*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.

### Correct Answer: A: Gain increases

- **Explanation**: 
  - The gain of a Yagi antenna is enhanced by adding directors because directors are elements that focus the radiated energy in a specific direction. 
  - A longer boom allows for more directors, which increases the antenna's directivity and overall gain. 
  - Gain is typically measured in decibels (dB) and is a measure of how well the antenna converts input power into radio waves in a specific direction compared to a reference antenna.

### Why the other choices are incorrect:

- **B: Beamwidth increases**: 
  - This is incorrect because increasing the number of directors and the boom length actually narrows the beamwidth. A narrower beamwidth means the antenna is more directional, which is a characteristic of higher gain antennas.

- **C: Front-to-back ratio decreases**: 
  - This is also incorrect. The front-to-back ratio, which measures how well the antenna rejects signals from the rear compared to the front, typically improves with more directors and a longer boom. This is because the added directors help to focus the energy forward, enhancing the front-to-back performance.

- **D: Resonant frequency is lower**: 
  - This statement is incorrect as well. The resonant frequency of a Yagi antenna is primarily determined by the length of the elements. While adding directors can slightly affect the resonant frequency, increasing the boom length does not lower it. In fact, longer elements (directors) can lead to a lower resonant frequency, but this is not a direct result of increasing the boom length alone.

### Useful Rules of Thumb:

- **Gain Increase**: Each additional director can increase the gain by approximately 1 dB, depending on the design and spacing.
- **Beamwidth**: As gain increases, beamwidth typically decreases. A high-gain Yagi may have a beamwidth of around 20-30 degrees, while a lower gain antenna may have a beamwidth of 60 degrees or more.

In summary, increasing the boom length and adding directors to a Yagi antenna primarily increases its gain while improving directivity and front-to-back ratio, making it a more effective antenna for focused communication.