*Question G0A02:
Which of the following is used to determine RF exposure from a transmitted signal?*  

*Correct Answer is  D: All these choices are correct*  

### AI Response: (may contain errors)  

To determine RF exposure from a transmitted signal, all the provided factors play a role. Let's break down each choice:

1. **Correct Answer: D - All these choices are correct**
   - **Duty Cycle**: This refers to the fraction of time a signal is transmitted. A higher duty cycle means more time transmitting, which can increase exposure.
   - **Frequency**: Different frequencies interact with the body in various ways. For example, higher frequencies can penetrate tissues differently than lower frequencies, affecting exposure levels.
   - **Power Density**: This is the amount of power per unit area (usually expressed in watts per square meter, W/m²). It directly relates to how much RF energy is present in a given area, influencing exposure levels.

2. **Why the other choices are not incorrect**:
   - **A: Its duty cycle**: This is important because it affects the average power output over time. A continuous wave (100% duty cycle) will have a different exposure level compared to a pulsed signal (lower duty cycle).
   - **B: Its frequency**: This is crucial as different frequencies can have different biological effects. For example, frequencies in the microwave range can cause heating effects, while lower frequencies may not.
   - **C: Its power density**: This is a direct measure of how much RF energy is present in a specific area, making it essential for assessing exposure.

3. **Useful formulas or rules of thumb**:
   - **Power Density Calculation**: The power density ($S$) at a distance ($d$) from an isotropic radiator can be calculated using:
   $$S = \frac{P_T \times G}{4\pi d^2}$$
   where:
   - $P_T$ = transmitter power in watts
   - $G$ = gain of the antenna (linear)
   - $d$ = distance from the antenna in meters

This formula helps in understanding how power density decreases with distance and how it is influenced by the antenna gain. 

In summary, all these factors—duty cycle, frequency, and power density—are essential for accurately determining RF exposure from a transmitted signal.