*Question E7C10:
Which of the following filters is used in a 2-meter band repeater duplexer?*  

*Correct Answer is  B: A cavity filter*  

### AI Response: (may contain errors)  

In a 2-meter band repeater duplexer, the correct filter to use is a **cavity filter**. Here's why:

1. **Why the correct answer is correct:**
   - **Cavity Filter**: These filters are specifically designed for high selectivity and are capable of handling high power levels, which makes them ideal for use in repeater systems. They allow the repeater to transmit and receive simultaneously on closely spaced frequencies by isolating the transmit and receive paths.

2. **Why the other choices are incorrect:**
   - **Crystal Filter**: These are used for very narrow bandwidth applications, such as in single-sideband (SSB) receivers, but they are not suitable for the high power and wide frequency separation needed in a repeater duplexer.
   - **DSP Filter**: Digital Signal Processing (DSP) filters are used in digital systems to process signals, but they are not used in the RF path of a repeater duplexer due to their inability to handle high power RF signals directly.
   - **L-C Filter**: Inductor-capacitor (L-C) filters are used for basic filtering applications, but they do not provide the high selectivity and power handling required for a repeater duplexer.

3. **Useful Information:**
   - **Cavity Filters**: These are resonant structures that use the physical dimensions of a cavity to determine the resonant frequency. They are highly selective and can be tuned to very specific frequencies, which is essential for separating the transmit and receive frequencies in a repeater.

In summary, cavity filters are the best choice for a 2-meter band repeater duplexer due to their ability to handle high power and provide the necessary selectivity to separate closely spaced frequencies.