*Question G9D01:
Which of the following antenna types will be most effective as a near vertical incidence skywave (NVIS) antenna for short-skip communications on 40 meters during the day?*  

*Correct Answer is  A: A horizontal dipole placed between 1/10 and 1/4 wavelength above the ground*  

### AI Response: (may contain errors)  

For NVIS (Near Vertical Incidence Skywave) communication, the goal is to have radio waves directed nearly vertically to the ionosphere, which then reflects them back to Earth over short distances. This is particularly useful for short-skip communications.

### Correct Answer: A

- **A horizontal dipole placed between 1/10 and 1/4 wavelength above the ground** is most effective for NVIS.  
  - **Reason**: At this height, the dipole radiates most of its energy upwards, which is ideal for NVIS. The low height above ground helps in achieving a high angle of radiation, which is necessary for the waves to be reflected back to Earth over short distances.

### Why Other Choices Are Incorrect:

- **B: A vertical antenna placed between 1/4 and 1/2 wavelength above the ground**  
  - **Reason**: Vertical antennas typically radiate energy at lower angles, which is not suitable for NVIS. They are better for long-distance communication rather than short-skip.

- **C: A horizontal dipole placed at approximately 1/2 wavelength above the ground**  
  - **Reason**: At this height, the dipole tends to radiate at lower angles, which is not optimal for NVIS. The higher the dipole, the lower the angle of radiation, which is contrary to the needs of NVIS.

- **D: A vertical dipole placed at approximately 1/2 wavelength above the ground**  
  - **Reason**: Similar to the vertical antenna, a vertical dipole at this height will not provide the high-angle radiation needed for NVIS. It will favor lower angle radiation, which is not suitable for short-skip communications.

### Useful Rule of Thumb:

- **NVIS Antenna Height**: For effective NVIS operation, a horizontal dipole should be placed low, typically between 1/10 and 1/4 wavelength above the ground. This ensures the majority of the radiation is directed upwards.

By understanding these principles, you can effectively choose and set up antennas for NVIS communication on the 40-meter band.