Q: I’ve been working in the industry for quite a while now, and recently, I’ve been repairing a lot of cable.
One thing I’ve never really had a grip on is that there seems to be lots of different ways to shield a cable.
Is any one way better than another? Does it even matter, or is it just the choice of the manufacturer.
A: A very interesting question!
Each cable is always shielded in a way that provides the maximum amount of RF-shielding possible given the circumstances in which it will be operating.
What does that mean? Well, some shields are super-flexible and good for in the field, while others not so, and better for installation.
The first step to answering this question is understanding the different types of shielding. Let’s get started!
Braided Shield: A cable shield that is applied by braiding bunches of copper strands called picks around the insulated, electrostatically shielded center conductor. The braided shield offers a number of advantages.
Its coverage can be varied from less than 50% to nearly 97% by changing the angle, the number of picks and the rate at which they are applied. It is very consistent in its coverage, and remains so as the cable is flexed and bent, unlike Serve Shields.
This can be crucial in shielding the signal from RFI, where there are very short wavelengths that can enter very small “holes” in the shield. This RF-shielding superiority is further enhanced by very low inductance, causing the braid to present a low transfer impedance to high frequencies.
This is very important when the shield is supposed to be conducting interference harmlessly to ground.
Drawbacks of the braid shield include restricted flexibility, high manufacturing costs because of the relatively slow speed at which the shield-braiding machinery works, and the laborious “picking and pigtailing” operations required to solder them to connectors or circuit parts.
Serve Shield: Also known as a Spiral-Wrapped shield, is applied by wrapping a flat layer of copper strands around the center in a single direction (either clockwise or counter-clockwise).
The serve shield is very flexible, providing very little restriction to the “bendability” of the cable. Although its tensile strength is much less than that of braided shields, the serve’s superior flexibility often makes it more reliable in “real-world” instrument applications.
Tightly braided shields can be literally shredded by being kinked and pulled, as often happens in performance situations, while a spiral-wrapped serve shield will simply stretch without breaking down. Of course, such treatment opens up gaps in the shield that can allow interference to enter.
The inductance of the serve shield is also a liability when RFI is a problem; because it literally is a coil of wire, it has a transfer impedance that rises with frequency and is not as effective in shunting interference to ground as a braid.
The serve shield is most effective at frequencies below 100 kHz. From a cost viewpoint, the serve requires less copper, is much faster and hence cheaper to manufacture, and is quicker and easier to terminate than a braided shield. It also allows a smaller overall cable diameter, as it is only composed of a single layer of very small (typically 36 AWG) strands. These characteristics make copper serve a very common choice for audio cables.
Foil Shield: A type of cable shield composed of a thin layer of mylar-backed aluminum foil in contact with a copper drain wire used to terminate it.
The foil shield/drain wire combination is very cheap, but it severely limits flexibility and indeed breaks down under repeated flexing.
It’s the only commonly available shield type that offers true 100% coverage, however, this advantage is somewhat compromised by its high transfer impedance (aluminum being a poorer conductor of electricity than copper), especially at low frequencies.
It does allow for very small cables and is often a preferred shield in permanent installations where space is much more of a concern than durability.
Conclusion: Now you have an understanding as to what specific uses each type of shield is intended, so which one is “better?”
The answer, as you’ve no doubt realized, is that it depends completely upon the needs of your specific project. Your best bet is to evaluate the needs of your project, and make a decision from there.
As always, we welcome input from the PSW community and would love to know your thoughts on shielding. Feel free to let us know in the comments below.
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