*Question E9H03:
What is receiving directivity factor (RDF)?*  

*Correct Answer is  D: Peak antenna gain compared to average gain over the hemisphere around and above the antenna*  

### AI Response: (may contain errors)  

The Receiving Directivity Factor (RDF) is a measure used to evaluate the performance of receiving antennas, particularly in terms of their ability to reject noise and interference from unwanted directions.

### Correct Answer: D

- **Peak antenna gain compared to average gain over the hemisphere around and above the antenna**:  
  RDF is defined as the ratio of the peak gain of an antenna to the average gain over the hemisphere surrounding the antenna. This metric helps in assessing how well an antenna can focus on signals from a desired direction while minimizing reception from other directions, which is crucial for reducing interference.

### Why Other Choices Are Incorrect:

- **A: Forward gain compared to the gain in the reverse direction**:  
  This describes the front-to-back ratio, which is a different measure used to evaluate how well an antenna can reject signals from the opposite direction of its main lobe.

- **B: Relative directivity compared to isotropic**:  
  This describes the directivity of an antenna, which is the gain of the antenna compared to an isotropic radiator. It does not specifically address the average gain over a hemisphere.

- **C: Relative directivity compared to a dipole**:  
  This describes the gain of an antenna compared to a standard dipole antenna, often referred to as "dBi" when comparing to an isotropic radiator or "dBd" when comparing to a dipole. It does not involve the average gain over a hemisphere.

### Useful Formula:

The RDF can be expressed as:

$$
\text{RDF} = \frac{\text{Peak Gain}}{\text{Average Gain over Hemisphere}}
$$

This formula highlights the concept that RDF is about comparing the focused gain of an antenna to its average performance in all directions, which is key for understanding its ability to reject unwanted signals.