*Question G9C05:
What is the primary effect of increasing boom length and adding directors to a Yagi antenna?*  

*Correct Answer is  A: Gain increases*  

### AI Response: (may contain errors)  

Increasing the boom length and adding directors to a Yagi antenna primarily affects its gain. Here's why:

1. **Correct Answer: Gain increases (A)**
   - **Explanation**: The gain of a Yagi antenna is directly related to its ability to focus energy in a particular direction. By increasing the boom length and adding more directors, the antenna can focus the radio waves more effectively, thus increasing its gain. The directors help to direct more energy in the forward direction, enhancing the antenna's ability to transmit and receive signals over longer distances.

2. **Why the Other Choices are Incorrect:**
   - **Beamwidth increases (B)**: This is incorrect because increasing the boom length and adding directors actually narrows the beamwidth. A narrower beamwidth is a result of higher gain, as the antenna focuses energy more tightly in one direction.
   - **Front-to-back ratio decreases (C)**: This is incorrect because adding directors typically improves the front-to-back ratio. The front-to-back ratio is a measure of how well the antenna suppresses signals from the rear, and more directors can enhance this suppression.
   - **Resonant frequency is lower (D)**: This is incorrect because the resonant frequency of a Yagi antenna is primarily determined by the length of the driven element, not the boom length or the number of directors. Adding directors does not inherently lower the resonant frequency.

3. **Useful Formulas or Rules of Thumb:**
   - **Gain and Beamwidth Relationship**: As a general rule, for directional antennas like the Yagi, increasing gain results in a narrower beamwidth. This is because the antenna is focusing more energy in a specific direction.
   - **Front-to-Back Ratio**: More directors can improve the front-to-back ratio, which is the ratio of the power radiated in the forward direction to the power radiated in the opposite direction.

In summary, increasing the boom length and adding directors to a Yagi antenna primarily increases its gain by focusing energy more effectively in the desired direction.