The FCP on 80 Meters: When a Counterpoise Gets Mistaken for an Antenna
The FCP on 80 Meters: When a Counterpoise Gets Mistaken for an Antenna
HamAnalyst’s model raises a useful question: which conclusions belong to the inverted-L, which belong to the folded counterpoise, and which require the missing feed network and a controlled comparison?
Source under discussion: HamAnalyst’s Modélisation de l’antenne FCP 80 mètres presents a 4NEC2 model, a broad low-angle pattern and an evening of FT8/PSKReporter results. It also reports a feed impedance of 717 Ω at a phase angle of 88.7° and 24.5% modelled efficiency, then argues that omitting the isolation transformer makes those feed and efficiency results unrepresentative while a real installation may reach roughly 70–80% efficiency. Those linked propositions—not the author—are what this article evaluates.
The FCP is a compact folded counterpoise developed by Guy Olinger, K2AV, for low-band installations where a dense, uniform radial field is impractical. It can be a very useful return system. That does not make it the sole radiator, make a transformer an automatic 50 Ω match, or allow a normalised pattern and one evening of spots to establish efficiency.
Central distinction: the complete structure radiates, but the aerial wire, folded counterpoise, feedline exterior, transformer and earth do different jobs. Attribute a claimed benefit by changing one of those parts at a time and comparing absolute performance at the same power reference plane.
The FCP Is a Counterpoise Inside a Radiating System
Calling the FCP “not an antenna” is too absolute. Any conductor carrying time-varying current contributes fields, and the entire inverted-L/FCP/feed structure is the antenna system. The useful engineering distinction is one of intended role and contribution.
In the configuration discussed by HamAnalyst, the aerial wire is an inverted-L. Its vertical and horizontal sections establish much of the far-field pattern. The FCP provides the intended return-current path and is folded so that some of its fields near the ground oppose one another. K2AV’s maintained technical summary describes the FCP as a deliberately non-resonant, 5/16-wave single-wire folded counterpoise whose geometry is intended to reduce net field at nearby lossy earth.
The FCP therefore matters electrically: its current, height, fold spacing, environment and coupling affect feed impedance, loss and pattern. But a low-angle lobe cannot be credited to the FCP alone. A meaningful vertical current component in the inverted-L already tends to produce useful low-elevation radiation, while the return system determines how much accepted power is lost, rerouted or radiated elsewhere.
Precise language: this is an inverted-L over an FCP, not an FCP acting alone. The FCP is part of the radiating system, but its design purpose is the compact return path and reduction of nearby ground-field loss.
What the Isolation Transformer Actually Changes
K2AV’s maintained guidance requires a specified isolation transformer at the FCP feedpoint. Its essential system role is to remove the direct metallic path from the antenna-side circuit to the coax-side circuit, so the feedline shield is not simply placed in parallel with the intended counterpoise. That controls an otherwise installation-dependent external return path.
Isolation and impedance matching are still separate specifications. An ideal 1:1 transformer provides isolation without changing impedance ratio. A transformer with another turns ratio can transform impedance, and every real transformer also has finite magnetising and leakage impedance, capacitance, winding resistance, core loss and voltage limits. Those properties can alter the measured input, but they must be represented by an actual network model or measurement; the word “isolation” does not guarantee 50 + j0 Ω.
K2AV’s historical NCJ article describes a 1:1 version and separate ways to adjust or match the installed impedance. Current K2AV material also documents application-specific transformer ratios and external matching networks. K2AV now warns readers to use the maintained website—not the 2012 article—for construction details because portions of the older technical guidance have changed.
Model boundary: a NEC voltage source placed directly between the aerial wire and the FCP already creates an ideal two-terminal excitation without a modelled coax shield. That is a useful idealisation of feedline isolation. It does not include transformer loss, leakage, capacitance, turns ratio or voltage stress. Adding an ideal lossless 1:1 element to that same reference plane cannot, by itself, convert an arbitrary reactive impedance to 50 Ω.
How to Read the Reported 717 Ω Result
If 717 Ω at 88.7° is a polar representation of the complex impedance, it describes a feedpoint dominated by reactance, with only a small resistive component. That result may indicate that the model geometry is far from resonance at its declared source plane. It does not establish that the solver failed, and it does not identify how much of the resistance is radiation, conductor loss or ground loss.
