Sloping a 160/80 m Inverted-L EFHW: What Really Changes?

I have seen a slope improve the match of a 160/80 m end-fed installation, and it remains a practical adjustment worth testing. The useful lesson is not “drop the wire and the impedance falls.” It is that changing the three-dimensional wire path changes the antenna, including its coupling, current distribution, return current and pattern.

A Long Wire Has More Than One Mode

A wire around 80 m long is near a half wavelength in the 160 m band and near a full wavelength in the 80 m band as a first free-space approximation. That is a useful way to anticipate different current modes, but it does not assign a fixed end impedance.

Insulation, conductor diameter, height, bends, slope, transformer capacitance, the return branch, feedline exterior, soil and nearby conductors all alter the installed electrical length. A half-wave-like mode tends to place a current minimum and a voltage maximum near an open end. A full-wave-like mode adds another current maximum and minimum along the wire. Real minima are not perfect zeros, and their positions move with the installation.

That makes a dual-band end-fed system especially sensitive to geometry. The same physical change can move the operating point toward a resistance that suits the matching network on one band while adding reactance or moving the operating point away from a useful load on the other.

What Changing the Slope Actually Changes

Lowering the far end changes more than height. It changes the coordinates and orientation of a conductor carrying position-dependent current and voltage. Depending on the site, that can change:

• capacitance between the wire, earth, structures and vegetation;
• mutual coupling between the vertical, horizontal and sloping sections;
• the installed electrical length and the frequencies of impedance extrema;
• current magnitude and phase along the radiator;
• current on the intended return branch and coax exterior;
• transformer terminal resistance and reactance; and
• the azimuth, elevation and polarisation of the radiated field.

A slope can therefore lower the transformed SWR at one measurement plane. It can also raise it, move the best point in frequency or trade a better 80 m match for a worse 160 m result. The outcome cannot be predicted from slope direction alone.

It is also misleading to say that a slope simply “reduces the voltage maximum.” An open end still tends to be a high-voltage region. Geometry changes the amplitude and position of the standing wave as part of the complete boundary-value problem; it does not provide a general mechanism that safely erases the end voltage.

End Impedance Is Not a Fixed Table

End-fed resistance can be high near a half-wave or full-wave mode, but a quoted kilohm value without frequency, reactance, geometry, ground, return path and reference plane is not a design result. The load is complex:

Zload = R + jX

Both terms matter. Near an impedance extremum, a small change in wire length, height or coupling can move resistance and reactance substantially. Loss can also make a match look broader or less extreme while reducing radiation efficiency.

A 68:1 Ratio Is Not a Sweet Spot

An ideal 68:1 impedance transformer refers the complete complex load by a factor of 68. It does not turn an arbitrary end-fed impedance into 50 ohms, and it does not cancel reactance. The real transformer adds magnetising impedance, leakage inductance, winding capacitance, conductor and core loss, and frequency- and load-dependent voltage and current stress.

If the antenna-side load were purely resistive, dividing resistance by 68 would be a useful first calculation. A real dual-band antenna rarely supplies that simplified condition at both selected frequencies. The correct test records the antenna-side complex impedance and, where possible, transformer insertion loss and temperature under representative loads and duty cycle.

A pleasant SWR at the radio is therefore only a match result at that plane. It is not proof that the transformer is comfortable, that the wire is efficient or that accepted power is being radiated in the useful directions.

The Return Path Changes with the Radiator

An end-fed antenna is not a one-terminal circuit. Current leaving one transformer terminal must return to the other through a deliberate counterpoise, a defined section of coax exterior, capacitive coupling, soil, station wiring or some combination. That return geometry is part of the antenna.

Changing the wire slope changes coupling to those conductors and can redistribute common-mode current. A lower end may couple more strongly to soil or nearby objects, but stronger coupling is not automatically beneficial: it may add displacement current, loss, detuning or an unintended radiating path.

