Quarter-Wave Fan Verticals: Match, Mutual Coupling and Pattern
Quarter-Wave Fan Verticals: Match, Mutual Coupling and Pattern
Several resonant wires can share one feedpoint successfully. The real design job is ensuring that the intended branch carries the intended current—not merely trimming every band until the analyser shows a pleasant SWR.
A fan vertical is the vertical cousin of a fan dipole: several conductors share a feedpoint, and the branch nearest resonance is intended to carry most of the useful current on its band. That simple picture is a good starting point. It is not a guarantee.
Matching and radiation are different tests. Trimming a branch or adding a tuner can restore the impedance presented to the transmitter while mutual coupling has already changed current amplitude, phase, loss and elevation pattern.
The Antenna Is a Coupled Network
Every branch has self-impedance, and every nearby branch contributes mutual impedance. Current on one wire therefore depends on the voltage at the common feedpoint, its own resonance and the currents induced on the other conductors. Spacing, conductor diameter, branch angle, support structure, radial field, feedline and nearby metal all participate.
That does not make fan verticals bad antennas. It explains why the radiators cannot be designed as independent quarter-wave wires and then assumed to remain independent after they are fastened close together. The complete structure must be tuned and measured as one antenna.
A Low SWR Can Hide the Wrong Current
A network analyser reports impedance at its calibration plane. It does not identify which branch carries current, where current maxima occur, how much power is lost in soil or matching components, or what elevation pattern the installed structure produces.
Suppose a new branch pulls an adjacent resonance and changes the current division. Shortening one or both wires may return the feedpoint to 50 Ω. A tuner can do the same transformation without touching the wires. Both actions can make the transmitter happy; neither proves that the intended quarter-wave current distribution or low-angle component survived.
| Observation | What it establishes | What still needs evidence |
|---|---|---|
| Low SWR at the feedpoint | Convenient impedance at that plane | Branch-current distribution, loss, efficiency and pattern |
| Resonance moved after adding a wire | The structure is coupled | Whether the change is useful or harmful |
| Tuner restores the transmitter load | The tuner completed an impedance transformation | Current, tuner/feedline loss and takeoff angle |
| One branch carries the dominant base current | Useful evidence about current selection | Current farther along the structure and the far-field pattern |
Why Nearby Bands Become the Difficult Case
Branches for widely separated bands are often sufficiently different in electrical length that one clearly dominates while the others remain far from resonance. Closely spaced bands make that separation smaller. Their conductors can present significant reactive loading to one another, and repeated trimming becomes a negotiation between several resonances rather than one adjustment per band.
More wires can still be made to work. The question is whether the extra resonators deliver enough bandwidth or operating convenience to justify the coupling, mechanical crowding and verification burden. If a tuner already covers a narrow nearby band acceptably, another dedicated wire may solve the match while adding little to the station—and may make the installed current system harder to control.
Takeoff Angle Comes From the Installed Current System
The low-elevation component of a ground-mounted vertical depends on the current distribution along the vertical structure, its electrical height, the radial and ground system, feedline common mode, surrounding conductors and the distant ground that shapes propagation. An SWR trace contains none of that geometry.
When parasitic current appears on neighbouring branches, it can change the effective radiating structure. The result may be small, useful or harmful; it cannot be declared from the match alone. This is why electromagnetic tools such as LLNL's NEC report conductor currents and radiation patterns separately from input impedance.
The VertX Choice: Fewer Resonators, Clearer Jobs
RF.Guru tested a denser one-resonator-per-band VertX geometry. It could be tuned, but the interaction and repeated adjustment did not earn their place in the current design. The present architecture keeps the VertX radiator set deliberately simpler:
- 20 m: the aluminium tube is the principal radiator.
- 15 m: one dedicated wire provides the resonant branch.
- 10 m: three staggered wires address the unusually wide 28.000–29.700 MHz Belgian allocation.
- 12 m and 17 m: tuner-assisted operation is preferred over adding two more closely coupled resonators.
Twelve metres sits between 10 m and 15 m; 17 m sits between 15 m and 20 m. Those are precisely the places where more dedicated conductors crowd an already coupled structure. The three 10 m wires solve a different problem. At the highest design frequency their modest physical-length differences create staggered resonances across a wide allocation while keeping the VertX geometry deliberately compact.
This is a VertX design conclusion, not a declaration that every dense fan antenna fails. Nor is tuner-assisted operation a promise that 12 m and 17 m reproduce the same efficiency or low-angle pattern as the directly resonant 20 m, 15 m and 10 m structures. Those outcomes require installed current and field evidence.
The Radial Field Is Not Leftover Wire
A ground-mounted quarter-wave vertical needs a deliberate return-current system. Rudy Severns, N6LF, measured both radial count and length rather than treating “more short wires” as a universal rule. His multiband and ground-system experiments show that a few long radials perform poorly, that very sparse fields are unstable, and that the value of shortening radials changes with radial count, soil and frequency.
For VertX, sixteen approximately five-metre on-ground radials form a credible compact baseline. The 24 × 5 m kit adds eight more useful-length conductors for an all-equal 120 m field. The 32 × 5 m kit extends that equal-length layout to 160 m of wire, but its higher wire count is not by itself proof of lower loss.
Measure Current Before Believing the Match
Start with all intended radiators, radials, feedline and support hardware installed. Calibrate the VNA at a declared reference plane. Then record complex impedance and clamp-current readings around each branch near the feed region and at repeatable positions farther along the conductors where accessible.
Use an A/B/A sequence: measure the complete antenna, remove or detune the questioned branch, then restore it. Repeat the impedance and current maps without changing coax routing or soil conditions. If the operating claim is about low-angle radiation, add controlled field-strength or pattern comparison. The restoration step matters because soil moisture, cable movement and connection repeatability can imitate a design improvement.
Primary engineering references
Mini-FAQ
- Can a fan vertical work well? Yes. It must be designed, tuned and verified as one coupled antenna rather than as independent wires sharing a connector.
- Does a tuner remove mutual coupling? No. It transforms impedance at its ports. It does not restore a particular branch-current distribution or radiation pattern.
- Does a low SWR prove a low takeoff angle? No. Takeoff angle depends on installed current distribution, geometry, ground, radials, feedline and surroundings.
- Why use three wires on 10 m? The Belgian 10 m allocation is wide. Staggered resonances can broaden useful coverage while keeping the VertX geometry deliberately compact.
- Why not add separate 12 m and 17 m wires? In the tested VertX geometry, those closely spaced resonators added coupling and tuning complexity that did not justify their place. Tuner-assisted operation was the chosen compromise.
- Are more short ground radials always better? No. Severns' measurements show that count, length, soil and frequency interact. Preserve adequate long radial coverage and verify the installed system.