*Question E9D11:
Why do most two-element Yagis with normal spacing have a reflector instead of a director?*  

*Correct Answer is  D: Higher gain*  

### AI Response: (may contain errors)  

To understand why most two-element Yagis have a reflector instead of a director, let's break down the concepts involved.

### Correct Answer: Higher Gain

- **Higher Gain**: A Yagi antenna typically consists of a driven element (the active element) and one or more passive elements (reflectors and directors). The reflector is placed behind the driven element and serves to reflect energy forward, enhancing the antenna's gain in the desired direction. This configuration allows the antenna to focus more energy in the forward direction, resulting in higher gain compared to a setup with a director.

### Why the Other Choices are Incorrect

- **A: Lower SWR**: 
  - SWR (Standing Wave Ratio) is a measure of how well the antenna is matched to the transmission line. While a well-designed Yagi can have a low SWR, the presence of a reflector does not inherently lower SWR compared to a director. The matching is more dependent on the design and tuning of the antenna rather than the choice between a reflector and a director.

- **B: Higher Receiving Directivity Factor**: 
  - The directivity factor refers to how well the antenna can focus energy in a particular direction. While a Yagi with a reflector does provide good directivity, the primary reason for using a reflector is to increase gain rather than directivity alone. Both configurations can achieve high directivity, but the reflector specifically enhances gain.

- **C: Greater Front-to-Side Ratio**: 
  - The front-to-side ratio measures how much stronger the signal is in the forward direction compared to the side lobes. While a Yagi with a reflector can improve this ratio, the main advantage of using a reflector is the increase in gain. The front-to-side ratio is influenced by the overall design and spacing of the elements, not just the presence of a reflector.

### Summary

In summary, the use of a reflector in a two-element Yagi primarily enhances the antenna's gain, making it more effective for transmitting and receiving signals in the desired direction. The other options, while related to antenna performance, do not directly address the primary reason for including a reflector in this configuration. 

### Useful Rule of Thumb

- **Yagi Antenna Gain**: The gain of a Yagi antenna can be approximated by the formula:

$$
G \approx 10 \log_{10}(N) + 1.76
$$

where $N$ is the number of elements (including the driven element). This shows that adding a reflector (or more elements) increases the gain of the antenna.