Rolling Up Coax: What Changes and What Does Not

A coaxial cable can carry more than one electromagnetic mode. The wanted transmission mode is supported between the centre conductor and the inside of the shield. A separate current can involve the shield exterior and an external return path. Rolling the cable affects those modes differently.

1. Wanted Transmission and Outside-Shield Current

For the dominant coaxial TEM transmission mode, current on the centre conductor is balanced by an equal and opposite current on the shield’s inner surface:

The associated electric and magnetic fields are concentrated in the dielectric between the conductors. Schelkunoff’s primary coaxial-line theory establishes this guided mode and the shielding role of the outer conductor.

An additional net current can flow on the shield exterior, returning through the antenna structure, earth capacitance, mast, station wiring, another cable or the surrounding environment. That exterior-current circuit is commonly called common mode in antenna and EMC work.

Real shields are not perfect. Braid transfer impedance, connectors, apertures and construction tolerances allow some coupling between the inside and outside problems. The mode distinction remains essential: differential insertion loss does not qualify common-mode suppression, and outside current does not prove a defect in wanted coaxial transmission.

2. What Happens When Slack Coax Is Rolled

The cable’s physical length—and therefore its nominal differential delay and matched-line attenuation—does not change merely because the same length is arranged as a roll. With a healthy cable, compliant connectors and a bend radius within the manufacturer’s limit, a loose storage loop normally leaves the wanted transmission mode close to its straight-run behaviour.

That statement has mechanical boundaries. A tight roll can deform foam dielectric, wrinkle a foil shield, fatigue a solid centre conductor, loosen a connector or exceed the cable’s repeated-flex limit. Different cable families publish different installation and repeated-bend radii; cable diameter alone is not a complete rule.

Placement also matters. A roll beside metalwork, wiring, a mast or another coil can couple to a common-mode field even when the same roll in free space does not. Water retention, crushing and connector strain are installation problems independent of RF mode.

3. When the Roll Enters the Common-Mode Circuit

Outside-shield current requires both excitation and a return path. It can be driven by feedpoint asymmetry, unequal antenna-arm coupling, an electrically small ground or counterpoise, intentional use of the feedline exterior, station bonding, nearby conductors or external fields.

A useful first-order circuit view is:

The rolled shield becomes a series coil in that exterior-current path. It can therefore change current magnitude and phase, the location of current maxima, feedpoint impedance, radiation pattern, received noise coupling and RF voltage near the station. If common-mode excitation is negligible, there is correspondingly little exterior current for the roll to modify.

The complete denominator matters. Adding a reactive element can create or move a system resonance rather than simply “blocking RF.” A lower current at one probe position does not guarantee lower current everywhere else, and an SWR change does not reveal whether common-mode current improved.

4. A Coax Roll Is a Distributed, Frequency-Dependent Choke

For common mode, several turns of cable have inductance. The turns also have capacitance to one another and to nearby objects, while the cable length and environment add distributed transmission-line effects. The resulting common-mode impedance is complex:

Below its first important resonance, an air-wound roll often appears mainly inductive. Near resonance its impedance may rise sharply. Above resonance it can become capacitive and can show additional resonances. Medhurst’s primary measurements of single-layer solenoids demonstrate why coil dimensions, turn spacing, high-frequency resistance and self-capacitance must all be considered.

A coax roll adds further variables:

• cable outside diameter, shield construction and jacket permittivity;
• number of turns, roll diameter, length and turn spacing;
• tight bundling, crossed turns and tie placement;
• distance to earth, metalwork, other cables and wet surfaces;
• connector pigtails and the uncoiled cable at both ends; and
• temperature, transmit power and installation movement.

That is why a turn-count recipe cannot be assumed across cable types, bands and installations. An air-core coax choke can be useful when its measured common-mode impedance covers the intended band and its mechanical and power limits are respected. It is not automatically broadband, and it is not qualified by appearance.

