Understanding Skin Effect in High-Frequency Cables

High-frequency cables behave very differently from the simple wires we learned about in basic circuits. In RF (radio-frequency) transmission lines and fast digital links, alternating current (AC) tends to “hug” the outer surface of the conductor rather than fill its cross-section. This skin effect means that as frequency rises, current crowds the edge of the wire, leaving the interior underutilized. The result is a smaller effective conductor area and higher resistance, which can degrade signal integrity and increase losses in cable runs.

Understanding the skin effect is essential for engineers and procurement specialists because it directly impacts RF cable design, cable loss budgets, and, ultimately, the performance of high-speed links. In this post, we’ll explain the skin effect, how skin depth varies with frequency, and what cable designers and buyers can do to mitigate its impact.

Table of Contents

What Is Skin Effect?

Skin effect is the tendency of AC to flow mainly near the surface of a conductor. When an AC signal flows, its changing magnetic field induces circular eddy currents (shown in red above) that oppose the primary current (blue) in the center of the wire. These opposing eddy currents cancel out the center current and push the net current toward the conductor’s skin. In other words, high-frequency current takes a “path of least resistance” along the outer shell of the wire, barely penetrating the core. This phenomenon is absent with DC: a steady DC distributes uniformly across the cross-section. However, the skin effect appears for any frequency above zero, becoming more pronounced as frequency climbs.

The skin depth (δ) quantifies this behavior. Skin depth is defined as the distance from the surface into the conductor at which the current density falls to about 37% (1/e) of its surface value. Practically, over 98% of the AC flows within just a few skin depths from the surface. For example, copper at 60 Hz has δ ≈ 8.5 mm, so most of the 60 Hz current still uses almost the entire conductor. But at 1 MHz, copper’s skin depth is only on the order of millimeters, and at 1 GHz it’s just a few microns. One engineering formula shows copper δ ≈ 7.6/√f (with f in MHz and δ in cm), highlighting that skin depth falls roughly as 1/√f. This means that RF and microwave frequencies confine current to a skinny surface layer, dramatically increasing the conductor’s AC resistance.

Skin Depth and Current Distribution

The illustration above shows a cross-section of a wire under high-frequency AC. The red area indicates where most of the current flows (nearly all current is near the surface, with only a tiny fraction inside the dotted line). This skin depth δ (marked by the dashed circle) is very small at RF. In copper, δ shrinks from about 8.5 mm at 60 Hz to only ~2.06 μm at 1 GHz. As a result, the interior of a thick conductor carries almost no current at high frequencies. This exponential falloff of current with depth is unique to AC: in contrast, a DC would be roughly uniform across the same cross-section. The graphic clarifies why the skin effect literally “skins” the wire.

The physics behind this is well understood: the time-varying magnetic field of the AC induces eddy currents that circulate inside the conductor and generate their opposing fields. These cancel the current in the interior and reinforce it at the surface. The net effect is a self-adjusting current pattern that “crowds” the edges. Engineers often say AC takes the path of least magnetic impedance, which happens to be right at the conductor boundary. The numerical result is that the wire’s effective cross-sectional area shrinks with frequency, and its AC resistance rises. Over 98% of the current is confined within roughly 4× the skin depth from the surface. Hence, even relatively thick conductors behave like hollow tubes at high RF.

Impact on High-Frequency Cables and Signal Integrity

As skin depth becomes small, RF cables (coaxial, microwave, and high-speed data cables) see a significant resistance increase and more loss per unit length. This has several important consequences:

Higher RF resistance and attenuation. With current confined to the surface, the cable’s conductor presents less cross-section and higher effective resistance. This causes greater Ohmic loss and attenuation per meter at RF. For example, a copper cable may have only a few micrometers of skin depth at GHz frequencies, so only a shell of that thickness carries most of the signal. The result is more heating and insertion loss in the cable.
Reduced signal integrity. The increased loss and phase shift from skin effect can degrade high-speed signals. One industry blog notes that “current is confined to a thin layer near the conductor’s surface, reducing the effective cross-sectional area and increasing the resistance. This can lead to signal degradation and higher attenuation, impacting the cable’s performance”. In other words, failing to account for skin effect may cause RF links to lose amplitude or bandwidth, hurting data integrity or RF transmission.
Material and surface dependence. Because skin depth depends on conductivity and magnetic permeability, cable performance varies with conductor material. Copper, silver, and gold have different δ at a given frequency. For example, at 1 GHz, copper’s skin depth is about 2.06 μm, gold’s is ~2.38 μm, and nickel’s (high-permeability) is only 0.17 μm. Thus, a copper conductor has much lower loss than nickel at the same frequency. Moreover, surface roughness matters: a rough or plated surface forces the current to take a longer path, further increasing loss. High-frequency signals are susceptible to the finish of the conductor surface – a good reason why RF cables use smooth, high-conductivity surfaces (often silver plating) for the inner conductor.

