*Question E4D03:
What creates intermodulation interference between two repeaters in close proximity?*  

*Correct Answer is  B: The output signals mix in the final amplifier of one or both transmitters*  

### AI Response: (may contain errors)  

Intermodulation interference occurs when two or more signals mix together in a non-linear device, such as the final amplifier of a transmitter. Let's break down the correct answer and the other choices.

### Correct Answer: B
**The output signals mix in the final amplifier of one or both transmitters.**

- **Explanation**: When two signals are transmitted simultaneously, if they pass through a non-linear component (like a final amplifier), they can interact and create new frequencies. This mixing can produce unwanted signals at frequencies that are combinations of the original frequencies, leading to intermodulation interference. The new frequencies can be calculated using the formula:

$$
f_{mix} = n_1 f_1 \pm n_2 f_2
$$

where:
- $f_{mix}$ = the mixed frequency
- $f_1$ and $f_2$ = the original frequencies
- $n_1$ and $n_2$ = integers (1, 2, etc.)

### Why the Other Choices are Incorrect:

- **A: The output signals cause feedback in the final amplifier of one or both transmitters.**
  - **Explanation**: Feedback refers to a situation where a portion of the output signal is fed back into the input, which can cause distortion or oscillation. This is not the mechanism that creates intermodulation interference; rather, it is the mixing of signals that causes this issue.

- **C: The input frequencies are harmonically related.**
  - **Explanation**: Harmonic relationships between input frequencies can lead to interference, but intermodulation specifically refers to the mixing of two or more different frequencies, not necessarily harmonically related. Therefore, this choice does not accurately describe the cause of intermodulation interference.

- **D: The output frequencies are harmonically related.**
  - **Explanation**: Similar to choice C, this option suggests that the output frequencies are harmonically related, which is not a requirement for intermodulation interference. Intermodulation can occur with any two frequencies, regardless of their harmonic relationship.

### Summary
Intermodulation interference is primarily caused by the mixing of output signals in non-linear devices, such as amplifiers. Understanding this concept is crucial for preventing interference in closely spaced repeaters.