Wim’s OT8M Inverted-L: Fitting 160/80 m to the Contest Station

The OT8M installation account described excellent results and reliable 160/80 m contest use. That practical success is the reason for telling the story. The important engineering choice was to get a long low-band conductor into the available space while keeping its feed hardware accessible. The operating assessment is qualitative: there is no calibrated gain comparison or contest-score analysis attached to it.

The Installed Geometry

OT8M’s wire was trimmed from approximately 81 m to 77.5 m while tuning the installation. About 12 m rose vertically from a feedpoint roughly 0.5 m above ground; the remaining 65.5 m continued as a slightly sloping horizontal section whose far end was around 15 m high. These are the dimensions of Wim’s installation, not a cut list for every garden.

The Inverted-L earns its place here through layout. It uses the available height and horizontal span together, and avoids the need to place the feed assembly at a high centre support. That is a real installation advantage. It also makes the location of current along the vertical and top sections important; the bend is not automatically a current maximum on both bands.

Those dimensions make the antenna electrically substantial on both bands. Near 1.8 MHz the total wire is in the half-wave class before end effects and environmental loading are included. Near 3.7 MHz it is closer to a full-wave-class conductor. Bending the wire into an Inverted-L changes the three-dimensional arrangement of its current regions, so the vertical and horizontal sections do not behave as two independent antennas labelled “DX” and “NVIS.” Their fields add vectorially.

The bend height, horizontal bearing, wire slope, ground, nearby conductors and intentional return path all affect the realised elevation and azimuth pattern. A low physical feedpoint is therefore neither an automatic handicap nor evidence of a low-angle pattern.

What the Analyzer Readings Establish

Wim optimised for both bands, giving particular attention to 80 m SSB operation. The installation account recorded an SWR minimum near 2.4:1 at 3.700 MHz and another near 3.6:1 at 1.875 MHz. That is the practical tuning outcome: two useful response regions from one installed wire route, rather than a promise of a perfect direct match across both bands.

A suitable tuner can make such a load usable by the transmitter when the actual impedance lies inside its matching range and the tuner, feedline and interface remain within their current, voltage and thermal limits. SWR magnitude alone does not establish those conditions. The recorded minima describe the system at the analyzer’s connection point; the surviving account does not fully document that reference plane.

The readings do not reveal where power is dissipated. The same input SWR can coexist with different transformer loss, conductor loss, soil loss and common-mode current. Tuning can change delivered power and operating currents, but obtaining a good match at the transmitter does not automatically remove downstream loss or redesign the antenna’s current paths.

The Low Feedpoint Question

Raising a feedpoint from roughly 0.5 m to 1.5 m reportedly produced little visible change in the local SWR trace. That is a useful observation about this installation, not a general proof that feedpoint height is irrelevant. An impedance trace may remain similar while loss distribution, exterior current or pattern changes.

At a low feedpoint, the hardware is easier to reach but electric-field stress, accidental contact, moisture, soil proximity and current through nearby conductors deserve extra attention. At high transmit power, safe clearance and component voltage cannot be inferred from transmitter watts or SWR alone. The installed system needs its own RF-exposure and electrical-stress assessment.

Return Current Must Be Deliberate

An end-fed radiator still needs a second RF-current branch. A short wire, ground coupling, support hardware and the outside of the coax may all participate. Connecting a stake does not make that current disappear into an ideal RF earth, and a transformer ratio does not define the return path.

A choke establishes a useful current boundary only when its common-mode impedance is appropriate across the bands and every parallel path is included. Its position cannot be certified from a fixed cable length. Map current along the coax exterior and attached conductors, then place or adjust the boundary so the intended return region is reproducible.

This is also why “the coax is not part of the antenna” is too simple. The differential signal inside a sound coaxial line and current on its exterior are different modes. The exterior can still join the antenna system when return current is allowed to flow there.

What I Would Carry Over to Another Station

I would carry over the design order: start with the wanted bands and operating range, fit a substantial conductor to the supports, provide a deliberate return path, and then tune the installed system. I would not abandon a workable low feedpoint merely to satisfy a height slogan. Its accessibility is valuable, provided access during transmission is prevented and the electrical safety requirements are met.

The comparison with Marc’s F4VUX installation in the Ardennes is useful precisely because the site is different. Copying Wim’s final wire length or a cable distance is less useful than understanding what each part of his layout is doing. For a new installation, these records make the choices repeatable:

• Record geometry completely: wire length, bend height, bearing, end height, feedpoint height, conductor diameter, nearby metal and ground conditions.
• Measure at declared planes: save calibrated complex-impedance sweeps at the antenna and station ends where practical, together with the feedline specification.
• Characterise the interface: measure differential transfer, representative-load loss, common-mode impedance and temperature rise of the finished transformer-and-choke system.
• Map exterior current: sample the coax and attached conductors at several positions on both bands, not only beside the feedpoint.
• Compare at equal accepted power: correct for line and matching loss before comparing field strength or remote reports.
• Protect against propagation drift: use simultaneous receivers or rapid A/B/B/A switching with a restored baseline, across several bearings and times.

A model can help separate hypotheses before field work. Use the actual bent geometry, finite ground and connected conductors, then test convergence and material assumptions. The model is useful when it predicts measurements; it is not a certificate printed by the antenna’s name.

The Engineering Conclusion

Wim’s objective was a usable 160/80 m contest antenna, with 80 m SSB given deliberate tuning attention. The 77.5 m Inverted-L provided a practical way to pursue that objective with the available height and span, and the installation was described favourably in operation. A feedpoint only half a metre above ground did not rule it out.

That is why I consider this layout worth considering at a similar station: it makes a substantial radiator and an accessible feed arrangement possible without demanding a high centre feedpoint. Keep the return current deliberate, match within the equipment’s limits and choose the wire route for the directions you want to work. The benefit is a workable engineering choice—not a magic 77.5 m formula.

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