10 m / 60 m Non-Resonant Trap Inverted-L with F1QM: Why We Finally Said “No”

This follow-up article is the second step in a collaborative project between Jean-Claude Ducasse (F1QM) and Joeri (ON6URE). In the first part, we explored a compact 10 m / 60 m inverted-L using a Non-Resonant Trap (NRT) and a 4:1 UNUN as a proof of concept.

The design worked on paper and can be made to work in practice. But after deeper analysis and more reflection, our conclusion is simple: for 10 m + 60 m specifically, a non-resonant trap in a single inverted-L is usually more hassle than it’s worth.

Quick recap: what we set out to build

The original goal was attractive on paper:

• One compact inverted-L that radiates efficiently on both 28.5 MHz (10 m) and 5.3 MHz (60 m).
• Single wire, with a Non-Resonant Trap part-way up the wire.
• Broadband 4:1 UNUN at the base, 70 cm above ground, in a waterproof enclosure.
• Manageable SWR on both bands with only a light touch from the tuner.

The prototype geometry we discussed was roughly:

• Feedpoint: 70 cm above ground, in a base box with a 4:1 UNUN.
• Vertical section: about 3.4–3.5 m from feedpoint to NRT.
• Horizontal or sloping section: roughly 12–13 m beyond the NRT.
• Total length: around 15–16 m, to allow trimming.
• Ground system: copper wire to a borehole plus optional short counterpoises / radials.

Electrically, we “designed around” 60 m and then tried to bend the same wire into a useful 10 m radiator with the help of a non-resonant trap tuned around the mean between both bands.

What a non-resonant trap is (and isn’t)

It can be made to work, and it can be reasonably efficient – but 10 m / 60 m is not a very “natural” pair for a non-resonant trap, and in practice it is usually more effort than it is worth. To see why, you need to distinguish two trap philosophies:

• Resonant trap: a parallel LC tuned right on a band. At that band the trap becomes high-impedance and effectively cuts off the wire beyond it, so the inner section behaves as its own antenna. Below that frequency, the trap mostly looks like an inductive loading coil.
• Non-resonant / mean-frequency trap: the LC is tuned somewhere between two bands. It never fully blocks current; it just adds reactance at both frequencies. The idea is to tweak the current distribution so the combination of “before” and “after” the trap works well on both bands.

This trick tends to work very smoothly for octave-spaced bands like 80/40, 40/20, or 20/10. In practice, non-resonant traps really do “like harmonic pairs”:

• If f₂ ≈ 2 × f₁, you can pick a mean frequency where the trap’s reactance balances nicely.
• Current distribution on both bands tends to come out symmetrical enough that SWR and pattern are well-behaved with just one trap.

Non-harmonic pairs can still work, but the further you move away from a 2:1 ratio, the more you are fighting the physics instead of riding along with it.

Why 10 m + 60 m is such an awkward two-band pair

Let’s put numbers on it:

• 60 m band ≈ 5.3 MHz → λ ≈ 56.6 m → ¼ λ ≈ 14.1 m.
• 10 m band ≈ 28.5 MHz → λ ≈ 10.5 m → ½ λ ≈ 5.25 m.

The frequency ratio is about 5.4:1, so:

• On 60 m you want the inverted-L to be around ¼ λ (or maybe ⅜–½ λ) for decent radiation resistance and a clean pattern.
• On 10 m that very same wire is already more than a wavelength long, with several current peaks and nulls along its length.

A single mean-frequency trap has to juggle two very different jobs:

• Act as a gentle “divider” so 60 m sees a nice inverted-L with a high, strong current region near the vertical.
• At the same time, shape the 10 m current so that the inner section behaves somewhat like a useful radiator instead of a random multi-lobe mess.

It is not that this is impossible – you can absolutely pick an LC combination and a trap position that yields:

• A clean resonance on 60 m with the full length.
• A tolerable SWR and radiation pattern on 10 m using mostly the inner part of the wire.

