Why EFHW Antennas Are Inherently Unstable – Especially on Low Bands

End-Fed Half-Wave (EFHW) antennas have gained popularity due to their simplicity and dual band capability. But one critical issue is often overlooked or misunderstood: the EFHW's high feedpoint impedance makes it inherently unstable, particularly on the lower HF bands (160m–80m).

The Problem with High Impedance Feedpoints

The typical EFHW antenna presents a feedpoint impedance in the range of 2.5k–5k ohms. Unlike low-impedance antennas (such as center-fed dipoles), these high-Z points are extremely sensitive to environmental factors:

  • Humidity
  • Soil conductivity
  • Nearby objects (fences, trees, wet structures)
  • Rain or morning dew

These conditions alter the capacitive and dielectric environment around the antenna. As a result, an EFHW that reads 3.3 kΩ in the morning may drop to 2.7 kΩ in the evening, or rise to 2.3 kΩ during rain. This impedance shift directly affects SWR, common-mode behavior, and matching performance.

Why a Choke Alone Is Not Enough

Many operators try to solve EFHW instability by adding a common-mode choke. While a choke is essential to prevent coax shield radiation, it is not sufficient on its own—especially on lower frequencies like 160m and 80m.

At these bands, the wavelength is long, and the voltage maxima at the feedpoint means that even small capacitive imbalances or poor current sinks result in large unwanted voltages on the shield. A choke can't "absorb" these voltages if there is no viable return path for the imbalance.

This is less of a problem on higher bands (e.g., 20m–10m) because:

  • The overall antenna structure is electrically longer relative to the environment, so stray capacitances and proximity effects are less significant
  • The return current path through the coax is shorter in terms of wavelengths, reducing the risk of common-mode resonance
  • Matching circuits (e.g., 49:1 or 64:1 transformers) will operate in a more controlled impedance range with lower Q, resulting in less sensitivity to environmental variation

As frequency increases, the effective reactance of parasitic elements (like buildings, trees, and moisture) decreases, making high-Z imbalances easier to manage.

Our Recommended Solution: Current Sink + Choke

For stable performance, especially from 160m to 80m, we recommend the following:

  1. Use a counterpoise wire:

    • Any length is better than none
    • Laid in a straight line from the transformer toward the ground
    • Terminate in a poor conductor to earth such as an RVS316 stainless steel ground rod

  1. Add a 1:1 current choke at 0.05 lambda from the feedpoint

    • Based on the findings of DC4KU (Werner Schnorrenberg)
    • This placement minimizes common-mode current on the coax shield

Reference: https://shop.rf.guru/pages/coax-length-before-the-choke-why-it-matters-for-efhw-antennas

Why the RVS316 Rod?

Unlike low-impedance copper grounding rods (which act as short circuits at RF), an RVS316 stainless steel rod offers:

  • High RF impedance to act as a current sink
  • Low galvanic activity (maintenance-free)
  • Enough leakage to bleed imbalance without attracting differential current

This creates a safe high-impedance sink for unwanted RF currents while preserving the intended feedpoint behavior.

Summary

EFHW antennas are voltage-fed, high-impedance systems. This makes them:

  • Environmentally sensitive
  • Unstable under changing weather
  • Prone to common-mode issues if improperly installed

A choke alone is not enough. For low-band EFHW antennas, combine:

  • A proper high-Z current sink (RVS316 rod)
  • A well-placed choke (0.05 lambda)

This combination ensures stable, quiet operation from 160m to 80m—and restores the EFHW’s potential as a high-performance dual band antenna.

 

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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.