Case Study 1: Waterfall Changes When Moving a Feedline

Symptom:

  • The SDR waterfall display shows fluctuations in noise levels when moving the feedline.

  • The noise is broadband but follows the motion of the cable.

Analysis:

  • This suggests common-mode noise, where the feedline is acting as an unintended antenna. Alternatively or additionally, the movement of a conducting material near the dipole may deform the EM field of the incident wave in the aperture of the antenna.

  • Possible sources: Poor shielding, improper grounding, or lack of a common-mode choke.

Mitigation Steps:

  1. Test Isolation: Insert a common-mode choke (ferrite on coax, Mix 31 for HF) near the antenna and near the receiver.

  2. Check Grounding: Ensure the feedline shield is grounded at a single point.

  3. Re-route the Cable: Keep coax away from noise sources (AC lines, routers, computers).

  4. Improve Shielding: Use higher-quality coax like LMR-400 or clamp ferrites on both ends.