*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 dBi and dBd, we need to know what each term represents:

- **dBi**: Gain relative to an isotropic radiator, which is a theoretical antenna that radiates equally in all directions.
- **dBd**: Gain relative to a dipole antenna, which is a common reference antenna in practice.

The key relationship to remember is:

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

This means that gain expressed in dBi is always 2.15 dB higher than gain expressed in dBd for the same antenna. This is because a dipole antenna has a gain of 2.15 dB over an isotropic radiator.

### Explanation of the Correct Answer:
- **Correct Answer (B): Gain in dBi is 2.15 dB higher**  
  This is correct because when you convert gain from dBd to dBi, you add 2.15 dB. Therefore, the gain in dBi is 2.15 dB higher than the gain in dBd.

### Explanation of Incorrect Choices:
- **Choice A: Gain in dBi is 2.15 dB lower**  
  This is incorrect because it reverses the relationship. Gain in dBi is actually higher, not lower, than gain in dBd.

- **Choice C: Gain in dBd is 1.25 dBd lower**  
  This is incorrect because it introduces an incorrect value (1.25 dB) and reverses the relationship. The correct difference is 2.15 dB, and it should be added to dBd to get dBi.

- **Choice D: Gain in dBd is 1.25 dBd higher**  
  This is incorrect because it uses the wrong value (1.25 dB) and suggests the wrong direction of the relationship. The correct difference is 2.15 dB, and it should be added to dBd to get dBi, not the other way around.

### Rule of Thumb:
- Always remember: **dBi = dBd + 2.15 dB**. This will help you quickly convert between these two units of antenna gain.