Dual-Band 10m / 60m Inverted-L with NRT: A Collaborative Design with F1QM

This design article is the result of an engaging technical exchange between Jean-Claude Ducasse, F1QM and Joeri, ON6URE. What started as a discussion about antenna tuners and baluns evolved into the idea of using a Non-Resonant Trap (NRT) to build an efficient dual-band 10 m / 60 m antenna on a single inverted-L wire.

Design Goals

The shared design target was a compact HF antenna that:

• Radiates efficiently on both 28.5 MHz (10 m) and 5.3 MHz (60 m)
• Uses a single wire in an inverted-L configuration
• Uses a 4:1 UNUN for broadband matching
• Offers manageable SWR on both bands with only a simple tuner

Physical Configuration

• Feedpoint: ~70 cm above ground in a waterproof PVC box
• Transformer: RF.Guru 4:1 UNUN (1 kW PEP, N-connector output)
• Vertical section: ~3.45 m from feedpoint to the NRT
• Horizontal/sloping section: ~12.3 m beyond the NRT
• Total wire length: 13.4–15.8 m (allowing room for trimming)
• Ground system: ~15 m earth copper wire to a borehole plus an optional short counterpoise

NRT Placement Logic

In classic dipoles using non-resonant traps, the trap position is often chosen using the geometric mean of the two band frequencies. In this case, the structure behaves more like a loaded vertical / inverted-L, so an arithmetic mean for the trap design frequency is used:

fNRT = (f10m + f60m) / 2
        = (28.5 + 5.3) / 2
        = 16.9 MHz

Around this frequency the NRT should present a high impedance, effectively blocking 10 m current from flowing into the rest of the wire, while remaining largely “transparent” at 60 m where the current distribution is dominated by the full wire length and ground system.

LC Design for the Non-Resonant Trap

The trap is designed as a simple series LC network, dimensioned so that its series resonance sits near 16.9 MHz. Using the standard resonance formula:

f = 1 / (2 · π · √(L · C))

For a chosen capacitance of C = 68 pF, the required inductance becomes:

L ≈ 0.427 µH

This inductance can be realized in several ways:

• 5–6 turns on an air-core former, or
• A low-loss toroid with a few turns, keeping RF current and voltage ratings in mind

The goal is a moderate-Q trap. If the Q is too high, the response becomes very sharp and sensitive to small parameter changes. With a moderate Q, the trap:

• Provides broadband blocking on 10 m (around 28.5 MHz)
• Remains comparatively “invisible” at 60 m (5.3 MHz)

Note: even though we call it “non-resonant,” the NRT is still based on a resonant LC combination. The “non-resonant” idea refers to the antenna wire, which is not cut as a classic half-wave resonant segment on either band.

Wire Length and Geometry

As a starting point, the electrical quarter-wave lengths (with a velocity factor of about 0.95) are:

• 10 m band: λ/4 ≈ 2.63 m × 0.95 ≈ 2.5 m
• 60 m band: λ/4 ≈ 14.15 m × 0.95 ≈ 13.45 m

That leads to the following practical geometry:

• Trap position: about 2.5 m from the feedpoint (seen by 10 m as the main radiator)
• Total wire length: 13.4–15.8 m so the 60 m tuning can be trimmed on site

The presence of the NRT adds some reactance and therefore makes the structure behave electrically a bit longer on both bands. The calculated lengths are therefore a starting point rather than a final prescription.

Expected SWR with a 4:1 UNUN

At the feedpoint, the inverted-L with ground system presents an impedance that is usually somewhere in the 100–200 Ω region on these bands. A 4:1 current UNUN is a good broadband compromise to bring this closer to 50 Ω over both 10 m and 60 m.

Typical SWR expectations (indicative):

• 10 m: around 1.5:1 to 2.5:1
• 60 m: around 2.0:1 to 3.0:1

A basic tuner in the shack will comfortably handle the remaining mismatch as long as losses in the feedline and matching transformer are kept under control.

Field Testing and Final Adjustment

As with any antenna involving reactive elements, real-world measurements are essential. The theoretical wire lengths above give a solid starting point, but:

• The trap introduces additional reactance that tends to make the antenna behave electrically longer on both bands.
• Nearby objects (buildings, trees, gutters), soil properties, and the exact ground system layout all shift the impedance curves.

In practice, expect to trim a little wire off the far end while watching a VNA or antenna analyzer. The comparatively broadband nature of the NRT (moderate Q) helps keep the SWR curves stable enough that small length tweaks won’t cause wild swings.

Conclusion

This non-resonant-trap inverted-L is an elegant and compact way to target two popular HF bands with one wire and a straightforward matching system. It avoids the extreme impedance swings and transformer ratios of some wideband EFHW approaches, while still staying practical for installations with limited space and complex terrain.

The concept and dimensions presented here grew directly from field constraints and design ideas shared between Jean-Claude, F1QM and Joeri, ON6URE, making this a realistic template for similar 10 m / 60 m experiments in real-world gardens and rooftops.

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