*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 the input power into radiated power. It is defined as the ratio of the radiation resistance to the total resistance of the antenna system.

### Correct Answer: B

**Explanation:**

- **Antenna Efficiency Formula:**

  $$ \text{Efficiency} = \frac{R_{\text{radiation}}}{R_{\text{total}}} $$

  where:
  - $R_{\text{radiation}}$ is the radiation resistance, which represents the part of the antenna's resistance that actually radiates energy.
  - $R_{\text{total}}$ is the sum of the radiation resistance and any loss resistances (such as ohmic losses in the antenna materials).

- **Why this is correct:** The efficiency is calculated by dividing the radiation resistance by the total resistance, which includes both the radiation resistance and any losses. This gives a measure of how much of the input power is actually used for radiation.

### Why the Other Choices are Incorrect:

- **Choice A: Radiation resistance divided by transmission resistance**

  - **Incorrect because:** Transmission resistance is not a standard term in antenna theory. The relevant factor is the total resistance, which includes all resistive losses, not just a hypothetical "transmission resistance."

- **Choice C: Total resistance divided by radiation resistance**

  - **Incorrect because:** This is the inverse of the correct formula. Dividing total resistance by radiation resistance would give a value greater than 1, which does not represent efficiency.

- **Choice D: Effective radiated power divided by transmitter output**

  - **Incorrect because:** This ratio is related to overall system efficiency, including the transmitter and feedline, not just the antenna efficiency. Antenna efficiency specifically deals with how well the antenna itself converts input power to radiated power.

### Summary:

Antenna efficiency is a key parameter in understanding how well an antenna performs. It is important to distinguish between the different types of resistances involved and to use the correct formula to calculate efficiency.