Deep Dive: E9A12
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.
Why Other Answers 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.
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 Example
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.
Source & Coverage
Question Pool: 2024-2028 Question Pool
Subelement: E9A
Reference: FCC Part 97.3
Key Concepts
Verified Content
Question from the official FCC Extra Class pool. Explanation reviewed by licensed amateur radio operators and mapped to the E9A topic.