Optimal Common-Mode RF Current and Noise Elimination for TX antenna

Eliminating Common‑Mode RF on TX Antennas

Horizontal Antenna Setup

Figure 1 shows an optimized horizontal system and cable path. Each block plays a role in suppressing common‑mode currents (CMC).

BAL (Green) — Current Balun

Use a current balun (choke type) to transition balanced antenna ↔ unbalanced coax. It blocks RF on the coax outer during TX and reduces received CMC noise during RX.

LI1, LI2, LI3 (Yellow) — Line Isolators

• LI1: At tower base or beneath the wire, bonded via the shortest, thickest strap to a good RF ground. Kills CMC induced in the near field.
• LI2: Optional on long runs; supplementary isolation mid‑line.
• LI3: At shack entry — the most critical isolator. It separates noisy safety‑ground side (radio) from the clean RF ground on the antenna side. Orient correctly: clean side toward antenna.

Note: LI1/LI2/LI3 are identical wideband coax chokes; only placement differs.

TRX (Blue) — Transceiver

The transceiver bonds to house safety ground, which is high RF impedance. Improve RF grounding by either adding a local RF ground near the radio or relying on the isolators further downline.

Vertical Antenna Setup

Figure 2 shows an optimized vertical system and cable path.

ZM (Green) — Impedance Matching

Loaded verticals often have low feedpoint Z (<25 Ω). Match with a narrowband L/C network or a wideband UNUN for acceptable VSWR.

LI1, LI2, LI3 — Line Isolators

• LI1: Immediately after the matcher, bonded with the shortest, thickest strap to RF ground. Prevents the coax shield becoming a “radial.”
• LI2: Optional on long runs.
• LI3: At shack entry — isolates safety ground from clean RF ground. Orientation: clean side toward antenna.

TRX (Blue) — Transceiver

As with horizontals, safety ground ≠ RF ground. Create a local RF ground or rely on the isolation chain.

General Considerations

RF vs Safety Ground

• Safety ground: Low resistance at mains frequency for shock protection.
• RF ground: Low impedance at RF — multiple rods bonded with short, thick conductors to antenna or line hardware.

Other CMC Paths

CMC can ride rotor/control cables that span both the antenna near field and the shack. Fit wideband chokes on these lines too.

Summary

• Place a current balun at the antenna feedpoint (horizontal systems).
• Use LI1 at base/matcher, LI3 at shack entry; add LI2 mid‑run if needed.
• Create/maintain a clean RF ground distinct from house safety ground.
• Don’t forget rotor/control cable chokes.

With strategic placement of baluns, isolators, and proper matching, you can eliminate common‑mode RF, reduce RFI, and improve overall station performance.

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