But unlike a harmonic pair (80/40, 40/20, 20/10), there is no “elegant sweet spot” where one trap naturally makes both bands happy at once. You end up doing more trial-and-error for a solution that is, in the end, rather compromised on 10 m.

What about efficiency and losses?

When you worry about efficiency in a trapped inverted-L, the usual suspects are:

1. Radiation resistance of the antenna on that band.
2. Ground / radial losses (by far the biggest effect on a vertical or inverted-L).
3. Losses in the trap (coil Q and capacitor ESR).

On 60 m, a ¼-wave inverted-L can have a reasonable radiation resistance (tens of ohms). If you build the trap with:

• A high-Q coil (air-core or low-loss ferrite/toroid).
• A decent RF capacitor with low loss.

…the trap’s equivalent series resistance is typically well under an ohm. That is in the “fraction of a dB” loss zone – small compared to what you lose in the ground system.

Where things go wrong is when:

• You use lossy coax traps or poor-quality capacitors, and the trap sits in a high-current region on 60 m → real I²R loss.
• You skimp on radials, so ground loss dominates everything anyway, trap or no trap.

So the non-harmonic ratio does not automatically kill your efficiency. The real problem is that you add complexity and tuning effort for a 10 m pattern that is still less clean than a dedicated 10 m radiator – without gaining much compared to simpler alternatives.

Practical alternatives that are usually better

After going through the numbers and looking at the practical trade-offs, our joint conclusion for this specific pair (10 m + 60 m) is: yes, the NRT inverted-L can be made to work, but it is rarely the best choice if those are your only two bands.

In practice, you almost always win by doing one of these instead:

• Option A – Make 60 m the “main” antenna and let the tuner grab 10 m.
  Build a good 60 m inverted-L with a serious radial or ground system. Put a quality ATU at the base (or in the shack with low-loss feedline) and let it pull the antenna onto 10 m as a multi-wave wire. The 10 m pattern will be multi-lobed, but 10 m is forgiving and you will still radiate well.
• Option B – Plain 60 m inverted-L + tiny dedicated 10 m radiator.
  10 m hardware is physically small and cheap. A separate 10 m ¼-wave vertical, a small dipole, or even a simple wire element along the same support is straightforward. You get a clean pattern on 10 m without any trap complexity in the 60 m system.

In other words: keep the clever non-resonant trap trick for harmonic pairs where it really shines, and do not feel obliged to apply it everywhere “just because you can.”

When non-resonant traps really earn their keep

None of this is a criticism of non-resonant traps themselves. On the contrary: used in their “natural habitat,” they are a very powerful tool. Some classic cases where they do make a lot of sense:

• 80 m / 40 m inverted-L or dipole with a single non-resonant trap.
• 40 m / 20 m or 20 m / 10 m dual-band wires with clean patterns and low SWR on both bands.
• Multiband dipoles where you deliberately trade a bit of complexity for fewer separate wires in the air.

For those pairs, the 2:1 (or near-2:1) ratio means you can pick a mean frequency where the trap’s reactance balances nicely for both bands, and the current distributions are naturally compatible. That is where the non-resonant trap idea becomes very elegant.

Our final verdict on the 10 m / 60 m NRT inverted-L

So, where did we land in this F1QM–ON6URE joint experiment?

• The original 10 m / 60 m NRT inverted-L is technically valid and can be made reasonably efficient.
• The limiting factors in real life will be ground / radials and trap Q, not the lack of harmonic relation.
• However, the amount of design effort and tuning time does not translate into clear on-air advantages over simpler options.

That is why, with the benefit of hindsight, our advice is:

• Use non-resonant traps primarily on harmonic (or near-harmonic) band pairs.
• For 10 m + 60 m, either:
  • build a robust 60 m inverted-L and let a tuner work 10 m, or
  • add a small dedicated 10 m antenna and keep the 60 m wire trap-free.

You can still treat the 10 m / 60 m NRT design as an educational project – it is an excellent way to learn about current distribution, trap Q, and the interaction between vertical and horizontal sections in an inverted-L. But if your goal is simple, reliable DX on both bands, the “boring” solutions win.

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