S/FTP and F/UTP constructions deliver measurably different shielding — lab testing quantifies the difference in decibels per frequency band

1. Why Shielding Effectiveness Testing Matters for STP

Shielded twisted pair (STP) cabling exists for one reason: to keep electromagnetic interference out of the signal path and to keep emissions from leaking out. In industrial environments — plants with variable frequency drives, welding equipment, motors, and power lines — the interference field can be strong enough to corrupt 10GBASE-T links that would otherwise run cleanly. The shield's job is measured as shielding effectiveness (SE): the attenuation, in dB, that the screen provides against an incident electromagnetic field.

The stakes are quantified in alien crosstalk data. When two unscreened Cat6 cables are laid parallel, alien crosstalk at 100 MHz measures about 55 dB; screened cables under identical conditions reach about 95 dB. Because alien crosstalk is not captured by standard link tests, it silently erodes the signal-to-noise margin of adjacent cables in the same bundle — exactly the failure mode seen in dense patch panels and high-density trays. This is why shielding effectiveness testing is not a theoretical exercise: it is the evidence that a screen design will hold in a real, interference-heavy installation. For the fundamentals of STP design, see our what is STP cabling guide and our STP vs UTP terminology guide.

2. SE Fundamentals: Decibels, Transfer Impedance, Coupling Attenuation

Shielding effectiveness is defined as the ratio of the incident field (E1) to the transmitted field (E2), expressed in decibels: SE (dB) = 20 log (E1 / E2). A 60 dB screen attenuates the field by a factor of 1,000; 100 dB by a factor of 100,000. But for cables, two related parameters carry the technical weight:

IEC 61156-5/6 define performance grades: transfer impedance Grade 1 (10 mΩ/m at 1 MHz, typical of braid-plus-foil S/FTP) and Grade 2 (50 mΩ/m, foil-only F/UTP); coupling attenuation from Type I at 85 dB (high shielding, industrial) through Type Ib 70 dB, Type II at 55 dB (minimum for screened), and Type III at 40 dB (alien crosstalk control for unscreened). For a practical comparison of UTP and S/FTP cable behavior, see our Cat6 UTP vs S/FTP performance guide.

3. Test Standards Landscape: IEEE 299, MIL-STD-285, and Beyond

Which standard you use depends on what you are testing: a shielded room, a planar material, or a cable.

For installed cabling, screening conformance is verified at the component level per IEC 61156 and in the field by testing the permanent link or channel end to end. For the field-testing side of the story, see our component vs channel testing guide and our how to read Fluke test reports guide.

IEEE 299 covers shielded rooms from 9 kHz to 18 GHz; IEC 61156 governs data cable screen parameters up to 1 GHz and beyond

4. Testing by Frequency Band: Magnetic, Electric, Plane Wave

A single SE number is meaningless without a frequency context, because the physics of shielding changes with frequency and field type. IEEE 299 divides testing into three regimes, each with its own antennas and measurement logic:

The distinction between near field and far field matters: close to the source, electric and magnetic fields have no fixed ratio and must be measured separately; in the far field they form a plane wave and SE is expressed as power attenuation. Far-field plane-wave testing typically applies from about 0.7 GHz to 40 GHz with broadband horn antennas. This frequency-aware approach is exactly why a "95 dB shielded cable" claim must be read with the frequency and field type attached. For guidance on shielding and grounding in real installations, see our shielded vs unshielded patch panel grounding guide.

SE testing splits the spectrum into magnetic, electric, and plane-wave regimes — each with its own antennas and physics

5. The Lab Test Set-Up: Dynamic Range and Equipment

A shielding effectiveness measurement is only as valid as its dynamic range — the single most common source of invalid results.

The equipment set for a full 10 kHz-18 GHz sweep includes a shielded enclosure (≥1 m³, absorber-lined, with ridge-waveguide horn antennas for 700 MHz-18 GHz), a double TEM cell for DC-3 GHz low-frequency work, RF signal generator, receiver or spectrum analyzer (9 kHz-26 GHz), power amplifiers (4-5 W), broadband preamplifier, low-loss coax, attenuators, and adapters. Between the sample and the test frame, the connection must be better than the sample itself — silver paint, silver-loaded silicone, finger stock, and copper tape are standard gap-sealing materials. For cable link testing in the field, see our professional cable testing guide.

