Deep Dive: T5B09
The correct answer is B: 3 dB. A power increase from 5 watts to 10 watts represents approximately 3 dB. This is because 10 watts is double (2×) the power of 5 watts, and doubling power equals a 3 dB increase. The decibel (dB) is a logarithmic unit used to express power ratios. The formula for power in dB is: dB = 10 × log(P2/P1). For a 2:1 power ratio (doubling), dB = 10 × log(2) = 10 × 0.301 = 3.01 dB, which rounds to 3 dB. This is a fundamental relationship: doubling power = +3 dB, halving power = -3 dB. This is one of the most important dB relationships to memorize.
Why Other Answers Are Wrong
Option A: Incorrect. 2 dB doesn't represent a 2:1 power ratio. The correct value for doubling power is 3 dB. Option C: Incorrect. 5 dB would represent a power ratio of about 3.16:1, not 2:1. 5 watts to 10 watts is exactly 2:1, which is 3 dB. Option D: Incorrect. 10 dB represents a 10:1 power ratio, not 2:1. 10 watts is only double 5 watts, which is 3 dB.
Exam Tip
Double power = +3 dB. Remember: Doubling power equals +3 dB. Going from 5W to 10W is doubling, so it's +3 dB. This is a key relationship to memorize.
Memory Aid
**D**ouble **P**ower = **3** **d**B (think 'DP = 3dB' = Double Power = 3 dB)
Real-World Example
You increase your transmitter power from 5 watts to 10 watts. This doubles your power, which is a 3 dB increase. On the receiving end, this 3 dB increase might make the difference between a barely readable signal and a solid copy. Understanding that doubling power = +3 dB helps you understand the impact of power changes.
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.