*Question E9A10:
Which of the following improves the efficiency of a ground-mounted quarter-wave vertical antenna?*  

*Correct Answer is  A: Installing a ground radial system*  

### AI Response: (may contain errors)  

To improve the efficiency of a ground-mounted quarter-wave vertical antenna, the correct choice is:

**A: Installing a ground radial system**

### Explanation:

1. **Why the correct answer is correct:**
   - A ground-mounted quarter-wave vertical antenna relies on the ground to complete its electrical circuit. Installing a ground radial system provides a low-resistance path for the return currents, which improves the antenna's efficiency by reducing ground losses. This is crucial for maximizing the radiation efficiency of the antenna.

2. **Why the other choices are incorrect:**
   - **B: Isolating the coax shield from ground**  
     Isolating the coax shield from ground does not improve efficiency. In fact, proper grounding of the coax shield is important to prevent unwanted RF currents on the feedline, which can lead to interference and reduced performance.
   
   - **C: Shortening the radiating element**  
     Shortening the radiating element would detune the antenna from its resonant frequency, reducing its efficiency and effectiveness. A quarter-wave vertical antenna is designed to be a specific length to resonate at the desired frequency.

3. **Useful Rule of Thumb:**
   - For a quarter-wave vertical antenna, the efficiency can be significantly improved by using a radial system with at least 16 radials, each a quarter-wavelength long. More radials can further improve performance, but with diminishing returns beyond a certain point.

In summary, installing a ground radial system is essential for enhancing the efficiency of a ground-mounted quarter-wave vertical antenna by minimizing ground losses and ensuring proper operation.