Three questions must be answered before attaching a physical explanation:
- Is the impedance reported as R + jX or as magnitude and phase, and what is the sign of X?
- Where is the source/reference plane, and were feedline, transformer and matching elements omitted or represented?
- Were the aerial, FCP, earth parameters, conductor loss, height and nearby conductors those of the claimed installation?
A transformer can become part of a matching network when its ratio and parasitics are known. That is different from assuming the missing device will turn the reported impedance into 50 Ω. The model should be rerun with a documented two-port or equivalent circuit, followed by an installed R + jX measurement at a calibrated plane.
Pattern Shape Does Not Establish Absolute DX Performance
A normalised NEC pattern shows how the model distributes its radiated power by direction. It does not show the absolute scale unless gain or realised gain is retained. Two antennas can display nearly identical normalised low-angle lobes while one radiates much less power because of ground, conductor, transformer or feedline loss.
The HamAnalyst low-angle pattern is therefore compatible with a useful DX configuration, but it does not isolate the FCP’s contribution. The vertical portion of the inverted-L, its height, the horizontal-wire direction, soil parameters and return-current geometry all influence elevation and azimuth patterns.
A complete model report should include:
- absolute gain or realised gain, not only a normalised pattern;
- radiation efficiency and the loss mechanisms included;
- source location, accepted power and impedance at that plane;
- wire geometry, diameter, conductivity and segmentation;
- ground model, conductivity and relative permittivity;
- feedline exterior, transformer or an explicit statement that they are excluded; and
- convergence and sensitivity checks for height, soil and nearby conductors.
The Lawrence Livermore NEC-2 User’s Guide documents the source, network, ground and radiation-pattern facilities that define this boundary. NEC solves the structure supplied to it; confidence in a field claim also depends on how faithfully that structure represents the installation.
The 24.5% Efficiency Is a Model Result, Not a Field Measurement
Radiation efficiency is the ratio of radiated power to net power accepted at the antenna port. Total efficiency also includes mismatch. The distinction is set out in the NIST paper Improved Antenna Efficiency Measurement Uncertainty in a Reverberation Chamber.
ηrad = Pradiated / Paccepted
ηtotal = ηmismatch × ηrad
For a wider station boundary, transformer and feedline losses must also be included at compatible planes. A real transformer cannot raise energy efficiency above the idealised lossless case; it can improve the system by preventing power from flowing on an unintended, lossy feedline path or by providing a deliberate impedance transformation. Those are changes to the modelled current paths and reference plane, not permission to substitute an assumed efficiency.
The reported 24.5% is neither automatically correct for the physical antenna nor disposable. It is the result for the published model’s geometry, ground, conductors and excitation. A 70–80% field value would require an auditable full-system model, a calibrated efficiency measurement, or a controlled comparison to a reference antenna with uncertainty. Transformer omission alone does not supply that evidence.
What K2AV’s Work Supports
K2AV’s current FCP project page makes a bounded case. The FCP was developed to reduce ground loss from inadequate radial systems under inverted-L and vertical aerial wires where a dense radial field is impractical. K2AV’s executive summary still calls a full-size, dense, uniformly distributed broadcast-style radial system the reference solution beneath a purely vertical antenna.
That supports the FCP as a compact alternative to compromised return systems—not a universal claim that it outperforms every radial arrangement. K2AV also treats the isolation transformer, FCP dimensions, height and surrounding loss mechanisms as an integrated installation. Changing one of those conditions changes the evidence boundary.
W8JI’s published comparison models are useful as an independent sensitivity study: with the same general inverted-L family, changing the return arrangement changes impedance, gain, voltage and efficiency. His often-cited 1.83 MHz examples are 160 m models, however, so they do not validate a specific 80 m percentage. They demonstrate why geometry and model inputs must travel with each number.
A Fair Comparison Has Two Different Questions
To determine what the FCP contributes, run a component-attribution comparison:
- keep the inverted-L wire geometry, height, frequency, soil and nearby conductors fixed;
- change only the return system;
- model the feedline exterior consistently in every case, or exclude it consistently;
- retune and match each system with documented, loss-accounted networks;
- compare at equal accepted power at the antenna reference plane; and
- report ground loss, conductor and transformer loss, current distribution, absolute gain and realised gain.