5. A Feedpoint Choke Is a Design Choice, Not a Universal Cure

Choke placement defines where exterior current is discouraged. The correct position follows the intended antenna current path:

• Balanced radiator with unwanted feedline current: a suitable feedpoint current choke is often a logical first boundary, provided the installed antenna remains balanced and the choke is qualified for the band and power.
• Feedline exterior intentionally used as a return conductor: a choke directly at the feedpoint changes that architecture. The intended exterior section, its length, routing and terminating choke position must be designed together.
• Station-entry control: a choke near the entry can reduce current continuing into station wiring, but it does not remove radiation, coupling or loss that already occurred on the upstream feedline.
• Multiple resonant paths: more than one boundary may be useful, but placement should follow a current map rather than a fixed distance rule.

A return path is not necessarily a set of radials, and common-mode current is not always evidence that a counterpoise is “missing.” Antenna geometry, mast and cable routing, nearby conductors and station connections can all drive mode conversion. The remedy can involve symmetry, routing, a defined counterpoise, bonding changes, choke placement or a combination.

6. Measure the Installed Modes Separately

1. Map outside current. Clamp an RF current probe around the complete coax so the wanted internal currents cancel in the probe. Record magnitude and, if possible, phase at several positions.
2. Measure before changing geometry. Record frequency, power, antenna configuration, tuner state, feedline route, bonds and nearby conductors.
3. Compare equal cable lengths. For a straight-versus-rolled test, keep the cable, connectors, termination, reference planes and total length unchanged.
4. Characterise the roll as a common-mode component. Use a suitable VNA fixture, calibration and de-embedding to obtain R+jX or common-mode S-parameters across the full frequency range.
5. Check differential transmission separately. Measure insertion loss, return loss and phase through the coax with its specified differential termination.
6. Inspect the field result. Where pattern or radiation is the claim, use a controlled field or antenna-range comparison. Constantin and Tamas experimentally show how feeder common-mode current can materially affect antenna radiation measurements.
7. Repeat after installation changes. Re-map current after moving the roll, adding a choke, changing a counterpoise or altering station bonds.
8. Verify power and temperature. At transmit power, monitor cable, turns, ties and connectors within their ratings; a small-signal impedance trace is not a thermal qualification.

7. Practical Installation Decisions

Manufacturer data provide the mechanical boundary. For example, Times Microwave’s current LMR guide publishes separate installation and repeated-bend radii by cable model. Use the actual cable specification rather than copying the diameter of somebody else’s roll.

8. Practical Conclusions

• Wanted coaxial transmission and outside-shield common mode are different current systems.
• A gentle roll normally leaves the cable’s differential length, delay and matched loss essentially unchanged.
• Tight bends, crushing, connector strain and shield damage remain real limits.
• A roll changes RF behaviour when outside-shield current or external coupling makes it part of the common-mode circuit.
• Air-wound common-mode impedance depends strongly on frequency, dimensions, capacitance and surroundings.
• A high impedance at one frequency does not establish broadband choking.
• Feedpoint, counterpoise and station-entry choke positions serve different current-path objectives.
• Current probes, common-mode impedance measurements and controlled field checks provide stronger evidence than SWR alone.

Primary Sources and Scope Anchors

• Schelkunoff, “The Electromagnetic Theory of Coaxial Transmission Lines and Cylindrical Shields”—guided coaxial transmission and shield theory.
• ITU-R Report SM.2158-3—in-force differential/common-mode decomposition, imbalance and current measurement.
• Bockelman and Eisenstadt, “Combined Differential and Common-Mode Scattering Parameters”—primary mixed-mode network measurement theory.
• Constantin and Tamas, “Evaluation and Impact Reduction of Common Mode Currents on Antenna Feeders in Radiation Measurements”—experimental feeder-current and radiated-field assessment.
• Medhurst, “H.F. Resistance and Self-Capacitance of Single-Layer Solenoids”—primary measured coil resistance and self-capacitance data.
• Times Microwave LMR Cable Guide—current manufacturer-specific bend, attenuation and construction limits.

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