TECHNICAL NOTE / RFI / FEED SYSTEMS
Unintended RF current on the outside of coax shield can alter an antenna pattern, couple RF into the shack and create receive interference. A low SWR reading is not proof that the feed system is free of common-mode problems.
Typical symptoms
- Antenna pattern or receive noise changes when the coax is moved.
- Computer, audio or control cables show RF sensitivity during transmit.
- Equipment cases or nearby metalwork carry measurable RF current.
- RFI shifts with band, power level or antenna direction.
A disciplined test
Use an appropriate RF current probe or clamp to compare common-mode current at consistent positions and operating conditions. Record frequency, power, antenna direction and test configuration. Change one variable at a time; compare results before and after adding a choke or changing cable routing.
Chokes and bonding
Choose choke impedance and thermal capability for the actual band and RF power. Keep station bonding, lightning protection and personnel safety distinct from the RF balance problem; a ferrite choke does not replace protective grounding.