Our 81 m EFHW: The Low-Height Trial and Inverted-L Choice

An antenna has to fit the station, not just the drawing. Operators sometimes hang our low-band wire as a sloper or a low span because that is what their masts and trees allow. I do not dismiss that choice. I do distinguish it from the inverted-L we intended—and I do not let one SWR reading make the whole decision.

The Trial That Prompted the Question

In our field trial, the 81 m wire had its feedpoint and far end both at approximately 8 m above ground. With the trial's 68:1 impedance transformer, we recorded approximately 3:1 SWR on both 160 and 80 m. That is an observation from this installation, not an SWR specification for every 81 m wire or a recommended mounting height.

Those two endpoint heights do not describe the complete wire route, its sag or any intermediate supports. In particular, equal-height endpoints do not establish a simple sloping span. The familiar “sloper” label is therefore not enough to reconstruct this trial's geometry. What we can discuss is the reported low-height deployment and why it differs from the intended inverted-L.

The record does not include a calibrated measurement reference plane, exact frequencies or a complete impedance sweep. I cannot turn that pair of approximate SWR readings into a measured feedpoint resistance, an efficiency figure or a radiation plot. It does, however, make the installation question real: changing how we hang the wire changes the load and the current geometry, even though the wire and transformer have the same names.

What the 3:1 Reading Actually Says

SWR describes mismatch relative to the reference impedance at the measurement plane. It does not tell us whether the resistance is too high, too low or accompanied by reactance. For example, in an ideal 50 Ω system, a purely resistive 150 Ω load and a purely resistive 16.7 Ω load both give approximately 3:1 SWR. Many complex loads give the same reading.

For an ideal 68:1 impedance transformation, 50 Ω corresponds to 3,400 Ω on the high-impedance side. That is a circuit relationship, not a measurement of this antenna. Real transformer loss and parasitics, the feedline and the antenna's return path all stand between a simple ratio calculation and a trustworthy terminal-impedance result.

At a plane showing exactly 3:1 SWR, the reflected-wave voltage magnitude is half the forward-wave magnitude, and reflected power is one quarter of forward power. That does not mean one quarter of the transmitter's output has been turned into heat. Reflection, actual component loss, transmitter foldback and the final radiated power are different parts of the power budget.

The Same Wire Supports Different Modes

Near 160 m the conductor is in its fundamental half-wave family; near 80 m it approaches a full-wave harmonic mode. The upper-band current has additional maxima and nulls, so its pattern cannot be inferred from the 160 m result.

An end-fed half-wave has low current and high voltage near its feed end; the substantial current maximum is farther along the wire. On the full-wave mode there is more than one current-rich region. Bending the wire does not automatically move a current maximum to the bend or turn the vertical leg into a conventional quarter-wave vertical with maximum current at its base.

This matters in the trial: both bands can show the same SWR while sending their strongest radiation in different directions. A largely horizontal low-band installation often favours higher elevation angles, but “low” does not mean “NVIS only.” On 80 m especially, the full-wave current phases can produce important lobes and nulls. Regional coverage still depends on the actual pattern and on the ionosphere supporting the path.

Why I Still Prefer the Inverted-L Starting Point

The current RF.Guru EFHW16080 is a deliberately bounded 160/80 m design with an inverted-L installation focus. The practical idea is a substantial vertical rise followed by a high, clear horizontal or gently sloping section. This uses height for a purpose: it gives part of the antenna current a vertical orientation while keeping more of the long wire away from lossy ground and nearby objects.

Compared with a predominantly horizontal low span, that vertical current component offers a route to useful low-angle radiation. It is the reason I start with the L for DX—not a claim that every L has one guaranteed take-off angle or beats every sloper. How much current occupies the rise, the bend position, horizontal height, soil and surrounding conductors determine the result on each band.

Nor is a lower SWR the mechanism that makes the pattern better. Changing the layout can improve the match, worsen it or shift its minimum. The radiation objective comes first; we then arrange the feed, return path and matching system for the load that the chosen layout presents.

Will Raising One End Help?

It can be a worthwhile improvement when it raises a current-rich part of the wire or reduces coupling to soil, buildings and foliage. Raising one end also creates a different slope and changes the impedance. There is no sound basis for promising that its SWR will remain in a narrow range—or that it must fall by a particular amount.

If the available supports cannot form a useful inverted-L, I would rather work with a well-positioned lower span and a suitable matching system than force a poor L into dense obstructions just for its name. This is a site choice, not a loyalty test for an antenna shape. For a regional station, the higher-angle opportunities of a low span may suit the intended paths; for DX, my priority is useful low-angle current geometry and clearance.

The Transformer and Return Path Still Have Jobs to Do

Terminal impedance moves with height and slope. The high-ratio transformer must suit those complex loads on both bands; a bigger impedance ratio is not automatically a better match. The outside of the coax, a deliberate counterpoise and environmental capacitance can all participate in the return network. A current choke restricts unwanted continuation towards the station, but it does not make the antenna-side return path disappear.

An external tuner may make this installation usable within its documented impedance, voltage, current and power limits. A shack-end tuner does not remove standing waves from the feedline beyond it, undo transformer loss or reshape the antenna pattern. At low-band frequencies, suitable coax can make a moderate mismatch tolerable, but the actual cable type, length, load and operating power determine the cost. I would not discard the antenna solely because its meter reads 3:1, nor declare it harmless solely because a tuner finds a match.

Keep the Comparison Useful

For a meaningful comparison with an inverted-L, record wire length and route, all support heights, return conductors, feedline and choke position. Measure both bands at the same stated reference plane. If the claim is better coverage, compare received field or rapid repeated on-air results in named directions under suitably stable conditions; a different SWR dip is not that comparison.

Change wire or cable routing only with the transmitter de-energized, then stand clear for low-power remeasurement. Never touch an energized antenna or feed system. Keep the whole installation, including erection and possible mast or wire fall, clear of overhead power lines; obtain the electricity network operator's advice where clearance is uncertain.

The lesson I take from our 8 m endpoint trial is practical: the approximate 3:1 readings describe a matching problem to accommodate, not a verdict on the entire antenna. Use the lower deployment when it fits the site and the paths you want. When I have the height and clear space for low-band DX, I still begin with the intended inverted-L—because of where it lets me put the current, not because its name promises a perfect SWR.

Questions, antenna-factor records or height trials to share? Contact RF.Guru.