*Question E9C13:
How does the radiation pattern of a horizontally polarized antenna vary with increasing height above ground?*  

*Correct Answer is  B: The takeoff angle of the lowest elevation lobe decreases*  

### AI Response: (may contain errors)  

When discussing the radiation pattern of a horizontally polarized antenna, especially in relation to its height above ground, it's important to understand how the takeoff angle and beamwidth are affected.

### Correct Answer: B
**The takeoff angle of the lowest elevation lobe decreases**

1. **Explanation:**
   - As the height of a horizontally polarized antenna above ground increases, the takeoff angle of the lowest elevation lobe decreases. This means that the main lobe of the radiation pattern is directed more towards the horizon, which is beneficial for long-distance communication.
   - This occurs because the ground reflects radio waves, and as the antenna is raised, the interaction between the direct and reflected waves changes, resulting in a lower takeoff angle.

### Why Other Choices Are Incorrect:

- **Choice A: The takeoff angle of the lowest elevation lobe increases**
  - This is incorrect because increasing the height of the antenna actually lowers the takeoff angle, not increases it.

- **Choice C: The horizontal beamwidth increases**
  - The horizontal beamwidth is primarily determined by the antenna's design and not significantly affected by height above ground. Increasing height affects the vertical pattern more than the horizontal.

- **Choice D: The horizontal beamwidth decreases**
  - Similar to Choice C, the horizontal beamwidth is not significantly influenced by the height above ground. The height affects the vertical radiation pattern, not the horizontal beamwidth.

### Useful Rule of Thumb:
- **Height and Takeoff Angle:**
  - As a general rule, for a horizontally polarized antenna, increasing the height above ground results in a lower takeoff angle. This is particularly important for HF antennas used in long-distance (DX) communication, where a lower takeoff angle can improve signal reach.

Understanding these principles helps in optimizing antenna placement for desired communication outcomes.