Why We Prefer a 4:1 UNUN for Wideband Multiband Wire Antennas Without Harmonic Relationships

When radio amateurs venture beyond the simplicity of harmonic multiband antennas (e.g., 80/40 or 40/20 EFHWs), they often run into a wall: impedance unpredictability. In particular, non-harmonically related bands like 60/40/30 meters are challenging to match efficiently using typical broadband transformers. For those using slopers or flat-tops around 13–14 meters long, the 4:1 UNUN has become the practical choice. Here's why.

The Problem With Non-Harmonic Multiband Use

Antennas such as the EFHW work efficiently across harmonically related bands because their feedpoint impedance maintains a somewhat predictable pattern. But the moment you choose bands like 60/40/30 meters — often required by modern operating preferences or contest bandplans — those harmonic relationships vanish.

As a result, the impedance seen at the feedpoint becomes more chaotic. You may see anything from 100 Ω to 1000 Ω depending on the frequency and wire geometry. Traditional 49:1 transformers are optimized for high-impedance loads (~2450 Ω), which only occur at resonance of an EFHW. In multiband slopers or flat-tops covering non-harmonic bands, the impedance is rarely that high and often sits in the 100–200 Ω range or even lower on some bands.

Why 4:1 UNUNs Fit the Role

A 4:1 UNUN converts 200 Ω down to 50 Ω, which hits the sweet spot for many non-resonant wire lengths used in slopers. In particular:

  • A ~14-meter wire often shows feedpoint impedances between 100–300 Ω on 60, 40, and 30 meters
  • A 4:1 UNUN brings these values within the range of modern ATUs (internal or external)
  • The result is less tuner stress and better power transfer efficiency

In contrast, a 9:1 UNUN is designed for extremely high or very low impedances and generally performs worse in this mid-range. And a 1:1 choke won't provide any impedance transformation at all.

The Role of Multiple Counterpoises

Using multiple counterpoises — typically 1/4 λ wire lengths per band — helps stabilize the feedpoint impedance by providing a more consistent ground reference. For example:

  • One wire ~5.3m for 60m
  • One wire ~3.5m for 40m
  • One wire ~2.6m for 30m

These can be tied to the UNUN's ground side, improving performance and reducing variability caused by changing soil or mounting height.

The Sweet Spot: 14m Wire for 60/40/30

A 14-meter wire isn’t resonant on any of the bands — and that’s exactly the point. It ensures no deep nulls or narrow bandwidths, and allows a tuner + 4:1 combo to smooth out the ride. Add multiple band-specific counterpoises and optionally a good common-mode choke further down the line, and you have a surprisingly effective 3-band system that fits in small gardens or rooftops.

That said, other lengths and layouts can offer equally compelling results:

  • 35.5m in inverted-L configuration: excellent for 160/80/60 meter operation, especially with three counterpoises (e.g., 26m, 13.5m, and 4m). This setup performs surprisingly well for low-band coverage.
  • 24.5m sloper: a solid alternative for 40/30m when space allows. This length tends to be more efficient for 40m, offering better radiation resistance and lower losses than shorter wires.

As with any antenna, the optimal solution depends on the use case and geometry. The wire height above ground, slope angle, and available space all play critical roles in determining performance. Always evaluate slopers, inverted-Ls, and flat-tops not just by band coverage but by their interaction with the environment.

Conclusion

The 4:1 UNUN is not just a compromise—it's a tailored solution for wire antennas operating over non-harmonic multiband ranges. When combined with smart wire lengths, band-specific counterpoises, and proper tuner use, it delivers performance and versatility where harmonic designs fall short.

If you're aiming to work 60/40/30m on a single 14m wire, or expand into 160/80/60 with 35m of wire, the 4:1 UNUN is your best friend.

 

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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.