Optimal Placement of Common-Mode Chokes for Various Antenna Types

A common-mode choke (also called a current balun, Mantelwellensperre in German, mantelstroomfilter in Dutch) is an essential part of modern HF antenna systems. Its correct placement reduces noise, RFI, and radiation pattern distortion. This guide explains where and why to install chokes for different antenna types.

Understanding Common-Mode vs. Imbalance Currents

Common-mode currents are often misunderstood. They are not simply “imbalanced return currents.” True CM currents occur when the entire coax shield becomes part of the radiating system due to coupling, poor feed transitions, or environmental asymmetry.

Separately, imbalance-driven shield currents happen when the differential-mode return (normally confined to the shield’s inner wall) leaks onto the outer wall due to skin effect and imperfect balance. Though technically differential, these outer-surface returns behave like CM by radiating and distorting patterns.

A good choke addresses both:

• Blocks true CM currents (induced or coupled noise).
• Suppresses imbalance-driven outer-surface currents from asymmetry or lack of counterpoise.

Where Should You Place a Choke?

Choke performance depends on placement. CM currents form standing waves along the shield. A choke at a current maximum achieves maximum suppression. Current maxima typically appear 0.05–0.25λ from the antenna feedpoint.

Additionally, always place a choke at the shack entrance to keep noise from entering equipment and to stop RF returning inside.

Practical 0.05λ Reference Distances

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

This simple rule of thumb avoids complex modeling. Place chokes 0.05–0.25λ from the feedpoint for high suppression efficiency.

Choke Placement by Antenna Type

End-Fed Half-Wave (EFHW)

• Choke at 0.05–0.15λ from feedpoint (avoid feedpoint directly due to high voltage).
• Additional choke at shack.

EFHW Inverted-L

• Needs shield or counterpoise as RF return.
• Do not choke at feedpoint—would block the return.
• Place choke only at shack.

End-Fed Off-Center (EFOC)

• No counterpoise: choke at 0.05–0.15λ.
• With counterpoise: choke may be at feedpoint.
• Always add a shack choke.

End-Fed Long Wire (EFLW)

• Usually fed via 9:1 unun; shield acts as return.
• Add counterpoise at unun or rely on shield.
• Choke at 0.05–0.15λ plus one at shack.

End-Fed Half-Square (EFHS)

• High-impedance feedpoint.
• Choke at 0.05–0.15λ plus shack choke.
• Avoid feedpoint choke.

Inverted-L

• Choke at feedpoint.
• Optional extra choke at 0.1–0.2λ.
• Always add shack choke.

Center-Fed Dipole (coax-fed)

• 1:1 current balun at feedpoint.
• Optional extra choke at 0.15–0.25λ.

Off-Center-Fed Dipole (OCFD / Windom)

• Feedpoint choke is essential.
• Optional extra choke at 0.1–0.2λ plus shack choke.

Vertical with Elevated Radials

• Choke at feedpoint.
• Optional extra choke at 0.1–0.25λ.

Ground-Mounted Vertical

• Choke at base.
• Optional shack choke.

Loop Antennas (Delta, Quad, Magnetic)

• Choke at feedpoint, especially for asymmetrically fed loops.

Summary Table

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

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