F1QM and ON6URE: The 10 m / 60 m NRT Decision
F1QM and ON6URE: The 10 m / 60 m NRT Decision
A useful collaboration does not have to end with a product or a construction drawing. Sometimes the best result is a clear reason to stop—and a simpler antenna that is easier to measure, tune and trust.
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 project began with a technical exchange between Jean-Claude Ducasse, F1QM, and me. The target sounded attractive: one compact inverted-L for 60 metres and 10 metres, one reactive section in the wire, a moderate-ratio UNUN at the base and only light help from a tuner. We followed the current path far enough to answer the real question. The antenna can be made to present useful impedances, but that does not make it the cleanest way to obtain useful radiation on both bands.
That is the collaboration worth publishing. We did not merely exchange a drawing. We separated the trap circuit from the antenna modes, checked the arithmetic, looked at the return path and asked whether the extra parts bought a predictable pattern. For this widely separated band pair, the simpler answer won.
Our decision: build the 60-metre inverted-L as a complete antenna with an intentional return system, measured impedance transformation and a separate common-mode choke. If a clean 10-metre pattern matters, add a dedicated 10-metre element. A tuner may also load the long wire on 10 metres, but then its multi-lobed installed pattern is part of the bargain.
The Original Engineering Target
The first sketch placed the feedpoint near ground, ran a vertical section upward and continued with a longer horizontal or sloping wire. A reactive network part-way along the conductor was intended to make the inner section behave as the principal 10-metre radiator while allowing the full wire and ground system to work on 60 metres.
The attraction is obvious. A quarter wavelength in free space is about 2.63 metres at 28.5 MHz and about 14.15 metres at 5.3 MHz. One support can therefore carry both a short upper-band current section and a much longer lower-band structure. But those lengths do not describe the installed antenna by themselves. Height, the bend, top loading, soil, radials or other return conductors, source position and the reactive network all move the impedance and current distribution.
The bands are separated by a frequency ratio of about 5.4:1. A wire sized around a 60-metre quarter wave is already roughly 1.4 wavelengths long on 10 metres. Unless the outer section is effectively isolated, several current maxima can appear and the azimuth/elevation pattern becomes a geometry-dependent multi-lobe result. Low SWR cannot tell us whether that pattern is the one the station wanted.
A Trap Must First Be the Right Circuit
The word trap is often used too loosely. A conventional wire-antenna trap is a parallel LC circuit inserted in series with the conductor. Near its parallel resonance it presents a high impedance, so relatively little current continues into the outer wire. Below resonance it behaves predominantly inductively; above resonance it behaves predominantly capacitively.
A series LC circuit does the opposite at its series resonance: its inductive and capacitive reactances cancel and the series impedance becomes small. It does not become a high-impedance blocker. That distinction must be settled before choosing a coil, capacitor or position.
For either ideal LC form, the resonance calculation starts with:
f₀ = 1 / (2π√LC)
With 68 pF and 0.427 µH, the calculated resonance is about 29.5 MHz. To place ideal resonance near 16.9 MHz with 68 pF would require roughly 1.30 µH. Real coils and enclosures add self-capacitance, lead inductance, resistance and voltage stress, so the finished network must still be measured in its fixture.
No magic mean frequency: the arithmetic or geometric mean of two bands is not a universal trap-design rule. Resonance, L/C ratio, component Q, trap position, wire lengths and installed geometry must be solved as one antenna. A mean can be a starting hypothesis; it is not evidence that both current distributions are useful.
Non-Resonant Does Not Mean Non-Reactive
In a non-resonant-trap design, the parallel LC network is deliberately operated away from its own resonance on one or both working bands. It then adds a frequency-dependent reactance rather than acting as a clean open circuit. This can be elegant when the two desired current distributions are compatible. It can also become a sensitive extra degree of freedom that moves the impedance without delivering the desired radiation pattern.
The 2014 IEEE CAMA paper by Parrini, Papi and Pieraccini describes the familiar trapped-dipole principle as parallel-resonant circuits that cut off part of the dipole branches, then experimentally develops a more complex double-resonance network to shape matching across two sub-bands. The lesson is not that traps are bad. It is that the network topology and measured antenna response matter; the label alone predicts neither current cutoff nor bandwidth.
Likewise, antenna input impedance can show both series and parallel resonances for modal reasons. Obeidat, Raines and Rojas explain these through characteristic modes. That is another reason not to infer the current distribution from one zero-reactance or SWR point.
