When the Better SWR Curve Lost the Field Test

The result is worth keeping because it attacks a seductive shortcut: a beautiful SWR curve can make a compact antenna look finished before its radiation has been tested. The result is not a licence to dismiss loading coils or folded conductors. It is a reason to measure the complete installed antenna.

The Two Fan Verticals

Antenna A used a separate straight wire for each band. Each element was cut and adjusted as a quarter-wave radiator without a loading coil or folded-back section.

Antenna B used the same broad fan-vertical idea, but some elements were shortened or packed into the available space with folds, small loading coils and more compact geometry.

The two antennas were compared through the same radio, RF switch and nominal coax length at the same transmitter power setting. Their SWR curves looked quite different. Antenna B stayed below about 1.5:1 on most of the tested bands. Antenna A was less tidy on some bands; on 15 and 17 metres its SWR approached 2.5:1, still within the matching range of the radio used for the test.

That first result would have made Antenna B the easy winner on paper. It was not the winner in the on-air comparison.

What the FT8 Comparison Reported

I transmitted FT8 and compared the signal reports returned by distant stations. Across the comparisons made at the time, the straight-element Antenna A was repeatedly reported more strongly than Antenna B. On bands where both antennas used straight elements, the reports were effectively alike.

That is the observation. It is stronger than an opinion about what an SWR plot ought to mean, but it is not a calibrated efficiency or pattern measurement. The useful conclusion is deliberately narrow:

The comparison does not, by itself, prove how many decibels separated the antennas, whether the difference was total radiation efficiency, or whether the straight build simply placed more energy at the azimuths and elevation angles sampled by those receive paths. Those are different questions.

What SWR Can and Cannot Say

SWR describes the magnitude of a reflection at a declared reference plane. In an ideal 50-ohm system, the reflection coefficient magnitude can be obtained from:

Those equations describe power accepted at that plane under the stated model. They do not divide accepted power into radiation and heat, and they do not describe where the radiated power goes. Conductor resistance, coil loss, ground loss and dielectric loss can all make input impedance look easier while reducing radiation efficiency.

The opposite mistake is also possible: a higher SWR antenna can lose more power in the feed line, trigger transmitter foldback or require a lossy tuner setting. The meter at the radio therefore cannot decide the comparison alone. The reference plane, line loss, matching-network loss and actual incident power must be known.

Why Compact Geometry Remains a Suspect, Not a Verdict

Folding a conductor back near itself changes mutual coupling and the phase and magnitude of current along the element. Close anti-parallel current sections can partly cancel their far fields. A loading coil can provide the reactance needed to resonate a short element, but its RF resistance dissipates power and its placement changes current distribution. Closely spaced fan elements also couple to one another, so adjusting one can move the impedance and current on another.

Any of those mechanisms could contribute to the reported difference between the two builds. Pattern change could also make one antenna stronger toward the sampled DX stations without giving it higher efficiency in every direction.

None of this makes loading or folding inherently poor engineering. A carefully designed shortened antenna can be efficient enough for its size, mechanically practical and repeatable. The test says that this particular collection of compacting choices did not earn its attractive match in the on-air comparison. It does not establish a universal ranking of straight and loaded elements.

Why FT8 Reports Need Care

WSJT-X expresses an FT8 report as signal-to-noise ratio in decibels referenced to a 2500 Hz noise bandwidth. It is a useful receiver observation, not a calibrated field-strength reading at the transmitter site.

Every remote report includes the wanted signal, propagation over that path, the receiving antenna pattern and polarization, local noise, receiver bandwidth and gain behaviour, and the decoder's estimate. Reports from different stations do not share one calibrated noise floor. A station that reports a numerically better SNR may simply have a quieter location or a better antenna in that direction.

Time matters as well. An A transmission and the following B transmission do not cross the ionosphere at the same instant. Fading, multipath, absorption and changing interference can alter the result between FT8 periods. ITU-R comparison guidance for HF systems notes that real paths vary uncontrollably and cannot be recreated later; simultaneous testing is preferred when the method allows it.

