Why the “One Wire-Spacing” Rule for HF Window Line Is a Myth
Why the “One Wire-Spacing” Rule for HF Window Line Is a Myth
The field around a parallel-wire line does not stop at one conductor spacing. Whether a route is acceptable depends on the complete cross-section, nearby materials, symmetry, moisture and the measured limits of the installed feed system.
The originating claim sounds practical: keep window line about one conductor spacing away from nearby objects, or put it in pipe insulation, and it can be laid on a roof, soil or inside conduit. Joeri’s challenge is direct: that is not a transmission-line rule. A low-loss spacer may be benign, while the structure beyond it still changes impedance, velocity, attenuation or balance.
There is no universal clearance threshold. Increase separation until measured installed behaviour converges sufficiently toward the free-air reference for the actual frequency, load, weather and acceptance limits.
One Spacing Is a Geometry Ratio, Not a Field Boundary
The electric and magnetic fields around a two-wire line decay continuously into the surrounding space. They do not end at a distance equal to the conductor separation. An object’s effect depends on more than its nearest distance:
- conductor diameter, separation and the line’s plastic web or supports;
- object size, orientation and length parallel to the line;
- permittivity, conductivity, magnetic properties and moisture;
- whether both conductors couple to it equally; and
- frequency, load, line length and the voltage/current distribution.
A small dry spacer touching both sides symmetrically can have little effect. A long wet roof membrane, metal flashing, reinforced concrete or earth at the same nominal distance can have a much larger effect. “One spacing” cannot describe both cases.
The Parallel-Wire Formula Is a Baseline
For two identical round conductors in a uniform homogeneous dielectric, sufficiently far from other conductors, the low-loss approximation is:
Z0 ≈ (120 Ω/√εr) cosh−1(D/d)
D is centre-to-centre spacing and d is conductor diameter. The formula shows why geometry and dielectric matter, but its assumptions are exactly what a roof, soil, an off-centre conduit or a discontinuous window-line web violates. A real line has a composite field through air, insulation, water films and nearby structures, so a single bulk εr may not exist.
The general distributed-line expressions are safer:
Z0 = √[(R + jωL)/(G + jωC)]
γ = α + jβ = √[(R + jωL)(G + jωC)]
Nearby dielectric generally changes capacitance and may change field-dependent loss. Nearby conductors can change both capacitance and inductance and introduce additional modes. In a uniform lossy dielectric, G is commonly approximated by ωC tanδ; a mixed roof/air/plastic/soil cross-section must instead be treated as a field-distribution problem or measured directly.
Effective Permittivity Changes Impedance and Velocity
In the low-loss approximation, Z0 is about √(L/C) and phase velocity is about 1/√(LC). If a dielectric intercepts more of the electric field, the effective capacitance normally rises, phase velocity falls and electrical length changes. The impedance shift depends on whether L changes as well.
This is why “450 Ω” is a nominal characteristic impedance under stated conditions, not an invariant value. Wes Stewart, N7WS, measured commercial window-line samples with an error-corrected VNA. His published 50 MHz results included the following:
| Sample and condition | Measured Z0 | Velocity factor | Matched loss |
|---|---|---|---|
| Wireman #551, dry | 405 Ω | 0.902 | 0.33 dB/100 ft |
| Wireman #551, wetted | 387 Ω | 0.864 | 5.8 dB/100 ft |
| Wireman #554, dry | 359 Ω | 0.928 | 0.41 dB/100 ft |
| Wireman #554, wetted | 343 Ω | 0.887 | 6.1 dB/100 ft |
Those were short sample measurements at 50 MHz using a wetting-agent procedure intended to create a severe wet condition. They are not loss specifications for every product or for HF. They do establish the relevant point: moisture can change characteristic impedance, propagation velocity and attenuation together, and the magnitude is construction- and test-specific.
A Benign Spacer Does Not Characterize What Lies Beyond It
A spacer should be judged as one component of the installed cross-section. Low density alone is not a complete RF specification. Material permittivity, loss tangent, water absorption, contamination, compression and geometry all matter.
| Installation feature | What it can change | Required boundary |
|---|---|---|
| Small symmetric dry spacer | Local C, velocity and a short discontinuity | Measure the actual material and support geometry; do not infer from appearance |
| Roof assembly | Dielectric loading, loss and asymmetry from membranes, moisture, flashing, mesh or rebar | Test the complete dry/wet route, including hidden conductors |
| Soil or damp masonry | Complex permittivity and conductivity that vary with frequency, composition, temperature and water content | Keep the line clear or verify the worst credible moisture condition |
| PVC or other insulating conduit | Dielectric loading; off-centre line position; coupling to the structure outside the conduit | Control centring and test the complete conduit installation |
| Metal conduit | A new multiconductor line with the conduit as a third conductor | Redesign and terminate it as a shielded structure; ordinary window line is not automatically 450 Ω twinax |
ITU-R P.527 models soil’s complex permittivity as a function of frequency, temperature, composition and volumetric water content. That is a much more realistic boundary than “rain always adds the same amount of loss.” Weather can materially change a route, but the direction and magnitude must be established for the actual materials and geometry.