Why 10 Metres and 60 Metres Fight the Same Wire
On 60 metres, the full inverted-L can be developed around one dominant low-band current region and a deliberate return system. On 10 metres, the same physical wire is electrically long. A reactive network can reduce current beyond one point, but its finite impedance, component loss, parasitic response and position all affect what remains on both sides.
| Design question | What must be established | What SWR cannot prove |
|---|---|---|
| Does the inner section carry the intended 10 m current? | Installed current distribution or validated full-wave model | That the outer wire is inactive or the pattern is simple |
| Does the trap remain useful on 60 m? | Loaded trap impedance, loss, voltage/current stress and full antenna efficiency | That a low feedpoint reflection means low trap or ground loss |
| Is a 4:1 ratio appropriate? | Complex feedpoint load across the intended band and transformer operating boundary | That the antenna is generally in a 100-200 ohm range |
| Is the coax outside the RF circuit? | Exterior-shield current map with the chosen return system and choke position | That an UNUN or balanced-looking wire automatically controls common mode |
| Is the two-band compromise worthwhile? | Pattern, accepted power, component temperature and restored-baseline field comparison | That two impedance minima equal two good antennas |
This is where the attractive compact drawing stops being an automatic recommendation. A design may be tunable and radiate on both bands while still being more installation-sensitive than two simpler radiators. On 10 metres, a dedicated quarter-wave element or dipole is physically small. That changes the trade: the reactive complexity has to beat an alternative that is inexpensive, pattern-predictable and easy to validate.
The UNUN and Choke Have Different Jobs
An inverted-L over a radial or ground system is unbalanced. If its measured complex feedpoint load calls for impedance transformation, an UNUN performs that job. A separate current choke defines how much of the coax exterior is allowed to participate in the return path. Combining the labels does not combine the physics.
We therefore do not prescribe 4:1 merely because the original idea used it. Measure the installed load at the intended reference plane, choose a transformation that keeps loss, voltage, current, temperature and tuner range under control, then map exterior-shield current and place the choke from that evidence. In many practical amateur installations, UNUN plus choke covers both the unbalanced transformation and common-mode boundary more honestly than treating a current balun as a universal cure.
The Simpler Antenna Plan
The collaboration ended with two practical options rather than one clever drawing.
Build the inverted-L with a serious, documented return system. Put the tuner where it avoids unnecessary feedline loss, measure the installed load and accept that 10-metre operation on the full wire may have a multi-lobed pattern.
A small vertical or dipole gives a much clearer current and pattern objective. It can share a support while remaining a separately measured radiator.
A third option remains valid for an experimenter: build the trapped version, but treat it as a development project. Sweep the trap in its fixture, model the complete wire and return system, measure current on both sides of the network, map coax current, log component temperature and compare field strength with the simpler baseline. The result must earn its complexity.
Why This Is a Successful Collaboration
Jean-Claude, F1QM, brought the real installation problem. I followed the matching, trap and current-path consequences. The first idea was plausible enough to calculate. The calculation exposed the topology and frequency questions. The antenna view exposed the pattern and return-path questions. The final decision became more useful than a parts list.
That is how RF.Guru wants collaborations to work: share the constraint, make the assumptions visible, calculate what can be calculated, measure what the environment controls and be willing to reject a complicated solution. “No” is not a failed prototype when it saves someone from building the wrong compromise.
Have an engineering constraint worth testing? Send the problem to RF.Guru and return to the RF.Guru collaboration notebook as measured decisions are published.
Bottom line: a 10 m / 60 m NRT inverted-L is possible, but possibility was not our standard. For this band pair, the trap and matching system add too many installation-sensitive variables compared with a well-built 60-metre inverted-L plus a small dedicated 10-metre radiator.
Sources and technical foundations
Mini-FAQ
- Can a 10 m / 60 m trapped inverted-L be made to work? Yes. The question is whether its installed current distribution, pattern, loss and tuning stability justify the extra network compared with two simpler radiators.
- Why does trap topology matter? A parallel LC branch presents high impedance near parallel resonance and can restrict current into the outer wire. A series LC branch presents low impedance at series resonance and does not act as that blocker.
- What is the resonance of 68 pF and 0.427 µH? The ideal calculation gives about 29.5 MHz. Parasitics shift a real trap, so measure the completed network in its fixture.
- Is the mean of the two bands the correct trap frequency? Not universally. Trap frequency, L/C ratio, position, wire lengths and installed geometry must be solved and measured together.
- Why not assume a 4:1 UNUN? The required ratio follows the measured complex feedpoint load and operating limits. Use an UNUN when transformation is needed and a separate choke to control the coax exterior.
- What would Jean-Claude and I build instead? A properly returned and measured 60-metre inverted-L, plus a small dedicated 10-metre element when a cleaner upper-band pattern matters.