ARRL Technician Practice Exam

What is the source of a high-pitched whine that varies with engine speed in a mobile transceiver's receive audio?

1. The battery

2. The alternator

3. The antenna

4. The engine fan

The correct answer is: The alternator

The high-pitched whine that varies with engine speed in a mobile transceiver's receive audio is primarily caused by the alternator. This phenomenon is commonly referred to as "alternator noise." As an engine runs, the alternator generates electrical power to charge the battery and supply current to the vehicle's electrical systems. The frequency of this whine correlates with the engine speed because the alternator's output frequency increases as the engine RPM increases. When the alternator operates, it can induce electrical noise into the vehicle’s power lines, which may manifest as audio interference in the transceiver. This noisy signal often becomes audible through the radio’s audio output, producing a whine that changes tone with the alternator's speed. While other components such as the engine fan or battery could potentially introduce noise, they do not typically produce a sound that is directly related to engine speed in the same manner that the alternator does. The antenna's role is primarily in receiving signals and would not generate such an audio frequency interference based on engine operation. Understanding the relationship between the vehicle's electrical system and radio interference is key to troubleshooting and resolving such issues.