Why a “perfectly symmetric” loop often isn’t, especially when it’s low

Updated: December 2025 — Practical antenna behavior explained from real-world installations.

The Myth of Symmetry

A lot of hams look at a loop antenna and assume it has “automatic symmetry” built in. The shape is symmetric, the wire comes back to itself, and the current “must” split evenly and behave nicely — almost like a loop has some built-in magic balancer.

That belief is the myth of symmetry.

A loop can look symmetric and still be electrically asymmetric. And the lower the loop is (especially below about ¼-wavelength), the more likely it is that the loop is not behaving like the clean, balanced textbook antenna people imagine.

This article explains what symmetry really means for antennas, why low loops break it, why the feed system matters as much as the wire, and what you can do if you want predictable behavior.

What “symmetry” really means (and what it doesn’t)

When hams say “my loop is symmetric,” they usually mean geometric symmetry:

• The loop has a nice shape (square, rectangle, circle).
• The feedpoint looks centered.
• Both sides are the same length.

That is geometric symmetry.

What actually matters is electrical symmetry:

• Equal and opposite currents in corresponding sections.
• Balanced voltage to ground on both sides of the feedpoint.
• Similar coupling to ground and nearby objects.
• No significant common-mode current on the feedline.

A loop is only symmetric if the entire system is symmetric: antenna, feed, and surroundings.

And the surroundings are where the myth usually collapses.

Why height matters so much: the ¼-wavelength guideline

Experienced loop users repeat a simple rule of thumb:

• Above ~¼ λ: behavior starts resembling free-space models.
• Below ~¼ λ: ground interaction dominates and symmetry breaks easily.

This isn’t a hard switch, but it’s a very useful dividing line.

What changes when the loop is low?

• Unequal ground coupling — each section has different capacitance and loss to earth.
• Image currents distort fields — current distribution and impedance shift.
• Environmental loss is uneven — wet soil, dry soil, roofs, fences all behave differently.
• The loop becomes a system of parasitic couplings, rarely symmetric.

Even if all wire lengths are equal, the impedances seen in each direction are not.

“But it’s a loop — aren’t loops self-balancing?”

This is the heart of the myth.

Loops can behave beautifully when they are high enough, in a uniform environment, and fed in a way that enforces balanced currents. But a loop does not magically force symmetry.

If the feed allows imbalance, the system will become imbalanced — especially when the loop is low.

The feedline problem: where symmetry really ends

Most “mystery loop behavior” is not the loop itself.

It is common-mode current on the outside of the coax.

Without an effective current choke at the feedpoint, the feedline becomes:

• a third radiator,
• an extra wire hanging off one side,
• a strong source of asymmetry.

Once common-mode flows, you no longer have “a loop.” You have a loop plus a random wire.

The transformer confusion: 4:1 UNUN, 4:1 BALUN, and SWR myths

A common misconception:

• “The loop is balanced.”
• “A 4:1 transformer matches it.”
• “The SWR is good, so it must be right.”

What often actually happens:

• The loop is electrically asymmetric.
• The feedpoint impedance is distorted by ground coupling.
• The transformer is acting as an SWR band-aid.
• Common-mode current still exists.

Matching is not the same as balancing.

A transformer can make the radio happy while the antenna system remains asymmetric.

Why low loops are rarely symmetric in practice

Below ~¼ λ, symmetry is broken easily by:

• house walls, gutters, and downspouts,
• driveways, fences, and metal roofs,
• unequal ground moisture,
• feedline routing and support hardware.

These effects matter far more when the antenna’s near field strongly interacts with its environment.

So what is symmetry?

A useful working definition:

A symmetric antenna is one whose currents are symmetric — not one whose geometry is symmetric.

Signs your “symmetric” loop is actually asymmetric

• SWR changes when you move or reroute the coax.
• RF in the shack or hot microphones.
• Noise level changes with feedline routing.
• Strong pattern pull not matching expectations.
• Performance changes after rain.

Practical ways to reduce the symmetry problem

Get height where it matters

What matters is electrical height, not absolute height. A loop that is too low on 80 m band may behave acceptably on 20 m band only because the same physical height represents a much larger fraction of a wavelength. The improvement is not due to the loop itself, but because the antenna becomes electrically higher at shorter wavelengths.

Control common-mode aggressively

• Use a proper current choke at the feedpoint.
• Route coax away at right angles.
• Avoid parallel runs close to the loop.

Don’t confuse matching with balance

SWR alone tells you nothing about current symmetry.

Accept that the environment is part of the antenna

If one side sees metal and the other doesn’t, the antenna is not symmetric.

Measure instead of guessing

A balanced system is far less sensitive to feedline movement.

The bottom line

Loops are excellent antennas — but they are not magic.

Symmetry is not guaranteed by shape. It is earned through height, controlled feed currents, and a reasonably symmetric environment.

Currents decide. The environment decides what currents are possible.

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