*Question T9B07:
Which of the following is true of PL-259 type coax connectors?*  

*Correct Answer is  C: They are commonly used at HF and VHF frequencies*  

### AI Response: (may contain errors)  

The correct answer to the question about PL-259 type coax connectors is:

**C: They are commonly used at HF and VHF frequencies.**

### Explanation:

1. **Why the correct answer is correct:**
   - The PL-259 connector is widely used for connecting coaxial cables in amateur radio, particularly at HF (High Frequency) and VHF (Very High Frequency) bands. Its design allows for good performance in these frequency ranges, making it a standard choice for many radio applications.

2. **Why the other choices are incorrect:**
   - **A: They are preferred for microwave operation.**
     - This is incorrect because PL-259 connectors are not ideal for microwave frequencies. At higher frequencies, connectors like the N-type or SMA are preferred due to their better performance and lower loss characteristics.
   
   - **B: They are watertight.**
     - This is also incorrect. While PL-259 connectors can be made to be weather-resistant with proper sealing and installation, they are not inherently watertight. Additional measures, such as using heat shrink tubing or weatherproofing, are necessary to protect them from moisture.
   
   - **D: They are a bayonet-type connector.**
     - This statement is incorrect because PL-259 connectors are not bayonet-type connectors. They use a screw-on mechanism for securing the connection, unlike bayonet connectors which have a twist-lock mechanism.

### Useful Rules of Thumb:
- **Connector Selection:**
  - For HF and VHF applications, PL-259 is a common choice.
  - For UHF and microwave applications, consider using connectors like N-type or SMA for better performance.

- **Weatherproofing:**
  - Always ensure connectors are properly sealed if used outdoors to prevent moisture ingress, which can lead to signal loss and corrosion.