*Question T7A01:
Which term describes the ability of a receiver to detect the presence of a signal?*  

*Correct Answer is  B: Sensitivity*  

### AI Response: (may contain errors)  

The correct answer to the question is **Sensitivity**.

1. **Why the correct answer is correct:**
   - **Sensitivity** refers to a receiver's ability to detect weak signals. It is defined as the minimum input signal level that can be detected by the receiver and is crucial for effective communication, especially in weak signal conditions. A more sensitive receiver can pick up signals that are lower in strength, which is essential for reliable operation in amateur radio.

2. **Why the other choices are incorrect:**
   - **A: Linearity**: This term describes how well a receiver can reproduce the input signal without distortion. While important for signal fidelity, it does not directly relate to the ability to detect weak signals.
   - **C: Selectivity**: This refers to a receiver's ability to isolate a specific signal from other signals or noise. While selectivity is important for avoiding interference, it does not measure the receiver's ability to detect weak signals.
   - **D: Total Harmonic Distortion (THD)**: This is a measure of the distortion in a signal caused by non-linearities in the system. It is not related to the detection of signals but rather to the quality of the output signal.

3. **Useful formulas or rules of thumb:**
   - There isn't a specific formula for sensitivity, but a common rule of thumb is that a receiver with a sensitivity of around $0.1 \, \text{µV}$ (microvolts) is considered very good for HF (high frequency) bands. Sensitivity can also be expressed in terms of the signal-to-noise ratio (SNR) required for effective communication.

In summary, sensitivity is the key term that describes a receiver's ability to detect the presence of a signal, making it the correct choice for this question.