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

Choices:  
A: The output signals cause feedback in the final amplifier of one or both transmitters  
B: The output signals mix in the final amplifier of one or both transmitters  
C: The input frequencies are harmonically related  
D: The output frequencies are harmonically related    

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

Correct answer: B — The output signals mix in the final amplifier of one or both transmitters.

Why B is correct:
- Intermodulation occurs when strong signals at two different frequencies, $f_1$ and $f_2$, enter a nonlinear device (like a repeater’s final PA). The nonlinearity “mixes” them, creating new spurious frequencies at sums and differences.
- With two repeaters in close proximity, each transmitter’s output can couple into the other’s PA. The PA’s nonlinearity generates intermod products that can land on nearby repeater inputs or other services, causing interference.

Why the others are incorrect:
- A: “Feedback” isn’t the mechanism. Intermod is due to nonlinear mixing, not feedback/oscillation within the PA.
- C: Input frequencies being harmonically related is not required. Intermod does not depend on harmonic relationships of inputs.
- D: Output frequencies being harmonically related is not required. Intermod can occur between any two strong, unrelated frequencies when a nonlinearity is present.

Useful formulas and rules of thumb:
- General intermod products from two fundamentals $f_1$ and $f_2$:
  $$f_{\text{IM}} = m f_1 \pm n f_2 \quad \text{(}m,n \in \mathbb{Z}^+\text{)}$$
- Most troublesome are 3rd-order products:
  $$f_{\text{IM3}} = 2f_1 - f_2,\quad 2f_2 - f_1$$
- Rule of thumb: 3rd-order IM products change about 3 dB for each 1 dB change in the fundamentals (3:1 slope). Reducing PA drive or improving isolation reduces IM quickly.
- Mitigation: ensure linear PAs, use isolators/circulators and cavity filters, good antenna/feeder isolation, and physical separation between repeaters.