Yagi Antennas: Common Mode Listening Machines

Yagi-Uda antennas, known for their directional gain—typically 6 to 13 dBi depending on the number of elements—and narrow beamwidth, often between 30° and 60° in the E-plane, are among the most widely used antenna types for HF and VHF operation. However, one major drawback often overlooked in real-world installations is their tendency to act as highly effective common mode pickup systems. Especially when installed without careful attention to feedline decoupling and grounding, a Yagi can become a powerful receiver for unwanted RF noise and interference.

Why Yagis Are So Susceptible

Yagis are inherently unbalanced antennas, typically fed with unbalanced coaxial cable. In transmission line theory, an 'unbalanced' antenna presents a different impedance to ground on each terminal of the feedpoint, causing unequal current paths. When fed with coax (which itself is unbalanced), this mismatch can lead to current flowing on the outside of the coax shield, forming common mode currents. A 'balanced' antenna, by contrast, has equal and opposite currents with respect to ground, ideally preventing such common mode effects. Unless a true current-mode balun or choke is used at the feedpoint, any imbalance in the system results in (return) current flowing back along the outside of the coax shield. This common mode current transforms the feedline itself into part of the antenna system.

What makes Yagis particularly susceptible is:

  • Element geometry and coupling: The driven element is surrounded by parasitic elements that tightly couple RF energy in specific directions. This also enhances pickup from the environment.
  • Asymmetric installation: Even small asymmetries, like the feedline hanging down one side, introduce imbalance.
  • Metallic surroundings: Rotators, masts, and booms all create capacitive paths to ground, encouraging CM current.
  • Long coax runs: The longer the unchoked feedline, the more effective it becomes as a receiving antenna on its own.

The result is a Yagi that not only receives signal via its elements, but also picks up noise and unwanted signals via the feedline. At lower frequencies (below 50 MHz), where the physical cable length can be a significant fraction of a wavelength, this effect becomes particularly severe.

Common Mode vs Differential Mode

To understand why this is problematic, it’s important to distinguish between differential mode (the intended signal transmission between inner and outer conductor) and common mode (unwanted signal appearing on the entire outer surface of the coax shield).

Common mode currents do not cancel out and can radiate or receive RF, as they propagate along the entire outer surface of conductors with respect to ground. Unlike differential currents that are confined within the coaxial structure, common mode currents couple directly to the environment, allowing the coax to behave as a longwire antenna. This coupling mechanism enables unwanted signal pickup and reradiation, even from parts of the system that were not intended to radiate, causing:

  • Unpredictable pattern distortion
  • Increased receive noise floor
  • False lobes or reduced F/B ratio

Frequencies Where the Problem Is Worst

  • HF (3–50 MHz): Common mode is most problematic here because coax lengths are typically between 0.2 and 0.8 wavelengths long. Without a proper choke, the cable acts like a long vertical or slanted radiator.
  • 6 meters (50 MHz): Still very sensitive. Many installations ignore CM control at 6m, but noise pickup and beam pattern degradation are real concerns.
  • VHF (144 MHz and above): Less sensitive to common mode because the physical cable length becomes too short (in wavelengths) to act as an efficient radiator. However, local noise and coupling can still occur, especially in stacked arrays or with long feedlines.

Ineffective Mitigations

  • Using double-shielded coax (e.g. RG400): While this reduces RF leakage, it also makes ferrite chokes ineffective because the common mode current may flow beneath the outer shield and become magnetically invisible to ferrite beads. Use for feedline purposes only!
  • Relying on grounding alone: A ground at the station end will not eliminate common mode current that originates at the antenna feedpoint.
  • Voltage baluns: These may transform impedance, but they do not block common mode currents effectively.

Proper Mitigation Techniques

1. Current Choke at the Feedpoint

  • Use a high-CMRR current balun or choke directly at the Yagi feedpoint.
  • Ferrite beads (mix 31 or 43) over coax with single braid shielding to allow proper coupling.
  • Winding the coax through toroids can also work well if space allows.

2. Feedline Routing

  • Route coax at 90 degrees away from the boom or elements for at least 1/4 wavelength before bending.
  • Avoid draping coax along the boom or under elements.

3. Additional Chokes Down the Line

  • For long feedlines, consider additional chokes every 1/2 wavelength to suppress standing common mode currents.

4. Use True Balanced Feed Systems

  • In advanced setups, use open wire or transformer-isolated systems to ensure differential-only signal flow.

Conclusion

Yagis are powerful antennas, but their performance can be seriously degraded by common mode currents if not properly addressed. Below 50 MHz, common mode is a real and measurable issue. For example, measurements comparing a Yagi fed with and without a proper current choke at 28 MHz often show CMRR improvements of 20–30 dB. CMRR plots across the HF and low VHF bands consistently show peak improvements when choke impedance exceeds 1 kΩ at the target frequency. that requires deliberate design choices: proper coax selection, effective chokes, and attention to physical layout.

Ignoring these aspects turns a high-gain beam into a high-gain noise receiver.

A well-constructed choke is not optional for serious HF and 6m Yagi systems. It is foundational to achieving the clean pattern and low noise floor that a directional antenna promises.

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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.