EFHW16080 as a Shallow Inverted-U: A Practical Two-Support Layout
EFHW16080 as a Shallow Inverted-U: A Practical Two-Support Layout
You have two useful supports, but no straight run long enough for the low-band wire. A shallow inverted-U can make the garden work: up, across, and down again.
RF.Guru working definition: Common-mode current is the non-cancelling phasor-sum current in a specified set of conductors, evaluated at a defined cross-section and using a declared current-direction convention. In the intended differential transmission-line mode, the outgoing and return currents are equal and opposite, so their phasor sum is zero. When they do not cancel, the remaining current must close through another reference or return path—such as the outside of a coax shield, a mast, equipment chassis, station wiring, nearby structures, earth, the operator, or distributed coupling through the environment.
This broader working definition is especially useful in practical antenna systems. On transmit, non-cancelling current on the outside of the coax can make the feedline and connected structures part of the radiating antenna system unless that path is intentional, clearly defined and properly controlled—for example by providing the required return path and placing a suitable common-mode choke at the correct boundary.
This is why I like the shallow-U option for an EFHW16080: it gives a long low-band conductor a practical route without asking for a perfectly straight garden. Put the useful central span up in the clear, use the rising and falling sections to complete the path, and keep access to the feed arrangement for maintenance. We are adapting the installation to the site—not claiming that bends are electrically invisible.
The useful compromise: Two supports and a broad, open U can accommodate the full wire path. Spend the available height on the current-carrying span and keep the ends clear. That is a reason to choose the layout; a guaranteed radiation pattern is not.
A Practical Shallow-U Starting Layout
Keep the transformer accessible near one end, rise toward the first support, carry the main span across to the second, then let the far section descend while remaining safely out of reach. As a geometry example, an approximately 81 m conductor with a main span around 10–15 m high and a far end several metres above ground illustrates the idea. That is not a current product cut length or a clearance certification: check the supplied wire and the current installation requirements.
The full wire path still has to fit without tight folds or long, close parallel sections. If the site is narrower, use gentle diagonals or longer slopes. Accessible for adjustment means accessible with transmission disabled; it never means a live high-voltage feedpoint or wire end within reach.
support support
/-----------------------\
/ \
feed end / \ far end
ground below
The Bends Are Electrically Visible
Parallel or nearby wire sections couple to each other. Vertical drops couple differently to ground than the horizontal crown. Bend angle, separation, height and nearby metal can move resonances and redistribute current on both intended bands.
Choose the Fold From Current, Not Symmetry
A neat-looking U is not automatically the best RF shape. Avoid bringing high-current sections close to lossy roofs, trees, gutters or each other. Keep voltage-sensitive wire ends well clear of people, supports and conductive structures.
On 160 m, the feedpoint and far end are current minima and voltage-sensitive regions; the broad current maximum lies farther along the half-wave conductor. On 80 m, the same wire supports approximately a full-wave mode with two principal current-rich regions. The exact locations move with the installed electrical length and coupling, so use the standing-wave picture to place the wire—not as centimetre-accurate survey data.
The 80 m Mode Needs Its Own Pattern Check
On 80 m the long conductor carries a higher-order mode. Folding can rotate, reinforce or cancel particular field contributions. The result may be useful, but a 160 m SWR trace cannot establish it.
Regional Coverage and DX Are a Trade, Not a Switch
The raised central span contributes a substantial horizontal component, while the rising and falling legs add differently oriented current. On top band, where a 10–15 m span is still electrically low, expect meaningful high-angle energy alongside whatever lower-angle lobes the installed slopes create. On 80 m, the extra current regions make the azimuth and elevation pattern more structured.
That mixed behaviour is the point of the shallow-U: it can fit a long low-band wire onto two practical supports and offer both regional and longer-path utility. It is not equivalent to a tall quarter-wave vertical over a serious radial field, and it should not be sold or tuned as if the shape alone proved a pattern.
Give the Feed System a Deliberate Boundary
End feeding does not remove the return path. The matching network still works against another part of the current system: a deliberately arranged return conductor, capacitance to the surroundings, or otherwise the feedline and station. Decide which conductors belong on the antenna side of that boundary, then use suitable common-mode choking to discourage the coax run towards the shack from joining in. A choke does not make the return current disappear, and a ground stake is not a substitute for that decision.
This is a useful distinction in Tom Rauch, W8JI's analysis of end-fed half-wave systems: even a resonant end-fed has a finite feed current and a return path. Keep the return arrangement intentional on both 160 and 80 m instead of treating the coax route as an afterthought.
Commission the Final Shape
- Record all segment lengths, heights, spacing and orientation.
- Sweep complex impedance on both bands at the same calibrated plane.
- Map current on the coax exterior and any intentional counterpoise.
- Check transformer heating at a conservative power before increasing it.
- Recheck after rain, support movement or a major feedline-route change.
My choice when the straight run will not fit: start with the broadest, clearest shallow-U the two supports permit. It is a practical way to keep a long low-band wire in service. The installation checks refine that choice; they are not the reason for the antenna.
Further Reading
- W8JI — End Fed Half Wave Antennas: finite end-feed impedance, current return and the influence of the surroundings.
- NEC-2 user manual — Introduction: conductor geometry, ground, calculated current and radiated fields belong in the same model.
- RF.Guru EFHW16080: the current 160/80 m product destination.
Mini-FAQ
- Can I use the same wire length as an inverted-L? Use it as a starting point, but the folded geometry changes coupling and normally requires installed retuning.
- How close may the vertical legs be? There is no universal spacing. Greater separation reduces strong mutual coupling; model and measure the actual layout.
- Does the inverted-U preserve the low-angle pattern? Not automatically. The vertical drops, crown, ground and harmonic current distribution all affect elevation and azimuth lobes.
- Where should the choke be placed? At the intended return-current boundary, verified with exterior-current measurements on both bands.