*Question G8C13:
What is indicated on a waterfall display by one or more vertical lines on either side of a data mode or RTTY signal?*  

*Correct Answer is  D: Overmodulation*  

### AI Response: (may contain errors)  

In a waterfall display, vertical lines on either side of a data mode or RTTY signal typically indicate overmodulation. Here's why:

1. **Correct Answer: Overmodulation (D)**
   - Overmodulation occurs when the modulation level exceeds the linear range of the transmitter, causing distortion. This distortion can create additional unwanted frequencies, known as sidebands, which appear as vertical lines on a waterfall display. These sidebands are a visual indication that the signal is being overdriven.

2. **Why the Other Choices are Incorrect:**
   - **Long path propagation (A):** This refers to a signal traveling a longer path around the Earth, often resulting in a delayed signal. It does not cause vertical lines on a waterfall display.
   - **Backscatter propagation (B):** This involves signals reflecting off irregularities in the ionosphere and returning to Earth. It might cause signal fading or distortion but not vertical lines specifically.
   - **Insufficient modulation (C):** This occurs when the modulation level is too low, leading to a weak signal. It would not create additional sidebands or vertical lines on a waterfall display.

In summary, vertical lines on a waterfall display are a clear indication of overmodulation, as they represent unwanted sidebands generated by excessive modulation levels.