MagLoop Efficiency Myths and What Really Determines Performance
Every few months a new “magic magnetic loop formula” circulates online — often claiming that taking 10% of the full-wave loop length and averaging the 40 m and 80 m values will magically produce a maximum-efficiency magnetic loop. It sounds tidy, but it is completely disconnected from the physics that govern small transmitting loops.
This article explains what those shortcuts are actually doing, why they are wrong, and what truly controls magnetic loop performance.
What the 10% Rule Is Really Doing
The full-wave loop estimate (1005 ÷ MHz) is a convenient heuristic for resonant wire loops. It has nothing to do with small transmitting loops (STLs). Declaring “a magnetic loop is 10 % of that,” and then averaging two band values, is numerology — not antenna theory.
What Actually Determines Magnetic Loop Efficiency
A magnetic loop’s efficiency comes from the balance between:
- Radiation resistance (Rrad) — extremely small on low bands
- Loss resistance (Rloss) — copper, joints, capacitor ESR, proximity losses
η = Rrad / (Rrad + Rloss)
The key is that radiation resistance depends strongly on loop area and wavelength, not on any percentage of a full-wave loop length.
Why Circumference Averaging Doesn’t Work
For the often-quoted “optimal” 21.55 ft loop:
- Rrad ≈ 0.11 Ω on 40 m
- Rrad ≈ 0.007 Ω on 80 m
Even with a very low-loss design (Rloss ≈ 0.1 Ω):
- 40 m efficiency ≈ 52 %
- 80 m efficiency ≈ 6 %
No arithmetic average of resonant loop circumferences can overcome the fact that radiation resistance scales with 1/λ⁴. This is why 80 m is always significantly less efficient on a physically small loop.
Better Ways to Design a Real MagLoop
- Start from your physical constraints: diameter, conductor type.
- Design for the lowest band first (80 m).
- Calculate inductance and required tuning capacitance.
- Compute Rrad and Rloss based on actual geometry and materials.
- Iterate size and conductor thickness to optimize efficiency.
Once designed for 80 m, the loop will automatically be more efficient on 40 m — that is simply how electrically small antennas behave.
Key Takeaways
- The 1005/f formula is not relevant for magnetic loops.
- “10 % of a full wave” is not an engineering principle.
- Efficiency depends on loop area and losses, not circumference myths.
- You cannot make 40 m and 80 m equally efficient on a small loop.
- Physics, not averages, determines magnetic-loop performance.
Mini-FAQ
- Is a larger loop always better? — Yes. Larger area increases radiation resistance dramatically.
- Can a compact loop be efficient on 80 m? — Only if built large and extremely low-loss.
- Is the 10% rule meaningful? — No. It is a hobby myth with no basis in small-loop theory.
- Why does 40 m always outperform 80 m? — Radiation resistance is ~16× higher on 40 m for the same loop size.
Interested in more technical content? Subscribe to our updates for deep-dive RF articles and lab notes.
Questions or experiences to share? Contact RF.Guru.