The Quarter-Wave Choke: A Necessary Necessity For The Monoband Antenna

Many antenna installations suffer from unpredictable noise pickup, unstable SWR behavior, or underperformance in receive sensitivity. In many of these cases, the root cause can be traced back to common-mode currents flowing on the outside of the feedline. One of the most overlooked—but incredibly effective—solutions to this problem is the quarter-wave choke.

What Is a Quarter-Wave Choke?

A quarter-wave choke is a simple transmission line stub, typically made by shorting the end of a coaxial cable and cutting it to one-quarter wavelength at the frequency of interest. When installed correctly, this stub reflects the common-mode current back toward the source as a high impedance, effectively acting as a notch filter for unwanted RF currents flowing on the outside of the feedline.

Why Is It Important?

Many hams assume that a 1:1 current balun or ferrite choke at the feedpoint is sufficient to prevent common-mode currents. While this may be true for balanced antennas (like dipoles with symmetrical feed), it often fails to fully suppress common-mode on verticals, end-fed wires, or asymmetrically fed antennas where imbalance is unavoidable.

In such cases, a single choke at the feedpoint may only offer partial suppression. A quarter-wave choke, placed further down the coax—typically at a current maximum—can complement the feedpoint choke and provide targeted suppression at specific frequencies.

What Happens If You Skip It?

Without a properly placed quarter-wave choke:

  • The coax braid becomes part of the antenna system, radiating and picking up noise
  • The radiation pattern of the antenna becomes distorted
  • Directionality and front-to-back ratios can degrade in phased arrays
  • The feedpoint impedance seen by the tuner or transceiver becomes frequency-dependent due to uncontrolled common-mode paths
  • RF in the shack becomes more likely, especially when operating QRO

How to Implement It

Measure or calculate the electrical quarter wavelength for your target frequency, taking velocity factor into account. For RG-58 or RG-213, this is typically around 0.66 to 0.70 of the physical wavelength. Trim a piece of coax, short the end, and connect the other end to the coax braid using a T-connector or soldered tap. Mount the stub near the location on the feedline where maximum common-mode current is expected (this often depends on antenna height and grounding).

Common Mistakes

  • Only using one ferrite choke at the feedpoint for a highly unbalanced antenna
  • Placing the choke too close or too far from the high-current region
  • Ignoring frequency-specific common-mode behavior
  • Using lossy cable for the choke stub (good coax matters)

Conclusion

The quarter-wave choke is not just an optional tweak—it is often a necessary component in serious HF installations, especially when dealing with verticals, end-feds, or complex phased arrays. It provides frequency-specific suppression of common-mode currents that can otherwise degrade your signal, increase noise, and make your station harder to control.

Integrating one into your system can be the difference between an average and an excellent signal. If you're chasing performance or troubleshooting unexplained RF behavior, don't overlook this simple but powerful tool.

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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.