Question
Which of the following filters is used in a 2-meter band repeater duplexer?
Answer Options
- A) A crystal filter
- B) A cavity filter
- C) A DSP filter
- D) An L-C filter
Correct Answer: B
Explanation
A duplexer is a critical component in a repeater that allows the transmitter and receiver to share a single antenna while operating simultaneously on two different frequencies separated by a small frequency shift (e.g., 600 \text{ kHz} on the 2-meter band). The duplexer must provide very high isolation between the powerful transmitter output and the highly sensitive receiver input.
To achieve the necessary isolation and high Q factor at VHF, the filter type used is a cavity filter. A cavity filter consists of large metallic resonant chambers (cavities) that are precision-tuned to either pass the transmit frequency and reject the receive frequency (Tx side) or vice versa (Rx side). They provide the steep skirts and deep notches required for simultaneous transmit/receive operation.
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