, and a dipole has 2.15 dBi, then gain over dipole = 6 - 2.15 = 3.85 dBd. The relationship is: Gain in dBd = Gain in dBi - 2.15. This conversion accounts for the fact that a dipole is already 2.15 dB better than isotropic, so when comparing to a dipole, you subtract that 2.15 dB advantage.
Exam Tip
dBi to dBd = Subtract 2.15. Remember: To convert gain from dBi (isotropic) to dBd (dipole), subtract 2.15. 6 dBi - 2.15 = 3.85 dBd.
Memory Aid
"**d**Bi **T**o **d**Bd = **S**ubtract **2**.**15** (think 'dTd = S2.15', 6 dBi = 3.85 dBd)"
Real-World Application
Your Yagi antenna has 6 dBi gain (6 dB compared to isotropic). A half-wave dipole has 2.15 dBi gain. So your Yagi has 6 - 2.15 = 3.85 dB more gain than a dipole. This is the gain in dBd (dipole reference). The 2.15 dB accounts for the dipole being better than isotropic.
FCC Part 97.3Key Concepts
Why Other Options Are Wrong
Option B: Incorrect. 6.0 dB would be the gain over isotropic, not over a dipole. You need to subtract the dipole's 2.15 dBi gain.
Option C: Incorrect. 8.15 dB would be adding instead of subtracting. The correct calculation is 6 - 2.15 = 3.85 dB.
Option D: Incorrect. 2.79 dB doesn't match the correct calculation of 6 - 2.15 = 3.85 dB.
题目解析
The correct answer is A: 3.85 dB. An antenna with 6 dB gain over an isotropic radiator has 3.85 dB gain compared to a half-wavelength dipole. This is because a dipole itself has about 2.15 dBi gain (2.15 dB compared to isotropic). Calculation: If gain over isotropic = 6 dBi, and a dipole has 2.15 dBi, then gain over dipole = 6 - 2.15 = 3.85 dBd. The relationship is: Gain in dBd = Gain in dBi - 2.15. This conversion accounts for the fact that a dipole is already 2.15 dB better than isotropic, so when comparing to a dipole, you subtract that 2.15 dB advantage.
考试技巧
dBi to dBd = Subtract 2.15. Remember: To convert gain from dBi (isotropic) to dBd (dipole), subtract 2.15. 6 dBi - 2.15 = 3.85 dBd.
记忆口诀
**d**Bi **T**o **d**Bd = **S**ubtract **2**.**15** (think 'dTd = S2.15', 6 dBi = 3.85 dBd)
实际应用示例
Your Yagi antenna has 6 dBi gain (6 dB compared to isotropic). A half-wave dipole has 2.15 dBi gain. So your Yagi has 6 - 2.15 = 3.85 dB more gain than a dipole. This is the gain in dBd (dipole reference). The 2.15 dB accounts for the dipole being better than isotropic.
错误选项分析
Option B: Incorrect. 6.0 dB would be the gain over isotropic, not over a dipole. You need to subtract the dipole's 2.15 dBi gain. Option C: Incorrect. 8.15 dB would be adding instead of subtracting. The correct calculation is 6 - 2.15 = 3.85 dB. Option D: Incorrect. 2.79 dB doesn't match the correct calculation of 6 - 2.15 = 3.85 dB.
知识点
Antenna gain, dBi vs dBd, Isotropic vs dipole, Gain conversion
Verified Content
Question from official FCC Extra Class question pool. Explanation reviewed by licensed amateur radio operators.