Efficient Multiband Verticals with a 4:1 UNUN Explained

"Only a vertical of exactly 7.2 meters will give a good match on all bands" — this idea, often attributed to the so-called Rybakoff length, has misled a generation of hams. But the real story is far more flexible, practical, and grounded in RF physics.

In the world of HF vertical antennas, many hams chase after “magic lengths” — 5.15 m, 6.55 m, 8.8 m, or the classic Rybakoff 7.2 m — assuming they’re the only lengths that give decent SWR when fed with a 4:1 UNUN. This is a misconception rooted in folklore, not in RF fundamentals.

The truth is: the length of your vertical should match your intended working bands in terms of physical wavelength, not a mythical chart. And with the right matching approach — namely, a proper 4:1 UNUN and a choke — you can get excellent multiband performance from “non-Rybakoff” lengths like 6 m or 9 m. (A 6 m aluminium pole is a standard metric length in Europe! Clamp it in an insulator fed it ... and done!)

Let’s break it down.

It’s All About Capacitive Coupling and Ground Return

A ground-mounted vertical is nothing more than a radiator above a lossy capacitor — the ground. The whole matching game is about adjusting feedpoint capacitance and current distribution to yield an impedance that lands within your tuner’s or UNUN’s useful range.

Here’s how you do that in practice.

Choose a Physical Length that Matches the Bands You Want

  • 9 meters (about 0.43λ on 15 m and 0.22λ on 40 m) is an excellent choice for covering 15–40 meters.
  • 6 meters suits 10–20 meters well.
  • These are physical lengths, not electrical lengths — forget the “end-fed half-wave” or “quarter-wave” obsessions. What matters is how the current tapers along the vertical and whether the feedpoint impedance lands in a usable range.

These lengths naturally give feedpoint impedances in the 100–200 Ω range across multiple bands — a perfect match for a 4:1 UNUN into 50 Ω coax.

Lay Down a Proper Radial Field

You’ll often hear “just toss out a few random wires.” No. If you want stable, predictable SWR and lower ground losses, install at least 4 radials per band you care about, each cut to 0.25λ for that frequency.

Example for a 9 m vertical:

  • Four 10.5 m radials for 7 MHz
  • Four 5.5 m radials for 14 MHz
  • Four 3.75 m radials for 21 MHz

You don’t need a dense mat — you need resonant return paths.

Tune the Feedpoint — It’s All About Height and Element Balance

There are two variables that dramatically affect your SWR across the bands:

  • Feedpoint Height Above Ground:
    A 4:1 UNUN works best when the feedpoint sees an impedance in the 100–200 Ω range. For a 9 m vertical, this sweet spot is often around 0.1–0.2λ above ground. Try varying the mounting height between 20 cm and 1.2 m and watch what happens to your SWR.
  • Feedpoint Position Along the Vertical Element:
    Shifting the feedpoint slightly up the element changes the current distribution, effectively tapering the high-impedance “whip” section and improving multiband balance. Even 20–30 cm makes a difference.

These two adjustments — together with a decent radial field — allow you to “tune the capacitor,” not the antenna. You’re optimizing the electrostatic balance and current return path — not chasing resonance.

Why SWR Drops Into Place

When you hit the right combination of:

  • Physical radiator length (matched to bands),
  • Return path (radials),
  • Feedpoint impedance (height & location),

…you naturally end up in a situation where the real part of the impedance hovers around 100–150 Ω, and the reactance is either capacitive or inductive, but not extreme. A 4:1 UNUN brings that nicely into the 20–50 Ω range your transceiver or tuner can handle.

No traps. No lossy matching boxes. No magical wire lengths. Just physics.

Bonus Tip: Add a Choke 0.05λ Down the Line

Don’t forget to kill common-mode current. Place a 1:1 choke (line isolator) about 0.05λ down the coax from the feedpoint — that’s around:

  • 6.6 m on 80 m
  • 3.5 m on 40 m
  • 1.25 m on 15 m

This prevents the coax braid from becoming an unintended counterpoise and gives you cleaner patterns and lower RFI.

Conclusion

The ham world’s obsession with “tuned lengths” and “magic formulas” has blinded many to the underlying principle: your vertical is just a radiator over a capacitive ground — and with a 4:1 UNUN and proper radial layout, almost any reasonable length works.

Choose a length based on the bands you want to work. Focus on feedpoint tuning and ground interaction. That’s the real art of making verticals sing.

And stop worshiping the Rybakoff chart. The real magic is in the dirt.

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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.