FT8 reports become persuasive antenna evidence when they are paired by the same receiving station, frequency region and short time interval, repeated through many A/B/A cycles and checked across several azimuths and distances. Save every timestamp, receiver identity, report and switch state. A median difference with its spread is more informative than a handful of best reports.

Equal Radio Settings Are Not Equal Accepted Power

Using the same radio, coax length and switch removes several obvious variables. It does not guarantee that the two antenna terminals accepted equal power.

The transmitter may reduce power into one load. An internal tuner has a different insertion loss for each transformation. A mismatched coax carries standing waves, and its additional loss depends on the cable's matched loss, electrical length and load. The switch, connectors and common-mode path can also differ once a different antenna impedance is attached.

A stronger test records forward and reflected power at a clearly stated plane for each switch position, verifies actual transmitter output, characterises switch and feed-line loss, and either measures or estimates tuner loss. For a gain comparison, normalise the received result to accepted power at the antenna terminal—not merely the power selected on the radio.

This is also why there is no universal answer to whether a shack-side tuner is harmless. The right calculation uses the actual complex load, feed-line type and length, matched attenuation, mismatch and tuner loss. Put the matching network where the measured system needs it, rather than treating either end of the coax as correct by habit. Reflected power is part of that transmission-line calculation, not a direct measurement of radiated power.

A Test That Can Separate the Causes

• Freeze the geometry. Record element lengths, folds, coil dimensions and placement, radial system, support height, feed-line route, choke position, nearby conductors and ground conditions.
• Calibrate at a declared plane. Save complex impedance and S11 at the antenna terminal as well as SWR at the radio. Measure the common coax and switch paths.
• Equalise accepted power. Monitor incident and reflected power, transmitter foldback and tuner state. Account for coax, switch and matching-network loss.
• Use rapid paired switching. Automate repeated A/B/A sequences, keep the same transmit frequency and waveform, and pair reports from the same receiver over the shortest practical interval.
• Keep all reports. Predefine the receiver and time filters. Do not select only distant stations or the strongest decodes after seeing the result.
• Sample more than one direction. Compare fixed receivers around the antenna or use a suitable antenna range. One DX path measures realised gain toward that path, not total efficiency.
• Measure current and heat. Map element, radial and feed-line-exterior current, and check loading coils and conductors thermally at a stated power and duty cycle.
• Compare with a physical model. A NEC model that includes the real folds, coils, ground and losses can test whether the observed direction is consistent with pattern change, loss or both.

A calibrated local far-field receiver or antenna range avoids ionospheric variation. It still needs adequate separation, controlled polarization, multipath assessment, a stable reference antenna and an uncertainty budget. Repeating the FT8 comparison remains useful because it tests the installed station on real paths; it should be treated as complementary evidence.

Primary and Authoritative Technical Sources

• WSJT-X User Guide—the definition and reference bandwidth of FT8 signal reports.
• ITU-R Recommendation F.1487-0—uncontrolled HF path variability and the need for simultaneous system comparison where practicable.
• IEEE Std 149-2021, Recommended Practice for Antenna Measurements—impedance, pattern, gain, efficiency, test-site and uncertainty practice.
• IEEE Std 145-2025, Definitions of Terms for Antennas—consistent distinctions among directivity, gain, efficiency and related antenna quantities.
• ITU-R Recommendation BS.705-2—HF transmitting-antenna characteristics, patterns and installation context.
• Keysight, Fundamentals of RF and Microwave Power Measurements—incident/reflected power, mismatch and measurement uncertainty.
• Lawrence Livermore National Laboratory, Numerical Electromagnetic Code v5—current, load, ground and radiation-pattern modelling for physical wire geometries.

Joeri's Bottom Line

I still trust the lesson from these two builds: the antenna with the prettier SWR curve was not the antenna that drew the stronger FT8 reports. The straight-element fan vertical won the comparisons I made, while the bands with equivalent straight geometry were alike.

I do not need to turn that observation into folklore. It does not prove that every coil or fold is bad, and it does not identify efficiency as the only cause. It proves that matching and radiation are different tests. Build for the site, measure accepted power, current and pattern, and then ask the air—not the SWR meter alone—which compromise worked.

Do not chase perfect SWR numbers alone. Optimise for the signal that leaves in the directions you need.

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