Deep Dive: E4D12
The correct answer is C: +8dB. What is the link margin in a system with a transmit power level of 10 W (+40 dBm), a system antenna gain of 10 dBi, a cable loss of 3 dB, a path loss of 136 dB, a receiver minimum discernable signal of -103 dBm, and a required signal-to-noise ratio of 6 dB is +8dB. Link margin = received power - (MDS + required SNR). Received = 40 + 10 - 3 - 136 = -89 dBm. Margin = -89 - (-103 + 6) = -89 - (-97) = +8 dB. For amateur radio operators, this is important for link budget calculations. Understanding this helps when planning communications.
Why Other Answers Are Wrong
Option A (-8dB): Incorrect. -8dB is negative margin - the correct calculation gives +8dB. -8dB is wrong. Option B (-14dB): Incorrect. -14dB is negative margin - the correct calculation gives +8dB. -14dB is wrong. Option D (+14dB): Incorrect. +14dB is too high - the correct calculation gives +8dB. +14dB is too high.
Exam Tip
Link margin calculation = received power - (MDS + required SNR). Think 'L'ink 'M'argin = 'L'eftover 'M'argin. Received = 40 + 10 - 3 - 136 = -89 dBm. Margin = -89 - (-103 + 6) = -89 - (-97) = +8 dB. Not -8dB, not -14dB, not +14dB - just +8dB.
Memory Aid
Link margin calculation = received power - (MDS + required SNR). Think 'L'ink 'M'argin = '+8' dB. Received = -89 dBm, minimum = -97 dBm, margin = +8 dB. Important for link budget calculations.
Real-World Example
Link margin calculation: Transmit 40 dBm + antenna 10 dBi - cable 3 dB - path 136 dB = -89 dBm received. MDS -103 dBm + required SNR 6 dB = -97 dBm minimum. Margin = -89 - (-97) = +8 dB. This is the link margin - +8dB.
Source & Coverage
Question Pool: 2024-2028 Question Pool
Subelement: E4D
Reference: 2024-2028 Question Pool · E4 - Amateur Practices
Key Concepts
Verified Content
Question from the official FCC Extra Class pool. Explanation reviewed by licensed amateur radio operators and mapped to the E4D topic.