*Question E9C14:
How does the radiation pattern of a horizontally-polarized antenna mounted above a long slope compare with the same antenna mounted above flat ground?*  

*Correct Answer is  B: The main lobe takeoff angle decreases in the downhill direction*  

### AI Response: (may contain errors)  

When a horizontally-polarized antenna is mounted above a long slope, the radiation pattern is affected by the slope's geometry. Here's how it works:

1. **Correct Answer: B - The main lobe takeoff angle decreases in the downhill direction**

   - **Explanation**: When an antenna is mounted on a slope, the ground acts as a reflector. The slope causes the reflected wave to combine with the direct wave in such a way that the main lobe of the radiation pattern is tilted downward. This results in a lower takeoff angle in the downhill direction compared to when the antenna is mounted on flat ground. The slope effectively "aims" the radiation more towards the horizon in the downhill direction.

2. **Why the other choices are incorrect**:

   - **Choice A: The main lobe takeoff angle increases in the downhill direction**
     - This is incorrect because the slope causes the takeoff angle to decrease, not increase. The slope directs the energy more towards the horizon, not upwards.

   - **Choice C: The horizontal beamwidth decreases in the downhill direction**
     - The horizontal beamwidth is not primarily affected by the slope in this context. The slope mainly affects the vertical takeoff angle, not the horizontal spread of the beam.

   - **Choice D: The horizontal beamwidth increases in the uphill direction**
     - Similar to Choice C, the horizontal beamwidth is not significantly altered by the slope. The main effect of the slope is on the vertical takeoff angle, not the horizontal beamwidth.

3. **Useful Concepts**:

   - **Takeoff Angle**: The angle above the horizontal at which the main lobe of the antenna's radiation pattern is directed. A lower takeoff angle means the signal travels more parallel to the ground, which can be beneficial for long-distance communication.

   - **Reflection and Refraction**: The slope acts as a reflective surface, altering the path of the radio waves and thus changing the takeoff angle.

Understanding how terrain affects antenna radiation patterns is crucial for optimizing communication range and quality. In this case, the slope helps direct the signal more effectively in the downhill direction by lowering the takeoff angle.