John Portune’s Window-Line Argument Fails at the Foundation

Portune’s practical idea deserves a fair test. Foam can keep the conductors off an abrasive or wet surface and can hold a more repeatable geometry. The engineering boundary is that the foam also becomes part of the dielectric environment, while nearby metal or earth can become part of the electromagnetic environment.

What Portune Actually Tested

The test used a VNWA-3e, a 9:1 Guanella transformer, a length of foam-covered window line and an open circuit at the far end. Portune compared the returned signal from about 18 to 22 MHz, with attention near the line’s first half-wave repetition around 19.8 MHz.

That is a legitimate reflection experiment. With a uniform single-mode line, an ideal open circuit and a measurement plane calibrated to the line terminals, the magnitude of the returned wave contains the round-trip attenuation. The same setup can also reveal changes in electrical length through reflection phase. In Portune’s fixture, however, the measured S11 includes the transformer, its transition to the line, the line, the termination and any energy converted into modes that do not return to the analyzer.

The result is useful as a paired fixture comparison: keep the hardware and geometry fixed, change one environment, and observe the difference. It is not by itself a complete characterization of window line routed through a building, across several bands or at transmitter power.

Spacing Changes Coupling; It Does Not Create a Shield

A two-wire line carries its intended signal as a differential mode. Much of its electric field lies between the conductors, but the field does not stop at a fixed distance beyond them. Conductor diameter, separation, dielectric supports and nearby objects all contribute to the per-unit-length inductance and capacitance.

Foam increases physical separation from a surface and may make the local environment more repeatable. That can reduce a particular perturbation. It does not create the continuous conductive boundary that confines the intended mode in coax. A claim that the field is “mostly” within a given distance is therefore not a universal clearance rule.

Symmetry matters as much as average clearance. If both conductors couple equally to a nearby structure, the differential mode may remain reasonably balanced even while its impedance and velocity change. If one conductor couples more strongly, part of the differential signal can convert to common mode. Gutters, rebar, window frames, wiring, foil insulation and irregular earth rarely produce the same geometry as a short, repeatable bench arrangement.

Characteristic Impedance Is More Than the Wire Spacing

For a general uniform transmission line, characteristic impedance and propagation are set by the distributed resistance R, inductance L, conductance G and capacitance C:

For a low-loss line, Z₀ is approximately √(L/C). Conductor spacing and diameter strongly affect L and C, but so do the plastic web, foam jacket, moisture and nearby conductive or dielectric material. “450 Ω” is therefore a nominal line property under specified conditions—not a fixed impedance presented to the transmitter and not a guarantee that the installed structure remains 450 Ω.

Wes Stewart, N7WS, measured commercial window-line samples for his paper Balanced Transmission Lines in Current Amateur Practice. At 50 MHz, his dry Wireman #551 and #554 samples measured about 405 Ω and 359 Ω, with velocity factors about 0.902 and 0.928. Those are results for those samples and that method, not new universal ratings. They demonstrate why the catalogue label cannot replace an installed measurement.

The impedance presented at the transmitter end is another quantity again. It depends on Z₀, load impedance, electrical length, loss and frequency. A 9:1 transformer is not automatically correct because a line is sold as 450 Ω; it must suit the actual differential impedance range, frequency, voltage, current and common-mode requirement at its reference plane.

Velocity Factor Moves with the Dielectric Environment

In the low-loss approximation, propagation velocity is approximately 1/√(LC). Adding dielectric around a line usually increases its effective capacitance, reduces phase velocity and changes its electrical length. Foam density, wall thickness, water uptake and the amount of field that crosses air versus plastic all matter.

A half-wave repetition near 19.8 MHz is therefore evidence about the electrical length of the tested assembly. It is not a material constant for all foam-covered window line. The useful measurement is phase over frequency, from which β, delay and velocity factor can be derived after fixture effects are removed. Repeat it dry, wet and near each intended surface rather than importing a single velocity factor into every installation.

What the Open-Circuit S11 Test Can Prove

For an ideal open-circuit termination on a uniform line:

After calibration or de-embedding to the balanced line terminals, and provided the energy remains in one differential mode, the one-way attenuation in decibels is approximately half the measured return loss. The reflection phase contains the round-trip electrical length. A half-wave condition is convenient because the load impedance repeats at the input of an ideal lossless line; it is not the only frequency at which “real loss” exists.

