Reflectivity of modern day's wideband radio pollution in cabling&interfaces

As I realize when high frequency RF is imposed and runs on a conductor surface it will in most cases encounter significant obstacle in straight transmission in many systems. If the conductor is terminated in a way that presents a high enough impedance to the RF transmission, it starts its bouncing dance between impedance pathways until it finds itself, well, somewhere.
I am talking of all possible signal conducting material found in any audio system. If meticulous care throughout a system was paid to allowing for the smoothest surface transmission for RF everywhere, it would exit cleanly, instead of bouncing around needlessly.
Preventing RF from ever nearly even entering the signal path is of course a goal and one that requires some meticulous shielding and grounding solutions. But not rocket science, costly maybe. Not even necessarily. Can be improved with a little DIYing with reasonable cost of shielding conduits and wire, etc, if you really know how to apply it. (I hope I know how to apply it someday! Or… afford it for that matter!)

I feel that in this type of analysis transmission line theory should be applied to any and all conductors. If we are so interested in removal of RF noise from our systems, shouldn’t we formulate a scaled model of transmission line theory principles, so adjusted to scale for these more “macroscopic” instances. And apply it to better understand RF behavior in audio systems as a whole.
AQ has made some claims of applying transmission line principles to speaker cabling and frankly some of those claimed accomplishments don’t seem scientifically valid.

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I’d wait to hear what @rower30 had to say about this, intriguing as it seems to be.

We are defining we have a problem with no proof it exists or hasn’t been addressed, first of all. Let’s get that out of the way.

Second, if we did have a problem what solves it? Our cases act like faraday cages by design. We also don’t want to use magnetic cases (Faraday cases are usually copper mesh such the they shield the proper wavelengths).

Faraday cages shield their contents from electromagnetic radiation. Basically, when an electromagnetic field hits something that can conduct electricity, the charges remain on the outside of the conductor rather than traveling inside.

When a metallic (stuff a magnet will stick to) material is charged and discharged it creates noise. The name of the phenomenom evades me 40 years since physics class. Remembered it - Barkhausen basic definition is the effect given to the noise in the magnetic output of a ferromagnet when the magnetizing force applied to it is changed. To remove this noise, we use non magnetic stuff that is STILL a low permeability to RF like aluminum. RF is easily attenuated through the thickness of the shield (case). Look at a case and stuff inside like a coaxial cable. Our circuits are on the inside, the case is the braided shield on the outside.

The ends of our cable braid (case) shield needs to be at the same potential as earth. This means the RF ground produces zero voltage (voltage = I * R). If we keep R at zero, we have no inductively coupled noise. That’s impossible since the shield has a resistance due to length.

We can’t adhere to the absolute ideal, so we measure shields ground differential using transfer impedance and SEED (Shield Effectiveness Evaluation Device) tests. A shield is measured for it’s isolation to ingress or egress in dB. That’s our induced voltage trying to get through, and how much it is attenuated. Numbers are typically 80 dB (single braid) to 110 dB (quad shields). Curiously, a shield also shields the same amount to stuff radiating OUT of the signal wire! Yes, we need to keep that to ourselves, too. Go back and look at the real basic definition of a Faraday cage. Our shields all looks like one and act like one from the basic principal of RF.

Alternatively, we can indirectly measure the induced voltage by the resitance at specific frequencies (shield transfer impedance is not linear to frequency, see the graphs in the .PDF) using transfer impedance in mill-ohm/mtr. The lower the better as this means a smaller induced voltage can be inductively coupling to the inner wire (circuit board). Notice the shield works better at higher frequencies as the B-field drops away to just an E-field.

One method measure the attenuation factor based on the shields properties, the other calculates a number that can allow a ratio to be used to differentiate shields across frequency.

RF is pretty easy to mitigate in proper designs. But you have to know the technicalities to do it. Guesses in the RF region are seldom accurate or right. RF is weird stuff, but it isn’t impossible to understand it even if it’s properties are aggravating.

Last, RF doesn’t have Return Loss on a Faraday cage or conductive case. RL is caused by VERY, VERY periodic ground changes caused by distance (diameter changes on a coaxial cable dielectric). That alters the Impedance because the capacitance is changing and this differs the cable’s impedance properties to the fixed resistive load. The variation in impedance causes the reflections.The Faraday cage or component case isn’t a transmission line, it is an end point ground. The RF stays on the outside and goes to ground. The higher the RF, the less it penetrates the shield. We’ll assume we are talking true electric E- fields and not magnetic B-fields, which are not the same problem at all.

Go here -



Yes, so basically the feat mainly deals with isolating noise from cabling outside the cases. For cases wouldn’t it work to run an “RF-dedicated” wide and smooth foil conductor to ground from the case instead of relying on the ground wire inside the power cable, which is what it is. A smooth highly conducting foil surface is what RF would love most to travel through?
To make for the cleanest and shortest exit for the imposed RF, to reduce the time it lingers and reflects on circuits?

And of course for casing it’s notable to address how conductors and circuit components are internally shielded from themselves.
Will twisting really do for adequate rejection since that’s seen in even best of hand wired tube amps, rarer to see shields utilized for internal conductors in such amps. Or most amps.

RF doesn’t “linger”. It is attenuated by the shield, and that’s that. The ratio of the shield’s attenuation is set, the RF is attenuated 80-110 dB by the shield. The low amplitude remainder is inductively coupled, as small as it may be. The longer the parallel distance, yes, the more it can couple. But this isn’t “lingering” unless parallel coupling is your definition of lingering.

RF is pretty weak, too. If there is any reflections n ot classic return loss at all, it is rapidly attenuated out. Short circuits actually WANT to design-in attenuation to remove reflections. Longer digital cables work better than shorter ones for that reason. Ethernet patch cable already uses 28 AWG wire versus 24 AWG to help attenuate out the high levels of RL at the near ends where it is worst.

The shield’s construstion and frequency are tied together. If you ONLY want to shield 100 MHz and up RF signal sure, a 1/3-mil foil works. Not so much below that where the B-field component starts to be an issue. Or, if you have electrostatic discharge that needs a low resistance shield. A one-third mil foil is not low resistance to ESD. You need a braid too.

Most circuits are designed to ignore RF, as they BW limit the circuits input frequency such that RF sees a high impedance path through the analog portion, and shunts RF to ground by a far lower impedance path designed into that circuit block.

This does not meant that amplifier analog circuits can’t be high BW designs with super fast slew rates and all the benefits. It does means the input is designed to the highest signal frequency the designer adopts. The benefits to high BW designs remains through analog audio. We remove the signals that would upset the amp’s ability to manage 10-20KHz. The old days of zero RF mitigation are gone.

Still, you haven’t shown that we have an RF problem with current measurements. RF is not ignored. The FCC requires a maximum amount of EGRESS out of our stuff (ingress is the same ratio) so there is a minimum level of shielding already to be UL FCC approved.

That’s just the minimum. Equipment designers do far, far more than you give them credit for to get the 110 dB S/N ratio we commonly see in good stuff. Noise is noise and will impact those measurements.