Question E4D04:
Which of the following is used to reduce or eliminate intermodulation interference in a repeater caused by a nearby transmitter?  

Choices:  
A: A band-pass filter in the feed line between the transmitter and receiver  
B: A properly terminated circulator at the output of the repeater’s transmitter  
C: Utilizing a Class C final amplifier  
D: Utilizing a Class D final amplifier    

Correct Answer is  B: A properly terminated circulator at the output of the repeater’s transmitter  

To reduce intermodulation (IM) from a nearby transmitter, you want to keep that external RF from getting into your repeater’s power amplifier (PA), where nonlinearity would mix signals and create spurs. A circulator (used as an isolator when the third port is terminated) provides exactly this protection.

Why the correct answer is correct (B)
- A circulator is a 3‑port non-reciprocal RF device. With the third port properly terminated in a 50‑ohm load, it acts as an isolator.
- It sends the repeater’s TX power to the antenna, but any reverse or off-frequency RF (including signals from nearby transmitters coupled back from the antenna/duplexer) is diverted into the load rather than into the PA.
- This prevents external signals from entering the PA and being mixed (nonlinearly) with the repeater’s own transmit signal, thereby suppressing IM products.
- Rule of thumb: each isolator stage provides roughly 20–30 dB of reverse isolation; crowded sites often use two in series.

Why the other choices are incorrect
- A: A band-pass filter in the feed line between the transmitter and receiver
  - A “band-pass between TX and RX” is not a standard configuration; duplexers/cavities separate TX/RX paths but do not reliably prevent off-frequency energy from a nearby transmitter from entering the PA via the antenna line. A band-pass alone does not provide the necessary reverse isolation an isolator gives.
- C: Utilizing a Class C final amplifier
  - Class C is efficient for constant-envelope modes (e.g., FM) but is intentionally nonlinear; it does not reduce intermodulation and can worsen it if subjected to strong off-frequency signals.
- D: Utilizing a Class D final amplifier
  - Class D is a switching topology unsuitable for linear RF service in this context and does not address intermodulation from external signals; it can generate more spectral products, not fewer.

Useful formulas and rules of thumb
- Third-order IM product frequencies for two signals at $f_1$ and $f_2$:
  $$f_{\text{IM3}} = 2f_1 - f_2 \quad \text{and} \quad f_{\text{IM3}} = 2f_2 - f_1$$
- Third-order behavior: a 1 dB increase in interfering signal level typically raises the IM3 products by about 3 dB.
- Site practice: use a properly terminated circulator (isolator) at the TX output; combine with good duplexer cavities and adequate physical/antenna isolation for best results.