Why the 80 m Lazy Loop aka Sky Loop Needs a 4:1 UNUN, Not a 4:1 BALUN
Why the 80 m Lazy Loop Needs a 4:1 UNUN, Not a 4:1 BALUN
The 80 m Lazy Loop — also known as a Skyloop, Horizontal Loop, or simply a full-wave loop — is a classic antenna: four sides of 21 m each, giving a perimeter of about 84 m, nearly one wavelength at 3.6–3.7 MHz. Fed in a corner at ~10 m height (often strung between trees), it may look like a perfectly balanced antenna on paper. In the real world, however, a corner-fed loop at low height behaves unbalanced. That’s why the correct transformer choice is a 4:1 UNUN, not a 4:1 BALUN.
Why the Loop Isn’t Truly Balanced in Practice
- Equal paths, unequal reality: A corner feed splits the loop into two equal 42 m paths. In theory this is balanced, but in practice current distribution is distorted by ground proximity and feedline coupling.
- Low height: At ~10 m (~0.12 λ), the loop strongly couples to the ground. Image currents and capacitive effects disturb symmetry even if the geometry is perfect.
- Feedline influence: The coax shield acts like a third conductor unless it is choked. This adds a common-mode component that no current BALUN can fully “fix.”
- Environment: Trees, tilts, and real-world geometry drift push the feedpoint impedance away from the ideal symmetric case.
Why a 4:1 BALUN Struggles
A 4:1 current BALUN assumes equal and opposite currents in its outputs. When asymmetry creeps in:
- The core must absorb the imbalance, causing heat and loss.
- High-duty modes like FT8 or RTTY risk saturation.
- The feedline can radiate, creating pattern distortion and noise pickup.
Why a 4:1 UNUN Works Better
A 4:1 UNUN is a voltage transformer with one side grounded. It doesn’t fight to balance currents — it just transforms impedance efficiently:
- Matches reality: handles variable, ground-influenced impedances more gracefully.
- Cooler cores: no wasted flux forcing symmetry.
- Cleaner design: common-mode suppression is left to a dedicated choke where it belongs.
(Add a 1:1 choke about 0.05–0.10 λ down the coax — ~4–8 m on 80 m — for proper common-mode rejection.)
Practical Results
- Smoother SWR across 80–10 m.
- Cooler cores under QRO duty.
- Reduced receive noise thanks to less feedline radiation.
Key Takeaway
The 80 m Lazy Loop may be called a Skyloop, Horizontal Loop, or full-wave loop, but when it’s corner-fed at ~10 m height it is geometrically balanced yet environmentally unbalanced. A 4:1 UNUN accepts this reality and performs the impedance step cleanly, while a dedicated 1:1 choke eliminates common-mode current. This division of tasks ensures efficiency, stability, and lower noise in real installations.
Mini-FAQ
- Isn’t the loop balanced? — Geometrically yes, but ground coupling and feedline effects make it unbalanced in practice.
- Where should the choke go? — About 0.05–0.10 λ down the coax (~4–8 m on 80 m).
- When is a BALUN valid? — On monoband, center-fed dipoles or loops where symmetry is preserved and the feedpoint impedance stays near its nominal value (typically with a 1:1 current balun).
- What about multiband use? — The UNUN handles impedance variation better; the choke keeps the feedline quiet.
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