Why RF.Guru Recommends Only Inverted L for 160/80M and 80M/40M with 68-70:1 Transformers for end fed antennas

At RF.Guru, we prioritize antenna designs that offer maximum efficiency, predictable impedance characteristics, and optimal radiation patterns for their intended bands. For 160m and 80m, as well as 80m and 40m, we recommend the Inverted L configuration with a 68-70:1 impedance transformer, while we advise against using traditional multiband EFHW (49:1) (56:1) or EFOC (End-Fed Off-Center) antennas in this arrangement. Below, we explain the technical reasons behind this recommendation.

Why RF.Guru Recommends Inverted L for 160/80M and 80M/40M

1. Impedance Considerations

  • An EFHW antenna typically presents an impedance of 3kΩ-5kΩ at the feedpoint when installed in a horizontal or sloping configuration. This matches well with a 49:1 transformer, which steps it down to 50-60Ω.
  • However, in an Inverted L configuration with the vertical segment originating from the feedpoint, ground effects and vertical current distribution reduce the impedance to around 4kΩ-5kΩ, requiring a higher-ratio transformer (68-70:1) for proper impedance matching.
  • Using a 49:1 or 56:1 on an Inverted L often leads to high SWR and reduced efficiency due to impedance mismatch.

2. Performance Benefits of an Inverted L for Low Bands

  • On bands like 160m and 80m, obtaining full horizontal length is often impractical. The Inverted L configuration provides a mix of vertical and horizontal radiation, optimizing both local and DX communication.
  • The vertical section contributes to low-angle radiation, which improves DX performance.
  • The horizontal segment ensures good Near Vertical Incidence Skywave (NVIS) propagation for regional contacts.
  • The radiation pattern of an Inverted L is more omnidirectional than that of a traditional horizontal EFHW, making it a better all-around antenna for low bands.

Why RF.Guru Does NOT Recommend Using a Standard EFHW (49:1) (56:1) as an Inverted L

1. Incorrect Feedpoint Positioning Affects Impedance and Performance

  • A standard EFHW, when installed as an Inverted L with the vertical section starting at the feedpoint, lowers the impedance unpredictably due to interaction with ground and surrounding structures.
  • The impedance at the feedpoint can drop significantly below 3kΩ, making the 49:1 transformer a poor matchand leading to high SWR and efficiency losses.

2. Radiation Pattern and Efficiency Issues

  • In traditional EFHW setups, the horizontal wire is the primary radiator, while the voltage-fed nature of the antenna makes it susceptible to common-mode currents if improperly grounded.
  • In an Inverted L where the transformer is at ground level, the antenna behaves more like a long-wire with poor matching rather than a half-wave resonator.
  • The first half-wavelength section is critical: placing a significant portion vertically at the feedpoint modifies the current distribution, shifting the radiation pattern in unintended ways and reducing efficiency on higher bands.

3. EFOC and Multiband EFHW Antennas Struggle in Inverted L Form

  • Multiband EFHW designs rely on harmonic resonance, but an Inverted L configuration disrupts current and voltage distribution, leading to less predictable resonances.
  • The EFOC (End-Fed Off-Center) antennas are particularly affected because their current peak is not optimized for a mix of vertical and horizontal segments, leading to inefficiencies.
  • The transformer loss increases on higher bands (20m, 15m, 10m) due to an impedance mismatch, making performance subpar compared to a standard horizontal or sloping EFHW.

Alternative Mounting for EFHW and EFOC: The Reversed Inverted L

While a standard Inverted L with the vertical section starting from the transformer is not ideal for EFHW (49:1) (56:1) or EFOC antennas (29 and 17M wire version), there is an alternative that can work well: the Reversed Inverted L.

How the Reversed Inverted L Works

  • The transformer and the horizontal portion are installed at the same height, ensuring the initial impedance conditions remain stable.
  • The vertical segment is at the far end, rather than at the feedpoint.
  • This preserves the expected current and voltage distribution, ensuring proper impedance matching and good radiation characteristics.
  • Works well for multiband EFHW and EFOC antennas, unlike the standard Inverted L.

This setup is a valid alternative where a full horizontal EFHW is impractical, and it maintains better performance and impedance stability than a traditional Inverted L.

Conclusion

At RF.Guru, our engineering experience and field testing confirm that:

  • For 160m/80m and 80m/40m, an Inverted L with a 68-70:1 transformer provides the best balance of impedance matching, efficiency, and radiation characteristics.
  • For standard EFHW antennas (49:1) (56:1), a horizontal or sloping installation is ideal, while a traditional Inverted L configuration is suboptimal due to impedance mismatches and radiation inefficiencies.
  • For EFHW and EFOC antennas, a Reversed Inverted L (with the transformer at the same height as the horizontal wire and the vertical drop at the far end) is a valid alternative, maintaining good impedance characteristics and radiation efficiency.

By adhering to these guidelines, RF.Guru ensures that radio operators get the best performance out of their antennas for both local and DX communications. If you need assistance selecting the best antenna for your setup, contact RF.Guru for expert recommendations tailored to your station!

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.