F4VUX's Ardennes Inverted-L: Good 80 m Results on a Real Site
F4VUX's Ardennes Inverted-L: Good 80 m Results on a Real Site
Marc, F4VUX, installed a long end-fed Inverted-L beside his house in the French Ardennes: limited vertical height, a low feedpoint near a wall and a long horizontal run. The reported operating experience was particularly positive on 80 m, with 160 m described as usable with a tuner. That is why this installation interests me: a practical low-band station taking shape around the site it actually has.
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.
Good on-air experience belongs in a field story. Marc's 80 m results were described as particularly good for regional work, including operation characterised as NVIS. I want to keep that practical success in view while explaining what the geometry and analyzer readings tell us. A positive operating assessment is useful context; it is not the same thing as a calibrated pattern or efficiency measurement.
The practical lesson: A constrained site need not stop a worthwhile low-band installation. Marc's case combines encouraging reported 80 m operation with a recognisable inverted-L arrangement and a workable matching problem. Keep the operating experience and SWR readings alongside each other: neither one, alone, measures the antenna's efficiency or proves why it worked.
The Installation Marc Built
Marc's installation record describes an approximately 8 m vertical rise followed by roughly 69.5 m of horizontal wire. The feed region was about 25 cm above ground, close to a house wall. A short conductor of about 1 m connected the feed region to a ground stake, while a common-mode choke was placed approximately 5 to 10 m along the coax.
The ground was described as acidic sandy or loamy soil over weathered schist and granite. That is useful site context, but geological description is not a measurement of RF conductivity or relative permittivity. Both quantities vary with frequency, moisture, temperature, composition and depth. The surrounding building, buried services, vegetation and terrain can also change the installed system.
At 3.6 MHz, an 8 m vertical rise is only about 0.096 free-space wavelength; at 1.8 MHz it is about 0.048 wavelength. Those ratios help us picture the geometry, but they do not isolate an 8 m radiator. Current flows on the complete wire and its return structure, and the bend couples the vertical and horizontal sections into one antenna.
Good 80 m Experience, with the SWR Record Alongside It
The encouraging part of this account is the assessment of good 80 m regional operation, not just the 1.1:1 dip. On 160 m, the installation was described more cautiously as serviceable with a tuner. There are no accompanying contact logs, signal comparisons or controlled before-and-after records from which to calculate coverage or an advantage over another antenna, so those assessments remain qualitative reports of this case.
The installation record reported these approximate SWR values:
| Band | Frequency | Reported SWR | What it establishes |
|---|---|---|---|
| 80 m | 3515 kHz | 1.1:1 | A close match at the measurement plane at that frequency and under those conditions |
| 80 m | 3800 kHz | 3.6:1 | The impedance changed substantially across the band |
| 160 m | 1800 kHz | 2.8:1 | A moderate mismatch at the measurement plane |
| 160 m | 2000 kHz | 6.6:1 | A larger mismatch toward the upper end of the measured range |
The field record does not define whether those values were measured at the feedpoint, after the transformer, at the far end of the coax or through a tuner. It also does not state the calibration plane, cable type and length, loss, weather or soil moisture. That missing context does not make the readings useless. It limits the conclusions we can attach to them.
The 3515 kHz reading gives a useful starting point for understanding the 80 m matching behaviour, while the higher reading at 3800 kHz shows why the chosen operating segment matters. The reported 160 m readings also explain why a tuner was part of the practical account; they do not establish that any tuner can handle that load at any power.
A low SWR means that the impedance at the stated reference plane is near the system impedance. It is not an efficiency measurement. Transformer loss, feedline loss, soil loss and current flowing on lossy structures can all make an SWR curve look smoother. Conversely, a useful radiator can present a mismatch that a suitable, low-loss matching system handles safely.
Ground Quality Cannot Be Read from the Curve
It is tempting to call rocky or sandy ground “poor” and then explain every measurement from that label. The RF system needs actual electrical parameters. Conductivity and permittivity influence fields in and close to the earth, while terrain affects the far-field pattern. Neither can be inferred reliably from soil colour, acidity or the apparent sharpness of an SWR minimum.
A dissipative return path can broaden an impedance response, but so can transformer behaviour, feedline attenuation, environmental coupling and multiple interacting resonances. A broad or convenient match is therefore not proof that the soil is helping, and a narrow match is not proof of high efficiency.
