Performance of Our 81m EFHW as a Sloper for 160m and 80m

Our 81-meter end-fed half-wave (EFHW) antenna was developed as an Inverted L primarily for 160m and 80m operation. However, many operators install it as a sloper, either due to height constraints or mast configuration. While this configuration is absolutely functional, it's important to understand the behavior of the antenna when used in this manner, especially regarding SWR.

Sloper Configuration: Only Suitable for NVIS

When deployed as a sloper, the 81m long EFHW still performs well for near-vertical incidence skywave (NVIS) communication on 160m and 80m. The absence of a vertical section means there is very little low-angle radiation, making this configuration unsuitable for DX. The radiation pattern favors high-angle takeoff, ideal for short- to medium-range contacts.

This antenna has been tested as a sloper with the feedpoint and far end both at 8 meters height, and confirmed to produce an SWR of approximately 3:1 on both 160m and 80m through a 68:1 transformer.

This is not a fault of the antenna—it is a natural consequence of how an EFHW behaves when its electrical length corresponds to a half-wave (160m) or full-wave (80m). At those lengths, the impedance at the end of the wire becomes very high, often between 2.5 kΩ and 5 kΩ.

Will a Higher Sloper Help?

Raising the end of the sloper (for example, to 10, 12, or even 15 meters) will influence the impedance slightly, but it will not significantly lower the SWR. The EFHW end impedance remains high due to the voltage maximum at the wire end. Although height improves efficiency and overall field strength, it does not fundamentally change the fact that this is a high-impedance point.

Expect the SWR to remain within the 2.5:1 to 3.5:1 range, depending on height and surroundings.

Inverted L Configuration: Better Match and Radiation

When installed as an Inverted L—with the feedpoint between 1 and 3 meters above ground, and the vertical portion of the antenna at least 15 meters high—the behavior changes significantly:

  • A large part of the antenna's radiating section is now vertical, enhancing low-angle radiation for DX.
  • The current maximum is no longer confined to a horizontal high-impedance region but is now distributed across both vertical and horizontal parts.
  • The feedpoint impedance aligns more closely with the optimal range for a 68:1 transformer, typically falling in the 3.2–3.6 kΩ range.

As a result, the SWR with a 68:1 transformer drops to around 1.5:1 to 2:1 on both 160m and 80m. This configuration provides a better match and significantly improved performance, especially on 160m, where the vertical section plays a major role in efficient radiation.

Why a 68:1 Transformer Still Can't Perfectly Match in Sloper Configuration

The 68:1 transformer is an improvement over the common 49:1, as it attempts to better match the higher feedpoint impedance on 160m and 80m. However, in the sloper configuration, even this ratio is not ideal for full-wave operation on 80m, where the impedance can rise above 5 kΩ.

At those values:

  • Efficiency is still affected by mismatch loss
  • SWR remains above 2.5:1 in most cases
  • The transformer's performance depends heavily on load variation and core properties

This is a limitation of end-fed designs in sloper form on low bands: the impedance swings are wide, and no single fixed-ratio transformer can provide a perfect match across multiple resonant bands in this layout. That’s why this antenna is specifically intended to be installed as an Inverted L—not as a sloper.

Conclusion: NVIS-Optimized as Sloper, DX-Ready as Inverted L

An 3:1 SWR is normal and acceptable for this sloper configuration on 160m and 80m. Most tuners can handle this easily, and the antenna still radiates well for its intended NVIS purpose.

Even on long feedlines, especially with high-quality coax like FlexBury 7mm, losses remain marginal at 3:1 SWR on these low bands. For example, with 25 meters of FlexBury 7mm, total loss stays well under 2 dB, confirming the practical viability of the sloper layout, even if not ideal from a matching perspective.

If you're targeting DX, install the antenna as an Inverted L with a substantial vertical section. In this layout, SWR improves to around 1.5:1 to 2:1, and the radiation pattern shifts toward low-angle takeoff for long-distance DX performance. For regional work and reliable low-band coverage, the 81m EFHW—whether sloped or L-shaped—remains a robust, efficient solution.

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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.