Why We Use a 4:1 UNUN Instead of a 4:1 BALUN

When designing wideband antenna systems, especially for field or portable use, it's tempting to grab whatever 4:1 transformer is at hand. But in our experience, the choice between a 4:1 UNUN and a 4:1 BALUN isn't trivial. It significantly impacts efficiency, return current behavior, common-mode suppression, and real-world power handling. Here's why we consistently use a 4:1 UNUN instead of a 4:1 BALUN for our systems.

Understanding the Core Difference

  • BALUN stands for BALanced to UNbalanced — it assumes you're feeding a balanced antenna, like a dipole or center-fed loop.
  • UNUN stands for UNbalanced to UNbalanced — it's made to feed unbalanced antennas, such as verticals, end-feds, or bottom-fed loops.

Many real-world antennas that look balanced on paper (like delta loops or symmetric geometries) are not truly balanced in operation due to environmental asymmetries, ground coupling, and feedpoint conditions. Assuming a balanced load in such cases leads to unintended current flow, core heating, and radiation pattern distortion.

Efficiency in Wideband Matching

A 4:1 UNUN typically outperforms a 4:1 BALUN when:

  • The antenna impedance is reactive or highly variable across bands
  • The antenna is not center-fed or lacks symmetry in its grounding
  • The environment introduces asymmetric capacitive or inductive coupling

From a transformer perspective, UNUNs:

  • Efficiently transfer power between mismatched impedances with minimal insertion loss
  • Operate with one winding grounded, reducing voltage stress and improving core utilization
  • Handle differential-mode currents only, avoiding the added burden of enforcing current balance

By contrast, BALUNs — especially current baluns — are required to absorb the common-mode component resulting from unbalanced conditions. This leads to excessive magnetic flux in the core, often beyond design limits, especially under high power or off-resonance.

This explains why improperly applied BALUNs often fail prematurely or exhibit high core heating: they are misused as combination devices (transformer + choke), which they are not.

The Delta Loop Example

One perfect case for wideband usage (like our DeltaRex) is a delta loop fed at the bottom vertex.

Although the delta loop is a closed system (often made of wire or aluminum, as in our case), when fed at the bottom vertex:

  • The current distribution becomes asymmetric due to physical height differences and proximity to ground
  • The antenna couples strongly to the earth, creating a return current path that is not symmetrical
  • Ground losses are inherently non-uniform — there is no such thing as "average ground" in physical reality (only in simulations)

In practical measurement, the current maximum shifts, the voltage reference floats, and feedpoint impedance varies strongly with mounting height and orientation. All of this makes the system unbalanced at RF, even if the shape is geometrically symmetric.

In this scenario:

  • A 4:1 BALUN performs poorly: more core loss, radiation asymmetry, and potential RF feedback
  • A 4:1 UNUN adapts to the true feedpoint condition: unbalanced, asymmetrical, and ground-referenced

Real-World Consequences

When using a 4:1 UNUN on a delta loop fed at the bottom vertex or a vertical:

  • SWR curves are smoother and impedance transitions are more manageable across bands
  • The core remains cooler under high duty cycle modes like FT8 or RTTY
  • Less return current and common-mode current appears on the outside of the coax braid when the transformer is correctly chosen and the system is properly choked

Misconception: Balun = Choke?

One common misconception is that a 4:1 current BALUN inherently provides sufficient suppression of common-mode and return currents. In reality, it does not. A current balun’s primary function is to enforce current balance between its output legs — assuming a balanced load. When the antenna is unbalanced, as is often the case in wideband or ground-coupled systems, the balun core ends up absorbing the imbalance, heating up and degrading efficiency.

To properly suppress common-mode currents on the feedline, a dedicated common-mode choke is still required. This choke should be placed at a strategic distance — typically between 0.05λ and 0.15λ from the feedpoint — to present high impedance to surface currents and prevent them from entering the shack or re-radiating.

This distinction is often overlooked in popular guides and commercial literature. However, differential-mode impedance transformation and common-mode rejection are orthogonal engineering problems — best solved with specialized components.

When to Use a BALUN

A 4:1 BALUN has valid use cases:

  • Center-fed symmetric dipoles or loops in free space or at high elevation
  • Balanced ladder-line-fed antennas with symmetric current paths — although in such cases a 1:1 current balun is more than sufficient, as no impedance transformation is needed
  • Experimental setups with tightly controlled symmetry and predictable reactance

Even here, it is crucial to match the BALUN type (voltage, current, transformer hybrid) to the specific impedance transformation and common-mode rejection needs. Many so-called wideband baluns fail when real-world loads deviate from the purely resistive 200 Ω they were modeled for.

In most unbalanced, wideband, and ground-coupled antenna systems, a 4:1 UNUN provides better performance, lower loss, and fewer surprises.

Conclusion

Transformer selection should reflect electromagnetic reality — not geometric symmetry.

We use 4:1 UNUNs over BALUNs in wideband portable and base antennas because they:

  • Match real-world feedpoint asymmetries
  • Prevent excess core heating and unintentional current imbalance
  • Allow separation of impedance transformation and common-mode suppression tasks
  • Enable robust performance across bands, terrain, and power levels

What may seem like reinventing the wheel is in fact a return to fundamentals. Many of the so-called 'rules' of thumb are outdated dogmas rooted in legacy assumptions. Our design choices are grounded in scientific principles — not anecdotal habits — and validated through empirical testing.

RF current doesn’t care what label is printed on the transformer — it follows the path of least impedance. So should we.

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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.