A common-mode choke places impedance in the coax-exterior branch. Its useful position follows the installed current path and the intended return boundary—not a universal fraction of a wavelength from the transformer. If the coax section between transformer and choke is intentional counterpoise, document it. Then measure exterior current before and beyond the choke on both bands.

Measure at the Right Reference Plane

A VNA displays impedance and SWR at its calibrated reference plane. A length of feedline between that plane and the transformer transforms the complex impedance, while real cable attenuation reduces the magnitude of the returned wave. The shack-end trace is useful for the transmitter, but it is not automatically the transformer-terminal impedance.

For a geometry experiment, calibrate at the plane you intend to compare or characterise the intervening cable and move the reference plane mathematically. Keep adapters and connectors unchanged. Save resistance, reactance and reflection data across both bands; a single SWR number hides too much.

A Better Slope Experiment

1. Define the purpose. Choose the wanted frequencies, bandwidth, power, duty cycle and paths before touching the wire.
2. Document the baseline. Record the complete radiator, transformer, return conductor, feedline, choke, ground and nearby structures.
3. Make one geometry change. Lower or raise the end by a measured amount while keeping wire length and every other conductor fixed.
4. Sweep both bands. Save complex impedance at the same calibrated plane and the same frequency points.
5. Map common-mode current. Check the coax exterior and accessible bonded conductors at low safe power before and after the change.
6. Check loss and stress. Compare transformer temperature and, where practical, insertion loss under representative complex loads.
7. Model the recorded geometry. Include lossy ground, the return branch and any conductor that carries material current; test segmentation and ground-model sensitivity.
8. Compare field behaviour. If performance matters, use controlled A/B/B/A observations at equal accepted power, several directions and contemporaneous propagation conditions.

Repeatable impedance movement after a controlled slope change is useful evidence. It supports the statement that geometry changed the installed load. It does not by itself identify capacitance, current redistribution or ground loss as the dominant mechanism; those require current, loss or field evidence.

The Pattern and NVIS Question

The vertical, horizontal and sloping sections contribute fields with different orientation, magnitude and phase. Moving one section changes the vector sum and its ground reflection. The pattern can therefore change even when the SWR movement is small.

A low horizontal component often contributes substantial high-angle radiation on 160 or 80 m, but “high angle” is not synonymous with a completed NVIS circuit. Ionospheric critical frequency, absorption, season, time, location, noise and required link margin determine whether that energy returns usefully.

Likewise, preserving a familiar SWR curve does not prove that low-angle gain, azimuth coverage or efficiency stayed constant. Use a full three-dimensional model and controlled field comparisons if the slope is being chosen for communications performance rather than match alone.

Primary and Authoritative Sources

• NIST, A Two-Port Model for Antennas in an Arbitrary Environment—a measured network framework for separating antenna loss, efficiency and environmental influence.
• Roy W. Lewallen, W7EL, Baluns: What They Do and How They Do It—original analysis and measurements of imbalance current and feedline participation.
• Keysight, Specifying Calibration Standards and Kits for Vector Network Analysers—calibration and the establishment of a fixed measurement reference plane.
• Lawrence Livermore National Laboratory, Antenna Modelling with the Numerical Electromagnetics Code—wire-current, ground and radiation-pattern modelling with verification limits.
• ITU-R P.341-7, The Concept of Transmission Loss for Radio Links—definitions that separate antenna, feeder, mismatch, polarisation and propagation effects.
• ITU-R P.533-14, Method for Predicting HF-Circuit Performance—the in-force framework for HF frequency availability, field strength, SNR and reliability.

Joeri’s Bottom Line

Sloping a 160/80 m Inverted-L EFHW is a valid practical experiment. It may move the 80 m load into a friendlier region and may also help or hurt the 160 m operating point. I would absolutely try it when the geometry permits—but I would measure what moved instead of assigning the result to a universal impedance drop.

The useful question is wider than “did the SWR fall?” Record the complex load at the correct plane, trace the return current, check transformer stress and verify that the installed pattern still serves the intended paths. A better match is welcome; a better understood antenna is the real result.

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