Where the Current Flows, the Signal Grows

Where the Current Flows, the Signal Grows

By Joeri Van Dooren, ON6URE – RF.Guru

If you want to know how well an antenna will play, trace the current. Not all parts of an antenna radiate equally. The signal is born where RF current is strong and that current is in the clear—up and away from lossy ground and clutter. That simple idea explains why some wires “punch above their weight” and others sound sleepy even with a perfect SWR.

This is a practical tour—no heavy math—of how current distribution shapes performance, and how to use that to your advantage.

Why Current Distribution Matters

• High-current regions are your powerhouses. Put those sections high and clear, and they radiate efficiently.
• Low-current / high-voltage regions are delicate. They stress transformers and insulators and are easily detuned by nearby objects.

Key idea: The more useful current you place where it can “see” free space (not soil), the stronger your signal.

Current-Fed vs. Voltage-Fed — and the Gray Zone Between

How you feed a wire shapes everything: losses, voltage stress, and how tolerant the antenna will be of its environment. There are three families of feed systems, and understanding which one you’re dealing with saves hours of troubleshooting later.

1. Current-Fed — the calm, efficient classic

Examples: center-fed dipoles and doublets.

• Feedpoint sits at a current maximum.
• Low feedpoint voltage — gentle on hardware and insulators.
• Low loss, predictable behavior, and broad bandwidth.
• Performs well even when not very high.

Current-fed systems are straightforward: the radio drives current directly into a balanced point. Radiation is efficient, SWR is stable, and the transformer (if any) runs cool.

2. Voltage-Fed — the demanding specialist

Examples: End-Fed Half-Waves (EFHW), long wires, and high-ratio ununs such as 9:1, 49:1, or 64:1 designs.

• Feedpoint sits at a voltage maximum and a current minimum.
• High impedance—often several kilohms.
• Requires a high-ratio transformer to bring impedance near 50 Ω.
• High stress on cores and feedpoint hardware.
• Highly sensitive to surroundings, feedline routing, and grounding.

These antennas can perform very well, but only if their transformers are designed and cooled correctly and the installation minimizes stray coupling and RF return paths.

3. The Gray Zone — the 4:1 Voltage Unun

This category is often misunderstood. A 4:1 unun (voltage transformer) used for an off-center-fed dipole sits halfway between the two worlds.

It’s still technically voltage-fed, but the impedance and voltage involved are moderate—typically transforming 200 Ω down to 50 Ω. The correct way to feed an OCF is with a 4:1 unun for the impedance step, followed by a 1:1 current balun or choke placed about 25–50 cm behind it. This separation of roles keeps losses low, maintains symmetry, and prevents feedline radiation. Combining both functions into a single “4:1 current balun” wastes power and drives the core into heating and imbalance.

In short: use the right tool for the job. A 4:1 unun efficiently matches a moderately unbalanced antenna to coax. A 1:1 current choke keeps that coax quiet. Higher-ratio ununs (9:1, 49:1, 64:1) are for true end-fed or high-impedance applications where voltage dominates. Knowing which world you’re in makes every installation behave better.

Translation: Feeding near a current maximum makes life easy. Feeding at a voltage maximum can work—but only if your transformer and choke strategy handle it properly.

EFHW (End-Fed Half-Wave)

Fed right at the tip where current is minimal and voltage is maximal. Efficient when executed carefully, but picky: it wants height, a well-designed transformer, and careful coax management to prevent stray transmit return currents on the coax shield and common-mode pickup in receive. When you can get the high-current belly of the wire high and clear, it performs impressively—especially on its true half-wave and full-wave bands.

An EFHW operates naturally on its half-wave fundamental and its even harmonics (for example, 80 / 40 / 20 m). On those bands the current distribution repeats cleanly, and the feedpoint impedance remains in the few-kilohm range that a 1:49–1:64 transformer can match efficiently. Those are the genuine resonances of the wire.

The so-called “extra” bands sometimes claimed in multiband EFHW designs—such as 30 m, 17 m, 15 m, 12 m, and 10 m—are not true resonances. They appear through an interplay of transformer leakage inductance, distributed wire capacitance, and environmental coupling. At those frequencies the system behaves as a self-resonant network rather than a real half-wave radiator. SWR may look reasonable, but efficiency falls sharply and pattern control is lost. A correctly designed EFHW should never need a parallel capacitor to “force” those bands into tune; if it does, you’re compensating for parasitic reactance, not improving radiation.

Above roughly the 17 m band, the antenna’s behavior becomes increasingly erratic. Self-resonance and core capacitance drive impedance swings, the current distribution fractures into multiple lobes, and small changes in height or surroundings detune the match. It will still radiate, but only by accepting a substantial efficiency penalty. That’s why the practical sweet spot for a broadband EFHW remains 80–40–20 m—the bands where physics and geometry truly cooperate.

Doublet (Center-Fed with Ladder Line)

Pure current-fed behavior. Feedpoint sits at a current maximum, and loss is minimal—especially with open-wire line and a proper transmatch (tuner). With the high-current region elevated and clear, a doublet remains one of the most efficient and broadband HF antennas ever built.

Radiation Resistance vs. Loss Resistance

Every antenna turns some power into radiation and some into heat.

• Radiation resistance is the “good” part—power leaving as a wave.
• Loss resistance is the “bad” part—wire loss, transformer heating, ground coupling.

Feeding near a current maximum and lifting that region boosts radiation resistance relative to loss—exactly what you want. A perfect SWR means nothing by itself: a dummy load has a perfect match and radiates nothing.

The “Height Is Might” Myth—With an Asterisk

“Height is might” is true for horizontal antennas, but it’s easily misunderstood with verticals.

Horizontals: their current maximum lies mid-span, so raising that belly lowers take-off angle and sharpens the pattern.

Verticals: they’re two-wire systems—radiator up, return path down. They need a proper reference plane. A tall radiator without tuned radials just invites loss.

• Ground-mounted vertical + many radials: reliable but lossy to soil.
• Elevated vertical + tuned radials: fewer losses, cleaner low-angle radiation.

Practical takeaway: height helps when it lifts current maxima into clear space without breaking geometry. For verticals, raise the whole system—radiator and radials together.

Quick Placement Wins

• Put current high, keep voltage away from clutter.
• Use proper chokes or baluns to block RF return paths.
• Monitor transformer heat—it’s feedback about system loss.
• “Good enough” height still matters; every meter helps.
• Four tuned elevated radials can rival a mat of on-ground wires.

A Simple Mental Model

Picture your antenna as a glowing wire. The glow is brightest where current is strongest. Your job: place that bright section high and free, and feed it in a way that avoids waste.

• Current-fed: simple, efficient, forgiving.
• Voltage-fed: powerful but demanding—treat it with respect.

Bottom line: Where the current flows, the signal grows.

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