Measuring cable shield efficacy

[E-Stat]

While recently looking at the data sheet for LMR 400 Coax (http://www.dxstore.com/download/lmr400UF.pdf), I think I may have found an answer to a question I've been asking for many years concerning how well a shield works in a cable. The data sheet says the shielding effectiveness is > 90 db. Does anyone know if that represents a standard measurement suite? Or is that just unique to that cable vendor? You just don't find many cables that attempt to quantify how well their shielding works.

rw

[Kevio]

There is BS going on here.

First thing that jumps out at me is the claim that 90 dB + 90 dB = 180 dB (The correct result is somewhere between 93 and 96 dB).

Second thing is that you can't give a useful spec for shield effectiveness without qualifying with length and frequency. Shields are useful at RF frequencies and beyond. They do nothing at audio frequencies. Regardless or how the shield is built, a long cable picks up proportionally more noise than a short cable of same construction.

To try and dodge your question of many years: Unless you live next to a radio transmitter, shielding effectiveness is a fairly small piece of the equation for low-noise interconnect. You see how logarithmic (decibel) math works - one weak link takes the whole thing down. Other typically more significant contributers are unbalanced connections, inadequate RF filtering and improper termination or grounding.

[E-Stat]

First thing that jumps out at me is the claim that 90 dB + 90 dB = 180 dB (The correct result is somewhere between 93 and 96 dB).
Yep. I was looking at the spec area where it just says > 90 db.


Second thing is that you can't give a useful spec for shield effectiveness without qualifying with length and frequency.
Which is exactly why I phrased the question the way I did.


Shields are useful at RF frequencies and beyond. They do nothing at audio frequencies.
Strictly speaking, I would agree. By definition radio frequencies are outside the realm of audio signals. Amplifiers, on the other hand, unfortunately do react to RFI in undesirable ways that manifest themselves in the audible band. An extreme case is an inexpensive Bellari phono preamp in the garage system that goes nuts if my cell phone is even close to it.


To try and dodge your question of many years: Unless you live next to a radio transmitter, shielding effectiveness is a fairly small piece of the equation for low-noise interconnect.
Inside my house, I have two 2.4 Ghz transmitters (80211.G access points) and when the wife uses the newest cordless, there is a 1.9 Ghz transmitter in use not to mention constantly being in range of the local cable company's broadcast Wi-Fi service. CDPs are themselves RFI generators in fairly close proximity to amplification stages. Indeed RFI is a fairly small piece, but it would be nice to know which products work more effectively.

Thanks for the reply.

rw

[Kevio]

By definition radio frequencies are outside the realm of audio signals. Amplifiers, on the other hand, unfortunately do react to RFI in undesirable ways that manifest themselves in the audible band. An extreme case is an inexpensive Bellari phono preamp in the garage system that goes nuts if my cell phone is even close to it.Exactly right. This behavior is most commonly due to improper termination of the shield connection to the internal ground rather than the external chassis. The problem is fairly common in audio equipment both professional and consumer. There's now heightened awareness of it because most of us are carrying around a 5 W cell phone. It is being addressed by manufacturers. A better cable will do nothing to improve this. You need to get into the equipment and terminate the shield properly.

[Smokey]

The data sheet says the shielding effectiveness is > 90 db. Does anyone know if that represents a standard measurement suite? Or is that just unique to that cable vendor?

As Kevio said, there is no mention of frequency, so Shield Effectiveness (ratio of received signal with shield and without the shield) might vary from one vender to next. But as a rule of thumb, as the shielding density increases there is a correlated increase in the shielding effectiveness value.

So a single braid shield coax will have SE of -55 dB, single braid + single foil coax will have SE of -90 dB and double braid + double foil coax will have SE of -110 dB.


Shields are useful at RF frequencies and beyond. They do nothing at audio frequencies.

Considering that most interference in a Home Theaters comes in range of audible frequency, that might be an over statement. I agree that standard shielding might not be too useful at lower frequency (the shield have to be pretty thick to be effective), but it does attenuate low frequency interference such as hum from close-by power cords.

Double Braid coax cable seem to be more effective at lower frequency protection than combination braid/foil shield coax cables.

[Kevio]

Another thing worth mentioning, I think, is that shields are effective at blocking electric fields. They don't do much against magnetic fields. The stuff that radiates from power cables is primarily a magnetic field (because there's current flowing in the power cable and the voltage is relatively low). To isolate from magnetic fields, you either need physical distance or a balanced line interconnect.

[Smokey]

I agree that physical distance or a balanced line interconnect is the best solution when having EMI (Electro-Magnetic Interference) problems--such as nearby PC cords, but the coax shield is also useful against such a interference. As long as the shield have low resistance and a good connection to ground, it can route most of EMI energy to ground as it hit the coax's shield.

[Kevio]

EM theory does tell us that at lower frequencies (i.e. 50-60 Hz AC line), low impedance and good ground connections are helpful but at these frequencies theory also says it doesn't matter whether the conductors are side-by-side (i.e. lamp cord) or one inside the other (i.e. shielded).

[Smokey]

EM theory does tell us that at lower frequencies (i.e. 50-60 Hz AC line), low impedance and good ground connections are helpful but at these frequencies theory also says it doesn't matter whether the conductors are side-by-side (i.e. lamp cord) or one inside the other (i.e. shielded).

Do you have any links to explain this further.

I don’t know if a Tube TV is considered a huge source of EMI or not, but had this nasty hum in my system and after many trail and error found out that it was that TV (had couple of IC cables running close to the back of TV).

And to cure that hum, had to upgrade those two run-of-the-mill IC cables to much heavier shielded coax. And the noise was greatly attenuated.

[E-Stat]

EM theory does tell us ...
Theory is always a great place to start, but not necessarily where to stop.

<a href="http://www.bluejeanscable.com/articles/humrejection.htm">EMI shielding</a href>

rw