How Good Is Your Cable Shield, Really? Quantifying Transfer Impedance and Shielding Effectiveness

Why Cable Shielding Matters More Than You Think

You've routed your sensitive analog signal through a shielded cable, connected the shield at both ends, and yet your EMC pre-scan still shows a nasty spike at 150 MHz. Sound familiar? The problem often isn't *whether* you have a shield — it's *how effective* that shield actually is at the frequencies that matter.

Cable shielding effectiveness isn't a single number stamped on a datasheet and valid across all conditions. It depends on the shield's construction (braid, foil, spiral), its DC resistance, the cable length, and critically, the frequency of the interfering signal. Understanding the interplay between these parameters is essential for passing radiated emissions and immunity tests.

The open the Cable Shield Effectiveness calculator lets you quickly estimate both the transfer impedance and the resulting shielding effectiveness for a given cable configuration — no spreadsheet gymnastics required.

Transfer Impedance: The Key Metric

Transfer impedance, $Z_T$, is the gold-standard figure of merit for cable shields. It quantifies how much voltage appears on the inner conductor per unit length when current flows on the outer surface of the shield. The formal definition is:

where $V_{inner}$ is the induced voltage on the inner conductor, $I_{shield}$ is the current flowing on the shield, and $L$ is the cable length.

At low frequencies (below a few MHz), the transfer impedance is dominated by the shield's DC resistance per unit length, $R_{DC}$. As frequency increases, two competing effects come into play:

1. Skin effect — Current concentrates on the outer surface of the shield, reducing the field that penetrates to the inner conductor. This *decreases* $Z_T$.
2. Porpoising and braid leakage — In braided shields, the weave pattern creates small apertures. At higher frequencies, magnetic field coupling through these apertures *increases* $Z_T$.

For a solid tubular shield, transfer impedance drops monotonically with frequency due to skin effect:

where $t$ is the shield wall thickness and $\delta$ is the skin depth at frequency $f$:

For braided shields, $Z_T$ typically reaches a minimum somewhere between 1 MHz and 30 MHz, then rises due to braid porpoising. This is why a cable that works beautifully at 10 MHz can be surprisingly leaky at 200 MHz.

Shielding Effectiveness From Transfer Impedance

Once you have $Z_T$, shielding effectiveness (SE) in decibels can be estimated by comparing the transfer impedance to the characteristic impedance or load impedance of the circuit. A common simplified expression is:

where $Z_0$ is a reference impedance (often 50 Ω in test setups or the actual circuit impedance) and $L$ is the cable length in meters. Higher SE means better shielding — 60 dB is decent, 80 dB is good, and 100+ dB is excellent.

Worked Example: Evaluating a 2-Meter Braided Shield Cable at 100 MHz

Let's say you're using a 2-meter cable with a tinned copper braid shield. The manufacturer specifies a shield DC resistance of 15 mΩ/m.

Inputs:

• Shield DC resistance: $R_{DC} = 15 \text{ mΩ/m}$
• Cable length: $L = 2 \text{ m}$
• Frequency: $f = 100 \text{ MHz}$

First, we estimate the skin depth for copper ($\rho = 1.68 \times 10^{-8}$ Ω·m) at 100 MHz:

For a braid with an effective thickness of roughly 0.1 mm (100 μm), the ratio $t/\delta \approx 15$, meaning skin effect is very significant. However, because this is a braid and not a solid tube, the porpoising effect adds a mutual inductance term. Typical braided cables at 100 MHz exhibit transfer impedances in the range of 10–100 mΩ/m, depending on optical coverage and braid angle.

Let's assume the calculator determines $Z_T \approx 50 \text{ mΩ/m}$ at 100 MHz (a realistic value for an 85% coverage braid). The total transfer impedance over the 2-meter length is:

Shielding effectiveness referenced to 50 Ω:

That's marginal for many EMC requirements. If your spec calls for 60 dB, you'd need to either shorten the cable run, switch to a higher-coverage braid (95%+), or move to a cable with braid-plus-foil (which can push $Z_T$ below 5 mΩ/m at 100 MHz, yielding SE > 74 dB for the same length).

Plug these exact values into the open the Cable Shield Effectiveness calculator and you'll see the results instantly — along with the ability to sweep frequency and compare different shield configurations.

Practical Tips for Improving Shield Effectiveness

• Increase braid coverage. Going from 85% to 95% optical coverage can reduce $Z_T$ by a factor of 3–5 at high frequencies.
• Use combination shields. A braid-over-foil construction gives you the low-frequency performance of the braid and the high-frequency sealing of the foil.
• Minimize cable length. Since $SE$ degrades linearly with length (in dB terms), shorter cables always win.
• Terminate the shield properly. A pigtail ground connection can add 10–20 mΩ of impedance at the connector — sometimes more than the cable shield itself. Use 360° backshell terminations wherever possible.
• Watch out for resonances. Cable lengths that are multiples of $\lambda/4$ at your problem frequency can create standing waves on the shield, dramatically reducing effectiveness at those specific frequencies.

When to Worry (and When Not To)

For low-frequency applications (audio, slow serial buses below 1 MHz), even a modest braid with 15 mΩ/m DC resistance provides excellent shielding because $Z_T$ is essentially just $R_{DC}$ and the total transfer impedance is small relative to circuit impedances.

The real challenges emerge above 30 MHz, where braid leakage dominates and transfer impedance can rise rapidly. If you're dealing with high-speed digital signals, switch-mode power supply harmonics, or radiated emissions in the 100 MHz–1 GHz range, you need to take shield quality very seriously.

Try It

Grab your cable's DC resistance spec and the length of your run, then open the Cable Shield Effectiveness calculator. Sweep across your frequencies of concern and see exactly where your shielding holds up — and where it doesn't. It's a 30-second check that can save you a failed compliance test and weeks of redesign.