Current & Voltage Distribution: Keys to Antenna Performance

Why Current and Voltage Distribution Define Antenna Behavior

When building high-performance antennas, it’s not just the length, height, or feedpoint that matters. The distribution of current and voltage along the elements shapes radiation patterns, defines polarization, and sets efficiency. Let’s dive into the fundamentals and then see why dipoles and verticals illustrate this better than anything.

Standing Waves and Energy Maps

Antennas support standing waves of RF current and voltage. Where current peaks, radiation is strongest. Where voltage peaks, impedance skyrockets. Every antenna is essentially a guided energy map of these two quantities:

• Current maxima: Drive radiation. These define gain, lobes, and take-off angle.
• Voltage maxima: Dictate feedpoint impedance, insulation stress, and losses.

Radiation comes from current. Voltage defines how you feed and where losses creep in.

Dipoles: Current Defines Everything

A half-wave dipole behaves like a standing-wave transmission line:

• Feedpoint at center = current maximum, voltage minimum → ≈70 Ω, easy to match.
• Current tapers symmetrically toward the ends → clean figure-eight radiation.
• Ends = current null, voltage maximum → sensitive to leakage and corona.

Symmetry matters: if one leg couples differently to ground (poor grounding, asymmetrical feed, or traps), polarization and pattern distort. Off-center feeding deliberately shifts the current peaks, creating multi-band behavior but also changing the lobe structure.

Verticals: Voltage Symmetry Is Key

Vertical antennas look simple but hide complexity in their distributions:

• A 1/4 λ vertical has a current maximum at the base → low impedance, but ground losses matter most here.
• A 1/2 λ vertical fed at the base is at a voltage maximum → high impedance, demanding a matching transformer.
• Elevated radials mirror voltage nodes → without symmetry, imbalance creates common-mode currents and skewed patterns.

Why Symmetry Protects Efficiency

Clean symmetry of current and voltage preserves predictable behavior:

• Dipoles: Symmetry ensures horizontal polarization and stable patterns.
• Verticals: Radial symmetry stabilizes impedance and minimizes common-mode current.
• End-feds: Feedpoint sits at a voltage maximum, forcing extreme impedance transformation and making common-mode suppression mandatory.

Polarization: The Direct Result of Current Flow

Polarization isn’t a guess; it’s defined by the axis of current:

• Horizontal current → horizontal polarization.
• Vertical current → vertical polarization.
• Asymmetry → mixed polarization and efficiency loss.

The E-field aligns with current direction, while the H-field wraps around it. This is why maintaining a clean distribution map matters more than obsessing over “SWR” alone.

Bottom Line: Respect the Map

Every antenna is a guided map of current and voltage. Ask yourself:

• Where is current strongest?
• Are voltage paths symmetrical?
• Does this preserve clean polarization?

Small changes—like shifting a feedpoint, raising a vertical, or rerouting radials—reshape that map, sometimes dramatically improving performance. Wires are not just conductors: they are standing-wave energy maps. And symmetry is the compass.

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