If ground loss matters to the conclusion, measure or bound it. Record moisture and weather, identify the soil layers that carry current, compare a declared return system under repeatable conditions and use a model whose ground assumptions are stated. The model is a hypothesis until field measurements support it.
The Ground Stake Is One Connection, Not the Whole Return Path
An end-fed wire still needs a complete current loop. In Marc's installation, current could divide among the short stake connection, displacement current into the surroundings, the exterior of the coax, building wiring and other nearby conductors. A 1 m connection to a stake does not make the rest of those paths disappear.
The resistance and inductance of the stake connection, the soil immediately around it and the coupling to the house all vary with frequency. Protective earthing and lightning bonding also have safety purposes that must not be altered just to improve an antenna measurement.
The practical test is a current map. Measure common-mode current on the coax exterior near the feed region, immediately before and after the choke, farther along the route and at the station entry. If possible, also measure current in the intentional return conductor. Repeat on both bands and at several frequencies, because a useful choke boundary on 80 metres may behave differently on 160 metres.
The Choke Defines a Boundary Only When the Current Data Agree
A choke located 5 to 10 m from the feedpoint can leave that length of coax exterior electrically available on the antenna side. That may be intentional, but the length is not a universal counterpoise formula. The choke's complex common-mode impedance and voltage, current and thermal limits must suit the installed load over both bands.
Impedance transformation and common-mode suppression are separate jobs. The matching network transforms the differential load presented to the feedline. The choke impedes current on the feedline exterior at its installation point. A good transmitter match does not prove that the choke is effective, and an effective choke does not make an unsuitable transformer lossless.
Nor can the choke be said to decouple the house wiring merely because the SWR is stable. Coupling can occur through the antenna's near field, feedpoint capacitance, bonding conductors and coax exterior on either side of the choke. Current measurements and a controlled before-and-after field or interference test are needed.
Why This Layout Can Be Useful for Regional and Distant Paths
The long horizontal run is a plausible contributor to the regional usefulness reported on 80 m: horizontal current can provide useful high-elevation radiation when its height and phase distribution suit the path. But a wire approaching a full-wave mode has more than one current-rich region, so the complete geometry can also create lobes and nulls. I would not attribute Marc's results to horizontal length alone.
NVIS means near-vertical incidence skywave: a high-angle signal returns through the ionosphere to support a regional path. Both the antenna pattern and the ionosphere must cooperate. Critical frequency, absorption, operating frequency, path length, power, noise and receiver performance still matter; a favourable SWR dip cannot establish that propagation mode.
The vertical rise may contribute useful lower-angle field, but an 8 m vertical section does not guarantee a DX take-off angle. The current magnitude and phase along the complete conductor, the return structure, height, ground parameters, terrain and nearby building determine the installed pattern. On the higher current modes of the wire, several lobes and nulls can form, and their directions need not align with the desired path.
Marc's positive operating assessment and the matching record answer different questions. The first tells us the installation was considered useful on air; the second shows how the load appeared at the analyzer. Neither needs to be discarded just because the other is not a complete measurement. To compare patterns, use a full-geometry model with measured or bounded ground, then verify selected bearings and elevation objectives with controlled field measurements. Keep accepted power constant, use fixed receiver settings or calibrated signal records and alternate configurations quickly enough that propagation changes do not dominate the result.
What This Field Case Can and Cannot Establish
| Observation | Reasonable conclusion | Conclusion that still needs evidence |
|---|---|---|
| Particularly good 80 m regional operation was reported | A positive qualitative operating assessment for this installation | Measured NVIS pattern, coverage probability or comparative gain |
| The long Inverted-L produced measurable responses on 160 and 80 m | The installed system has useful impedance structure on both bands | Efficiency, safe power or superiority over another antenna |
| SWR was lowest near 3515 kHz in the recorded 80 m points | The measurement-plane impedance was closest to the line impedance there | Best field strength, best DX frequency or best NVIS frequency |
| The feedpoint sat low beside a wall | The building and local return environment deserve attention | That the wall has no effect or that the system is balanced |
| A choke was installed down the coax | An intentional common-mode boundary was attempted | Suppression across both bands or isolation of house wiring |
| The site was described as sandy or loamy over weathered rock | The soil context should be recorded and measured | A numerical RF ground quality or a loss value |
What I Would Record Next at Marc's Site
I would keep Marc's installed layout and operating conditions as the baseline, then connect the on-air assessment to repeatable electrical and field records:
- Freeze the geometry: record wire lengths, height profile, bend, feedpoint, coax route, choke position, stake and nearby conductors.
