Deep Dive: T5B10
The correct answer is C: -6 dB. A power decrease from 12 watts to 3 watts represents approximately -6 dB. This is because 3 watts is one-quarter (1/4) of 12 watts, and quartering power equals a -6 dB decrease. The relationship: halving power = -3 dB, so quartering power (halving twice) = -6 dB. Alternatively, using the formula: dB = 10 × log(3/12) = 10 × log(0.25) = 10 × (-0.602) = -6.02 dB, which rounds to -6 dB. This is another fundamental dB relationship: quartering power = -6 dB, quadrupling power = +6 dB.
Why Other Answers Are Wrong
Option A: Incorrect. -1 dB represents a power ratio of about 0.79:1, not 0.25:1. 3 watts is one-quarter of 12 watts, which is -6 dB. Option B: Incorrect. -3 dB represents halving power (2:1 ratio), not quartering. 3 watts is one-quarter of 12 watts, which requires -6 dB. Option D: Incorrect. -9 dB would represent a power ratio of about 0.126:1, not 0.25:1. The correct value for quartering is -6 dB.
Exam Tip
Quarter power = -6 dB. Remember: Quartering power (dividing by 4) equals -6 dB. Going from 12W to 3W is quartering, so it's -6 dB.
Memory Aid
**Q**uarter **P**ower = **-6** **d**B (think 'QP = -6dB' = Quarter Power = -6 dB)
Real-World Example
You reduce your transmitter power from 12 watts to 3 watts (perhaps to reduce interference or extend battery life). This quarters your power, which is a -6 dB decrease. On the receiving end, this might make your signal weaker, but it's still usable if conditions are good. Understanding that quartering power = -6 dB helps you understand the impact of power reductions.
Source & Coverage
Question Pool: 2022-2026 Question Pool
Subelement: T5B
Reference: FCC Part 97.313
Key Concepts
Verified Content
Question from the official FCC Technician Class pool. Explanation reviewed by licensed amateur radio operators and mapped to the T5B topic.