*Question E9A09:
What is antenna efficiency?*  

*Correct Answer is  B: Radiation resistance divided by total resistance*  

### AI Response: (may contain errors)  

Antenna efficiency is a measure of how effectively an antenna converts input power into radiated electromagnetic waves. It is defined as the ratio of the radiation resistance to the total resistance of the antenna.

The correct answer is:

**B: Radiation resistance divided by total resistance**

### Explanation of the Correct Answer:
- **Antenna Efficiency Formula**:  
  $$ \text{Antenna Efficiency} = \frac{R_r}{R_t} $$  
  where:
  - $R_r$ = radiation resistance (the part of the antenna's resistance that contributes to radiation)
  - $R_t$ = total resistance (the sum of radiation resistance and loss resistance)

- This formula shows that antenna efficiency is a measure of how much of the power supplied to the antenna is actually radiated as radio waves, compared to how much is lost as heat or other forms of energy.

### Why the Other Choices are Incorrect:
- **A: Radiation resistance divided by transmission resistance**  
  - This is incorrect because transmission resistance is not a standard term in antenna theory. The relevant comparison is between radiation resistance and total resistance.

- **C: Total resistance divided by radiation resistance**  
  - This is incorrect because it inverts the relationship. The efficiency should be a ratio of radiation resistance to total resistance, not the other way around.

- **D: Effective radiated power divided by transmitter output**  
  - This is incorrect because while it relates to the performance of the antenna, it does not define antenna efficiency. This ratio would give you a measure of how much power is effectively radiated compared to what is input, but it does not directly relate to the resistance components.

### Summary:
- Antenna efficiency is crucial for understanding how well an antenna performs.
- Remember the formula:  
  $$ \text{Antenna Efficiency} = \frac{R_r}{R_t} $$  
- Focus on the definitions of radiation resistance and total resistance to grasp the concept fully.