- Declare reference planes: calibrate the VNA at the plane being reported, or characterise and de-embed the intervening cable within a stated uncertainty.
- Record complex impedance: save resistance and reactance across both bands, not only SWR points, with tuner state and weather documented.
- Measure return current: map coax-exterior and intentional-return current before and after the choke at representative frequencies.
- Account for loss: measure or bound feedline, transformer, choke and ground-system dissipation, including equilibrium temperature at the intended power and duty cycle.
- Test the field: compare equal accepted power by bearing with rapid A/B/B/A switching, simultaneous receivers where possible and a restored baseline.
- Qualify coverage claims: relate regional or distant results to the ionospheric state, receiver noise and path geometry rather than to SWR alone.
Low Feedpoints Need Deliberate Safety Engineering
A feed region only about 25 cm above ground is accessible. End-fed matching networks and wire ends can carry high RF voltage, while return conductors and coax shields can carry appreciable current. Weatherproofing does not make an energised enclosure safe to touch.
Never touch or adjust an energized antenna or feed system. Make routing changes with the transmitter de-energized, then stand clear for low-power remeasurement. Keep people and animals outside the controlled area, provide mechanical strain relief, protect terminals against accidental contact and maintain clearances from the wall, vegetation, utilities and combustible material. Commission at low power first, then increase power in controlled steps while watching temperature, arcing, connector heating and unexpected exterior current. RF exposure assessment must use the actual power, duty cycle, accessible geometry and installed pattern.
Bottom line: Marc built a practical low-band Inverted-L around the supports and surroundings available in the Ardennes, and the reported 80 m experience was encouraging. That useful result is the heart of this case. For a similarly constrained station, I would keep this layout on the shortlist, with the return path, matching range and accessible feedpoint treated deliberately. The four SWR readings help explain commissioning; they do not explain away the operator's experience or prove the soil, choke or antenna pattern caused it.
Primary technical references
- Keysight — Reflection measurements, scalar SWR and complex reflection data
- Recommendation ITU-R P.527-6 — Electrical characteristics of the surface of the Earth
- Recommendation ITU-R BS.705-2 — HF transmitting and receiving antenna characteristics and diagrams
- Recommendation ITU-R P.533 — Method for the prediction of HF circuit performance
- Rudy Severns, N6LF — Experimental determination of ground-system performance, Part 1
- Roy Lewallen, W7EL — Baluns: What They Do and How They Do It
- ICNIRP — Guidelines for limiting exposure to electromagnetic fields, 100 kHz to 300 GHz
Mini-FAQ
- What was the practical result at F4VUX? The installation was reported to give particularly good 80 m regional results, with 160 m described as usable with a tuner. This is a qualitative operating assessment, separate from the four SWR readings and not a measured pattern or efficiency claim.
- Does the 1.1:1 reading prove high efficiency on 80 metres? No. It establishes a close match at the measurement plane; feedline, transformer, ground and environmental losses still have to be measured or bounded.
- Does rocky or sandy soil automatically mean poor RF ground? No. RF behaviour depends on conductivity, permittivity, frequency, moisture, temperature and layered structure, not on a geological label alone.
- Is the 1 m wire to the stake the complete counterpoise? Not necessarily. Current can also divide onto the coax exterior, surrounding capacitance, bonding conductors and nearby structures.
- Does a choke 5 to 10 m from the feedpoint isolate the house? Not by placement alone. Its complex common-mode impedance and stress limits must be suitable, and currents should be measured on both sides across both bands.
- Does the low horizontal wire guarantee NVIS? No. The installed pattern must support high-elevation radiation, while the ionosphere, frequency, absorption, noise and link budget must support the path.
- What measurement would add the most confidence? A declared-reference-plane impedance sweep combined with a band-by-band return-current map, loss and thermal tests, and controlled equal-accepted-power field comparisons.