Foil ribbon cables & skin effect

I didn’t really manage to find proper info on this via Google.

How does a cable constructed from ribbons of metal foil (maximizing conductive surface area) affect the skin effect? Let’s say the foil is thinner than the skin depth of 100kHz in the material.

Also, how does such a conductor geometry handle mains transfer given lots of surface area? Let’s limit the number of parallel ribbons per conductor to 100 and the ribbon width to 10cm.
(I’m assuming it’s sensible to stack foil ribbon like this with dielectric gaps, if the hot and neutral sections are separated to avoid an overly capacitive cable, might be wrong…)

Please explain to me if radial gauge can be replaced with efficiently packed minimal-thickness foil ribbon in high-power applications.
If not, is there still sense to use foil in interconnects?

Why would you want to IMPROVE the RF properties of a conductor for 60Hz AC use? At 50/60 Hz the signal is diffusion coupled throught the wire so it’s shape isn’t as important. at RF, yes you want more surface are and why a RG11 or 6 has a LARGER signal wire than an RG59, to lower attenuation at RF with more surface area. The innards of the wire don’t count at true RF frequencies.

Another issue is that thin, especially aluminum, foil is far to easy to work hardened and to be SAFE in a power cable. When foils ARE used, it is an overall FOIL layer to shield RF only and does NOT carry any AC current. The foil tape is grounded to the GREEN safety ground and doesn’t carry high current, ever. Even in a fault, the current follows the heavier green ground wire path as it is low impedance for 50/60 Hz.

The thin film aluminum foil tape, like you correctly noted, acts like a low impedance to ground that the RF energy follows. That’s how you want to use thin foil tape. You don’t want AC power on a thin tape.

The EDGE corners of square shaped conductors also concentrates the EM field and makes dielectric stresses worse, too. Look at the magnetic field models of this shape…not so good for high power AC use, especially as frequencies go up with high power applied.

For the AC power conductors themselves, you want a HIGH impedance as far as RF “sees it” and this means making it’s job HARDER to travel in the conductor, not easier. LESS surface area is the objective and MORE CMA area for the AC current. And, REMOVE dielectric stresses so you don’t fry and ionize the insuation and get a dielectric fault from what is called “treeing” stresses. Those sharp edges will aggravate treeing big time.

The above means SMOOTH and uniform wire shapes with LESS surface area to mitigte RF conductance. A low pass filter in other words. AC goes down the cable, RF can’t so much is ideal.

Best,
Galen Gareis

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Thanks, very clear and comprehensive answer.
I always miss something really obvious… An RF-sponge as an AC cord, what a great idea, hah.

But are there any applications where foil (in a spiral pattern maybe? Some geometry to avoid the mentioned issues…) would work as a maximal area signal conductor (not shield) and not be superfluous? I guess not in unbalanced interconnect cables unless they have good additional RF protection, like AudioQuest does with their fairly thick copper outer shields?
Digital…?

Even if foil’s only proper place as of now is in shielding, I’m still interested in how skin effect exactly behaves in minimal thickness conductors.

Omega Mikro uses ultra-thin strips of copper - 300 micro inches - as conductors along with an active shield (battery powered) to reject RF.

https://www.omegamikro.com/planar-analog-ebony--lcx-.html

Skin effect will follow the self inductance inside the wire, and that effect is what pushes the signal to the surface of the wire. The signal just doesn’t “go there” it is FORCED to go there as the wire center gets higher and higher impedance, or resistance, as frequency goes up. This is called wire self inductance.

A broad flat “wire” will see the same skin depth as a round wire at the same frequency EXCEPT at the edges where eddy currents cause some confusion.

True RF does not need “thick” copper shields as the thickness is unused. Spiral shields of any material or thickness makes an inductor. Bad. This is why good spiral shields to improve flex also have SHORTING folds built-in to make the shield look like a metal TUBE.

Heavy copper tape shields are used where inductively coupled energy can get onto the shield. Think cell phone tower cable and lightening as an example. Typical RF doesn’t leverage the thick copper tapes real purpose.

Proper shields have no current if they are grounded right. A shield needs to look like zero ohms resistance to the frequency you want to shield. This is called transfer impedance. We don’t want RF on our SIGNAL wire so we make that a HIGH impedance at the frequency we want to block. Where we WANT the RF to go we make it LOW impedance to say to the RF, “Hey, OVER HERE!”.

