Why We Primarily Use Full-Wave and Half-Wave End-Fed Antennas

The Problem with Non-Resonant Sections in End-Fed Antennas

End-fed antennas are a popular choice among amateur radio operators due to their simplicity, efficiency, and versatility. However, the most effective configurations tend to be either full-wave or half-wave, with other lengths often resulting in inefficiencies. One common issue arises when trying to use an End-Fed Half-Wave (EFHW) antenna in a multi-band configuration where a section of the wire is approximately a quarter wavelength long at some operating frequencies. This is where efficiency problems become apparent.

Understanding the Quarter-Wave Issue in EFHW Antennas

A standard EFHW antenna is typically designed to be a resonant half-wave dipole, just fed from the end rather than the center. The main reason we avoid non-half-wave lengths, especially quarter-wave segments, is due to the impedance and radiation efficiency issues that arise.

For example, a 41-meter-long EFHW is sometimes promoted as a multi-band solution covering 160m, 80m, and 40m. However, this configuration presents a significant problem:

  • At 160 meters (1.8 MHz): The wire is only a quarter-wave long, which results in a very low radiation resistance at the feedpoint, making it inefficient.
  • At 80 meters (3.5 MHz): The wire is close to a half-wavelength, making it efficient with a reasonable impedance.
  • At 40 meters (7 MHz): The wire is close to a full wavelength, again leading to good efficiency.

The main issue occurs on 160 meters, where the 41-meter length is a quarter-wave. Unlike a true EFHW, which has a high impedance at the end and can be matched efficiently with a high-ratio transformer, a quarter-wave end-fed antenna has a very low impedance at the feedpoint. This presents several problems:

  1. Extremely Low Radiation Resistance: The radiation resistance of a quarter-wave end-fed wire is typically below 10 ohms, often closer to 2–5 ohms. Since efficiency is determined by the ratio of radiation resistance to total resistance (including ground losses), a low radiation resistance means that ground losses can consume most of the power, reducing overall efficiency.
  2. High Current Near the Ground: Unlike a half-wave EFHW, which has a voltage peak at the end and a current peak near the middle of the wire, a quarter-wave wire has a current peak near the feedpoint. If this feedpoint is close to the ground, significant energy is lost in ground resistance, further degrading efficiency.
  3. Poor Matching to a High-Ratio Transformer: EFHW antennas are typically matched using a 49:1, 64:1, 68:1, or 70:1 transformer, which assumes a feedpoint impedance of 2,000 to 5,000 ohms. However, a quarter-wave end-fed wire has a much lower impedance (often less than 10 ohms), making it a poor match for these transformers. This mismatch results in significant losses.

Why We Prefer Half-Wave and Full-Wave End-Fed Antennas

Half-wave and full-wave antennas naturally have high feedpoint impedances, which work well with common matching networks such as 49:1, 64:1, 68:1, and 70:1 impedance transformers. The benefits of these resonant lengths include:

  • Efficient Radiation: The current distribution ensures minimal ground losses, as the highest currents are well above the ground level.
  • High Feedpoint Impedance: This results in better matching to high-ratio impedance transformers, reducing mismatch losses.
  • Stable Pattern Across Bands: Unlike non-resonant lengths, half-wave and full-wave antennas maintain predictable radiation patterns across multiple bands.

Conclusion

While it may be tempting to use an arbitrary wire length for multi-band operation, the efficiency and performance of an end-fed antenna are highly dependent on its length relative to the wavelength of operation. Quarter-wave sections suffer from low impedance, high ground losses, and poor matching characteristics, making them inefficient. This is why experienced operators prefer full-wave or half-wave end-fed antennas, as they provide the best balance of efficiency, impedance matching, and predictable radiation characteristics across multiple bands.

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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.