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, walls, gutters, masts, or shack wiring, the stronger your signal.

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

How you feed a wire shapes everything: losses, voltage stress, common-mode current, and how tolerant the antenna will be of its environment. There are three broad 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: centre-fed dipoles and doublets.

• Feedpoint sits at or near a current maximum.
• Feedpoint voltage is relatively low.
• Hardware, insulators, and matching components are less stressed.
• Behaviour is usually predictable and repeatable.
• Losses are often easier to keep low.

Current-fed systems are straightforward: the radio drives current directly into a balanced point. Radiation is efficient, SWR behaviour is usually understandable, and the transformer or balun, if used, normally has an easier job.

2. Voltage-Fed — the demanding specialist

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

• Feedpoint sits near a voltage maximum and a current minimum.
• Feedpoint impedance may be high, often in the kilohm region.
• A transformer or matching network is normally required.
• Voltage stress can be high at the feedpoint.
• The system is more sensitive to surroundings, feedline routing, grounding, and counterpoise behaviour.

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

3. Off-Centre-Fed Dipoles — transformer first, choke always

Examples: off-centre-fed dipoles, windoms, and related multiband dipole variants.

An off-centre-fed dipole is not simply a centre-fed dipole moved a little sideways. By feeding the wire away from the centre, the feedpoint impedance can fall into a range that is useful on several bands. In many practical OCF designs, this impedance is somewhere in the few-hundred-ohm region, but it changes with frequency, height, wire split ratio, nearby objects, and feedline routing.

Because the feedpoint is neither perfectly balanced nor 50 Ω, the feed system normally has two separate jobs:

• Impedance transformation: converting the feedpoint impedance toward something the coax and transmitter can use, often with a 4:1 or 6:1 transformer depending on the design.
• Common-mode suppression: stopping the outside of the coax shield from becoming an uncontrolled third wire in the antenna system.

This is where many OCF discussions become confused. A 4:1 voltage balun, a 4:1 current balun, a Guanella transformer, a Ruthroff transformer, an autotransformer, and a separate 1:1 choke do not all do the same job in the same way. Some mainly transform impedance. Some also help with isolation. Some provide useful common-mode suppression only over part of the HF range. Some heat badly when the impedance, power level, duty cycle, or common-mode current is not what the designer expected.

So the safe rule is not “always use a 4:1 voltage unun” or “never use a 4:1 current balun.” A well-designed 4:1 current transformer can work well in some OCF installations. A voltage-style impedance transformer followed by a separate high-impedance 1:1 choke can also work well. What matters is whether the complete feed system transforms the impedance, keeps common-mode current under control, and stays cool at the intended power and duty cycle.

For many practical OCF installations, a good approach is to use a properly rated impedance transformer at the feedpoint and add a serious common-mode choke on the coax side. The choke may be directly at the feedpoint or a short distance away, depending on the mechanical design and measured behaviour. There is no magic universal 25–50 cm spacing rule. The correct location is the one that keeps the coax from radiating and does not over-stress the transformer or choke.

In short: an OCF dipole needs both impedance transformation and common-mode control. Do not judge the system only by SWR. Check for feedline radiation, RF in the shack, pattern distortion, and heating in the transformer or choke.

Translation: Feeding near a current maximum makes life easy. Feeding an OCF away from the centre can give useful multiband behaviour, but the feedline must be controlled. The transformer handles impedance; the choke keeps the coax from becoming part of the antenna.

EFHW (End-Fed Half-Wave)

An End-Fed Half-Wave is fed near the end of a wire that is approximately a half wavelength long on its lowest design band. At that end, current is low and voltage is high, so the feedpoint impedance is high, often in the few-kilohm region. A transformer, commonly 49:1 or 64:1 in many amateur HF designs, is used to bring that impedance closer to 50 Ω.

This makes the EFHW a voltage-fed antenna at the feedpoint. That does not make it bad, but it does make it more demanding. The transformer, insulation, enclosure, capacitor, wire end, and nearby objects can all see significant RF voltage.

An EFHW also still needs a return path. The missing half of the circuit may be a short counterpoise, the outside of the coax shield, the mounting structure, stray capacitance to the environment, or some combination of these. If that return path is not controlled, the coax shield can become part of the antenna. That can change the tuning, bring RF into the shack, increase receive noise, and make the antenna behave differently every time the feedline is moved.

