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

*Correct Answer is  D: 1/2 wavelength*  

### AI Response: (may contain errors)  

A Yagi antenna's driven element is typically a dipole, which is designed to be resonant at the operating frequency. The length of a dipole is approximately half of the wavelength of the frequency it is designed to operate on. Therefore, the correct answer is:

**D: 1/2 wavelength**

### Explanation:

1. **Correct Answer: 1/2 wavelength**
   - A dipole antenna, which is the driven element in a Yagi, is resonant when its length is approximately half of the wavelength of the frequency it is designed to receive or transmit. This is because a half-wavelength dipole has a natural resonance that allows it to efficiently radiate or receive radio waves.

2. **Why the Other Choices are Incorrect:**
   - **A: 234 divided by frequency in MHz**
     - This formula is used to calculate the length of a quarter-wave vertical antenna, not a half-wave dipole. It is not applicable to the driven element of a Yagi.
   - **B: 1005 divided by frequency in MHz**
     - This formula is used to calculate the length of a full-wave loop antenna, not a half-wave dipole. It is not relevant to the driven element of a Yagi.
   - **C: 1/4 wavelength**
     - A quarter-wavelength is typically used for vertical antennas or monopoles, not for the driven element of a Yagi, which is a half-wave dipole.

### Useful Formula:

To calculate the length of a half-wave dipole (driven element) in meters, you can use the formula:

$$
L = \frac{150}{f}
$$

where:
- $L$ is the length in meters,
- $f$ is the frequency in MHz.

This formula is derived from the speed of light and the relationship between frequency and wavelength. It provides a good approximation for the length of a half-wave dipole.