*Question E9D05:
Approximately how long is a Yagi’s driven element?*  

*Correct Answer is  D: 1/2 wavelength*  

### AI Response: (may contain errors)  

To determine the length of a Yagi's driven element, we need to understand its relationship to the wavelength of the signal it is designed to transmit or receive.

### Correct Answer: D: 1/2 wavelength

- The driven element of a Yagi antenna is typically designed to be approximately $1/2$ wavelength long. This length allows the antenna to effectively resonate at the frequency of operation, maximizing its efficiency and performance.

### Why the Other Choices are Incorrect:

- **A: 234 divided by frequency in MHz**  
  This formula is used to calculate the length of a dipole antenna in feet, not specifically for a Yagi's driven element. The correct formula for a half-wave dipole is:
  $$ \text{Length (feet)} = \frac{468}{f \text{ (MHz)}} $$
  This does not apply directly to the Yagi's driven element.

- **B: 1005 divided by frequency in MHz**  
  This formula is not standard for calculating antenna lengths. It does not correspond to any common antenna design and is not relevant for a Yagi's driven element.

- **C: 1/4 wavelength**  
  While a quarter-wavelength element can be used in some antenna designs (like a ground plane), the driven element of a Yagi is specifically designed to be $1/2$ wavelength long to ensure proper resonance and efficiency.

### Useful Formulas and Rules of Thumb:

- For a half-wave dipole (and thus the Yagi's driven element):
  $$ \text{Length (meters)} = \frac{150}{f \text{ (MHz)}} $$
  
- For a half-wave dipole (in feet):
  $$ \text{Length (feet)} = \frac{468}{f \text{ (MHz)}} $$

Understanding these relationships will help you accurately determine the dimensions of various antenna elements, including the driven element of a Yagi antenna.