*Question E9A01:
What is an isotropic radiator?*  

*Correct Answer is  C: A hypothetical, lossless antenna having equal radiation intensity in all directions used as a reference for antenna gain*  

### AI Response: (may contain errors)  

An isotropic radiator is a theoretical concept used in antenna theory. Here's why the correct answer is correct and why the other choices are incorrect:

1. **Correct Answer: C**  
   An isotropic radiator is a **hypothetical, lossless antenna** that radiates power equally in all directions. It serves as a reference point for measuring antenna gain. Since it is hypothetical, it does not exist in reality but is used as a standard to compare real antennas.

2. **Why Other Choices Are Incorrect:**
   - **Choice A:** A calibrated, unidirectional antenna is not isotropic because it does not radiate equally in all directions. It is used for precise measurements but is directional, not omnidirectional.
   - **Choice B:** An omnidirectional, horizontally polarized antenna can radiate in all horizontal directions but not equally in all directions (3D space). It is not isotropic because it has polarization and is not lossless.
   - **Choice D:** A spacecraft antenna used to direct signals toward Earth is directional, focusing energy in a specific direction, which is the opposite of isotropic behavior.

3. **Useful Concept:**
   - **Antenna Gain:** The gain of an antenna is often expressed in decibels relative to an isotropic radiator (dBi). The formula for gain in dBi is:
     $$ G_{\text{dBi}} = 10 \log_{10} \left( \frac{P_{\text{antenna}}}{P_{\text{isotropic}}} \right) $$
   - Here, $P_{\text{antenna}}$ is the power radiated by the actual antenna, and $P_{\text{isotropic}}$ is the power that would be radiated by an isotropic antenna with the same input power.

Understanding the concept of an isotropic radiator is crucial for grasping how antenna gain is measured and compared.