*Question T3A06:
What is the meaning of the term “picket fencing”?*  

*Correct Answer is  B: Rapid flutter on mobile signals due to multipath propagation*  

### AI Response: (may contain errors)  

The term "picket fencing" refers to a phenomenon observed in mobile radio communications, particularly when signals experience rapid fluctuations in strength due to multipath propagation. Let's break down the correct answer and the other choices:

### Correct Answer: 
**B: Rapid flutter on mobile signals due to multipath propagation**

- **Explanation**: 
  - Picket fencing occurs when a mobile signal reflects off various surfaces (like buildings or hills) and arrives at the receiver via multiple paths. This can cause rapid changes in signal strength, resembling the appearance of a picket fence on a signal strength meter. The variations in the signal can lead to a fluttering effect, making it difficult to maintain a stable communication link.

### Why the Other Choices are Incorrect:

- **A: Alternating transmissions during a net operation**
  - This choice refers to the practice of different operators taking turns to transmit during a net, which is unrelated to the concept of picket fencing. Picket fencing specifically describes signal behavior, not transmission protocols.

- **C: A type of ground system used with vertical antennas**
  - This choice pertains to grounding techniques for antennas, which is a separate topic. Picket fencing does not involve grounding systems but rather the effects of signal propagation.

- **D: Local vs long-distance communications**
  - This choice discusses the differences in communication range and does not relate to the rapid signal fluctuations caused by multipath propagation. Picket fencing is specifically about the variations in signal strength experienced in mobile environments.

### Summary:
Picket fencing is a term that describes the rapid fluttering of mobile signals due to multipath propagation, making **B** the correct answer. The other options do not accurately reflect this phenomenon and pertain to different aspects of radio communication. 

Understanding these concepts is crucial for the Extra Class exam, as they relate to practical radio operation and signal behavior.