Multiband Fan Verticals and Why Mutual Coupling Isn’t a Big Deal

Multiband fan verticals are popular antennas among HF operators who want a no-tuner, single-feed solution that covers multiple bands. Whether you're chasing DX on 20 meters or checking into a net on 40, a well-designed fan vertical can get the job done. But there’s a recurring concern that often comes up in forums and coffee chats:

"What about mutual coupling between the vertical elements?"

In this article, we’ll break down what mutual coupling is, and why, in practical terms, it’s really not that big of a deal.

What’s a Fan Vertical, Really?

A fan vertical consists of multiple quarter-wave vertical elements, each cut for a different band, all connected at a single feedpoint. They "fan out" upward or outward from a shared base, typically with a shared radial field. Each element is tuned to its own band, and the idea is simple: when you're transmitting on, say, 20 meters, the 20-meter element resonates, while the others appear largely reactive and don’t significantly interact.

This works remarkably well in practice. Even though the other elements are physically close, they behave like open stubs or high-impedance elements on the active band.

The Mutual Coupling Concern

Mutual coupling is the interaction between adjacent elements where energy from one induces current in another. On paper, this sounds like it should ruin the SWR or distort the radiation pattern. In reality, the effects are minimal for most fan verticals used by amateurs.

Why?

  1. Spacing and detuning: The non-resonant elements are off-frequency enough that they don't efficiently re-radiate energy.
  2. High impedance at non-resonant frequencies: An element that isn’t resonant on the operating frequency often presents a high impedance, so little energy is absorbed or re-radiated.
  3. Real-world construction tolerances: Most fan verticals have a little variation in angle, length, and mounting that actually helps prevent problematic resonances.

Performance Near Resonance

Even when you're slightly off the resonant point of an element (say, 14.050 MHz when your 20m leg is cut for 14.200), the antenna still works, and your tuner handles the match. This "close enough" performance band is typically forgiving.

Even in this near-resonant operation zone:

  • The dominant element still carries most of the current.
  • Coupling into non-resonant legs is minimal and does not seriously affect tuning.
  • SWR curves may shift slightly, but usually not outside the range a typical tuner can handle.

What About Close Bands Like 17m and 20m?

Adjacent bands like 17 meters and 20 meters (or 10m and 12m) are indeed closer in both physical length and frequency, which increases the potential for interaction. But in practical builds, the effects are still manageable:

  • Minor Detuning Can Occur
    A 17m element may slightly shift the resonant point of a 20m element, but this typically results in a small SWR minimum shift, not a major mismatch.
  • Dominant Current Still Flows Where It Should
    When operating on 20m, the 20m element remains the main radiator. The 17m element is reactive and picks up little current.
  • A Bit of Physical Separation Helps
    Fanning the elements at slightly different angles or spacing the top ends a bit (20–30 cm) often eliminates measurable effects.
  • Trimming Is Rarely Necessary
    As long as the dips in SWR occur near or just below the band of interest, most tuners will easily match the impedance. There is usually no need for fine-tuning or trimming unless you're aiming for exact resonance without a tuner.

In short: close band interactions are real but minor. They're easy to tame with careful layout, and if the dips are in the right neighborhood, the tuner will take care of the rest.

A Word on SWR and Performance

Many hams focus heavily on SWR, but it's important to understand that SWR is not a measure of how well an antenna radiates. It is simply an indicator of how efficiently power is transferred from the transmitter to the antenna without a tuner.

With a tuner in place, the transmitter sees a proper load, and even if the SWR at the antenna feedpoint is not ideal, the system can still radiate effectively. As long as losses in feedline and matching components are low, overall performance remains excellent.

Myth-Busting FAQ

Q: Does low SWR mean my antenna is working well?
A: Not necessarily. Low SWR just means your transmitter is efficiently transferring power. It says nothing about how well your antenna radiates.

Q: Is high SWR always bad?
A: Not if you're using a tuner. High SWR at the feedpoint can be easily matched, and the antenna can still radiate just fine.

Q: Do I need exact resonance on each band?
A: Absolutely not. Perfect resonance is often misunderstood in amateur radio. It's an electrical condition, not a performance guarantee. As long as the antenna presents a load your tuner can work with, you'll make contacts. Don't chase an imaginary ideal. As long as you're in the ballpark, most tuners will match the impedance. Perfect resonance is nice, but not essential.

Q: Will mutual coupling ruin my pattern or cause loss?
A: In practical fan verticals, mutual coupling causes minor shifts at most. Radiation efficiency remains high.

Bottom Line

Multiband fan verticals are efficient, elegant solutions for covering multiple HF bands with minimal fuss. While mutual coupling sounds scary in simulations, it rarely causes problems in real-world use.

If your elements are reasonably spaced and properly trimmed — or simply resonant near the intended bands — the antenna will perform well not only at resonance but also throughout the tuning range where you actually operate.

So build with confidence. Fan verticals work — and mutual coupling just isn’t the monster some make it out to be.

Interested in more technical content like this? Subscribe to our notification list — we only send updates when new articles or blogs are published: https://listmonk.rf.guru/subscription/form

Questions or experiences to share? Feel free to contact RF.Guru or join our feedback group!

Written by Joeri Van Dooren, ON6URE – RF, electronics and software engineer, complex platform and antenna designer. Founder of RF.Guru. An expert in active and passive antennas, high-power RF transformers, and custom RF solutions, he has also engineered telecom and broadcast hardware, including set-top boxes, transcoders, and E1/T1 switchboards. His expertise spans high-power RF, embedded systems, digital signal processing, and complex software platforms, driving innovation in both amateur and professional communications industries.