Without de-embedding, a reduction in returned magnitude can include:

• conductor and dielectric loss in the line;
• loss and mismatch in the 9:1 transformer and launch;
• radiation from the fixture or termination;
• differential-to-common-mode conversion; and
• instrument directivity, calibration and connector uncertainty.

Those mechanisms all remove energy from the reflected differential wave seen at the analyzer. Calling the entire difference “line loss” requires the other terms to be characterized or shown negligible.

The Transformer Is Part of the Measurement

Portune’s 9:1 Guanella transformer enabled a 50 Ω instrument to drive the nominally high-impedance balanced port. Its turns ratio, insertion loss, leakage inductance, interwinding capacitance and common-mode impedance vary with frequency and load. A good current transformer can provide a useful balanced test port, but it does not disappear from the error budget.

A stronger measurement either calibrates at the balanced terminals, de-embeds a separately characterized fixture, or uses a multiport VNA method that forms differential and common-mode S-parameters. The same transformer should remain in every paired comparison, at the same orientation and distance from nearby objects.

Balance Needs Its Own Measurement

Return loss describes reflection in the observed mode at a stated reference plane. It does not prove that the two-wire line carries only differential current. With both conductor currents defined in the same longitudinal direction:

An ideal differential mode has equal-and-opposite conductor currents and therefore zero I_CM. Unequal coupling to earth or nearby metal can create a common-mode path even when the differential return loss looks respectable. That external current can alter radiation, receive noise and the impedance seen when the feedline or instrument is moved.

Measure balance with mixed-mode S-parameters or with calibrated current probes on both conductors and, where possible, around the pair. Repeat along the line because common-mode current can form its own standing-wave pattern.

Metal Conduit Creates a Different Transmission-Line Structure

Putting a two-wire pair inside conductive conduit does not merely add spacing. It creates a multiconductor line with the conduit as a third conductor. A carefully designed, centred pair inside a continuous bonded shield can support a useful shielded differential structure, but its characteristic impedance and modes are set by the complete geometry.

Ordinary window line pulled through EMT is not automatically 450 Ω twinax. Off-centre routing, bends, joints, discontinuous bonding and different terminations can change impedance and convert modes. The conduit exterior can also carry current relative to the surrounding structure. Burial adds water ingress, corrosion, electrical bonding and code requirements that a small-signal reflection result does not address.

A Good Match Does Not Make the Feedline Invisible

An antenna can preserve its intended current distribution and pattern with many feedline types if the transition is correct and the feedline remains sufficiently isolated from the radiating system. A matched impedance at one plane is necessary for some designs, but match alone does not establish that isolation.

If foam-covered line couples asymmetrically to a roof, tower, wiring or earth, the surrounding structure and line can become part of the antenna system. A tuner may still present a comfortable impedance to the transmitter. That says nothing by itself about common-mode current, pattern, efficiency, receive-noise pickup or RF voltage on accessible conductors.

A Fair Test of the Broader Claim

Portune’s paired comparison can be extended without discarding its practical insight:

• calibrate or de-embed to the balanced line terminals and characterize the transformer separately;
• measure Z₀, γ = α + jβ, delay and velocity factor across every intended band;
• record differential reflection and differential-to-common-mode conversion, not S11 alone;
• repeat dry, wet, near concrete, near asymmetrical metal and inside conduit with controlled geometry;
• use several line lengths and repeated assemblies to separate a local discontinuity from distributed behaviour;
• compare antenna pattern, receive noise and line current with the installed antenna attached; and
• at transmitter power, verify voltage clearance, current, temperature, moisture and insulation within safe test arrangements.

Results should be stated for the tested product, frequency range, length, routing, fixture and acceptance limits. That turns “it worked here” into engineering evidence another station can evaluate.

The Defensible Conclusion

Portune demonstrated that a particular foam-covered line-and-transformer fixture could show modest changes in its returned differential signal under several adverse placements. That is useful evidence for a narrow installation technique. It supports trying a short, controlled foam-protected section when the alternatives are impractical, followed by measurements on the installed system.

Joeri’s disagreement remains: foam spacing is not shielding, and the experiment does not establish that window line can generally be routed like coax, buried in conduit without redesign, or judged by attenuation alone. Characteristic impedance, velocity factor, differential loss, balance and system interaction must each be measured on their own terms.

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