*Question E4E12:
What causes interference received as a series of carriers at regular intervals across a wide frequency range?*  

*Correct Answer is  A: Switch-mode power supplies*  

### AI Response: (may contain errors)  

Switch-mode power supplies (SMPS) are known to cause interference across a wide frequency range due to their operation. Here's why:

1. **Why the correct answer is correct:**
   - **Switch-mode power supplies (SMPS)**: These devices convert electrical power efficiently by rapidly switching the current on and off. This switching action generates harmonics and spurious emissions that can appear as a series of carriers at regular intervals across a wide frequency range. The interference is often due to the high-frequency switching components and inadequate filtering.

2. **Why the other choices are incorrect:**
   - **Radar transmitters**: While radar systems can cause interference, they typically operate on specific frequencies and are not characterized by a series of carriers at regular intervals across a wide frequency range.
   - **Wireless security camera transmitters**: These devices operate on specific frequencies allocated for video transmission and do not produce a wide range of carriers. Their interference is usually limited to their operating frequency bands.
   - **Electric fences**: Electric fences can cause interference, but it is usually impulsive noise rather than a series of carriers. The interference is often heard as a repetitive clicking sound rather than a wide frequency range of carriers.

3. **Useful information:**
   - **Harmonics**: The switching frequency of an SMPS and its harmonics can cause interference. Harmonics are integer multiples of a fundamental frequency, and they can extend across a wide frequency range if not properly filtered.
   - **Filtering**: Proper filtering and shielding are essential in SMPS design to minimize interference. This includes using components like ferrite beads, capacitors, and inductors to suppress unwanted emissions.

Understanding the nature of interference from different sources helps in identifying and mitigating these issues in amateur radio operations.