This post is part of Building Your First Ham Radio Station.
If you’ve run HF mobile, you know the sound: a whine that rises and falls with engine RPM, showing up right on top of weak signals. That’s alternator whine, and it’s one of the most common noise problems in mobile installs.
Where It Actually Comes From
Alternators generate ripple on the DC output as they switch between diode pairs, and that ripple frequency scales directly with RPM, which is why the pitch tracks your engine speed. It gets into your radio two ways: conducted through the power leads, or radiated through the air and picked up by your antenna and its feedline.
Fixing Conducted Noise
Start here, it’s usually the bigger contributor. Run your radio’s power leads directly to the battery terminals, not to a convenient point further down the harness, and keep them as short as practical. A quality DC noise filter in-line on the power leads, or ferrite chokes clamped near both ends of the run, knock down conducted noise significantly. Cheap aftermarket power leads with thin gauge wire make this worse, not better.
Fixing Radiated Noise
If filtering the power leads doesn’t fully clear it, the noise is likely getting into your antenna system directly. Check that your antenna mount has a solid ground to the vehicle body, add a common-mode choke on the feedline near the radio, and route coax away from the engine bay and alternator wiring wherever your install allows. In stubborn cases, a shielded or braided ground strap between the engine block and the chassis reduces the noise at its actual source.