The Challenges of Multiband End-Fed Half-Wave (EFHW) Antennas and Ferrite Losses

Multiband End-Fed Half-Wave (EFHW) antennas are a popular choice among amateur radio operators due to their simplicity and ability to cover multiple bands with a single wire. However, there are inherent efficiency challenges when using a single ferrite core-based transformer to match impedance across a broad frequency range, particularly from 80 meters (3.5 MHz) to 10 meters (28 MHz). This article explores the reasons behind these efficiency losses and why it is unrealistic to expect excellent performance on both lower and higher bands with a single-core transformer.

The Role of the Ferrite Core in EFHW Transformers

An EFHW antenna requires an impedance transformer, typically a 49:1 or 64:1 unun, to match the high impedance of the antenna (typically around 2500–5000Ω) to the 50Ω feedline. This transformer is usually built using ferrite toroidal cores, which operate by transferring energy through magnetic coupling.

The choice of ferrite material significantly impacts performance. Different ferrite compositions have optimal frequency ranges due to their permeability and loss characteristics. No single ferrite material offers high efficiency over the entire HF spectrum without trade-offs.

Frequency-Dependent Losses in Ferrite Cores

Ferrite materials exhibit frequency-dependent characteristics, leading to efficiency losses when attempting to cover a wide range of bands with a single core. These losses manifest due to the following factors:

Low-Frequency Losses (80M and 40M bands)

  • At lower frequencies, ferrite cores experience higher core losses due to increased hysteresis and eddy currents.
  • The reactance of the transformer windings is lower at these frequencies, causing increased magnetizing currents that contribute to heat generation and reduced efficiency.
  • Permeability variations with temperature can also degrade performance over time.

High-Frequency Losses (20M to 10M bands)

  • At higher frequencies, ferrite cores suffer from skin effect and increased resistive losses.
  • The impedance transformation may become less effective due to stray capacitance and leakage inductance, leading to poor matching and energy dissipation.
  • Saturation effects become more pronounced at higher power levels, reducing efficiency further.

High Power Factor and I²R Losses

Another critical factor affecting multiband EFHW transformers is power dissipation due to resistive losses and high power operation:

High Power Factor and Core Saturation

  • At high power levels, ferrite cores are more prone to saturation, particularly at lower frequencies where the magnetizing currents are higher.
  • Saturation leads to increased hysteresis losses, which manifest as heat, further reducing efficiency.
  • The core material’s ability to handle high power without excessive heating is a key design consideration.

I²R Losses in Windings

  • The windings of the transformer contribute additional losses due to resistance (I²R losses), which increase with higher currents.
  • At lower frequencies, the required turns ratio results in longer wire lengths, increasing resistance and subsequent heat dissipation.
  • At higher frequencies, skin effect causes the current to concentrate on the outer surface of the wire, reducing the effective conductive cross-section and increasing resistance.
  • Using high-quality Litz wire or silver-plated copper conductors can help mitigate these losses.

Trade-Offs in Core Selection

To illustrate why a single-core solution is inefficient, consider common ferrite materials used for EFHW transformers:

  • Fair-Rite Mix 43: Works well at lower frequencies (3–10 MHz) but suffers significant losses at higher bands like 15M and 10M.
  • Fair-Rite Mix 52: More suitable for mid-to-high frequencies (7–30 MHz) but does not perform efficiently at 80M.
  • Fair-Rite Mix 61: Optimized for high-frequency operation (10–30 MHz) but with poor low-frequency performance.

Since no single ferrite mix can efficiently cover the entire HF spectrum, multiband operation with a single-core transformer leads to compromises. The reality is that a transformer optimized for 80M will not provide the same efficiency at 10M, and vice versa.

Practical Solutions to Improve Efficiency

Using Multiple Cores with Different Mixes

  • Implementing a dual-core or stacked-core approach with different materials optimized for distinct frequency ranges can improve efficiency.
  • Example: Using a combination of Mix 43 and Mix 61 cores allows better performance across a wider spectrum.

Segmenting EFHW Antennas for Specific Bands

  • Instead of attempting full HF coverage, designing separate EFHW antennas or using switchable transformers for low and high bands can yield better efficiency.

Optimizing Winding Techniques

  • Adjusting the number of turns, winding configuration, and spacing can reduce losses caused by stray capacitance and leakage inductance.
  • Bifilar and trifilar winding techniques can help mitigate losses at higher frequencies.
  • Using thicker wire or Litz wire reduces I²R losses, especially at high power levels.

Conclusion

While EFHW antennas are convenient for multiband operation, expecting a single-core transformer to perform efficiently across a broad frequency range from 80M to 10M is unrealistic. The frequency-dependent behavior of ferrite materials introduces losses that cannot be ignored. Additionally, high power operation exacerbates core saturation and I²R losses, reducing efficiency further. For optimal performance, radio amateurs should consider multiple core materials, dedicated band-segmented antennas, or alternative matching methods tailored to their specific operating needs. Understanding these limitations allows for better design choices and improved efficiency in HF antenna systems.

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.