*Question G9C04:
How does antenna gain in dBi compare to gain stated in dBd for the same antenna?*  

*Correct Answer is  B: Gain in dBi is 2.15 dB higher*  

### AI Response: (may contain errors)  

To understand the relationship between antenna gain expressed in dBi and dBd, we need to clarify what these terms mean:

- **dBi**: This is the gain of an antenna compared to an isotropic radiator, which is a theoretical antenna that radiates equally in all directions.
- **dBd**: This is the gain of an antenna compared to a dipole antenna, which is a real-world antenna that has a specific radiation pattern.

The key relationship between these two measurements is:

$$
\text{Gain in dBi} = \text{Gain in dBd} + 2.15 \text{ dB}
$$

### Explanation of the Correct Answer:
- The correct answer states that "Gain in dBi is 2.15 dB higher." This is true because when you convert from dBd to dBi, you add 2.15 dB. Therefore, if an antenna has a gain of, say, 5 dBd, its gain in dBi would be:

$$
5 \text{ dBd} + 2.15 \text{ dB} = 7.15 \text{ dBi}
$$

### Why the Other Choices are Incorrect:
- **Choice A: Gain in dBi is 2.15 dB lower**  
  This is incorrect because it misrepresents the relationship; dBi is always higher than dBd by 2.15 dB.

- **Choice C: Gain in dBd is 1.25 dBd lower**  
  This choice is incorrect because it does not accurately reflect the conversion between dBi and dBd. The difference is not 1.25 dBd.

- **Choice D: Gain in dBd is 1.25 dBd higher**  
  This is also incorrect for the same reason as choice C. The correct difference is 2.15 dB, not 1.25 dB.

### Summary:
- Remember the conversion:  
$$
\text{Gain in dBi} = \text{Gain in dBd} + 2.15 \text{ dB}
$$
- This means that dBi is always higher than dBd by 2.15 dB, which is crucial for understanding antenna specifications and performance.