RG402 Common-Mode Chokes: Cable, Ferrite and Bend Limits

I would not reach for RG402 first when I need a compact, multi-turn HF common-mode choke. That is a practical decision, not a law of electromagnetics. Semi-rigid coax is harder to route repeatedly through a toroid without violating the cable maker's forming limits. If a straight, single-pass or carefully preformed layout gives the required complex common-mode impedance, differential loss and thermal margin, RG402 can be entirely valid.

First Confirm What “RG402” Means

M17/130-RG402 is a nominal 0.141-inch, 50-ohm semi-rigid coaxial construction. Representative qualified or manufacturer-specific versions use a PTFE dielectric and a continuous copper or plated-copper outer tube. That solid outer conductor gives stable geometry and strong shielding for RF and microwave interconnections.

The label is also used loosely for conformable 0.141-inch cables. Those products may use copper tape, composite braid or a tin-filled outer conductor instead of a seamless tube. They have different forming, attenuation, temperature and connector instructions. Before discussing a choke, read the exact cable data sheet and identify the construction, plating, permitted bend radius and whether the stated limit applies to one-time installation or repeated flexing.

The Ferrite Sees Net Current, Not Braid Porosity

In the intended coaxial differential mode, current on the centre conductor is opposed by return current on the inner surface of the shield. Their external magnetic fields largely cancel. A separate current on the outer surface of the shield does not have that internal opposing partner, so a ferrite surrounding the complete cable sees its net magnetising current.

That physics does not require holes in a braided shield. A continuous copper tube carrying exterior current still produces magnetic field outside the tube, and a ferrite around it can add common-mode impedance. The tube does not starve the ferrite of flux merely because it also provides excellent shielding between the cable's inner and outer regions.

Skin effect applies to any sufficiently high-frequency conductor current. In coax it helps separate the shield's inner-surface differential return from exterior-surface current. It neither guarantees nor prevents a good choke. The completed common-mode circuit—cable exterior, ferrite, winding geometry, enclosure, load and surrounding conductors—sets the result.

Where Semi-Rigid Cable Becomes Awkward

A compact HF toroidal choke often obtains useful impedance by passing the cable through the core several times. RG402's tube can be formed, but it must stay within the exact manufacturer's tooling and bend instructions. Kinking, flattening, work-hardening or repeatedly reforming the outer conductor can alter the coax geometry, create local stress and compromise an assembly.

The minimum bend radius is not a universal RG402 number. Published limits differ among true semi-rigid, conformable and supplier-specific variants, and a one-time forming radius is not permission for repeated flexing. Connector launches and soldered joints need their own strain relief. A stiff loop can also transfer force into a brittle ferrite core or enclosure mounting point.

Flexible braided coax is usually easier to route through a small core more than once, but “flexible” does not mean any radius is acceptable. Its jacket, braid, dielectric and centre conductor still have minimum bend, pull, temperature and installation limits. The useful comparison is between two exact data sheets and two buildable geometries—not between the words tube and braid.

One Pass Can Be a Legitimate Design

A cable through one ferrite aperture is a one-turn common-mode choke. More passes can increase low-frequency inductive impedance approximately with the square of turns while the structure remains in the appropriate lumped region. At higher frequencies, parasitic capacitance, leakage, lead length and self-resonance make that simple relationship unreliable.

If RG402 cannot make the required multi-turn geometry safely, alternatives include a larger core, multiple cores in series along a straight section, a different ferrite geometry or a cable whose data sheet permits the intended bends. None is automatically superior. A string of single-pass cores may occupy more length; a multi-turn winding may add capacitance and concentrate voltage; a larger core changes both magnetic path and mechanics.

Fair-Rite publishes cable-core impedance for a stated material, part, test frequency and single-turn fixture. Those values are useful starting evidence, not the installed choke result. HF work needs a material and core size selected for the actual frequency range, followed by measurement of the completed winding.

