Deep Dive: E4D03
The correct answer is B: The output signals mix in the final amplifier of one or both transmitters. What creates intermodulation interference between two repeaters in close proximity is that the output signals mix in the final amplifier of one or both transmitters. Repeater outputs can mix in the final amplifiers, creating IMD. For amateur radio operators, this is important for repeater operation. Understanding this helps when troubleshooting repeater interference.
Why Other Answers Are Wrong
Option A: Incorrect. Output signals causing feedback in final amplifier isn't the cause - output signals mixing in final amplifier is the cause. Feedback isn't the cause. Option C: Incorrect. Input frequencies being harmonically related isn't the cause - output signals mixing in final amplifier is the cause. Input frequencies isn't the cause. Option D: Incorrect. Output frequencies being harmonically related isn't the cause - output signals mixing in final amplifier is the cause. Harmonic relationship isn't the cause.
Exam Tip
Intermodulation between repeaters = output signals mix in final amplifier of one or both transmitters. Think 'I'MD between 'R'epeaters = 'I'nterference 'M'ixing in 'D'evice (final amp). Repeater outputs can mix in the final amplifiers, creating IMD. Not feedback, not input frequencies, not harmonic relationship - just mixing in final amplifier.
Memory Aid
Intermodulation between repeaters = output signals mix in final amplifier of one or both transmitters. Think 'I'MD between 'R'epeaters = 'M'ixing in 'F'inal amp. Repeater outputs can mix in the final amplifiers, creating IMD. Important for repeater operation.
Real-World Example
Intermodulation interference between two repeaters in close proximity: The output signals mix in the final amplifier of one or both transmitters. When repeaters are close, one repeater's output can enter another's final amplifier and mix, creating IMD products. This is what creates the interference - mixing in final amplifiers.
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.