Case Study 4: Repeating Spectral Humps at 1.8MHz Intervals

Symptom:

  • Wavy noise peaks appear every 1.8 MHz, gradually decreasing in amplitude up to 9 MHz.

  • The pattern also appears on another antenna/receive system at the same site.

Analysis:

  • This pattern suggests harmonics of a switching power supply.

  • Many cheap wall warts, LED drivers, and computer chargers generate harmonic noise.

Mitigation Steps:

  1. Identify the Device:

  • Unplug switching power supplies one at a time and observe changes in the waterfall.

  1. Replace or Shield:

  • If a device is the culprit, swap it for a linear power supply.

  • If replacement isn’t possible, add ferrite chokes (Mix 43 for VHF, Mix 31 for HF) to the DC output.

  1. Filter the Power Line:

  • Use an AC line filter to block conducted emissions.