You are using an HF off-centre-fed (OCF) unbalanced antenna. When you transmit on SSB, distorted audio and noise are heard from an outboard amplified speaker. What device could you install in the transmission line to mitigate this problem?

Off-centre-fed (OCF) antennas are naturally unbalanced and can easily induce common-mode currents on the outside of the coaxial cable shield. This effectively turns your feedline into part of the radiating antenna, allowing RF energy to travel straight back into your radio room. Once inside, this RF energy can be picked up by unshielded audio cables or the circuitry in outboard amplified speakers, leading to distorted audio or buzzing. This disruptive phenomenon is commonly known as 'RF in the shack'. Installing a common-mode choke on the transmission line restricts these unwanted currents from flowing down the outside of the coax shield. While a low-pass filter stops harmonic interference and an antenna tuner matches impedance, neither solves the issue of common-mode feedline radiation.