Optimal Placement of Common-Mode Chokes for Various Antenna Types

A common-mode choke (also known as a current balun, or Mantelwellensperre in German, mantelstroomfilter in Dutch) is an essential component in modern HF antenna systems. Its correct placement can greatly reduce noise, RF interference, and pattern distortion. This guide explains where and why to place these chokes effectively for various antenna types.

Understanding Common-Mode and Imbalance Currents

Common-mode currents are frequently misunderstood. They are not caused by return current imbalance. Instead, they result from the entire coaxial cable—particularly the outer surface of the shield—becoming part of the radiating system. This can occur due to capacitive or inductive coupling, poor feedpoint transitions, or environmental asymmetries.

In contrast, differential-mode currents flow in equal and opposite directions on the coax inner conductor and the inner surface of the shield. These are the desired signal-carrying currents. However, when an antenna system is asymmetrical or lacks a proper return path, a portion of the differential current may spill onto the outer shield due to skin effect. These imbalance-driven surface currents, while technically differential, behave similarly to common-mode currents and can cause radiation, pattern distortion, and RF feedback.

A good current choke addresses both issues:

  • It blocks true common-mode currents (induced noise or external coupling).
  • It impedes imbalance-driven outer shield currents that arise from improper symmetry or grounding.

Where Should You Place a Choke?

The effectiveness of a choke depends heavily on placement. Common-mode currents form standing wave patterns along the outer surface of the coax. Installing a choke at a current maximum ensures maximum suppression. These peaks typically occur in the range of 0.05 to 0.25 wavelengths (λ) from the antenna feedpoint.

Additionally, placing a choke at the shack entrance is always good practice—it prevents common-mode noise from entering the antenna system or returning into your equipment.

Practical Tip: Use 0.05–0.25λ Spacing for Choke Placement

Here are some practical 0.05λ reference distances:

Band 0.05λ Distance
80m ~4 meters
40m ~2 meters
20m ~1 meter
10m ~0.5 meter

This rule of thumb allows for effective choke placement without complex modeling. The spacing range of 0.05–0.25λ reflects where current maxima occur along the feedline. Installing a choke in this range ensures high suppression efficiency.

Choke Placement by Antenna Type

End-Fed Half-Wave (EFHW)

  • Choke at 0.05λ to 0.15λ from the feedpoint (not directly at feedpoint due to high voltage).
  • Additional choke at the shack is recommended.

EFHW Inverted-L

  • Requires a solid RF return path to ground, typically via coax shield.
  • Do not place choke at feedpoint—this would block the return path.
  • Place choke at the shack only.

End-Fed Off-Center (EFOC)

  • Similar to EFHW, but off-center-fed for multiband operation.
  • With no counterpoise: choke at 0.05–0.15λ.
  • With counterpoise: choke can be placed at feedpoint.

  • Choke at the shack is always recommended.

End-Fed Long Wire (EFLW)

  • Often fed via 9:1 unun; shield acts as return path.
  • Add a counterpoise at the unun or rely on the coax shield.
  • Place choke at 0.05–0.15λ, and another at the shack.

End-Fed Half-Square (EFHS)

  • High-impedance feedpoint like EFHW.
  • Choke at 0.05–0.15λ is recommended.
  • Choke at shack also useful.
  • Avoid placing choke at feedpoint.

Inverted-L

  • Choke at the feedpoint.
  • Optional second choke at 0.1–0.2λ.
  • Shack choke always advisable.

Center-Fed Dipole (with coax)

  • 1:1 current balun or choke at the feedpoint.
  • Optional second choke at 0.15–0.25λ.

Off-Center-Fed Dipole (OCFD / Windom)

  • Choke at the feedpoint is essential.
  • Additional choke at 0.1–0.2λ is beneficial.

Vertical with Elevated Radials

  • Choke at the feedpoint.
  • Optional second choke at 0.1–0.25λ.

Ground-Mounted Vertical

  • Choke at the base.
  • Optional second choke at the shack.

Loop Antennas (Delta, Quad, Magnetic)

  • Choke at the feedpoint, especially for delta loops with asymmetric feed.

Summary Table

Antenna Type Recommended Choke Positions
EFHW 0.05–0.15λ + shack
EFHW Inverted-L Shack only (preserve return path)
EFOC 0.05–0.15λ or feedpoint + shack
EFLW 0.05–0.15λ + shack
EFHS 0.05–0.15λ + shack
Inverted-L Feedpoint + optional 0.1–0.2λ + shack
Center-fed Dipole Feedpoint + optional 0.15–0.25λ
OCFD / Windom Feedpoint + 0.1–0.2λ + shack
Vertical (elevated) Feedpoint + optional 0.1–0.25λ
Vertical (ground-mount) Feedpoint + shack
Loops (Delta, etc.) Feedpoint

 

Let us know if you need help determining exact choke placement for your setup. RF.Guru offers high-impedance broadband chokes for all typical scenarios.

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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.