*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)  

To understand why a cavity filter is used in a 2-meter band repeater duplexer, let's break down the options:

### Correct Answer: B - A cavity filter

- **Cavity filters** are specifically designed for use in VHF and UHF applications, including the 2-meter band. They are highly selective and can effectively separate the transmit and receive frequencies in a repeater system. This selectivity is crucial to prevent interference between the two channels, allowing for clear communication.

### Why the Other Choices are Incorrect:

- **A: A crystal filter**
  - Crystal filters are typically used in lower frequency applications and are not suitable for the wide bandwidths required in VHF/UHF systems like the 2-meter band. They are more common in HF applications where narrow bandwidth and high selectivity are needed.

- **C: A DSP filter**
  - Digital Signal Processing (DSP) filters are used in modern communication systems for signal processing but are not physical filters like cavity filters. They process signals in the digital domain and are not used as standalone components in duplexers.

- **D: An L-C filter**
  - L-C filters (inductor-capacitor filters) can be used for various filtering applications, but they do not provide the same level of selectivity and performance as cavity filters in the VHF/UHF range. They are more commonly used in simpler applications and may not handle the power levels or frequency separation required in a repeater duplexer.

### Summary

In summary, cavity filters are the preferred choice for 2-meter band repeaters due to their ability to provide high selectivity and power handling, which is essential for effective operation in a duplexer setup.