In practical cable designs, skin effect is part of the signal integrity budget. Cable loss specifications must include skin-effect-induced attenuation for digital RF connectors or high-speed analog signals. Shielding and cable geometry also interact: for example, coaxial cables confine the RF current to the inner conductors and inner surface of the shield, where skin effect still applies. The skin effect is one reason many coax cables use copper-clad steel or silver-plated copper: the core provides strength and lower cost, while the high-conductivity surface carries the RF current.

Mitigation and Design Strategies

Engineers use several strategies to counteract the skin effect in RF cable design:

High-conductivity materials (silver plating): Since skin effect losses are proportional to surface resistance, a better conductor helps. For example, silver-plated copper or solid silver conductors are popular in RF cables because silver’s higher conductivity reduces skin-effect losses. (Silver also resists oxidation.) In practice, the outer coating is silver or gold while the bulk can be copper to balance cost.
Litz and stranded conductors: Although more common in power transformers, Litz wire (many finely insulated strands) is used in some RF applications (like inductors or winding) to force current more evenly into all strands. The same idea applies to multicore or braiding strategies in cables: using multiple small wires in parallel, each sees more of the RF current, reducing overall resistance. Wikipedia notes that specialized Litz wires can mitigate skin effect by making each strand thinner than the skin depth.
Tubular and hollow conductors: Why waste metal if the interior carries little current? Manufacturers sometimes make large conductors hollow (tubular) to save weight and cost without changing RF performance. Similarly, flat or foil conductors with corrugated surfaces can increase surface area for the same volume.
Optimized geometry and shielding: In coax and cable assemblies, design choices (conductor diameter, braid density, dielectric support) can minimize the impact of skin effect. For instance, a tightly braided copper shield provides more surface for return currents than a loosely braided one. Smooth plating and properly treated surfaces help.
Frequency-appropriate sizing: Procurement teams should specify cable dimensions so that conductor thickness is not excessive for the intended bandwidth. A very thick center conductor offers no benefit above a specific frequency, since only a few outer microns are used. Selecting an appropriate outer diameter and inner conductor size (often guided by impedance and power requirements) is key.

Key Takeaways and Material Considerations

Skin depth falls rapidly with frequency. At RF, skin depth is tiny (micrometers at GHz). Engineers often remember rough values (e.g., copper ≈8.5 mm @60 Hz, ≈0.21 mm @100 kHz, ≈0.066 mm @1 MHz, ≈0.0021 mm @1 GHz).
99% of the current is very near the surface. Because of the exponential decay, almost all high-frequency currents live in the conductor’s skin.
Conductor choice matters. Use high-conductivity metals (silver, copper) and consider permeability (avoid magnetic metals). Silver plating significantly lowers skin-effect loss compared to bare copper.
Surface finish is critical. Rough surfaces amplify skin-effect loss. High-end RF cables use polished or plated surfaces to minimize this extra resistance.

Accounting for these factors can preserve signal integrity. Cable and connector makers provide data on attenuation vs. frequency that inherently includes skin-effect losses. In practice, designers and buyers ensure cable attenuation is acceptable for the intended frequency range.

Choosing and Specifying RF Cables

When selecting a high-frequency cable or coax assembly, engineers should explicitly consider skin effect in the specs. Key questions include: What conductor material and plating are used? Is the braid density sufficient? How smooth is the inner conductor surface? Does the manufacturer report loss at the highest relevant frequency? Buying decisions should balance cost (silver plating and tight braids cost more) against performance.

Procurement professionals will find that reputable cable manufacturers address the skin effects through construction. For example, many RF cable suppliers list “silver-plated oxygen-free copper” conductors and high-coverage shields in their datasheets. Cable makers also design connectors and terminations to account for skin effect (often using silver on contact surfaces, etc.).

Understanding skin effect is essential for RF cable design and signal integrity. By choosing the right materials and geometry, and working with experienced manufacturers, engineers ensure that high-frequency signals pass through cables with minimal additional loss. Manufacturers like Romtronic specialize in custom cable solutions and can tailor conductor materials and cable construction to minimize skin-effect losses. Consulting with such cable experts can help you get a cable assembly that maintains signal integrity even at the highest operating frequencies.

Sam Wu

Sam Wu is the Marketing Manager at Romtronic, holding a degree in Mechatronics. With 12 years of experience in sales within the electronic wiring harness industry, he manages marketing efforts across Europe. An expert in cable assembly, wiring harnesses, and advanced connectivity solutions, Sam simplifies complex technologies, offering clear, actionable advice to help you confidently navigate your electrical projects.