VertX Fan Vertical: Why Matching Is Not Pattern Control
VertX Fan Vertical: Why Matching Is Not Pattern Control
A multiband vertical can be tuned until every band looks polite at the feedpoint. That is not the same as proving that every branch still carries the current—or produces the elevation pattern—we designed it for.
VertX testing forced us to choose between a seductive specification table and a cleaner current system. One resonator for every band sounds complete. In the measured structure, it also meant more interaction, more repeated trimming and less confidence that the SWR dip represented the radiation we wanted.
Our design rule: do not add a resonator merely to print another “no tuner required” line. Add it only when its current and pattern contribution earn the coupling and mechanical complexity it introduces.
Every New Wire Changes the Existing Antenna
Fan verticals do not contain isolated monoband antennas. Their radiators share a feedpoint, radial return, nearby space and often a support structure. Voltage applied at the feedpoint drives current according to every branch's self-impedance and the mutual impedance between branches.
When one wire is trimmed, several resonances can move. When a new wire is added, current can appear on neighbouring conductors. That is ordinary electromagnetic coupling, not a construction defect. The mistake is treating the final input impedance as a complete description of what happened.
You Can Tune Out the Symptom
If interaction moves the impedance, wire trimming can put the dip back. An antenna tuner can transform the remaining load at its own reference plane. Both operations may restore low SWR. Neither reverses coupling as if the other conductors had disappeared.
The transmitter sees the impedance delivered to it. Radiation comes from current distributed over the complete installed structure. Those are linked problems, but they are not the same measurement.
| Question | Evidence required |
|---|---|
| Can the transmitter deliver power? | Complex impedance and tuner state at declared reference planes |
| Which branch is doing the work? | Repeatable current measurements on the radiators |
| Did the coax join the antenna? | Common-mode current map around the complete feedline |
| Did efficiency survive? | Loss accounting, heating and controlled field comparison |
| Did the low-angle component survive? | Installed model validated by current data or a controlled pattern/field test |
The Nearby-Band Crowding Problem
The hardest additions were not random. A 12 m resonator sits close to the 10 m and 15 m structures. A 17 m resonator sits close to the 15 m wire and 20 m tube. In those gaps, the adjacent electrical lengths are similar enough that strong interaction is unsurprising.
Yes, we can trim the collection again. The question is what we have achieved after doing so. If the branch-current balance becomes opaque, if tuning one band repeatedly moves another, or if the low-elevation pattern needs a fresh proof for every compromise, the extra “resonant band” is not free.
Why 10 m Is the Deliberate Exception
Belgium's 10 m amateur allocation spans 28.000–29.700 MHz. That is a wide operating range for one short resonator. Three slightly different 10 m wires let VertX distribute resonances across that range.
At this highest design frequency, the differences in physical wire length are modest relative to the complete vertical structure. RF.Guru's testing found that this bandwidth strategy kept the intended vertical-current geometry compact enough to justify it. That statement is deliberately narrow: it does not publish universal branch lengths, exact resonant frequencies, band-edge SWR, gain or efficiency.
The Current VertX Architecture
- 20 m: the aluminium tube is the principal radiator.
- 15 m: one dedicated wire.
- 10 m: three staggered resonant wires for the wide allocation.
- 12 m and 17 m: tuner-assisted operation, without pretending that match alone proves the same pattern as a dedicated quarter-wave branch.
The feedpoint sits as low as mechanically practical above the deliberate radial connection. That gives the mechanics and return-current system a clear relationship. It is not a magic height, and moving a feedpoint down does not automatically create gain.
The current VertX product page is the ordering reference. This article is the engineering boundary: the radiator set is deliberately simple because current control is more valuable than an impressive count of resonant wires.
The A/B/B/A Comparison Favoured the Simpler Architecture
RF.Guru compared VertX with a DX Commander at the same site using an A/B/B/A receive sequence. VertX was consistently favoured, most obviously on 10 m, 12 m and 15 m. On 17 m and 20 m, the two antennas were broadly on par.
That observation supports the design choice; it does not isolate the mechanism. Reduced mutual coupling and better-preserved branch currents are the working explanation. The site did not permit RF.Guru's drone-borne oscillator far-field method, so the result is not presented as a numerical gain or takeoff-angle measurement.
Radials Must Carry Return Current
The vertical radiator is only one side of the RF circuit. The on-ground radial field carries intended return current and reduces the temptation for the coax exterior, mast or nearby wiring to become the accidental other half.
Rudy Severns' measurements are more useful than the slogan “throw down as many short wires as possible.” Four long radials are too sparse for a stable general solution. Sixteen useful-length radials form a credible compact baseline. The 24 × 5 m kit adds eight more equal conductors; the 32 × 5 m kit extends that field to 160 m of useful-length wire. Neither choice is automatic proof of lower loss.
That is why the larger current VertX kits use equal five-metre radials rather than building their count from shorter conductors. Where the site permits, more useful-length coverage remains a sensible direction.
What “Low Angle” Must Mean
A vertical often has a strong low-elevation component, but the actual elevation pattern belongs to the complete installation: radiator currents, electrical height, ground, radial field, feedline, mast and surrounding conductors. Low SWR does not measure it.
For 20 m, 15 m and 10 m, the architecture aims to retain the intended vertical current path through directly selected radiators. For tuner-assisted 12 m and 17 m, no equal-angle promise is made without the corresponding installed evidence. That is more honest than using the word “DX” as a substitute for a pattern measurement.
The Test That Decides
Measure the complete installation at a declared VNA plane. Map current on each accessible radiator and on the complete coax. Compare the full set with a questioned branch removed or detuned, then restore it. If the claim is about takeoff angle, use a controlled field or pattern comparison, not only an S-meter and not only a feedpoint sweep.
That process can show that a dense fan works well. It can also show that the extra branch earns nothing beyond a nicer dip. VertX follows the result rather than defending the first drawing.
Primary engineering references
Mini-FAQ
- Can a dense fan vertical be tuned? Often, yes. The remaining question is whether the intended branch currents, loss and pattern survived the tuning.
- Why does a tuner not settle the argument? A tuner transforms impedance at its ports. It does not measure or restore a particular antenna-current distribution.
- Will too many branches always cripple the pattern? No universal count defines failure. The risk rises with coupling and crowding; the installed currents and pattern decide.
- Why keep three 10 m wires? They address the wide Belgian 10 m allocation with staggered resonances while keeping the tested VertX geometry deliberately compact.
- Are 12 m and 17 m guaranteed to have the same low-angle pattern? No. They are tuner-assisted bands, and equal pattern or efficiency needs separate installed evidence.