6. What the Numbers Mean for STP Cable Selection

Once you can read an SE report, the next question is what specification actually protects your network. The hierarchy looks like this:

The selection logic follows the environment: general commercial spaces can often run F/UTP (Type II, 55 dB); industrial and medical environments where VFDs, motors, and imaging equipment generate strong interference demand S/FTP (Type I, 85 dB) with proper grounding at both ends. Note that a shield is only as good as its termination — a floating shield provides almost no protection. For grounding and installation guidance, see our shield continuity checklist, our STP grounding best practices, and our shielded cable requirements for PoE++. For alien crosstalk in 10G runs, see our Cat6a alien crosstalk mitigation guide and our 10G over Cat6 AXT limits guide.

7. Best Practices and Common Pitfalls

Whether you are running SE tests in a lab or evaluating a supplier's shielding data, the same discipline applies.

Finally, remember that the best screen in the lab fails in the field if termination, grounding, and continuity are sloppy — the three leading causes of real-world shield failure are improper drain-wire termination, floating shields, and breaks at the connector. For installation-side protection, see our termination failure analysis for PoE runs, our shielded vs unshielded patch cable guide, and our shielded vs unshielded cable selection guide.

Key Questions

Q1: What is shielding effectiveness (SE)?

Shielding effectiveness is the attenuation a shield provides against electromagnetic interference, expressed in decibels: SE (dB) = 20 log (E1 / E2). Shielded rooms target roughly 100 dB; 60-80 dB is accepted for many applications; STP cable constructions show 55-95 dB of alien crosstalk suppression depending on design.

Q2: Which standard should I use to test STP cable shielding?

For STP data cables, IEC 61156 defines screening parameters: transfer impedance per IEC 62153-4-3 and coupling attenuation per IEC 62153-4-5. For shielded rooms, use IEEE 299-2006 (superseding MIL-STD-285), covering 9 kHz-18 GHz. For planar materials, ASTM D4935 covers 30 MHz-1.5 GHz.

Q3: What is the difference between transfer impedance and coupling attenuation?

Transfer impedance (mΩ/m) measures the interference voltage a screen current induces on the inner conductor — the classic low-frequency screen parameter for screened cables only. Coupling attenuation (dB) measures the combined screen plus balance performance with absorbing clamps and applies to screened and unscreened cables. IEC 61156-5 grades both (Grade 1/2 transfer impedance; Type I 85 dB to Type III 40 dB coupling attenuation).

Q4: Why is shielding tested at different frequencies?

A shield behaves differently across the spectrum: magnetic field dominates below ~20 MHz, electric field in the mid range, and plane waves above ~300 MHz. IEEE 299 splits testing into magnetic (9 kHz-20 MHz), electric/resonant (20-300 MHz), and plane wave (300 MHz-18 GHz) regimes, each requiring different antennas and methods.

Q5: What is dynamic range and why does it matter?

Dynamic range is the difference between the maximum received signal (no barrier) and the noise floor (metal barrier), minus safety margin. The system's dynamic range must exceed the expected attenuation of the sample; otherwise a sample better than the test chamber yields invalid results. Always verify dynamic range before measuring.

Q6: How many dB of shielding does STP cable provide?

At 100 MHz, alien crosstalk between parallel unscreened Cat6 cables measures about 55 dB, while screened cables reach about 95 dB — a 40 dB improvement. IEC 61156 coupling attenuation grades run from Type I at 85 dB (industrial) down to Type III at 40 dB. Cat7 cable is commonly specified at ≥70 dB at 600 MHz.

Q7: How often should shielding effectiveness be verified?

For permanently installed shielded rooms, re-verify every 1-2 years or after any modification (new penetrations, doors, cable bulkheads). For cable shielding, conformance is verified at manufacture per IEC 61156 and validated in the field by testing the installed permanent link or channel.

Q8: What is the difference between near-field and far-field tests?

In the near field, electric and magnetic fields are measured separately because their ratio is not fixed. In the far field, the wave is planar and SE is expressed as power attenuation. Far-field plane-wave testing applies from roughly 0.7 GHz to 40 GHz with broadband horn antennas; below that, separate H-field and E-field measurements are required.

AMPCOM Technical Team

Shielding and EMC specialists with 17+ years in shielded cabling design, EMC testing, grounding practice, and industrial network infrastructure