Your Body Is Not a Line Isolator

This is a perfect example of why a visible effect is not yet an explanation. Touching the connector changes the RF system. The analyser notices. From that alone we still do not know which current changed, how much it changed, where it was flowing or whether the hand increased loss, supplied another return path, shifted a resonance or moved the measurement reference.

The Useful Question Behind the Demonstration

The video asks a valid station question: is current on the outside of the coax affecting the antenna, the indicated impedance, received noise, equipment behaviour or RF exposure? Those effects are real possibilities. They deserve a real measurement.

The problem begins when several different things are collapsed into one story:

• a mismatched load and reflected differential-mode power;
• mode conversion at an asymmetric antenna, connector, transition or installation;
• current on the shield exterior and its environmental return path;
• an SWR trace that changes when a person touches the system; and
• loss, capacitance and radiation along a coax run near ground.

These can interact, but they are not synonyms. A demonstration that changes all of them at once cannot tell us which one caused the new trace.

Reflected Power Does Not Jump to the Outside

In the intended coaxial transmission-line mode, the electric and magnetic fields are concentrated mainly between the centre conductor and the shield. The associated centre-conductor current and inner-shield-surface current are equal and opposite at a declared cross-section. That remains the intended mode whether the load is perfectly matched or produces a large reflection.

A reflection reverses the direction of a travelling wave. It does not, by itself, move that wave from the inside of the coax to the outside. A mismatched but geometrically symmetric load can support forward and reflected waves while the intended conductor currents still cancel.

Exterior-shield current needs a coupling mechanism and a complete external circuit. Antenna asymmetry, feed-line departure, an unbalanced transition, nearby conductors, shield-transfer imperfections or external fields can provide that mode conversion or excitation. ITU-T K.136 defines converted common-mode current as asymmetrical current converted from differential mode by cable or network unbalance. That is a much more useful model than “the SWR was not perfect, so reflected power came back on the outside.”

Skin Effect Describes Distribution, Not Cause

The video says RF always flows on the surface of a conductor, then uses the inner and outer shield surfaces to explain the unwanted current. Skin effect is real, but the slogan is too blunt. Current density penetrates a finite skin depth, conductor loss depends on frequency and construction, and electromagnetic power is transported by fields—not by treating RF as a liquid that chooses one metal surface and then spills onto another.

NIST Technical Note 2255 describes the coaxial TEM field and its voltage/current relationship. The shield’s inner and outer surfaces can support largely separate field problems when the construction is adequate. Skin effect helps that separation; it does not prove that appreciable exterior current exists in a particular installation.

A Human Body Adds a Path; It Does Not Isolate One

The video’s central statement is that the human body is one of the best line isolators because touching the coax absorbs some RF energy. That reverses the circuit description.

A line isolator or current choke is intended to add a characterized impedance in series with the unwanted common-mode path while leaving wanted differential transmission substantially intact. A person touching the cable does something very different: the body adds distributed capacitance, resistance and coupling to the room, equipment, wiring and earth. It can become another branch of the RF return network.

Some energy may be dissipated in that new branch, but the hand can also detune the external circuit, move a current node, change cable position, alter enclosure coupling or create a stronger return path. A lower SWR after the touch is not evidence that reflected power was harmlessly absorbed. It is evidence that the load seen at the analyser changed.

An SWR Change Is a Clue, Not a Current Reading

Touching the coax while an analyser is sweeping can reveal that the installation is sensitive to its environment. That may be useful as a crude perturbation test. It still cannot identify or quantify common-mode current.

Even at analyser power, the test changes several variables at once:

• body capacitance and loss are added to the external circuit;
• the cable and connector may move;
• the operator’s position changes near-field coupling;
• the instrument enclosure gains another capacitive reference; and
• the antenna’s complete boundary condition changes.

A 4–5 W carrier version does not improve the selectivity of the test. It only raises the energy available to an uncontrolled contact. Nor does an unchanged meter prove the coax exterior is quiet: a hand placed near a current minimum, or a low-sensitivity meter, can hide current that exists elsewhere.

The direct method is to clamp a characterized RF current probe around the complete coax. Equal-and-opposite wanted currents largely cancel inside the aperture; the remaining net current is the exterior/common-mode component for that declared conductor set.

The approximation still needs a known probe transfer factor, frequency response, instrument loading, cable position and uncertainty. The Rohde & Schwarz EZ-17 documentation, for example, treats a clamp-on current probe as a transformer with a frequency-dependent transducer factor. That is an instrument model. A palm has none.

Coax Near Ground Does Not Make Current Vanish

The video also suggests that a long coax run lying near the ground may dissipate most or even all of the common-mode return current through capacitance to earth. Capacitance is a return path, not automatically a loss mechanism. An ideal capacitor stores energy and returns it; it does not burn watts.

A real exterior-current circuit includes distributed capacitance, soil and dielectric loss, conductor resistance, radiation, nearby metal and terminations at both ends. Current amplitude at the shack may indeed be smaller than at the antenna. That can result from loss, radiation, current division, a standing-wave minimum or a different reference—not proof that the ground “absorbed” all return current.

There is a second trap: the same cable-to-environment coupling that changes transmit current can also receive local interference. A small station-end transmit-current reading does not prove the cable is irrelevant on receive, and a noisy receive path does not prove the transmitter’s reflected power caused it.

A Ferrite Core Is Not Automatically a Broadband Choke

Winding the complete coax through ferrite can add common-mode impedance while the wanted coaxial currents ideally cancel their magnetizing effect. But “a large 43-mix core with several turns” is a construction description, not a verified HF specification.

Core dimensions, exact material, turn count, winding spacing, cable capacitance, lead length and fixture parasitics shape the impedance and its resonances. Fair-Rite’s current 43-material data publishes frequency-dependent material behaviour and distinguishes part-specific impedance tests from a generic material label. The completed choke must be measured across every intended band.

Even a high measured impedance does not promise the same current reduction everywhere. ITU-T K.37 explains the key boundary: a common-mode choke raises impedance in the unwanted mode, while the achieved reduction depends on the original common-mode circuit. Placement, source impedance, external-path impedance, power, duty cycle and temperature remain part of the answer.

Measure the Current Path Without Becoming Part of It

1. Freeze the installation: record frequency, antenna state, tuner state, bonds, cable route, instrument and test power.
2. Declare the analyser plane: calibrate at the connector under test or de-embed the known fixture and line.
3. Record the differential result: save complex S11 or R+jX as well as SWR.
4. Map exterior current: use a characterized clamp-on probe around the complete coax at repeatable points along its route.
5. Change one thing: insert a choke with measured R+jX at the intended boundary while preserving the rest of the geometry.
6. Repeat A/B/A: compare current maps and impedance, then restore the baseline to expose drift.
7. Verify the real outcome: measure wanted signal and noise separately for receive work; check RFI, accessible RF potential and choke temperature for transmit work.

If the SWR changes while measured exterior current falls, both observations matter. If the SWR changes but the current map does not, look elsewhere. If current falls at one point and rises at another, the choke redistributed the mode rather than making it disappear.

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