Differential Loss and Common-Mode Impedance Are Separate

The wanted RF travels inside the coax as differential mode. Its loss depends on the exact cable length, conductor and plating, dielectric, frequency, temperature, connectors and bend quality. Semi-rigid cable may provide excellent differential transmission in a correctly formed microwave assembly. That does not predict its common-mode choking impedance.

The unwanted exterior current sees the ferrite-loaded common-mode path. Its complex impedance depends on core material, core dimensions, number and spacing of passes, cable position, winding capacitance, frequency, enclosure coupling and the impedances on both sides of the choke.

Thermal and Stress Limits Must Be Proven Together

A choke can heat from differential cable loss, connector loss and ferrite dissipation caused by common-mode current. The dominant mechanism changes with frequency, mismatch, duty cycle and installation. PTFE dielectric or a high cable temperature rating does not establish the ferrite temperature limit, solder-joint margin or enclosure safety.

Common-mode voltage across the choke can also be large. Turn-to-turn capacitance and spacing affect how that voltage distributes, especially near resonance. A stiff semi-rigid winding may hold spacing well, while an incorrectly formed tube may press directly on a core edge. A flexible winding may reduce mechanical force but still needs insulation, restraint and repeatable spacing.

Run powered tests with the intended waveform, frequency, duty cycle, load mismatch, ambient condition and airflow. Record cable, connector and ferrite temperatures separately where practical. Recheck small-signal impedance and insertion loss after the thermal cycle; a choke that survives briefly is not yet a verified operating envelope.

Measure the Assembly You Intend to Install

• Verify the cable identity. Record the manufacturer, part number, shield construction, plating, attenuation, bend instructions and temperature limits.
• Define the current boundary. State which exterior conductor current the choke is intended to impede and where the permitted return structure ends.
• Characterise complex common-mode impedance. Save both resistance and reactance versus frequency with fixture parasitics corrected or bounded.
• Check differential transmission. Measure insertion loss and return loss through the complete cable, connectors and winding at the declared reference planes.
• Check mode conversion. Where the fixture permits, use mixed-mode measurements to find differential-to-common conversion rather than inferring it from a two-port trace.
• Map installed exterior current. Compare current on both sides of the choke and along the feed line on every required band.
• Inspect the mechanics. Reject kinks, flattening, cracked plating, stressed solder joints, damaged jackets, sharp core contact and unsupported connector loads.
• Run the thermal test separately. Use representative RF power and mismatch, record temperature and drift, then inspect and remeasure the assembly.

Primary and Authoritative Technical Sources

• US Defense Logistics Agency, MIL-DTL-17 specification sheet 130—the M17/130-RG402 semi-rigid cable definition and controlled construction.
• Amphenol RF, Coaxial Cable Guide—RG402 as nominal 0.141-inch semi-rigid copper-shield cable and the distinction from conformable cable.
• Times Microwave Systems, TFlex-402—manufacturer data for a conformable 0.141-inch cable, including its stated bend radius.
• Belden 84316 RG-316 data—an example flexible PTFE coax construction, braid and manufacturer-specific bend limit.
• Fair-Rite, Ferrite Cores for Low-Frequency EMI Cable Suppression—a cable through a ferrite as a one-turn common-mode choke and the need to select material for frequency.
• Fair-Rite 17th Edition Catalogue—core dimensions, complex impedance, material behaviour and measurement conditions.
• Keysight, Impedance Measurement Handbook—ferrite-loaded cable shield impedance and common-mode-current suppression.

Joeri's Cable-Choice Rule

For a compact multi-turn HF choke, I usually choose a cable that can make the required turns cleanly, remain within its bend limits and leave the connectors unstressed. That often favours a suitable flexible PTFE coax over true semi-rigid RG402.

But the solid tube is not the electromagnetic villain. If RG402 suits a straight-core stack, a large-radius preformed winding or another mechanically valid layout, judge it by measured common-mode impedance, differential loss, current reduction and temperature. The right cable is the one that lets the complete choke meet those requirements without violating its mechanical data sheet.

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