Once we get the RF onto the shield, it tries to go to ground. E=I*R so we want zero ohms times the RF to ground current to generate a small inductive coupled voltage. Both ends of the shiled need to be near the same potential to STOP current flow in the shield. If each end is the same zero resistance and with zero potential DCR difference we can’t generate a voltage as we have no resistance to drop the voltage across.

If you have terrible grounds lifting one end, called a SPG - single point ground, wall outlet ground end is best will create a current by default but MIGHT offer less noise than a true shield. This is an ANTENNA and yes, they MOVE a CURRENT from A to B by design. There is no way to get around that. It is a stop gap “easy” way to initially mitigate noise.

NEVER use a signle point ground shield unless your ground buss is BROKEN (high DCR between end points). A SPG MAKES a current to ground and inductively couples energy to the signal wires underneath by design. It just MAY be less noise than a crappy ground system in specific situations. Better is to fix your ground bus. There are strict rules on ground resistance such that shields work right.

Ethernet networks are strictly tested for RF shield performance both ingress and egress. Many feel shields are a problem when the problem is poor ground design as so many use UTP cabling that allows you to build poor ground differental networks.

Once you see how all this works, you can see the trade-offs designers have to make.

best,
Galen Gareis

Alright now this will take some time to digest all in all. But I understand with some surplus common sense even though my skills at progressing through an electronics textbook are horrendous with the pacing and getting stuck at the simplest examples.
I guess that’s why I’m asking questions on a forum for a start, my question itself might be off but comprehensive conceptually explained answers are what I’m after and this forum is a goldmine for those. I’m grateful.

“A broad flat “wire” will see the same skin depth as a round wire at the same frequency EXCEPT at the edges where eddy currents cause some confusion.”

This is something I don’t quite understand, how come essentially just a longitudinal slice of a round conductor exhibits the same self inductance characteristics? How does this allow a cable design like the mentioned Omega Mikro to work so well as intended?

The internal self inductance determines the skin depth, same as a round wire. If you go high enough in frequency, only the SURFACE of the tape or thin plate will move the signal, not the inner area.

At DC the ENTIRE wire cross section is used, shape be damned.

The hard part is when we are BETWEEN the DC and true RF frequencies. This transition zone can get real messy.

Go here; http://fermi.la.asu.edu/w9cf/articles/equiv/index.html

And here;
https://www.ntmdt-si.com/resources/spm-theory/theoretical-background-of-spm/2-scanning-force-microscopy-(sfm)/27-magnetic-force-microscopy-quantitative-results-treatment/279-magnetic-field-of-rectangular-conductor-with-current

The “complex” approximations are here but it still is based on the RF skin depth. And, the complexity of the magnetic fields. We have both “E” electric fields and "B"or magnetic fields. Neither can exist without the other but the influence switches with the frequency, “B” at the low frequencies and shifting to the “E” at the high frequencies.

The way signals travel in the wire requires intimate ability to figure out how the “ends” or points of concentration distort the fields…and as you can see, it is a tough equation to solve.

In the world of solid object, finite element analysis helps determine how stresses are distributed in a solid. Magnetic flux lines are sort of like this idea in an electric field moving in a wire. The EM magnetic fields determine the wire inductance. Changing those fields also changes the capacitance but always in the opposite direction.

Flat or rectangular stacked conductors are difficult to keep capactance in check (too close a wire surface spacing), but can lower inductance (wires need to be close as you can get them). There is no free lunch. A proper balance is necessary based on the intent of the frequencies in the cable. Extreme designs on ONE reactive variable, L or C, are seldom the best solution. The uncontrolled variable will always be there to cause you design pain.

Note: this is rectangular wires, not HOLLOW rectangular RF waveguides, these aren’t the same situation. Waveguides trap the RF sinusoidal signal inside the “waveguide” inner surface. The signal is in “air” between two metal plates. This is an ideal situation for Xmission loss mitigation at very specific frequencies as there is zero plastic dielectric.

Best,
Galen Gareis

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“essentially just a longitudinal slice of a round conductor…”

This half round conductor is an EM wave nighmare. Again, the self inductance is the “B” field flux line concentration. No, this wire will NOT act like round or rectangular wire at all. It is it’s own complex animal.

Best,
Galen Gareis