Understanding Current Taper in Antennas

Current taper refers to the gradual decrease in current amplitude along the length of an antenna conductor, moving away from the feedpoint. It is a direct consequence of how standing waves form on antennas, and plays a crucial role in determining radiation resistance, efficiency, and the overall radiation pattern.

The Physics Behind It

In a resonant half-wave dipole or monopole, the feedpoint is typically located at a current maximum. From there, the current amplitude tapers off symmetrically toward the ends, following a sine distribution in ideal free-space conditions. Mathematically, for a center-fed, lossless half-wave dipole:

I(x) = I0 · sin(k(L/2 - |x|))

where:

  • I0 is the current maximum at the feedpoint,
  • k = 2π/λ is the wave number,
  • x is the distance from the center.

This taper is not due to conductor loss, but to the boundary conditions at the antenna ends. Since the ends are open-circuited, they enforce a current null, forcing the taper to follow the standing wave envelope.

Dielectric Influence: Minor but Present

While the taper arises fundamentally from boundary conditions and standing wave formation, the surrounding dielectric medium (typically air) can have a secondary influence. The dielectric constant affects the local wave propagation speed, which in turn slightly modifies the electrical length of the antenna. In free space or typical above-ground installations, this effect is minimal. However, antennas placed very close to high-dielectric materials or near lossy ground may exhibit subtle shifts in current distribution due to altered velocity factors and impedance loading. Still, these are second-order effects — the defining cause of current taper remains the open-ended boundary enforcing a current minimum.

Why It Matters

Current taper determines where and how efficiently an antenna radiates. Radiation strength is proportional to current magnitude and distribution, so regions of low current (such as the ends of the antenna) contribute little to radiation. Electrically short antennas show steep taper, concentrating radiation near the feedpoint and leading to low radiation resistance.

Multiband and asymmetric antennas — such as EFHWs or OCFs — exhibit non-symmetric taper patterns. In these systems, current taper also depends on feedpoint placement, impedance transformation, common-mode behavior, and ground proximity. A poorly chosen feedpoint can shift the taper unfavorably, reduce efficiency, and distort the pattern.

High-performance designs often manipulate the taper by feeding the antenna closer to regions of higher current — not necessarily at the end or at the exact center. This is particularly important in ground-mounted or vertical systems, where placing the feedpoint too low can result in feeding a current minimum.

Historical Blind Spot

Surprisingly, major reference works like Rothammel's Antenna Book, the ARRL Handbook, and even ON4UN's Low-Band DXing provide little direct attention to current taper. While they present current distributions graphically, they rarely name or isolate the phenomenon or explore its consequences in design or matching. Instead, their focus remains on resonance, impedance, and pattern — leaving taper as an implied result rather than a tool.

In the case of ON4UN, the concept does emerge more clearly in his treatment of traveling wave antennas such as Beverages and terminated folded dipoles. He illustrates and discusses how current decays along the wire toward the termination — a key attribute of traveling wave behavior. However, he does not explicitly label this decay as "current taper," nor does he generalize it to resonant or multiband systems. The description is functional and accurate, but the broader design significance of taper is left unarticulated.

This oversight has led many hams to miss an important design lever. By adjusting the feedpoint to intersect a region of higher current — even if it's not the center — one can improve coupling, reduce matching losses, and enhance usable bandwidth.

Final Thought

Current taper is not an academic curiosity. It defines the effectiveness of your radiator. Designing with taper in mind bridges the gap between theoretical resonance and real-world performance. If you're not thinking about where your current flows — and how fast it drops off — you're not really designing the antenna.

"It’s not where the wire is — it’s where the current flows that makes the antenna radiate."

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Written by Joeri Van Dooren, ON6URE – RF, electronics and software engineer, complex platform and antenna designer. Founder of RF.Guru. An expert in active and passive antennas, high-power RF transformers, and custom RF solutions, he has also engineered telecom and broadcast hardware, including set-top boxes, transcoders, and E1/T1 switchboards. His expertise spans high-power RF, embedded systems, digital signal processing, and complex software platforms, driving innovation in both amateur and professional communications industries.