Lowering SWR on a Dipole or Inverted-V by Making It Asymmetric

A dipole or inverted-V looks perfectly symmetrical on paper—two equal legs, equal heights, and a centered feedpoint. Real life rarely follows that drawing. Trees, gutters, roofs, the slope of the ground, and the feedline itself all disturb that balance. These small environmental asymmetries shift the feedpoint impedance away from what your coax wants to see, raising SWR.

By adding a small intentional asymmetry—usually making one leg slightly longer or lower—you can offset the natural imbalance. This controlled tweak shifts how current divides between the legs, nudging the feedpoint impedance toward 50 Ω and lowering SWR. You’re essentially balancing out the environment’s imbalance.

SWR in Plain Words

SWR rises when the antenna’s feedpoint doesn’t look like the coax. A good match needs two things at the operating frequency:

• Right resistance (close to what your coax expects).
• Little or no reactance (the antenna isn’t behaving like a capacitor or inductor).

Adjusting total length tunes resonance (reactance). Adjusting asymmetry influences how the resistance part behaves—often the key to lowering SWR without shifting frequency.

What Asymmetry Means Here

You can apply asymmetry in three main ways:

• Length asymmetry: One leg slightly longer than the other.
• Geometry asymmetry: One leg lower, closer to the roof or ground, or bent differently (for inverted-V: unequal droop or end heights).
• Feedpoint placement asymmetry: Moving the feedpoint away from center (classic off-center feed). This raises impedance but is less useful for small coax-fed SWR tweaks.

For day-to-day tuning with plain coax, stick to length and geometry asymmetry.

Why Asymmetry Changes SWR

Think of current and voltage standing waves along the wire. A perfectly centered feedpoint sits at a current maximum. If you change one leg’s length or height, you “slide” that current peak relative to the feedpoint. That changes how much resistance and reactance appear there.

• Lengthening one leg shifts the current distribution, often raising the feedpoint resistance slightly—good if it was too low.
• Lowering a leg increases coupling to ground or nearby metal, also changing the current balance and the effective impedance.
• The result: the SWR dip may deepen (lower SWR) without moving much in frequency.

A helpful mental model: altering leg length is like moving your feedpoint along the wire. Closer to a current maximum → lower resistance. Closer to a voltage maximum → higher resistance. Small shifts make controllable changes.

Why It’s Especially Effective on an Inverted-V

Inverted-Vs already have a lower feedpoint resistance than flat dipoles due to their geometry and ground coupling. That’s why they often show a shallow SWR dip (e.g. 2:1 minimum). Adding a little asymmetry raises the feedpoint resistance toward 50 Ω, reducing SWR. You can do this by lengthening one leg, lowering one end, or adjusting droop asymmetrically. Even a few centimeters can make a measurable difference.

A Practical Workflow (Words Only, No Math)

• Start symmetrical and measure. Use a proper 1:1 current choke at the feedpoint to prevent the coax from radiating, then make an SWR sweep.
• Adjust total length first to bring resonance where you want it.
• If SWR minimum is too high, try asymmetry next.
• See-saw method: Lengthen one leg slightly while shortening the other by the same amount. Total length stays nearly constant, but feedpoint resistance shifts. Continue small steps until SWR stops improving.
• Try geometry tweaks: Lower or raise one leg a little, or route it differently relative to ground or structures. Measure after each change.
• Secure and re-check once you find the best configuration.

What’s Really Being Fixed

• Environmental skew: Asymmetry counterbalances unavoidable left-right differences from surroundings and feedline.
• Resistive vs. reactive balance: Total length handles reactance (resonance). Asymmetry fine-tunes the resistive part.
• Bandwidth and harmonics: Small asymmetries may gently reshape the SWR curve or harmonic behavior—usually beneficial for single-band antennas.

Important Cautions

• Always use a current choke at the feedpoint. Without it, coax radiation can fake SWR “improvements.”
• Expect pattern tilt: Small asymmetries can skew the radiation pattern slightly—typically modest but real.
• Don’t over-tune: Once SWR is reasonable, chasing perfection can worsen performance.
• Asymmetry isn’t magic: If your antenna is too low or near metal, fix geometry before fine-tuning symmetry.

Special Note on Off-Center Feeding

Off-center-fed dipoles intentionally raise feedpoint impedance for multiband work using a 4:1 or 6:1 unun. That’s a different design goal. For single-band coax-fed antennas, keep the feedpoint centered and use mild asymmetry only for fine matching.

Takeaway

Symmetry is tidy in drawings but rare in nature. By introducing a small, deliberate asymmetry—preferably with the see-saw method—you can balance real-world imbalances, bring the feedpoint impedance closer to 50 Ω, and lower SWR without disturbing resonance. Think of it as coax diplomacy: making both sides meet halfway.

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