15 m or 20 m Mast for a 160/80 m Inverted-L?

The practical question is familiar: if an approximately half-wave wire on 160 m is bent into an Inverted-L, is a 20 m support worth the extra structure compared with 15 m? My answer is to model the actual bend and current path before awarding either mast the trophy. The five-metre change can matter, but it does not act alone.

What Actually Changes When the Bend Moves Up

With a feedpoint near ground, changing from a 15 m to a 20 m mast usually lengthens the rising section and shortens the horizontal remainder. It also raises the horizontal wire. Both changes alter the amplitude, phase, direction and environmental coupling of the current along the conductor.

The height change is electrically different on each band. Across 1.8–2.0 MHz, 15 m is roughly 0.09–0.10 wavelength and 20 m roughly 0.12–0.13 wavelength. Across 3.5–3.8 MHz, the same supports are about 0.18–0.19 and 0.23–0.25 wavelength. Those fractions describe the support height, not a promised current maximum or elevation angle.

On 160 m, both versions are still low structures in wavelength terms. The horizontal section can contribute substantial high-angle radiation while the rising section adds vertical current components. On 80 m the wire is electrically longer, so additional current maxima and phase reversals can create a more structured azimuth and elevation pattern. Moving the bend may strengthen one useful lobe and move a null into another direction.

The 20 m Mast Has Opportunities, Not Guarantees

A 20 m support can place more of the rising conductor farther from nearby fences, vegetation and buildings. It can raise the horizontal section and change the balance between vertically and horizontally oriented current. In a particular model or site, that may improve realised gain at some lower elevation angles.

It can also produce a less useful result. The higher bend changes the entire current distribution, not only the amount of “vertical wire.” A wanted lobe can rotate, split or acquire a null. A longer unsupported rise can change wire sag and coupling. The taller structure may require stronger guys, larger anchors, greater clearance and a more demanding installation procedure.

The 15 m Mast May Be the Better Station Choice

A 15 m support is often easier to install, guy, lower and inspect. That matters when the antenna must survive weather and be repeatable after maintenance. It leaves more wire in the horizontal section, which may support useful high-angle radiation on 80 m for regional paths. Whether that becomes working NVIS still depends on ionospheric critical frequency, absorption, noise and link margin.

The shorter mast is not automatically “the NVIS version,” just as the taller one is not automatically “the DX version.” At some sites, the 15 m layout can put more realised gain toward a wanted low-angle bearing because its lobe happens to align better. At another site, nearby loss or an unintended feedline current can dominate the comparison.

Compare the Whole Installed System

Keep the feed system identical when comparing supports. Record the transformer topology and complex load at its antenna port, the feedline type and length, the tuner reference plane, and any intentional return conductor. If those variables change between trials, the result is no longer a mast-height comparison.

The Return Path Can Rewrite the Pattern

An end-fed wire still needs return current. Depending on the installation, that path may include an intentional conductor, capacitive coupling, the support, the transformer enclosure and the coax exterior. The feedline can therefore become another radiating branch until a defined common-mode boundary stops it.

I do not put every choke at one memorized fraction of a wavelength. Map exterior feedline current at repeatable positions and on both bands. Use a separately specified choke at the boundary where intentional antenna or return current should end, then verify its complex common-mode impedance, differential loss, voltage and temperature in the relevant frequency and power range.

Model Before You Buy Five More Metres

A useful NEC comparison includes the actual wire coordinates, bend radius, sag, feedpoint, intended return branch, feedline exterior where relevant, ground model and nearby influential conductors. Compare realised gain over a range of azimuth and elevation angles rather than one peak-gain value. Also inspect current magnitude and phase along the wire; the five-metre change matters most where it moves current-rich sections.

Run more than one plausible ground model. Ground constants are rarely known precisely, and a model that wins only under one optimistic value is not a robust design. If guys or a metal mast are close enough to carry induced current, include them or treat them as an uncertainty.

Verify the Decision at the Site

• Record both geometries. Measure feedpoint, bend and end coordinates, sag, feedline route, guys and nearby conductors.
• Use the same reference planes. Compare impedance, accepted power and loss at declared points in the feed system.
• Map exterior current. Use the same calibrated clamp-current method at the same cable positions.
• Check loss and stress. Measure transformer, feedline and choke behaviour under representative power and duty cycle.
• Compare useful field results. Use simultaneous references or an A/B/B/A sequence with the same frequency, accepted power, receiver bandwidth and short time interval.
• Inspect the mechanics. Include mast rating, guys, anchors, clearances, lowering procedure and local structural/electrical requirements.

If the 20 m version repeatedly improves the wanted low-angle directions without introducing a troublesome null or an unacceptable mechanical burden, the extra height has earned its place. If the 15 m version gives the useful pattern and is easier to keep in the air, it is the better antenna system. That is a result worth trusting.

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