OT8M’s 160/80 m Inverted-L: What the Measurements Say
OT8M’s 160/80 m Inverted-L: What the Measurements Say
Wim, OT8M, installed a 77.5 m end-fed Inverted-L with a very low feedpoint. The useful lesson is not that one dimension guarantees contest performance. It is how geometry, return current, matching, feedline loss and pattern must be read together.
This is a real installation, not a universal recipe. The wire route and analyzer readings are worth keeping because they give the discussion a physical reference. They do not, by themselves, establish radiation efficiency, transformer loss, take-off angle or a repeatable advantage at another site.
The OT8M case in one sentence: a low feedpoint did not prevent useful 160/80 m matching in this installation, but the result belongs to the complete antenna—wire, earth coupling, intentional return, coax exterior, transformer, choke boundary and feedline—not to feedpoint height alone.
The Installed Geometry
OT8M’s wire was trimmed to approximately 77.5 m. 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.
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
The installation log recorded an SWR minimum near 2.4:1 at 3.700 MHz and another near 3.6:1 at 1.875 MHz. These readings locate two useful system-response regions at the analyzer reference plane. They show that the installed wire, transformer, return structure and feedline form a load that can be considered for matching on both bands.
They 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. A tuner can transform the impedance seen by the transmitter while leaving every loss and current path on the antenna side unchanged.
Reference-plane discipline: record complex impedance, not only SWR, at the same calibrated plane for every comparison. If a feedline separates the instrument from the antenna, its propagation constant and characteristic impedance must be known before the feedpoint condition can be reconstructed.
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.
How to Turn the Case Study into Evidence
- 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.
What remains open: without calibrated interface loss, exterior-current maps and installed-pattern or controlled field data, this installation cannot prove a universal transformer ratio, choke position, efficiency, soil advantage, DX pattern or power rating. Those are measurements still to be made.
The Engineering Conclusion
OT8M’s 77.5 m Inverted-L demonstrates something more useful than a slogan: a low, accessible feedpoint can be part of a workable two-band installation when the complete current path is handled deliberately. The recorded SWR minima are starting evidence for matching. They are not substitutes for loss, current and pattern measurements.
Keep the geometry and the station story. Then ask the harder questions. Where does return current actually flow? What is dissipated in the interface and ground? Which elevation and azimuth regions receive the accepted power? That is how one contest-station build becomes reusable engineering rather than another magic length.
Primary technical references
- IEEE Std 145-2025 — Standard for Definitions of Terms for Antennas
- Recommendation ITU-R BS.705-2 — HF Transmitting and Receiving Antenna Characteristics and Diagrams
- Lawrence Livermore National Laboratory — Numerical Electromagnetics Code
- NIST — A Two-Port Model for Antennas in an Arbitrary Environment
- Rudy Severns, N6LF — Ground-System Performance: Test Setup and Instrumentation
- Recommendation ITU-T K.52 — Guidance on Complying with RF-EMF Exposure Limits
Mini-FAQ
- Does OT8M’s low feedpoint prove that feedpoint height never matters? No. It shows that this installation retained useful impedance regions. Height can still alter coupling, safety, current distribution and pattern.
- Do the two SWR minima prove high efficiency? No. SWR describes a match at one reference plane and does not separate radiation from transformer, conductor, soil, feedline or common-mode loss.
- Is the horizontal section only for NVIS? No. Every current region contributes to one three-dimensional field. Frequency, height, ground, bearing and surroundings determine the realised pattern.
- Does a ground stake provide the whole RF return? Not necessarily. Return current may also use capacitance, nearby conductors and the coax exterior. Measure the installed current paths.
- Can a fixed choke distance be copied to another station? No. Choke placement follows the intended return boundary and measured exterior current across the used bands.
- What is the best next measurement? Save calibrated complex-impedance sweeps, map coax-exterior current and compare field strength at equal accepted power with a restored baseline.