Symmetry Controls Mode Conversion
Balanced operation requires more than equal conductor spacing. Both conductors must have sufficiently equal coupling to the surrounding structure. Using the same longitudinal current reference direction:
IDM = (I1 − I2)/2
ICM = (I1 + I2)/2
In the intended differential mode, the conductor currents are equal and opposite. Unequal capacitance or impedance to a roof, gutter, mast, soil or conduit can convert some differential energy into common mode. ITU-R SM.2158 describes the same general mechanism: balanced differential and common modes propagate separately in a balanced structure and couple at imbalances.
Common-mode current can make the feedline and connected wiring participate in radiation and reception. It can change the pattern, local-noise pickup and the impedance seen when routing or station connections move. A satisfactory differential SWR reading does not prove that this external mode is absent.
Roof, Soil and Conduit Need Different Answers
A Roof Is an Assembly
A dry nonconductive roof with a short perpendicular crossing may cause little disturbance. A long parallel run over a wet membrane, metal deck, foil insulation, flashing or reinforcing mesh is a different cross-section. Inspect construction drawings where possible, test dry and wet, and avoid treating the visible surface as the only material present.
Soil Is a Variable Lossy Medium
Direct contact with soil is difficult to keep uniform, dry and symmetric. Even if a foam sleeve prevents abrasion, the soil beyond it remains part of the field environment. Elevate the pair when practical. Where a near-ground section is unavoidable, measure seasonal and wet-condition changes rather than relying on a dry-day tune.
Insulating and Metal Conduit Are Not Equivalent
Oversized insulating conduit can protect the line mechanically, but it does not shield it from nearby metal or earth. The pair must remain centred and symmetric, and the conduit material and supports must be included in the test.
Metal conduit can provide shielding only as part of a deliberately engineered multiconductor line. Pair position, shield continuity, bonding, bends and terminations determine its modes and impedance. Pulling ordinary window line loosely through metal tube does not preserve its free-air characteristic impedance or guarantee balance.
Replace a Distance Rule with an Installed Convergence Test
Begin with the intended line suspended in a repeatable low-coupling reference arrangement. Then reproduce the proposed support, clearance and nearby structure over the actual parallel run length. Test at band edges and with representative loads, not only with a 50 Ω termination.
Define acceptance limits before choosing the route:
- Differential impedance: allowable Z0 or time-domain impedance departure and discontinuity length;
- Velocity: allowable delay/electrical-length change across bands;
- Loss: allowable excess Sdd21 or delivered-power reduction, including wet conditions;
- Mode conversion: allowable Scd21 and measured common-mode current;
- System behaviour: acceptable antenna input transformation, pattern and receive-noise change; and
- Power boundary: conductor/support voltage, current, heating, insulation and accessible-part safety at the intended transmitter power.
Increase distance, shorten the parallel run, change orientation or improve symmetry until the measured values converge within those limits. The resulting clearance is valid for that installation—not a new universal multiple of conductor spacing.
Measurement Methods That Separate the Effects
- Use a four-port VNA or characterized balanced fixture to measure mixed-mode S-parameters. Sdd describes differential reflection/transmission; Scd exposes differential-to-common-mode conversion.
- Use time-domain transformation to locate impedance changes at supports, roof crossings, bends and conduit transitions.
- Measure line delay or phase slope to detect velocity-factor changes.
- Use calibrated current probes on the pair or matched probes on both conductors to map common-mode current at several positions.
- Repeat reference and installed measurements dry, after controlled wetting where safe, and through realistic temperature/weather states.
- Cross-swap fixtures and line orientation to separate environmental asymmetry from instrument or balun error.
Do not make high-power adjustments live. Balanced lines can carry high differential RF voltage even when common-mode current is low. De-energize before touching or moving the line, and use supports and clearances rated for the actual electrical and environmental stress.
Bottom line: the one-wire-spacing folklore fails because distance alone does not specify the electromagnetic cross-section. A benign spacer can remain useful, but the roof, soil or conduit beyond it must still pass impedance, delay, loss, balance and power checks in the installed state.
Primary technical references
- NISTIR 4487 — Parallel-Wire RLGC Parameters and Environmental Limits
- Wes Stewart, N7WS — Balanced Transmission Lines in Current Amateur Practice
- ARRL — Transmission Line for Windows Documentation and Measured Line Models
- ITU-R P.527-6 — Electrical Characteristics of the Surface of the Earth
- ITU-R SM.2158-3 — Differential/Common-Mode Current and Imbalance
- Rohde & Schwarz — Measuring Balanced Components and Mixed-Mode Parameters
- Keysight — Time-Domain Analysis Using a Network Analyzer
Mini-FAQ
- Is one conductor spacing a safe universal clearance? No. The effect depends on the complete geometry, material, run length, symmetry, frequency, load and moisture.
- Can pipe insulation be an electrically mild spacer? Sometimes. Its permittivity, loss, water absorption and geometry must be known or measured, and it does not characterize the roof, soil or conduit beyond it.
- Why can rain change window-line behaviour? Water and wet materials can change the cross-section’s complex permittivity, conductivity, impedance, velocity and loss; the result depends on the actual construction.
- Can window line run inside PVC conduit? It can be tested as an installation, but PVC adds dielectric loading and does not shield the pair from the world outside. Centring and symmetry remain important.
- Can window line run inside metal conduit? Only as a newly designed multiconductor structure. The conduit becomes a third conductor, so impedance, modes, bonding and terminations must be engineered.
- How should acceptable clearance be chosen? Increase distance or change routing until impedance, delay, excess loss and mode conversion converge within predefined installed-system limits.