Understanding Common Mode Buildup and the Need for Multiple Chokes

Common-mode current buildup is a widespread and often misunderstood phenomenon in RF systems. It arises when unwanted currents appear on the outside of coaxial cables or other transmission lines, flowing in the same direction on all conductors. Unlike differential signals, these currents do not contribute to signal transmission; instead, they often lead to unwanted radiation, RF interference, and degraded antenna performance.

At the root of common-mode buildup is imbalance. When a transmission line system lacks symmetry in its impedance paths, or when antennas are not perfectly balanced, the differential-mode return current can no longer cancel out completely inside the shield. This imbalance results in some of the return current flowing on the outer surface of the coax shield—effectively turning the feedline into an unintended radiator.

Induced Common Mode: A Summation of Couplings

Common-mode current is not a single event; it accumulates along the line. It can be induced by various sources:

  • Asymmetry in the antenna (e.g., end-fed, off-center-fed designs, Yagis, Hexbeams, and other asymmetrical multi-element arrays)
  • Proximity to metallic structures
  • Ground loops
  • Electric field coupling from the antenna to the feedline
  • Magnetic field coupling from nearby radiating elements

Each of these interactions contributes to the net common-mode current. The problem is cumulative: small induced voltages and currents at different points add up along the cable, particularly in the presence of standing waves on the shield.

A special note should be made for antennas that are inherently unbalanced or asymmetrical by design—such as end-fed half-wave antennas and Yagi/Hexbeam arrays. Both act as efficient generators of common-mode currents. End-feds present a high voltage and asymmetrical feedpoint that easily excites the outside of the coax. Yagis/Hexbeams, due to their asymmetric element geometry and exposed feed arrangements, often pick up and reradiate common-mode noise unless aggressively choked.

Why One Choke Is Rarely Enough

A single common-mode choke, correctly placed, can provide significant suppression of shield currents. However, in many practical installations, one choke is insufficient due to the distributed nature of the sources of common-mode induction. When you place a choke at one point, it suppresses the current at that location, but not necessarily the induced current that appears further along the line. In some cases, new common-mode currents are generated downstream of the choke, especially if the feedline runs through or near an active RF field.

The impedance presented by a choke is typically frequency-dependent and may only be effective in a narrow band if poorly designed. Also, the choke's effectiveness is limited by its placement relative to the source of imbalance or coupling. For example, a choke placed only at the feedpoint might stop some common-mode currents from entering the feedline, but if the coax continues for 30 meters through an attic or along metal pipes, additional induced currents can reappear well after the initial choke.

Multiple Chokes: Strategic Placement

To mitigate these cumulative effects, multiple chokes are recommended. Strategic placement includes:

  • At the feedpoint: To isolate the antenna from the feedline
  • At cable transitions or bends: Where coupling is likely
  • At building entry points: To prevent shield currents from coupling into indoor wiring
  • At the transceiver or amplifier: To protect equipment and suppress residual current

Each choke acts as a localized barrier, attenuating the voltage gradient and stopping the propagation of common-mode energy beyond that point. The idea is not just to block the current once, but to segment the path into multiple short sections where buildup is interrupted.

Scientific Basis: Transmission Line Theory and Kirchhoff’s Law

Kirchhoff's Current Law (KCL) dictates that all current entering a junction must leave it. In an unbalanced antenna system, some of the RF current returning via the feedline finds no symmetrical counterpart, violating the assumptions of differential operation. The coax shield becomes a return path, but with a twist: it carries common-mode current, which does not cancel due to asymmetrical conditions.

Transmission line theory tells us that differential-mode signals are confined between conductors (e.g., inner conductor and shield), but common-mode signals see the entire cable as a single conductor with respect to some undefined return reference (usually "ground"). These common-mode signals are thus highly sensitive to environmental coupling, and the only way to deal with them effectively is to insert high-impedance barriers at several points.

Breaking the Chain of Accumulation

Common-mode current is not a one-time event, but a continuous risk along the transmission path. It builds up from multiple weak couplings and is best addressed by using multiple chokes at key locations. Each choke helps restore balance and reduce unwanted radiation and noise pickup. The more complex and RF-rich the environment, the more important it is to treat the feedline as a potential antenna and defend it accordingly.

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Written by Joeri Van Dooren, ON6URE – RF, electronics and software engineer, complex platform and antenna designer. Founder of RF.Guru. An expert in active and passive antennas, high-power RF transformers, and custom RF solutions, he has also engineered telecom and broadcast hardware, including set-top boxes, transcoders, and E1/T1 switchboards. His expertise spans high-power RF, embedded systems, digital signal processing, and complex software platforms, driving innovation in both amateur and professional communications industries.