Used carefully, an EFHW can work very well. The trick is to get the high-current part of the wire high and clear, use a transformer that is efficient at the intended power and frequency range, and manage common-mode current with a suitable choke strategy.

Harmonic operation: not only “even harmonics”

A common misunderstanding is that an EFHW works only on its fundamental and even harmonics. The more accurate statement is this:

An end-fed wire has a high feedpoint impedance when its length is close to an integer number of half wavelengths.

That means the useful condition is:

• 1 × half wave
• 2 × half waves
• 3 × half waves
• 4 × half waves
• and so on

Both odd and even multiples can matter. The key is not whether the harmonic number is odd or even. The key is whether the physical wire length is close to n × λ/2 on that band.

For example, a 40 metre EFHW cut near 7 MHz is approximately:

• 1 half wave on 40 m
• 2 half waves on 20 m
• 3 half waves on 15 m
• 4 half waves on 10 m

That is why a well-built 40 metre EFHW can often be usable on 40, 20, 15, and 10 metres. The exact match still depends on wire length, end effect, height, transformer design, compensation, feedline, counterpoise, and surroundings.

The WARC bands are different. On a 40 metre EFHW, bands such as 30 m, 17 m, and 12 m are not clean integer half-wave multiples of the 40 metre wire length. They may still show a usable SWR in some installations, especially with a tuner or transformer compensation, but they should not be described as natural EFHW resonances in the same sense.

Transformer compensation and “extra” bands

Some EFHW transformers include a small compensation capacitor. That is not automatically a sign of a bad design. A compensation capacitor can improve the high-band match by correcting transformer leakage inductance and stray capacitance.

However, compensation does not magically turn every band into an efficient half-wave resonance. It can improve the impedance match seen by the transmitter, but the real antenna performance still depends on current distribution, transformer loss, common-mode current, and radiation pattern.

As the wire becomes several half wavelengths long, the radiation pattern develops more lobes and nulls. This is normal physics, not a defect. On the higher bands, the antenna may still radiate well, but it becomes more directional and more sensitive to height, slope, surroundings, and feedline behaviour.

Practical takeaway: an EFHW is strongest and most predictable on bands where the wire is close to an integer number of half wavelengths and the transformer is operating efficiently. A good SWR on another band is useful, but it is not proof that the antenna is radiating efficiently.

Doublet (Center-Fed with Ladder Line)

Pure current-fed behaviour. The feedpoint sits at a current maximum on the fundamental, and loss can be very low — especially with open-wire line and a proper transmatch. 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 an electromagnetic wave.
• Loss resistance is the “bad” part — wire loss, transformer heating, tuner loss, ferrite loss, ground loss, and unwanted coupling into nearby objects.

Feeding near a current maximum and lifting that current region into clear space improves the ratio of useful radiation to unwanted loss. A perfect SWR means nothing by itself: a dummy load has a perfect match and radiates almost nothing.

The “Height Is Might” Myth — With an Asterisk

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

Horizontals: their current maximum is often near the middle of the span, so raising that current belly usually lowers take-off angle, improves far-field behaviour, and reduces ground loss.

Verticals: they are two-wire systems — radiator up, return path down. They need a proper reference plane. A tall radiator without a suitable radial or counterpoise system simply invites loss and feedline interaction.

• Ground-mounted vertical + many radials: practical and reliable, but soil loss still matters.
• Elevated vertical + tuned radials: often lower loss and cleaner low-angle radiation, but mechanically and electrically more sensitive.

Practical takeaway: height helps when it lifts useful current into clear space without breaking the antenna geometry. For verticals, raise the whole system — radiator and radials together — or provide a return system that is actually part of the design.

Quick Placement Wins

• Put current high and clear.
• Keep high-voltage ends away from gutters, walls, trees, wet ropes, and metalwork.
• Use proper chokes or baluns to control RF return paths.
• Monitor transformer heat; heat is feedback about loss.
• Do not trust SWR alone as a performance measurement.
• Every metre of height can help, but current placement matters more than height slogans.
• Four tuned elevated radials can rival a much larger mat of on-ground wires in some installations.

A Simple Mental Model

Picture your antenna as a glowing wire. The glow is brightest where current is strongest. Your job is to 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 voltage, transformer loss, and return current with respect.
• Off-centre-fed: useful multiband behaviour, but only when impedance transformation and common-mode control are both handled properly.

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

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