Feeding a Resonant Dipole With 600 Ω Open‑Wire

— Why No Choke Is Needed, and How Line Length Really Matters

A precise, monoband-focused guide with a short contrast to the multiband “doublet”.

TL;DR

• A center-fed resonant dipole and 600 Ω open-wire are both balanced; with symmetric geometry, feed it directly — no feedpoint choke required.
• Z_a (antenna) and Z₀ (line) are independent; mismatch creates SWR on the line, not extra radiation. With low-loss open-wire, efficiency stays high even with significant SWR.
• Near-field geometry dominates: keep the first 0.1–0.2 λ of feedline orthogonal to the dipole and clear of conductive surfaces; then route as needed.
• For monoband with no tuner, put a 1:1 current balun at the base, then short 50 Ω coax to the shack. Avoid odd ¼-wave line lengths and aim for an electrical ½-wave so the coax SWR is modest (~1.4:1 typical).

Why No Choke Is Needed at the Feedpoint

A coax-fed dipole often needs a choke because the coax shield is an unbalanced conductor and can carry stray return currents. An open-wire line is balanced: the two conductors carry equal and opposite currents so their fields cancel and the line does not radiate when the geometry is symmetric.

• Make both dipole legs the same electrical length and height.
• Let both conductors leave the feedpoint together, at right angles to the dipole for the first 0.1–0.2 λ.
• Keep that early run in free air, several line spacings away from metal and wet surfaces.

Small asymmetries can still induce a little stray return current on the line. That’s why we put a 1:1 current balun at the bottom—it scrubs any residual current before the coax.

Antenna Impedance vs. Line Impedance — Two Different Things

They don’t have to match. If Za ≠ Z0, you get SWR on the line, but with low-loss open-wire this adds only a small fraction of a dB for typical HF lengths. The energy still flows efficiently from antenna to your balun/coax.

Why Line Length Matters

Coupling & Balance (Does the Line Radiate?)

The reactive near field of a thin half-wave dipole extends to roughly ~0.2 λ. If the feedline leaves at right angles and stays clear for the first 0.1–0.2 λ, differential currents dominate and the line remains “quiet.” This is a geometry problem—not a magic cutoff length.

Impedance Transformation (What the Balun/Coax Sees)

The input impedance of a transmission line is length-dependent:

Zin = Z0 · ( ZL + j Z0 tan βℓ ) / ( Z0 + j ZL tan βℓ )

• At ¼ λ, impedance inverts (high ↔ low).
• At ½ λ, impedance repeats (Zin ≈ ZL).

Practical takeaway: For monoband into a 1:1 balun then coax with no tuner, avoid odd ¼-wave feedline lengths and aim for an electrical ½-wave (or any integer multiple). Staying within roughly ±2 % of ½-wave typically keeps coax SWR ≤ 2:1.

Practical Build for a Lightweight Monoband Dipole

• Antenna: Center-fed resonant dipole or inverted-V (e.g., 40 m or 20 m).
• Feedline: True wide-spaced open-wire (~600 Ω) or “ladder line” (~400–500 Ω).
• Transition: 1:1 current (Guanella) balun at ground → short 50 Ω coax to the shack.
• Routing: Keep the first 0.1–0.2 λ orthogonal and clear of other conductors.

What Spacing Really Gives “600 Ω”?

For two round wires in air, Z0 ≈ 120·acosh(S/d), where S is spacing and d conductor diameter.

“450 Ω ladder line” often measures 400–500 Ω in practice.

How This Differs From a Multiband Doublet

Design Cookbook (Monoband, No Tuner)

1. Cut the dipole for resonance near band center: L[m] ≈ 143/f[MHz].
2. Choose feedline Z₀: 400–600 Ω typical.
3. Plan open-wire length: make electrical length ≈ n·½-wave. L½[m] ≈ (150·VF)/f, VF≈0.98–0.99.
4. At the base: 1:1 balun → short 50 Ω coax. If SWR > 2:1, trim line or touch up with ATU.
5. Static & weather: 100 kΩ–1 MΩ bleeder across feedpoint helps with charge.

Half-Wave Open-Wire Reference Lengths (VF = 0.99)

Troubleshooting & Gotchas

• RF in the shack: Ensure bottom balun ≥ 3 kΩ choking Z; add coax choke if needed.
• High SWR: Likely near ¼-wave length. Trim toward ½-wave.
• Rain detuning: Wet wood/leaves change Z₀; keep line clear and elevated.
• Inverted-V: Lower Z_a (40–60 Ω). Still fine — check your ½-wave window.
• Ferrite heating: If operating near ¼-wave Z peaks, reduce power or adjust length.

Special Case: 160 m — Short Open-Wire Sections

Short answer: At 160 m, a 16 m line is only 0.10 λ, so you’re far from the ¼-wave points. SWR on the line stays constant; input impedance changes only slowly with length because phase rotation is small at 1.8–2 MHz.

• SWR on the line is length-independent: SWR = (1+|ΓL|)/(1−|ΓL|).
• Only phase changes with ℓ: Γin=ΓL·e−j2βℓ.
• At 1.9 MHz (λ≈158 m), 16 m = 0.10 λ → rotation ~28°, far from 90° inversion.
• Consequence: Below 0.1 λ, impedance varies slowly and efficiency remains high.
• Contrast: On 80 m, same line = 0.2 λ (144° rotation); on 40 m = 0.4 λ (288° rotation).

Bottom line: A resonant monoband dipole fed with open-wire is simple and efficient. Keep the early section symmetric, avoid odd ¼-wave lengths, use a solid 1:1 current balun at the base—and enjoy clean, stable signals.

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