To choose the best practical antenna for a restricted property, run a system comparison instead. Allow each candidate to be optimised within the same footprint, support height, wire budget, safety limits and transmitter power, then include every matching and isolation component. The winner of that test may differ from the winner of the one-variable experiment. Both are legitimate; they answer different questions.
| Evidence | What it supports | What remains unresolved |
|---|---|---|
| Normalised low-angle NEC plot | Pattern shape for the modelled currents and geometry | Absolute gain, efficiency and which component caused the shape |
| Modelled 24.5% efficiency | Efficiency for the stated model and loss assumptions | Efficiency of the physical installation |
| Installed 50 Ω match | Low reflection at the measurement plane | Radiation efficiency and external feedline current |
| One evening of FT8 spots | The links closed under those conditions | Absolute efficiency, gain and superiority to another antenna |
| Repeated rapid A/B comparison | Relative field performance on sampled paths | Absolute efficiency unless a calibrated reference and full loss budget are used |
FT8 Results Are Operational Evidence, Not an Efficiency Meter
An evening of PSKReporter or FT8 spots is valuable field evidence that the station closed paths under the prevailing propagation and noise conditions. It cannot separate antenna efficiency from transmit power, path opening, receiving-station sensitivity, polarisation, local noise or the inverted-L pattern.
A stronger on-air comparison rapidly switches between the FCP return system and a reference arrangement while preserving the aerial, power and timing. Compare reports from the same receiving stations over many alternating cycles, paths and nights. Record missing reports as well as successful ones and publish the distribution, not only the best spot.
The Defensible Verdict on the HamAnalyst Propositions
HamAnalyst’s model and FT8 results support a useful, practical conclusion: an 80 m inverted-L over a compact FCP can make DX contacts from a constrained site, and its model can show a broad low-angle lobe.
They do not, on their own, show that the FCP is the principal cause of that lobe, that an omitted isolation transformer automatically resolves a 717 Ω reactive input, or that the installed system has 70–80% efficiency. Those propositions require separate evidence: a controlled return-system comparison, an explicit feed-network model and a measurement or validated full-system loss analysis.
That boundary does not diminish the FCP. It gives the design credit for what K2AV built it to do: provide a compact, controlled return path and reduce loss when a high-quality radial field is not available.
Sources and model context
- HamAnalyst — Modélisation de l’antenne FCP 80 mètres
- K2AV — Maintained FCP+ project page and current guidance
- K2AV — The FCP among low-band counterpoise systems
- K2AV — Why use an isolation transformer?
- K2AV / NCJ — Historical 2012 FCP article
- W8JI — Folded counterpoise comparison models
- Lawrence Livermore Laboratory — NEC-2 User’s Guide
- NIST — Radiation and total-efficiency definitions and uncertainty
RF and electrical safety: elevated counterpoises, reactive feedpoints and isolation/matching networks can develop high RF voltage. Keep people and animals clear, use components and insulation qualified for the actual voltage, current, duty cycle and weather, and treat lightning protection and protective bonding as separate system requirements.
Mini-FAQ
- Is the FCP the main radiator? The whole structure radiates. In this system the inverted-L is the intended aerial, while the FCP is the designed return path whose current and ground coupling still affect loss and pattern.
- Does an isolation transformer automatically turn 717 Ω into 50 Ω? No. Isolation controls the feedline return path. Matching requires a documented turns ratio or network, including the real transformer’s parasitics and loss.
- Does a low-angle NEC lobe establish good efficiency? No. A normalised lobe describes pattern shape; absolute gain and efficiency require the loss scale and accepted-power reference plane.
- Is the modelled 24.5% necessarily the installed efficiency? No. It applies to the model’s geometry, ground, conductors and excitation. Replacing it with another percentage requires a complete model or calibrated measurement.
- Does an evening of FT8 spots prove the FCP beats other return systems? No. It proves useful links occurred. Superiority requires repeated, rapid A/B comparisons with equal power and common receiving paths.
- What is the fairest FCP comparison? Keep the inverted-L and environment fixed, change only the return system, model the feed path consistently, retune each case and compare loss plus absolute realised gain.